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Icarus 169 (2004) 140–174www.elsevier.com/locate/icaru

Lava lakes on Io: observations of Io’s volcanic activity from GalileoNIMS during the 2001 fly-bys

Rosaly M.C. Lopes,a,∗ Lucas W. Kamp,a William D. Smythe,a Peter Mouginis-Mark,b

Jeff Kargel,c Jani Radebaugh,d Elizabeth P. Turtle,d Jason Perry,d David A. Williams,e

R.W. Carlson,a S. Douté,f and

the Galileo NIMS and SSI Teams

a Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USAb Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, HI 96822, USA

c U.S. Geological Survey, Flagstaff, AZ 86001, USAd Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA

e Department of Geological Sciences, Arizona State University, Box 871404, Tempe, AZ 85287, USAf Laboratoire de Planetologie de Grenoble CNRS, Batiment D de Physique, B.P. 53, 38041 Grenoble cedex 9, France

Received 2 May 2003; revised 27 October 2003

Abstract

Galileo’s Near-Infrared Mapping Spectrometer (NIMS) obtained itsfinal observations of Io during thespacecraft’s fly-bys in August (I31and October 2001 (I32). We present a summary of the observations and results from these last two fly-bys, focusing on the distthermal emission from Io’s many volcanic regions that give insights into the eruption styles of individual hot spots. We include a comof hot spot data obtained from Galileo, Voyager, and ground-based observations. At least 152 active volcanic centers are now known on I104 of which were discovered or confirmed by Galileo observations, including 23 from the I31 and I32 Io fly-by observations presenWe modify the classification scheme of Keszthelyi et al. (2001, J. Geophys. Res. 106 (E12) 33 025–33 052) of Io eruption stylesthree primary types: promethean (lava flow fields emplaced as compound pahoehoe flows with small plumes< 200 km high originating fromflow fronts), pillanian (violent eruptions generally accompanied by large outbursts,> 200 km high plumes and rapidly-emplaced flow fieldand a new style we call “lokian” that includes all eruptions confined within paterae with or without associated plume eruptions).maps of active paterae from NIMS data reveal hot edges that are characteristic of lava lakes. Comparisons with terrestrial analogIo’s lava lakes have thermal properties consistent with relatively inactive lava lakes. The majority of activity on Io, based on localongevity of hot spots, appears to be of this third type. This finding has implications for how Io is being resurfaced as our resuthat eruptions of lava are predominantly confined within paterae, thus making it unlikely that resurfacing is done primarily by extenflows. Our conclusion is consistent with the findings of Geissler et al. (2004, Icarus, this issue) that plume eruptions and deposits,the eruption of copious amounts of effusive lavas, are responsible for Io’s high resurfacing rates. The origin and longevity of islanionian lava lakes remains enigmatic. 2003 Elsevier Inc. All rights reserved.

Keywords: Io; Volcanism; Satellites of Jupiter

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1. Introduction

A major objective of the Galileo Mission to Jupiter wto study volcanism on Io. During its 7-year mission, Galiobtained a wealth of observations of Io’s surface using

* Corresponding author.E-mail address: [email protected] (R.M.C. Lopes).

0019-1035/$ – see front matter 2003 Elsevier Inc. All rights reserved.doi:10.1016/j.icarus.2003.11.013

prime remote sensing instruments, the Solid State ImaSystem (SSI, e.g., McEwen et al., 1998a, 2000; Keszthet al., 2001; Turtle et al., 2001, 2004, this issue), the NInfrared Mapping Spectrometer (NIMS, e.g., Carlson et1997; Lopes-Gautier et al., 1999, 2000; Lopes et al., 20and the Photopolarimeter Radiometer (PPR, e.g., Spencal., 2000; Rathbun et al., 2004, this issue). McEwen e(2003) and Kargel et al. (2003a) have reviewed the mresults from Galileo pertaining to Io’s surface expressi

http://www.elsevier.com/locate/icarus

Lava lakes on Io 141

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of volcanism. We summarize the observations obtainedNIMS during the last two successful fly-bys of Io, which ocurred during Galileo orbits I31 (August 2001) and I32 (Otober 2001). We present an up-to-date compilation ofactive volcanic regions as known at the end of the Galmission, and focus our analysis on the thermal mappinkey volcanic features on Io imaged by NIMS. We compthe thermal emission detected at particular hot spots duthe I31 and I32 fly-bys with earlier data to investigate temporal variability. The combination of high spatial resolutiNIMS data (revealing the detailed thermal structure ofspots and temporal variability) and the images obtainedSSI (morphological and color information) forms a powerdata set for studying the styles of volcanic activity at dferent hot spots and for drawing comparisons with volcafeatures on Earth.

1.1. NIMS Io observations

Observations of Io by Galileo fall into two categoriefly-by observations (during which NIMS spatial resolutiranged from� 1 km (NIMS pixel)−1 to about 100 km (NIMSpixel)−1) and distant observations (during which most NIMobservations had spatial resolutions of 200–600 km (NIpixel)−1). The NIMS results from the first three Io flyby(orbits I24, I25, and I27 in, respectively, October 1999, Nvember 1999, and February 2000) were discussed by LoGautier et al. (2000), Lopes et al. (2001), and Douté e(2002).

Orbits I31 and I32 were Galileo’s last two successfulfly-bys for NIMS and SSI. The NIMS observations duriI31 and I32 were planned to target specific hot spots, mof which had not been seen before at high spatial resolutRegional-scale observations, which had proven particulvaluable in the first three fly-bys (e.g., Lopes-Gautier et2000), were also planned. Our observations fell into twoegories:

(i) Regional scale observations, ranging in spatial restion from 22 to 93 km (NIMS pixel)−1, were designedto study the global distribution of hot spots and SO2 onIo. Overlap of NIMS fields of view was minimized sthat large areas could be covered, unlike the prevobservations of this kind (I24 through I27) that coverrelatively small regions around Bosphorus Regio (Loet al., 2001). During I31 and I32, an important objetive was to obtain data at high latitudes to search forspots that might have goneundetected because of threlatively low spatial resolution of observations priorthese fly-bys.

(ii) Observations of key regions, ranging in spatial resotion from 1.5 to 30 km (NIMS pixel)−1, were designedto target diverse types of terrains, mostly hot spotshad exhibited varied volcanic styles in previous obsvations. Nearly every observation was done in collaration with SSI measurements, so that both instrum

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.

observed the same target almost simultaneously.SSI observations are described by Turtle et al. (20this issue). NIMS data were navigated onto SSI imagewhenever possible, so that associations could be mbetween features and colors seen in the visible imageand the thermal maps obtained from NIMS data.

This paper discusses results from the analysis of theemission detected by NIMS. Comparisons are also mwith certain terrestrial lava lakes, most notably the Kuianaha lava lake of Kilauea volcano, Hawaii. Results frthe spectroscopic analysis of the distribution of sulfur diide on Io’s surface from the I31 and I32 observationspresented by Douté et al. (2002).

2. The NIMS instrument

The NIMS instrument has been described in detailCarlson et al. (1992) and Smythe et al. (1995). NIMScluded a spectrometer with a scanning grating and spathe wavelength range 0.7–5.2 µm, therefore measuringreflected sunlight and thermal emission. The instrumformed spectra using 17 detectors in combination withmoving grating. The 17 wavelengths (spaced across twavelength range) obtained for each grating position wacquired simultaneously. During the Galileo nominal msion and the Galileo Europa Mission (GEM), two of tNIMS detectors stopped working and the sensitivity of tothers (at the shortest wavelength end) was considerabduced. Prior to the first NIMS observation in I24, gratimotion ceased, probably from radiation damage to the eletronics. The grating stopped at the short wavelength enthe scan.

Therefore, the observations during all Io fly-bys obtain13 fixed infrared (IR) wavelengths (in the range from 1.04.7 µm) instead of the planned 360 (see Lopes et al., 20This anomalous operation resulted in some advantagethermal mapping, as it provided greater sampling freque(24 samples instead of 1) at each wavelength. This increthe signal/noise ratio substantially and mitigated the probof radiation spikes in the data, thus increasing the reliabof results. The reduced number of wavelengths is also wsuited for band ratio mapping (SO2 and the absorption banin the 1 µm region, see Lopes et al., 2001, and Douté e2004).

The I31 and I32 NIMS observations are listed in Tble 1. NIMS obtained 11 regular observations during Iand 10 during I32. Additional observations, at reduced stial sampling, were obtained while SSI acquired data. Thare listed in Table 1 as “ride-along observations.” Obsetions were obtained both during day-time and night-timThe gain state (GS) was set at nominal (GS 2) for day-tobservations, and twice the nominal gain (GS 3) for obvations near the terminator. During operation prior to oI27 (February 2000), night-time observations were obtai

142 R.M.C. Lopes et al. / Icarus 169 (2004) 140–174

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Table 1NIMS observations in I31 and I32

Observation Target Resolution Detections, commentsname (km (NIMS pixel)−1)

NIMS observations in I3131INTHRMAL01 Pele 2–4 Night-time observation of Pele (Fig. 9). Thermal emission detected from Pe

considerable saturation in observation31INHSISUM01 Isum 1–2 Night-time observation of Isum hot spot. Thermal emission detected fr

Isum, considerable saturation in observation31INSO2MAP01 North polar region 0.5 A small area near closest approach was targeted. No thermal emission

tected, SO2 shows some variation within targeted area31INTVASHT03a Tvashtar 2.5 Ride-along observation with SSI31INTVASHT01 Tvashtar 3–5 Day-time observation of Tvashtar. Several hot spots detected (Fig. 6), S2

variations mapped (Douté et al., 2004, this issue)31INAMRANI02a Amirani 7 Ride-along observation with SSI31INGISHBR01 Gish Bar 7–11 Day-time observation of Gish Bar. Two hot spots detected (Fig. 12). SO2 vari-

ations mapped (Douté et al., 2004, this issue)31INAMRANI01 Amirani and Maui region 15–19 Day-time observation of Amirani flow, Maui and Dusurra hot spots. Therm

emission detected from numerous regions (Fig. 7). SO2 variations mapped(Douté et al., 2004, this issue)

31INREGION01 Pole to pole observation 22–46 Day-timeobservation covering all latitudes, longitude range∼ 80◦ W–172◦ W. Numerous hot spots detected (Fig. 1)

31INREGION02 Intended to be global ob-servation, only partly suc-cessful

72 Day-time observation of part of northern hemisphere. Isum is shown inness, near terminator

NIMS observations in I3232INTHPELE01 Pele 12 Night-time observation of Pele, 3 hot areas detected. Observation sho

siderable saturation32INTHLOKI01 Loki 3.5–5.5 Night-time observation of southern part of Loki Patera. Thermal emission de

tected from low-albedo areas (Fig. 8)32ISPELE_01a Pele 3 Ride-along observation with SSI32INTHPELE02 Pele 1.5–3 Night-time observation of Pele, 3 hot areas detected (Fig. 9). Obse

shows considerable saturation32INTHERML01 South polar region 1–2 Small mosaic targeting area near closest approach. No thermal emission

tected, but 1 µm absorber and SO2 are present in the spectra32ISEMAKNG02a Emakong 3 Ride-along observation with SSI32INTOHIL_01 Tohil region 2.4 Ride-along observation with SSI. Low light (near terminator). Radegast h

spot detected (Fig. 11)32INEMAKNG01 Emakong 3–5.5 Day-time observation of Emakong Patera and surroundings mapped (

see also Douté et al., 2004, this issue)32INITUPAN01 Tupan 7–9 Day-time observation of Tupan and surroundings (Fig. 13, see also D

al., 2004, this issue)32INTVASHT01a Tvashtar 9 Ride-along observation with SSI32INICHAAC01 Chaac and Baldur 9.5–12 Day-time observation of Chaac/Baldur region. Hot spot detected at Cha

SO2-rich area inside Baldur detected in I27 (Lopes et al., 2001) not deteat this lower spatial resolution

32INHTSPOT01 I31A hot spot 13–16 Day-time observation of site of major eruption first detected in I31. Coable saturation in observation (Fig. 5)

32INTERMIN01a Terminator, various ∼ 18 Ride-along observation with SSI32INTERMIN02a Terminator, various ∼ 20 Ride-along observation with SSI32INREGION01 Pole-to-pole coverage 24–39 Day-timeobservation covering all latitudes, longitude range∼ 90◦ W–165◦

W. Numerous hot spots detected (Fig. 2)32INREGION02 Global coverage 83–93 Day-time observation covering all latitudes, longitude range∼ 120◦ W–260◦

W. Numerous hot spots detected (Fig. 3)

a Denotes observation was a “ride-along” withthe Solid State Imaging experiment.

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using 4 times the nominal gain (GS 4). However, very hspatial resolution observations of hot spots in I24 andshowed considerable saturation when GS 4 was usedcause the average temperature within a pixel at thesespatial resolutions exceeded the instrument’s dynamic raThe gain state was reduced to GS 2 during the Pele ntime observation in I27 (Lopes et al., 2001), but the d

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still showed saturation. For I31 and I32 observations of Pat very high spatial resolution (1–4 km (NIMS pixel)−1 onIo’s night-side), the NIMS gain state was further reduceGS 1 in an attempt to reduce saturation. Unfortunately, csiderable saturation was still present in these observatespecially at wavelengths in the middle of the NIMS ranmaking temperature determination particularly difficult. The


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high eruption temperatures detected by SSI and NIMS a(e.g., McEwen et al., 1998b) had not been known to existhe time the instrument was designed.

3. Thermal mapping

NIMS high spatial resolution data from the Io fly-ballow us to map the thermal structure within hot spots,ing observations obtained both during day and night. Ttype of mapping was done previously using NIMS data frorbits I24, I25, and I27 and the techniques, which are smarized here, were described in detail by Lopes et al. (20Thermal mapping that resolves individual hot spotsonly be done from fly-by observations, in which the NIMspatial resolution typically ranges from 1–10 km (NIMpixel)−1. At other times, Io was observed by Galileo fromuch greater distances, resulting in large sizes of pixelsNIMS observations (resolutions typically ranging from 12500 km (NIMS pixel)−1), which did not resolve individuahot spots, and which usually contained more than onespot in the field of view. Therefore, mapping that showsinternal structure of individual hot spots has only been dfor a small number of volcanic regions on Io and the resreported here constitute a major addition to the NIMSsults previously reported by Lopes-Gautier et al. (2000)Lopes et al. (2001).

Following the procedure outlined in Lopes et al. (200thermal mapping was done using different methods forday-time and night-time observations. The majority ofobservations during the fly-bys were obtained during dtime and, therefore, the albedo associated with individuahot spots had to be modeled before temperatures couextracted.

For day-time observations, a single temperature wastained for each pixel. Although a NIMS pixel showing vocanic activity most likely contains lavas at a range of dferent temperatures, in the presence of sunlight, it ispossible to use two-temperature fits to the Planck funcin order to derive the “hot component” because the signathe shorter wavelengths is dominated by reflected sunlTherefore, for the day-time observations obtained duringfly-bys, fitting more than one temperature is not feasiA single temperature can thus be regarded as a lowermate of the temperature within the pixel.

For night-time observations, we used two-temperafits to the Planck function and expect that the hot coponent will be a reasonable approximation of the holavas within the pixel, according to models and results obtained for some terrestrial lavas (Crisp and Baloga, 19Flynn and Mouginis-Mark, 1992; Rothery and Pieri, 199Flynn et al., 2000; Harris et al., 2000; Wright et al., 200However, it must be noted that the hotter component isa lower estimate of the liquidus temperature of the lava,cause

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(i) lava cools quickly once it reaches the surface,(ii) small areas with very high temperatures may repres

too small a fraction of a pixel to be detected by NIMand

(iii) the observing geometry of NIMS may preclude viewihotter material within cracks in the surface layer.

3.1. Color temperature

A single color temperature,Tc, represents the physictemperature of a black body that is assumed to modelcomponent of a hot spot. The defining equation is:

(1)B(λ,Tc) = I (λ)/f,

whereB is the Planck function,λ is the wavelength, andIis the corrected radiance, andf is the fractional pixel areaall for a given spatial pixel. We assume surface emissivitbe 1.

We use a corrected radiance defined as:

(2)I (λ) = Io(λ) − AF(λ),

whereIo(λ) is the observed radiance,F(λ) is incident so-lar flux at that wavelength, andA is the albedo (bidirectionareflectance). The albedo of ahot spot was estimated asfunction of the shorter wavelengths (shorter than 2.5 µmthe NIMS range where the radiance from thermal emissis negligible compared with the radiance from reflected slight. At wavelengths between 3 and 5 µm, the contributfrom reflected sunlight is no greater than 20% of the thmal signal, so slight errors in albedo estimates have seffect on the derivation of temperature. For night-timeservations,I = Io.

We apply Eq. (1) to a set of bands at a given spatial pand solve forTc using a least-squares method. Our alrithm minimizes the usual “chi-squared” quantity, i.e., tsum of the squares of the differences between computedobserved spectra divided by the estimated error. The fidone using the method of “simulated annealing” describy Press et al. (1986). This method performs a stochasearch of the solution space, initially allowing large radom jumps, but gradually narrowing the range (henceanalogy to an annealing metal); it is particularly well-suifor highly non-linear problems with many local minima. Fdaytime observations, this method was applied to each pfor which at least 4 wavelengths were observed in the ra2.5–5 µm (which contains up to 8 NIMS wavelengths).this range, thermal emission dominates reflected sunlightemperatures greater than about 300 K. For night-time obvations, the full wavelength range (down to 1 µm) was us

3.2. Power output at 4.7 µm

In addition to temperature, we also compute the poat 4.7 µm. This wavelength was chosen for two reasFirst, it is the longest wavelength available in the fly-by d


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set and it has the lowest signal from the solar flux in dtime observations. Second, it facilitates comparisonsground-based observations (e.g., Veeder et al., 1994).power (emitted into a hemisphere) was derived using:

(3)P (4.7 µm) = (πRA)/cos(E),

whereP (4.7 µm) = power at 4.7 µm (in units of W µm−1);R = observed radiance (W m−2 ster−1 µm−1), corrected forbackground radiation; andA = emitting area (m2) = S2.FOV, whereS is the slant distance and FOV is the fieldview of the NIMS pixel.E is the emission angle. Eqution (3) was summed over all pixels showing a givenspot after the background was subtracted. The backgris the median of the radiances from a large area arounhot spot (10–20 pixels depending on the observation).assume that the albedo at 4.7 µm is constant over the bground and the hot spots. If the hot spots are darker at 4.than their surroundings, then the power reported will beunderestimate.

4. NIMS observations during I31 and I32

NIMS observations of Io in orbit I31 were performedAugust 6, 2001 between 04:33:31 GMT and 07:58:47 Gcorresponding to, respectively, 00:25:16 hours before cest approach to 02:59:59 hours after closest approacorbit I32, NIMS started observing on October 16 at 00:21GMT (01:05:44 hours before closest approach) and end07:37:00 GMT (06:14:07 hours after Io closest approaThe range of the spacecraft from Io varied between 100174,000 km for the I31 observations and 2000 to 186,000for the I32 observations. Details of the NIMS observatioincluding the spatial resolution of the data for each site ana summary of preliminary findings, are given in Table 1.will focus our discussion on two different types of obsertions as outlined above: regional-scale observations shothe distribution and locations of hot spots on the surfand high spatial resolution observations showing the thestructure of individual volcanic regions.

4.1. Regional-scale observations: locations of hot spotsand temporal changes

Hot spots and plumes had been detected and moniby NIMS and SSI from distant observations prior to thefly-bys (e.g., Lopes-Gautier et al., 1997, 1999; McEwenal., 1998a). Galileo’s first three Io fly-bys revealed 16ditional hot spots (Lopes et al., 2001), 14 of which wdetected in NIMS regional-scale observations (spatialolution 15–30 km (NIMS pixel)−1). Our observational straegy for regional and global-scale observations in I31 andwas to cover significantly more area at the expense ooverlapping NIMS fields of view that are used to improthe signal to noise ratio. Thus, while regional-scale obvations in I24, I25, and I27 covered only about 5% of I

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area, these observations in I31 and I32 cover about halsurface. The objectives were to examine the global distrtion of hot spots on Io, to compile as complete an invenas possible of the active regions, and to attempt to detectlatitude hot spots. The decrease in signal to noise rationot significantly affect the detection of hot spots or the thmal mapping, however, it was somewhat detrimental tomapping of SO2, as explained by Douté et al. (2004).

4.1.1. Locations of hot spotsTable 2 (updated from Lopes et al., 2001) lists all the

tive volcanic regions known by us on Io. Detections wmade from Galileo, Voyager, ground-based, and HST obvations, up to the end of the Galileo mission in 2003. We nknow of at least 152 active volcanic regions on Io (Tableand 18 other locations that arepossibly active volcanic centers but await confirmation (Table 3). Fourteen additiohot spots (not listed in Table 2) were detected from GalPPR observations and are reported by Rathbun et al. (2this issue), bringing the total to at least 166. The NIMSand I32 fly-by observations detected 22 hot spots not prously known and one that was unconfirmed. The two disobservations obtained during orbit C30 (23 May 2001 at stial resolutions of 171–192 km (NIMS pixel)−1) detected 3previously unknown or unconfirmed hot spots that havebeen reported before. Previously undetected hot spotseither have been too faint for earlier observations to de(particularly if the previous observations were of signcantly lower spatial resolution), or may be new or renewactivity at sites where volcanic activity had not previoubeen detected, despite earlier fly-by data at similarly highspatial resolutions (in the case of the REGION type obvations, 25–50 km (NIMS pixel)−1).

The I31 and I32 NIMS observations, covering Iohigh latitude and polar regions at spatial resolutions un100 km (pixel)−1 for the first time during the Galileo mission (Figs. 1–3), did not reveal any previously unknownspots at high latitudes. Tvashtar at 65◦ N remains the highest latitude hot spot detected by Galileo NIMS and SSI.unnamed hot spot at 76◦ N was detected by Galileo PPRwhich is sensitive to lower temperatures than NIMS or(Rathbun et al. 2004, this issue). Nemea, detected byager at 78◦ S, is the highest latitude hot spot so far detecon Io.

The paucity of detections of hot spots at high latitudenot unexpected; this result is consistent with several prevstudies, all of which indicated that there are fewer activecanic centers at high latitudes than at the equatorial regThese earlier results included mapping of volcanic cen(active or not) from Voyager and Galileo imaging data (Cet al., 1998; Schenk et al., 2001), and the observed gdistribution of paterae (Radebaugh et al., 2001). Radebet al. (2001) concluded that there are fewer and largerterae at high latitudes, suggesting that the character oeruptions may be different nearer the polar regions thathe equator.


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Table 2Active volcanic centers on Io: detections of plumes and hot spots by Galileo, Voyager, HST, and ground-based observations

Volcanic Location Detected by: Detected Plume Surface Notescenter of Galileo Galileo Voyager from detected? change

candidate SSI? NIMS? IRIS? ground or detected?surface HSTfeature NICMOS?

1 2 3 4 5 6 7 8 9

RuwaPatera

0.5 N,2.7 W

Yes No No 9812A? No No Faint hot spot detected by SSseveral orbits

NuskuPatera

64.4 S,4.9 W

No No Yes? Keck(12/2001)

No No Detected from Keck (de Pateral., 2004)

MbaliPatera

31.4 S,6.8 W

No No Yes No No Red deposits

Unnamed(Keck “S”)

Yes(11 S,11 W)

15± 1 S,10± 1 W

No No Detected by SSI in C21 anfrom Keck (12/2001, de Pater eal., 2004)

Unnamed 2.8 S,13.3 W

Yes No No No No Detected by SSI in several orb

Unnamed(Keck “U”)

No No No 31± 1 N,14± 1 W

No No Detected from Keck (12/2001de Pater et al., 2004)

Unnamed 11.5 S,14 W

Yes No No 9606C?9906A?

No No Detected by SSI in several orbi

KareiPatera

2 N, 16 W Yes No No 9608A?9812A?

No No Detected by SSI in G8

Unnamed 6 S, 19 W Yes No No No No Detected by SSI in several oUnnamed 1 N, 21 W Yes No No No No Detected by SSI in several orUnnamed 1 S, 23 W Yes No No No No Detected by SSI in several oUnnamed 5 N, 23 W Yes No No No No Detected by SSI in G8UtaPatera

35.3 S,24.5 W

No No Yes? 9606C?NICMOS15?, Keck(12/2001)

No No Very low albedo. Repeateground-based detections (01998 and 12/2001 from Keckalso detected by C. Dumas)

Unnamed 9 S, 27 W Maybe 7± 3 S,34± 3 W

No No No Faint hot spot in SSI G8, C10and E15. Detected by NIMS inC30


Yes No No 9606C? No No Detected by SSI in several or

Unnamed(N. Polar)

69 N,30 W

No No No 9610A No Yes N. Polar changes seen by Sunclear if location consistenwith ground-detected hot spoError on ground-observed hospot∼ 15◦

Kanehekili,N&S

16 S,38 W

14.5 S,33.4 W;17.2 S,35.5 W

12± 10 S,34± 4 W

No Numerousground-baseddetections,N5, Keck(12/2001)

G Yes Detected numerous times frothe ground and by NIMS. Twoactive areas (N and S) detecteby SSI

Unnamed(Keck “W”)

No No No 46± 1 N,41± 3 W


JanusPatera

3 S,42.5 W

Yes 2± 3 S,39± 3 W

No 9606A?,N2, D,Keck(12/2001)

No No Detected several times from thground (including by Keck on12/2001). Detected by NIMSand SSI in several orbits. NIMSC30 data suggests 2 hot sposecond at 7± 3 S, 34± 3 W

Unnamed(Keck “V”)

No No No 34± 1 N,51± 4 W


Unnamed 11 N,59 W

Yes No No 990930B? No No Detected by SSI in one or(C10)

(continued on next page)


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1 2 3 4 5 6 7 8 9

Masubi 48 S,60 W

Yes 45± 2 S,56± 2 W

No 9808A?,Keck(12/2001)

V Yes New plume deposits, hot spdetected by SSI and NIMS iE11, I31. Hot spot detected bJ. Spencer on 98/08/29 (faded98/08/31). Detected from Kecon 12/2001

Unnamed 60± 15 N,60± 15 W

No No No 002A,NICMOS14?

No No Detected by J. Spencer & R. Hwell at 60± 15 N, 60± 15 W

Shamshu 9.8 S,63.6 W

No 10± 4 S,67± 4 W

No No No

Unnamed(NIMSC30A,“Tejeto”)

48.9 S,69.4 W

No 49± 1 S,68± 1 W

No 9808A?9509A?

No No Possible site of outburst detecton 99/08/02 by R. Howell. Detected by NIMS in orbit C30I31, I32

ZalPatera

40.5 N,74.9 W

Yes 37± 3 N,78± 3 W

No No Yes Bright red deposits. DetectedSSI and NIMS in several orbitsincluding NIMS in I31 and I32

Tawhaki 3.1 N,75.1 W

Yes 3± 3 N,76± 3 W

No 9908A? No No Detected during several orbby SSI and NIMS, including byNIMS in I31 Possible site of outburst detected on 99/08/02 bR. Howell. Hot spot detected bNIMS before outburst (C21)

Unnamed No 37± 3 S,79± 3 W

No Yes? No No Detected by NIMS in E11 anI31

Hi’iakaPatera

3.1 S,79.8 W

No 1± 4 S,76± 4 W

No Yes No Yes Detected multiple times from tground and by NIMS. Plumedeposits detected by SSI1996/1997

EstanPatera(NIMS I31Fand I31M)

21 N,87 W

No 21± 2 N,87± 2 W;20± 1 N,81± 1 W

No No No Detected by NIMS in I31, I32

Unnamed(NIMSI32J)

18.6 S,87.5 W

No 19± 1 S,87± 1 W

No No No Detected by NIMS in I32. Possbly same as “Poliahu” hot spot

Ekhi 28.3 S,87.6 W

Yes No No No No Detected by SSI in one orb(G8)

Gish BarPatera

15.6 N,89.1 W

Yes 16± 4 N,89± 5 W

No 9908A? No No Detected by NIMS during seeral orbits, including I31, I32Possible site of outburst detecteon 99/08/02 by R. Howell. Detected by Keck on 12/2001

Unnamed(NIMSI31E,“AlunaPatera”)

43.9 N,90.7 W

Yes 44± 2 N,91± 2 W

No No No Detected by SSI in E15 and bNIMS in I31, I32


No No No 39.6± 5.7 S,91.2± 5.5 W

No No Detected by from Keck (Marchiet al., 2004)

Unnamed(NIMSI32K)

5.8 N,96.7 W

No 7± 1 N,95± 1 W

No No No Detected by NIMS in I32

SigurdPatera

5.9 S,97.4 W

No 5± 4 S,100± 4 W

No 990930I?,991124F?

No No Detected by NIMS in several obits, including I31

Itzamna 15.2 S,97.7 W

No 15± 3 S,97± 3 W

No 990930I? No No Detected by NIMS in C10, I3I32

ArushaPatera

39 S,100.7 W

No 39± 2 S,100± 2 W

No 9503A? No No Possible site of outburst detecby J. Spencer in March 1995Hot spot detected by NIMS inI31, I32.



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Table 2 (continued)

1 2 3 4 5 6 7 8 9

CathaPatera

53.6 S,100.9 W

No 53± 1 S,105± 1 W

No No Yes Detected by NIMS in C30, I31I32

MonanPatera

20.3 N,103.8 W

Yes 20± 1 N,103± 1 W

No No Yes? Detected by NIMS in several obits, including I31, I32. Detectedby SSI in E15. Plume possibldetected by SSI in E4. SSI images suggest 3 main active are

Unnamed(“Ah PekuPatera”)

10.8 N,108 W

Yes 9± 1 N,105± 1 W

No No No Detected by SSI in orbit E15, bNIMS in I32

Unnamed(NIMSI31D)

Possiblypart ofMonanPateracomplex

No 20± 2 N,106± 2 W

No No No Detected by NIMS in I31, neaMonan

AltjirraPatera

35 S,108 W

No 35± 2 S,108± 2 W

Yes—same asMalik?

No Yes Bright red deposits. Detected bNIMS in several orbits, includ-ing I31, I32

Unnamed(NIMSI32G)

47.1 S,108.1 W

No 48± 2 S,109± 2 W

No No No No Detected by NIMS in orbit I32

Unnamed(NIMSI32F)

69.1 S,108.3 W

No 69± 2 S,109± 2 W


Unnamed(NIMSC30B)

24 N,109 W

No 24± 1 N,109 ± 1 W

No No No No Detected by NIMS in orbit C30

Unnamed(NIMSI32M)

40 N,118.6 W

No 37± 2 N,118± 2 W

No No No No Detected by NIMS in I31(fainter), I32

Unnamed(NIMSI27E,NW ofAmirani)

31.1 N,115.9 W

No 31± 0.5 N,117± 0.5 W

No No No No Detected by NIMS in I27, I31I32

Amirani 23.2 N,116.3 W(locationofcaldera)

Yes 27± 4 N,112± 4 W(veryextended)

Yes Yes V, G Yes Bright red deposits. NIMS dtects thermal emission alonwhole flow. Persistent hot spodetected by NIMS and SSI inseveral orbits, including NIMSin I31, I32. Detected from Keckin 12/2001

Unnamed(NIMSI31J,in TvashtarCatena)

59.5 N,117.9 W

No 59± 1 N,117± 1 W

No No No No Detected by NIMS in I31. Activ-ity in SW corner of caldera lo-cated to the SE of Tvashtar lavfountain site

Dusurra 37.1 N,118.5 W

No 39± 7 N,125± 7 W

No No No No Detected by NIMS in orbits C21I25, I27, I31, I32

Emakong 3 S,120 W

No 3± 1 S,119± 1 W

No No No No Detected by NIMS in orbits I25I27, I32

Unnamed(NIMSI31L, NETvashtarCatena)

67 N,125 W

No 67± 1 N,125± 1 W

No No No No Small caldera to the northeastTvashtar, detected by NIMS iI31

TvashtarCatena(lavafountainsite)

61.5 N,120.2 W;62 N,123 W

Yes 62± 1 N,123± 1 W

No 9911A No Yes Detected by NIMS in I25, I27G29, I31, I32. Detected by SSI iI25 and G7. Lava fountain seein I25. Possible site of 990930Aand of outbursts in 11/13/00 an
12/16/00


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Table 2 (continued)

1 2 3 4 5 6 7 8 9

Unnamed(NIMSI31K,in TvashtarCatena)

60.5 N,120.4 W

No 61± 1 N,120± 1 W

No No No No Detected by NIMS in orbit I31(I31K). Small caldera SE oTvashtar lava fountain site

MauiPatera

16.2 N,123.8 W

No 16.5± 1 N,124± 1 W

Yes—same asAmirani?

No V Yes? Voyager plume site was at tend of Amirani flow Hot spot detected by NIMS in several orbitprior to I27, I31, and I32, but position uncertain

TvashtarCatena(Flow site)

64.8 N,126 W

Yes 65± 1 N,126± 1 W

No Yes ? Yes Yes Hot spot detected by SSI inbits G7, I27. Hot spot detecteby NIMS in orbits I27, I31,I32 Plume detected by Cassi12/200000-01/2001. Hot regionseen in “dolphin-shaped” flow incaldera

Unnamed(NIMSI31H)

11 S,128 W

No 11± 1 S,127± 1 W

No No No No Detected by NIMS in orbits I31I32

MalikPatera

34 S,129 W

No 34± 2 S,128± 2 W

Yes No No No Bright red deposits. Hot spot dtected by NIMS in several orbitsincluding I31, I32

Unnamed(NIMSI27B,“MajuPatera”)

19.5 N,131.1 W

No 20± 1 N,130± 1 W


Unnamed(NIMSI31A,“Thor”)

39 N,131–135 W

No 38± 1 N,131± 1 W,39± 1 N,135± 1 W

No 0108A,Keck 12/20

Yes Yes Large outburst. Hot spot detectby NIMS in I31, I32. Active flowdetected by NIMS in I31, I31Large plume detected by SSII31 and I32.

YawPatera(NIMSCamaxtli C)

9.3 N,132 W

No 9.5± 1 N,132± 1 W


Unnamed(S SethPatera,NIMSI25B)

5 S,132 W

No 5± 1 S,132± 1 W

No 991124D? No No Detected by NIMS in I25, I2C30, I31, I32. Seth Patera is2S, 133 W

Tien MuPatera(NIMSCamaxtlieast)

12N,133.9 W

No 12± 1 N,134± 1 W

No No No No Detected by NIMS in I24, I27I31, I32

CamaxtliPatera

15 N,136.4 W

Yes 14.5± 1 N,136± 1 W

No No No No Detected in E15 by NIMS, SSDetected by NIMS in I24, I27I32

Unnamed(NIMSI31B)

35.2 N,137.2 W

No 35± 1 N,137± 1 W

No No No No Detected by NIMS in I31 anI32, probably related to I31A

RuaumokoPatera(NIMSCamaxtliWest)

14.5 N,139.3 W

No 15± 1 N,139± 1 W

No No No No Detected by NIMS in I24 and I2

Unnamed(“ChorsFluctus,”NIMSI32H)

45 S,140 W

No 45± 1 S,139± 1 W

No No No No Detected by NIMS in I32



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Table 2 (continued)

1 2 3 4 5 6 7 8 9

TupanPatera

19 S,141 W

No 17± 1 S,141± 1 W

No No No No Bright red deposits. Persistehot spot detected by NIMS inseveral orbits, including I31, I32High resolution NIMS and SSobservations in I32

Unnamed(N. Polar)

66 N,144 W

Yes No No No No No Detected by SSI in orbit G7

Unnamed(NIMSI24A,near Surya)

22 N,145.6 W

No 22± 1 N,145± 1 W

No No No No Detected by NIMS in I24, I27I31, I32

Cuchi Pat-era (NIMSI25A)

0.6N,145.8 W

No 2± 1 S,144± 1 W

No No No No Detected by NIMS in I25, I32

Unnamed(NIMSI32C“Thor Fluc-tus”)

26 S,147 W

No 26± 1 S,147± 1 W

No No No No Detected by NIMS in I31 (faint)I32

ArinnaFluctus

32 N,147 W

No 30± 1 N,147± 1 W

No No No Yes Extensive, bright red deposiDetected by NIMS in several orbits, including I31, I32

Unnamed(I31I),possibly“Chor Fluc-tus”

E-W fissure∼ 46 S,144 W; mainflow ∼ 45 S,141 W

No 37± 2 S,149± 2 W


SoboFluctus(NIMSI24B)

14 N,150 W

No 14± 1 N,150± 1 W

No No No No Detected by NIMS in I24, I27and I32. Possibly 2 hot spots dtected in I32

Surya(NIMSI27A)

21.3 N,150.9 W

No 22± 1 N,152± 1 W

No No No Yes Detected by NIMS in I27. Suface change detected by SSI

ShamashPatera

35 S,152 W

No 34± 1 S,153± 1 W;36± 1 S,151± 1 W

Yes—same asMalik?

No No No Detected by NIMS in several obits, including I32, when NIMSdetected thermal emission frompatera and flow (I31I)

PrometheusPatera

0.5 N,153 W

Yes 1± 3 S,155± 3 W

No No V,G Yes Bright red deposits. Volcanic ativity along flow. Persistent hospot detected by NIMS and SSin several orbits, including I31I32. Plume moved between Voyager and Galileo

Chaac 11.8 N,157.2 W

No 10 N,157 W

No No No No Bright green deposits on caldeflour. Hot spot detected by NIMSin I25 and I27

RadegastPatera

26.9 S,159.2 W

No 27± 0.5 S,160± 0.5 W

No No No No Detected by NIMS in I32—smacaldera near Tohil

CulannPatera

19.9 S,161.5 W

Yes 18± 3 S,163± 3 W

No No G Yes Bright red deposits. Persisteplume and hot spot. Hot spot detected by NIMS in several orbitsincluding I32, and by SSI in E11

TsuiGoabFluctus(NIMSI27D)

0.0, 163.3 W No 0, 164 W No No No No Detected by NIMS in I27 (I27DI31, I32

Unnamed(NIMSI32E)

65.9 S,168.6 W

No 68± 1 S,166± 1 W

No No No No Detected by NIMS in orbits C3(faint), I32



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Table 2 (continued)

1 2 3 4 5 6 7 8 9

MichaboPatera(NIMSI31G)

3 N,168.8 W

No 2± 2 S,169± 2 W


Zamama 18 N,174 W

Yes 17± 2 N,172± 2 W

No Keck12/01

G Yes Bright red deposits. Detectefrom Keck 12/2001. Persistenhot spot detected by NIMS anSSI in several orbits, includingI32

Unnamed(NIMSI32D)

42 S,175 W

No 45± 2 S,172± 2 W


AidnePatera

2 S,178 W

No 2± 3 S,178± 3 W

No No No Yes Detected by NIMS in several obits, including I27

Volund 25 N,184.3 W

Yes 25± 3 N,174± 3 W

Yes No V, G Yes Detected by NIMS anSSI.Prometheus-type plumand lava flow

DonarFluctus

24.3N,186.2 W

Yes No No No No No Detected by SSI in E11

Haokah 20.7 S,187 W

No 19± 3 S,185± 3 W

No No No No Bright green deposit in SSimages. Hot spot detected bNIMS in E11, E14

Unnamed 28.1 N,192 W

Yes No No No No No Detected by SSI in G1

Fo Patera 40.9 N,192.6 W

Yes 39± 3 N,191± 3 W

No No No Yes Detected by NIMS and SSIseveral orbits

SethlausPatera

52 S,194 W

No 50± 3 S,195± 3 W

No No No No Red deposits. Hot spot detectby NIMS in several orbits

RataPatera

35.2 S,199.2 W

Yes 35± 3 S,199± 3 W

No No No No Red deposits. DetectedNIMS in several orbits, by SSin E11

Gabija 52 S,203 W

No 52± 3 S,204± 3 W

No No No No Hot spot detected by NIMS iE14, I24

Lei-KungFluctus

38 N,204 W

Yes 37± 3 N,206± 3 W

No No No No Bright red deposits. DetectedSSI and NIMS in several orbitand by PPR in I27 (Spenceral., 2000)

IsumPatera,N&S

28 N,209 W

32.9 N,204.7 W;30.3 N,206.8 W

31± 3 N,207± 3 W

Yes 9510A ? No No Bright red deposits. SSI dtected two hot spots. Kec12/2001. Activity detected byNIMS in several orbits, including I31

Marduk 28.4 S,209.9 W

Yes 27± 2 S,211± 2 W

Yes No V,G Yes Bright red deposits. DetectedNIMS and SSI in several orbitsby PPR in I27 (Spencer et a2000)

Unnamed 65 N,215 W

No No No No No No Detected by PPR in I25. Posble Lei-Kung source

Ot 0.9 S,217 W

No 2± 3 S,218± 3 W

No No No No Detected by NIMS in several obits including I24



MulunguPatera

17.2 N,217.5 W

Yes 17± 3 N,219± 3 W

No 9510A? No No Detected by NIMS in several obits, by SSI in G1

KurdalagonPatera

50 S,218.4 W

No 47± 3 S,219± 3 W

No No No No Red deposits. DetectedNIMS in several orbits

Susanoo 22.3 N,219.3 W

No 21± 3 N,222± 3 W

No 9510A? No No Hot spot detected by NIMSE14 and I24

Unnamed(NIMSI32A)

31 N,222 W

No 28± 2 N,227± 2 W




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Table 2 (continued)

1 2 3 4 5 6 7 8 9

WaylandPatera

32.2 S,225.5 W

No 33± 2 S,223± 2 W

No No No No Hot spot detected by NIMS iE14. Detected by Cassini ISS o01/01/01

ReidenPatera

13 S,236 W

Yes 11± 2 S,234± 2 W

No No No No Detected by SSI in G1, bNIMS in I24 and I32

Girru 22 N,240 W

Yes 22± 3 N,238± 3 W

No No No No Detected by NIMS in several obits, by SSI in E11

Unnamed(NIMSI32B,“Llew”)

12.1 N,241.8 W

No 10± 2 N,240± 2 W




PillanPatera

12 S,244 W

9.5 S,242.7 W;11.5 S,242.2 W

13± 3 S,244± 3 W

No Keck12/2001

G Yes Major eruption in 1997. Plumdetected by SSI and HST. Pesistent hot spot detected bNIMS since 1996 (G2). Calderafissure vent, lava flows identifieby SSI

PyerunPatera

55.4 S,251.1 W

No No Yes—same asMithra?

No No No Voyager 1 detection

Pele 18.4 S,255.7 W

Yes 20± 3 S,255± 3 W

Yes No V, G Yes Large, bright red depositPlume detected also by HSTVery persistent hot spot detecteby NIMS and SSI numeroutimes

SvarogPatera

48 S,265.5 W

Yes 42± 5 S,269± 5 W

Yes Keck,12/2001

No No Detected by NIMS and SSI iseveral orbits

ShakuraPatera

23.1 N,266 W

No No Yes—same asDaedalus?

No No No Very low albedo. Detected bPPR in I27 (Spencer et al., 2000

MithraPatera

58.6 S,266.7 W

54.3 S,268.6 W

No Yes—same asPyerun?

No No Red deposits. SSI detected hspot north of patera

BabbarPatera

39.4 S,271.8 W

No 37± 4 S,283± 8 W

Yes No No No Detected by NIMS in several obits, by PPR in I27 (Spencer eal., 2000)

DaedalusPatera

19 N,275 W

No 18± 3 N,273± 3 W

Yes 990929E?,991030C?,991125A?,980905B?,0112G?

No Yes Red deposits. Detected numous times from ground. Detected as a hot spot by PPRI25, I27

Unnamed 49.9 N,278.6

No No No Yes, Keck12/2001

No No Observed by Keck on 12/2001

ViracochaPatera

61.4 S,281 W

No No Yes No No No Detected by Voyager

UlgenPatera

40.4 S,287.7 W

No 41± 9 S,291± 9 W

Yes N6?, D?,Keck(12/2001)

No No Very low albedo, detected bNIMS in C22. Detected by Keck12/2001

HephaestusPatera

1.9 N,290.1 W

No No No Keck No No Detected By PPR in I27. Dtected from Keck on 12/2001

DazhbogPatera

54.3 N,301.1 W

No No No N13, Keck12/2001

No Yes Detected by NICMOS (66.4± 8N,310.6± 14W). Red plume deposits observed by SSI in I31I32. Hot spot detected by PP(Rathbun et al., 2004, this issu

Unnamed(Keck “I,”“Rarog”)

41.4 S,304.9 W

No No No 44± 1 S,302± 2 W

No No Detected from Keck (12/2001de Pater et al., 2004). Large paera

SengenPatera

32.5 S,304 W

No No No 9506J?, N6?,D?, Keck(12/2001)

No Yes



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Table 2 (continued)

1 2 3 4 5 6 7 8 9

Unnamed(Keck M”)

No No No 61± 2 S,305± 2 W


MihrPatera

16.2 S,305.7 W

Yes No No Keck(12/2001)

No No Detected by SSI in orbits C9E11. See also Rathbun et a(2004), this issue

AmaterasuPatera

36.3 N,306.2 W

No 40± 4 N,309± 4 W

Yes Maybe No Yes Detected by NIMS in severalbits, by PPR in I27 (Spencer eal., 2000)

LokiPatera

12.7 N,308.8 W

Yes 9± 7 N,309± 7 W

Yes Numerousground-basedobserva-tions, N1

V Yes Detected multiple times fromground and by NIMS. Twoplumes observed to the northcaldera by Voyager. Hot regionin caldera observed by NIMSand PPR at high resolution (I24I27, I32)

AtenPatera

48.2 S,310.5 W

No No Yes N9, D? No Yes Pele-type plume deposits, rdish

KinichAhau

50.4 N,311 W

No No No N11 No No Detected by NICMOS (50.3±5 N, 318.8± 8 W)

MazdaCatena

9.4 S,314.9 W

No No Yes 9606H? N7?,D?

No No Red deposits

Nemea 78 S,320 W

No No Yes No No No

ManuaPatera

35.2 N,321.6 W

Yes No No 06/1997? No No Detected by SSI in orbit E6 aby UH AO on 06/97

Ra Patera 8.3 S,325.2 W

No No No No G Yes Major brightening and suface change observed by HSbetween 3/1994 and 3/199(Spencer et al., 1997). Plumdetected by SSI in orbit G1, E4

Unnamed(Keck “L”)

PossiblyFuchi orManua?

No No No 34± 1 N,326± 1 W



Yes 36± 9 S,324± 9 W

No 9606G? No? No Detected by NIMS in C2NIMS hot spot could also bfrom feature at 40.5 S,326.3 W

FuchiPatera

28.3 N,327.7 W

Yes No No 9606G?,N4, D, Keck12/2001

No No Red deposits, hot spot detectby SSI in several orbits

Huo ShenPatera

15 S,330 W

No No No No Yes HST changes (Spencer et1997)

Unnamed 16.6 N,332 W

Yes No No No No No Detected by SSI in orbits CE11

AcalaFluctus

11 N,336 W

Yes No Yes N3, D G Yes Detected by SSI in E14, PPRI27 (Spencer et al., 2000)

Surt 44.9 N,337.1 W

No No No 9606E?,N12,0102A, Keck12/2001

No Yes Pele-type plume deposits oserved by Voyager 2. Outbursobserved on 02/2001

CreidnePatera

52.4 S,343.2 W

No No Yes N8? No Yes Tentative identification of hspot location

Unnamed 3.1 N,350.4 W

Yes No No No No No Detected by SSI in several orb

TiermesPatera

22.2 N,350.4 W

9507A

EuboeaFluctus

45 S,352 W

No No No 9606F?,N8?, D?,Keck(12/2001)

No Yes Pele-type plume deposits, brigred



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thely

Table 2 (continued)

1 2 3 4 5 6 7 8 9

Unnamed(Keck “R”)

PossiblyMama Pateraat 10.6 S,356.5 W

No No No 7± 1 S,353± 3 W


Unnamed 4.8 N,356.1 W

Yes No No No No No Detected by SSI in several orb

FjorgynnFluctus

12 N,358 W

Maybe(16.0 N,3.8 W)

No No 9606D?,N10, D,Keck “N”(9± 1 N,1± 1 W)

No Yes Possibly detected by SSI in obit E15. Detected from Keck(12/2001; de Pater et al., 2004)

Note and sources. Names in quotes are authors’ working names and have not been approved by the IAU. Sobo Fluctus has provisional name statusIAU.

(N) NICMOS hot spots detected by Goguen et al. (1998).(D) hot spots detected by C. Dumas et al. in1997 and/or 1998 (personal communication).Keck are hot spots detected by de Pater et al. (2004) and Marchis et al. (2004) from Keck telescope using Adaptive Optics.(V, G, C) indicate Voyager, Galileo, or Cassini detection.Other ground-based hot spots detected by Spencer et al. (1997).Galileo PPR detections from Spencer et al. (2000). See also Rathbun et al. (2004, this issue) for additional detections by PPR.Galileo SSI detections of hot spots, plumes, andsurface changes from McEwen et al. (1998a, 2000), Geissler et al. (1999, 2004, this issue), Keszi et

al. (2001) and Turtle et al. (2004, this issue).Galileo NIMS detections prior to orbit C30 from Lopes-Gautier et al. (1997, 1999) and Lopes et al. (2001).Locations of surface features are approximate center of caldera or feature identified by J. Perry.

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The suggestion of fewer hot spots at high latitudesalso put forward by Lopes-Gautier et al. (1999) basedmapping hot spots from distant Galileo NIMS observatioThis previous study discussed the possibility that hot spohigh latitudes may be harder to detect because of spaceviewing geometry. The subspacecraft point for all regionscale and global-scale Galileo observations is close toequator, therefore, hot spots at high latitudes are viewelower spatial resolutions than those at the equatorial regIn addition, there is the possible effect of topography. A lalake in a patera will be increasingly obscured by the pera wall at higher emission angles (see also Radebaual., 2004, this issue). The obscuration will depend ondepth and the diameter of the patera, and the emission aErupting lava fountains or hot spots surrounded by exsive, hot or cooling lava flows could be detected by NIMdespite higher emission angles. A more subtle topograeffect involves molten material at the bottom of small incdescent cracks in material too cool to be detected by NIMthe surface. Lopes-Gautier et al. (1999) reported detectionumerous hot spots near Io’s limb (at high emission angat low and mid latitudes, indicating that the effect of viewigeometry might not be substantial. They found no corrtion between the number of hot spots detected by NIMSthe emission angle. The decrease in the fluxes of hot ssuch as Pele and Prometheus as a function of emissionshowed that they could be easily detected at high emisangles. Lopes-Gautier et al. (1999) concluded that it islikely that there is a large number of hot spots undetecby NIMS at high latitudes that are of comparable brigness to those detected at lower latitudes. The possibilitymains that hot spots at higher latitudes were not sufficiebright to be detected in the NIMS observations, or that er

ft

.

t

.

e

tions are more sporadic, perhaps predominantly of pillantype. This second suggestion is consistent with findingsGeissler et al. (2004, this issue), who suggested that exsive polar eruptions on Io tended to be episodic, while that lower latitudes are usually persistent and smaller.

4.1.2. Temporal changesThe areas covered by the regional and global-scale N

observations in I31 and I32 overlapped considerably (seeble 4) and also overlapped with the areas around BosphRegio covered by observations at the same scale in I24,and I27. Several hot spots were detected in more thanorbit, making it possible to study temporal variations. Wassessed temporal variations by obtaining the power ouat 4.7 µm for individual hot spots, following the proceduoutlined above and comparing our results for I31 and(Table 4) with those obtained earlier (for I24, I25, and I2and reported by Lopes et al. (2001). All of these observatwere obtained under similar conditions and at similar sparesolutions, making comparisons straightforward. Somespots were detected in only one orbit, despite the factthe sites were observed in both I31 and I32. In these cwe give upper limits for the power based on the deteclimit of radiance in the NIMS unresampled pixels.

The results from Table 4 were combined with those fr(Lopes et al., 2001) for I24, I25, and I27 observationsplotted in Fig. 4. Given the lack of NIMS observationscomparable spatial resolutions between February 2000and August 2001 (I31), it is unclear what happened duthis long interval of time. The results, however, show sogeneral trends.

Of the 16 hot spots for whichP (4.7 µm) was obtained omore than one orbit, 6 showed no significant change w


umerous

as fohat

Fig. 1. This 4.4 µm map of the first NIMS regional-scale observation of Io during orbit I31 (I31 region 01, Table 1) shows thermal emission from nhot spots. The observed intensity is rendered in pseudocolor according to thecolor bar shown. (The units are arbitrary, as the image was heavily stretched inorder to bring out details outside the hot spots). Thetop arrow points to the I31A (“Thor”) hot spot, which was discovered from this observation. The hot spotwas the source of Io’s tallest plume detected so far (Geissler et al., 2004, this issue; Turtle et al., 2004, this issue). The central area of the hot spotis shown inblack because the pixels were saturated at this wavelength. The lower arrowpoints to the hot spot Gish Bar, which was faint during I31 (compare with Fig. 2).The area observed by NIMS is outlined in the SSI image on the right, which was acquired before the I31Aeruption. Io’s radius is 1821.6 km.

Fig. 2. This 4.7 µm map (right) was obtained by NIMS during the I32 fly-by (I32 region 01, Table 1). The observed intensity is rendered in pseudocolorrFig. 1. Nine previously unknown hot spots were detected from this observation. The area observed by NIMS is shown on the SSI image on the left. Note tGish Bar (on the right hand side edge of the observation) is very bright, considerably more so than in orbit I31 (Fig. 1). Io’s radius is 1821.6 km.


lor ass

the NIMS

Fig. 3. This 4.7 µm map (right) was obtained by NIMS during the I32 fly-by (I32 region 02, Table 1). The observed intensity is rendered in pseudocofor Figs. 1 and 2. Io’s terminator is located near the center of the image, the left hand side of the disk was in darkness during the observation. Numerouhotspots are shown as bright regions. An SSI image is shown on the left for comparison. Note that the hot spot I31A (“Thor,” near the top right ofobservation) is the brightest hot spot on Io at this wavelength. Io’s radius is 1821.6 km.

retput

tionslyat thes;s) forrom

andd

ndthan-IMS

om

ofMS

ivelloworalnionsPilla-nts

ma-

om01;ue).ng-n-

).onay

ol-typeell

Fig. 4. Variations in power output at4.7 µm of hot spots observed in mothan one Io fly-by. Gish Bar shows a dramatic increase in power oubetween orbits I31 and I32 (note the break in the vertical scale). Variaat other hot spots are also apparent, although several have stayed relativeconstant (see text for discussion). A representative error bar is showntop of the legend (± 0.5×109 W µm−1, most errors were smaller than thierror bars that exceeded this value are shown). Data (including errorI31 and I32 are shown in Table 4, data for I24, I25, and I27 orbits are fLopes et al. (2001).

errors are taken into account. These are Maui (I27, I31,I32), NIMS I24B (now called Sobo Fluctus; I24, I27, anI32), I32M (I31 and I32), I27B, Zal, and Tvashtar (I31 aI32). Most of the other hot spots showed changes of less50%, including NIMS Monan, Malik, Itzamna, Amirani, Tupan, and Prometheus. Significant changes occurred at NI27E (from 0.9±0.2×109 W µm−1 down to 0.2±0.1×109

W µm−1), I25B (from 2.9± 0.3× 109 W µm−1 in I27 downto 1.5± 0.3× 109 W µm−1 in I32); I24A (from 5.3± 0.5×

109 W µm−1 in I27 down to 0.5 ± 0.2 × 109 W µm−1 inI32); and, most dramatically, Gish Bar (an increase fr0.9±0.2×109 W µm−1 in I31 to 36.4±0.6×109 W µm−1

in I32). Below we interpret the styles of activity at severalthese hot spots using the analysis of high resolution NIand SSI observations.

4.2. Local-scale observations: different types of volcaniceruptions

The NIMS local-scale observations of individual eruptcenters, together with SSI images of these locations, aus to investigate the thermal, morphological, and tempcharacteristics that represent signatures of different eruptiotypes and rates. We identify three major types of eruptbased on their surface characteristics and processes. “nian” eruptions (Keszthelyi et al., 2001) are intense evesometimes accompanied by the expulsion of pyroclasticterials, usually including giant plumes (> 200 km high).They may correspond to the Io “outbursts” observed frEarth (e.g., Stansberry et al., 1997; Howell et al., 20Marchis et al., 2000, 2001; de Pater et al., 2004, this iss“Promethean” eruptions (Keszthelyi et al., 2001) are lolived and relatively gentle effusions of lava flows, with geerally smaller plumes (< 200 km high) that emanate fromflow fronts (McEwen et al., 2000; Milazzo et al., 2001A third type, which we name here “lokian,” is an eruptiof a lava lake or lava flows confined within a patera that mor may not include plumes. Loki, as the most powerful vcano on Io, may be an extreme example of this eruption(e.g., McEwen et al., 2003), but it illustrates extremely w


f

icate

of

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ame

ts

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y SSI

SSI

sits,

y

red

otsf

G8ydt

Table 3Identifications of possibly active volcanic centers

Volcanic Location of Galileo Ground- Surface Notescenter candidate SSI/NIMS observed? change?

surface feature tentative detection? HST NICMOS?

Unnamed 15.3 N, 4.7 W 9606D?Cataquil Patera 23.5S, 18.2 W No 9906A? No Tentative identification o

ground observed hot spotUkko Patera 32 N, 20 W No 9508A? Yes Surface changes ind

activityUnnamed 19.4 N, 23.3 W No 0011A? No Tentative identification

ground observed hot spotUnnamed 13.5 S, 23.9 W SSI 9606C? No Faint spot in SSI G8,

imagesLei-Zi Fluctus 14 N, 45 W No No Yes New plume deposits d

tected by SSI in orbit C9NIMS I32 NIMS at 39± 1 N,

69± 1 WNo No Possibly detected b

NIMS in I32, very faintUnnamed 55 N, 73.8 W No 0011B? No Dark patera. May be s

hot spot as above“Poliahu” 19.4 S, 81.8 W No Yes No Reported at 22± 5 S,

79 ± 5 W by Goguen etal. (1998) as very brigheruption in 1986. Same aI32J?

Shango 31.7 N, 99.7 W Yes No No Faint spot in SSI ecliimage

NIMS C30 53 S, 148 W NIMS No No Possibly detectedNIMS in C30, very faint

Unnamed 25.7 S, 168.2 W SSI at 22 S, 168 W No No Faint spot detected bin E11

Namarrkun 10.1 N, 175.7 W SSI No No Identification based ondata

Kami-Nari Patera 8 S, 234 W No Yes Pillan-type plume depodetected by SSI in C21I24

NIMS I32 NIMS at 23± 3 N,248± 3 W

No No Possibly detected bNIMS in I32—very faint

Unnamed 38 S, 291 W No No No Low albedo and brightmaterials

Khalla Patera 5.7 N, 303.4 W No Yes No Probably site of hot spobserved by University oHawaii AO 06/1997

Unnamed 2 S, 352 W Yes Yes? No Faint spot in SSIeclipse image. Possiblsame as hot spot detecteby C. Dumas on 6/3/98 a6± 3 S, 358± 3 W and byKeck (Keck “R”)

ned

toandaledail.ingcen-ifiedurce

in00;

ial

Iotile

ox-ikeande toingagetion,pes

that powerful and persistent eruptive activity can be confiwithin patera walls.

Although these three eruption types show similaritieseruption styles seen on Earth (fire fountains, lava flows,lava lakes, respectively), significant differences are revewhen the activity at individual centers is examined in detFor example Loki, which may be a periodically overturnlava lake (Rathbun et al., 2002), has an island in theter of the patera that appears to have remained unmodsince Voyager 1 imaged the patera in 1979, and the sofor the Prometheus plume has migrated by about 80 kmtwo decades (McEwen et al., 1998a; Kieffer et al., 20

Phillips, 2000). Thesephenomena have no known terrestrequivalents.

The factors leading to the different eruptions types onare not known, although the tapping of near-crustal volareservoirs of various compositions (sulfur and sulfur diide) may play a role (Williams et al., 2004, this issue). UnlEarth, Io does not exhibit evidence for plate tectonicsthe implied variations in lithospheric strength. The degrewhich Io’s different eruption types contribute to resurfacis not yet known, as their distribution and rates of coverhave not yet been examined on a global scale. In addiIo’s plumes have long been grouped into two major ty


n)n)

n)

Table 4Power output at 4.7 for hot spots detected in I31 and I32

Name I31 (×109 W µm−1) I32 (×109 W µm−1)

Tejeto 5.4± 0.8 Not observedMasubi 21.0± 1.5 Not observedCatha 1.0± 0.3 Not observedArusha > 0.1 (lower limit, at edge of observation) 0.7± 0.5Malik 2.0 ± 0.4 5.1± 0.4Itzamna 1.4± 0.3 3.9± 0.4I32C “Thor Fluctus” > 0.2 0.2 ± 0.1

(lower limit, at edge of observation)Tupan 4.0± 0.3 7.6 ± 0.3Tawhaki 1.0± 0.3 Not observedPrometheus 8.2± 0.4 10.9 ± 0.3Gish Bar 0.9± 0.2 36.4 ± 0.6Tien Mu > 0.1 Not detected (< 0.3)Maui 0.3 ± 0.2 0.2 ± 0.1Monan/I27D 1.4± 0.3 2.5± 0.4Amirani 9.1 ± 0.3 12.5± 0.4I27E 0.9 ± 0.2 0.2 ± 0.1Zal 3.2 ± 0.6 2.4 ± 0.8I32M 1.0 ± 0.3 1.2 ± 0.3I31E (Aluna) 1.7± 0.5 > 0.3Arinna 0.3 ± 0.1 0.5 ± 0.2I31A (“Thor”) > 23.2 (middle pixels saturated, value in-

cludes I31B as pixels overlap)51.7 ± 0.4

Tvashtar (several hot spots incatena unresolved)

2.0 ± 0.6 2.0± 0.5

Altjirra Not detected (< 0.3) > 0.1I32I Not detected (< 0.3) 0.2± 0.1Shamash Not observed > 0.1I32J Not observed > 4.2 (on edge of observatioCulann Not observed > 1.9 (on edge of observatioI25B Not detected (< 0.3) 1.5± 0.3Ah Peku Not detected (< 0.3) 0.4± 0.2I24B (Sobo) Not detected (< 0.3) 0.8± 0.2Estan Patera 0.4± 0.2 > 0.1Zamama Not observed > 1.3 (on edge of observatioI27B (Maju) Not detected (< 0.3) 0.7± 0.2I24A Not detected (< 0.3) 0.5± 0.2I31B Detected but pixels overlapped I31A’s. See

I31A above0.4 ± 0.3

983)rennionsd a

ons.pes

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-

p-a;-

1).shtar01;as-

ces--metionpil-ity.o-

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(promethean and pelean; McEwen and Soderblom, 1and these do seem to be directly associated with the diffeeruption types as described above. In general, prometheatype plumes are associated with promethean-type eruptbut the larger pelean type plumes have been observethe same locations as both pillanian and lokian eruptiGeissler et al. (2004, this issue) discuss the different tyof plumes on Io. In this paper, we focus on the surface cacteristics of eruptions.

4.2.1. Observations of eruptive centers showingpillanian-type activity: Thor and Tvashtar

“Pillanian” eruptions (Keszthelyi et al., 2001; McEwenet al., 2003) appear to have a short-lived intense episshowing high temperatures and fissure-fed eruptions,produces both extensive pyroclastic deposits and rapemplaced lava flows. Plumes> 200 km in height usu

t

,t

,

ally occur, although the plume from the 1997 Pillan erution was only∼ 140 km in height (McEwen et al., 1998Williams et al., 2001b). The hot spot Pillan is the prototypical locale for this eruption type (Keszthelyi et al., 200Pillanian-type eruptions have also been observed at Tva(e.g., McEwen et al., 2000, 2003; Wilson and Head, 20Keszthelyi et al., 2001). Extensive plume deposits aresociated with pillanian eruptions, but these are not nesarily emplaced simultaneouslywith the high thermal emission phase of the eruption. At Tvashtar the extensive pludeposit occurred several months after the main erup(Keszthelyi et al., 2001). Eruptive centers that producelanian eruptions have also exhibited other types of activFor example, lava flows followed the Tvashtar event of Nvember 1999 (e.g., McEwen et al., 2000; Williams et2001a, 2001b). Pillan itself showed less violent activity cfined to a patera (possibly a lava lake) before the 1997 e


ig-Io.py-

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be) or

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fly-lly

WSolu--by.lavaingnaler-so

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eenfore01.

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t ofew

th-1B,twonicmay

. 5)m-e).ich

sin-

ra-ra-ab;ob-eli-IMSthatltra-

ng00;i etedthe

(Lopes-Gautier et al., 1999). It is not yet known what trgers the particularly violent pillanian-type eruptions onThe 1997 event at Pillan produced a 400 km diameterroclastic deposit rich in Mg-rich orthopyroxene (Geissleal., 1999; Williams et al.2000, 2001b), whereas the planian 1999 event at Tvashtar (discussed below) proda 1200 km diameter red ring, similar to Pele’s, that maycolored by short-chain sulfur (e.g., Spencer et al., 1997Cl2SO2 (Schmitt and Rodriguez, 2003), suggesting theping of volatile regions of different compositions.

NIMS observed the Tvashtar eruption during I25 and(Lopes-Gautier et al., 2000) and the waning phase oferuption during I31 and I32, as discussed below. A newlanian eruption from a previously unknown volcanic cenwas serendipitously observed by NIMS during the I31by. This volcanic center, which has not yet been officianamed (proposed name Thor), is designated I31A.

“Thor” (I31A). This is located at 39 N, between 131and 135 W, northwest of the Amirani lava flow field. NIMobserved this region numerous times at low spatial restion, but no hot spot was detected there before the I31 flySSI images show preexisting bright flows and new, darkflows resulting from this eruption in images acquired durI32 (see Turtle et al., 2004, this issue). The NIMS regioscale observation in I31 (Fig. 1) showed two nearly ovlapping hot spots (I31A and B, see Table 2). I31A wasbright that its central region saturated NIMS at most walengths, including 4.7 µm. Using the nonsaturated pixelower limit for the power output was calculated: 1.4× 1011

W µm−1. For comparison, the power output of Pillan4.7 µm during its main eruption phase (orbit C9) was oforder of 3× 1011 W µm−1 (Davies et al., 2001).

It is not known when the I31A eruption started, but oservations of Pillan and Tvashtar suggest that the vigoous, pillanian phase of the eruption is short-lived (McEwet al., 2003). It is therefore likely that NIMS captured tevent close to its beginning. This region was observed twby NIMS a few months before during orbit C30 (Ma23, 2001). No hot spot at the location of I31A was dtected by NIMS in either of the two C30 observatioThe NIMS 30INGLOBAL01 observation imaged Io in rflected sunlight at 170 km (NIMS pixel)−1, while the NIMS30INECLPSE01 observation imaged Io while eclipsedJupiter (spatial resolution was 190 km (NIMS pixel)−1). Anevent with the power output detected in I31 would have beasily detected in either of those observations. We thereconclude that the pillanian event started after May 23, 20However, it is possible that a faint hot spot could have bpresent at this location prior to I31 and not detected byvious NIMS observations at low spatial resolution.

A major component of the I31 pillanian event was the 5km high plume observed by Galileo SSI that resulted iwhitish plume deposit (Turtle et al., 2004 this issue; Geiset al., 2004, this issue). The NIMS observation in I31tected SO2 in the plume deposits (Douté et al., 2004). It

interesting that the three plume deposits associated withlanian type eruptions observed by Galileo all have differcolors (dark at Pillan, red at Tvashtar, whitish at I31A), raing the possibility that they may be associated with plumof different materials, perhaps silicate-rich (Pillan), sulfrich (Tvashtar) and SO2-rich (I31A; see also Williams et al2004, this issue). These deposits and their contributionIo’s resurfacing are discussed by Geissler et al. (2004,issue).

The I31A hot spot was observed by NIMS again durthe I32 fly-by, both at regional and local scales. The hot swas still very bright at NIMS wavelengths (Figs. 2 andbut no longer showed saturation. The power output at 4.7had decreased to 0.37×1011 W µm−1, still the highest valuefor all of the hot spots observed in I32.

NIMS was able to observe I31A at high spatial resotion during I32, thanks to the willingness of the Galilproject staff to allow the re-targeting of an observationshort notice, following the discovery of the I31A hot spA thermal map made from the NIMS 32INHTSPOT obsvation (Fig. 5) is compared with a color SSI image ofregion obtained in orbit C21 (2 July 1999), prior to the ertion, and with the SSI image obtained in I32, after the mevent (Turtle et al., 2004, this issue). The NIMS thermmap clearly shows two major, distinct active areas. I31Athe main eruption site and its location coincides with thaorange-colored lava flows in the SSI image from C21; nflows appeared at the same location in an SSI image fromI32 (Fig. 13 in Turtle et al., 2004, this issue). To the souwest of I31A, another hot spot, which we designated I3is located at a small patera. The relationship between thehot spots is unclear. The renewal of activity at both volcalocations at apparently the same time suggests that theybe part of the same volcanic system.

The hottest areas shown in the thermal map (Figcoincide with the region where SSI observed newly eplaced lava flows in I32 (Turtle et al., 2004, this issuThe highest temperature detected by NIMS is 800 K, whis high for a single-temperature fit. For comparison, agle temperature fit to the spectrum of Pillan+ Pele dur-ing its main eruption (orbit C9) yields a similar tempeture (∼ 725 K, Davies et al., 2001). The hotter tempeture component at Pillan (> 1800 K) was derived fromjoint NIMS-SSI 3-component fit (McEwen et al., 1998Davies et al., 2001). Since the I31A observation wastained in reflected sunlight, it is not possible to obtain rable thermal data from the shorter wavelengths in the Nrange for a 2 or 3 component fit. However, it is possiblethe highest temperatures at I31A were similar to the umafic range temperatures detected at Pillan.

Tvashtar. The pillanian-type eruption at Tvashtar duri1999–2000 was described previously (McEwen et al., 20Lopes-Gautier et al., 2000; Howell et al., 2001; Keszthelyal., 2001). The NIMS observations in I31 and I32 providan opportunity to track the waning activity, and to map


I32

fromight.

SSIof the

bliqu

Fig. 5. The hot spots I31A (“Thor”) and I31B as viewed by NIMS and SSI during I32. The NIMS thermal map (middle panel) obtained during orbit(I32HTSPOT01, see Table 1). Areas shown in black represent pixels for which no temperature fit could be obtained,mostly due to lack of signal in the thermalchannels except for some pixels in the central part of the hot spot I31A, which were saturated. SSI images are shown for comparison. An SSI imageorbit C21 (left) shows orange-yellow flows that have been covered over by flows from the new eruption, which are shown in the SSI image on the rInterpretation from SSI images is discussed by Turtle et al. (2004, this issue).

Fig. 6. Tvashtar Catena was imaged by SSI (top) during I32 and by NIMS (bottom) a few months earlier during the I31 Io fly-by. The contours on theobservation indicate areas that are 300 K or higher in the NIMS temperature map. NIMS shows that several areas are active, including the location1999“lava curtain” fissure just south of the main patera, and the southern and eastern parts of the main patera. The scale bar is approximate due to the oeviewing angle of this observation.


d a1)SSIhe

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thermal structure of the hot spot. Tvashtar was observehigh spatial resolution by NIMS during I31 (August 200and the thermal map is shown in Fig. 6, alongside animage acquired during the I32 fly-by in October 2001. TNIMS observation shows that the Tvashtar Catena recontains several active sites. It is unknown whether thsites are part of the same volcanic system.

NIMS detected activity at 5 main sites at Tvashtar, w550 K being the highest temperatures detected. Thislower limit, as this observation was obtained in reflecsunlight. The relative intensity of the activity as shownthe thermal map suggests a migration of activity fromsoutheast to the northwest. The southernmost activity icated at a patera (I31J, Table 2) at 59.5◦ N, 117.9◦ W. NIMSdetects faint activity from the southwestern part of this pera’s interior (Tc < 300 K), an area that appears darker ththe rest of the interior in SSI images prior to the I32 fly-(Plate 2 in Keszthelyi et al., 2001). This patera shows acity that is confined to the interior, similar to paterae thatdiscussed below. Moving towards the northwest, the secactive location is a small patera (I31K in Table 2) at 60.5◦ N,120.4◦ W. This patera is surrounded by flow deposits butobservation shows that the flows were not active duringand that activity was confined to the interior of the patera

The most active regions detected by NIMS are furthethe northwest, at three locations. Continuing from southto northwest, NIMS detects activity (Tc > 300 K) at the pat-era where an explosive lava curtain along a fissure was ain the I25 fly-by (Tvashtar Catena “lava curtain site” in Tble 2 and Plate 2 in Keszthelyi et al., 2001), and at the pawhere the “dolphin tail” flow was active during I27 (TvashCatena “flow site” in Table 2 and Plate 4 in Keszthelyi et2001). Between November 1999 and February 2000, actmigrated northwest from the “lava curtain site” to the largpatera (“flow site”). The SSI image from I32 shows an aof low albedo material between the two eruption sites, incating that new flows may have spilled out of the “flow sipatera at this location. The NIMS thermal map from I31 cfirms that this area is active, and has pixels showing somethe highest temperatures in the area (Tc > 500 K).

The fifth location where NIMS detects activity is a smalow-albedo region in the SSI image that is surroundedlight-colored materials (I31L in Table 2). The light colormaterials are clearly seen in the SSI image obtained inand also in the SSI color image obtained in orbit C21 (J1999). The light-colored materials are likely plume depothat have been embayed by subsequent lava flows. The2distribution map made from the same NIMS observa(Douté et al., 2004) shows a band rich in SO2 just to thesouth of the low albedo, high thermal emission areaappears to coincide with these light-colored materials. Hever, the oblique viewing geometry of both the NIMS aSSI fly-by observations makes fine correlation between tobservations difficult.

Although few temporal data points are availableTvashtar, there was clearly a decline in the level of

ttivity after the “lava curtain” phase in I25 and the flophase in I27. The total power output at 4.7 µm could noobtained from I25 data because the NIMS observationgeted only a small part of the active region. In I27, a lararea was targeted, but the activity had shifted to the wand again the NIMS observation missed a large parthe active area. A lower limit for the I27 power outp(which we estimate could be more than 50% lower tthe total) was obtained from the NIMS 27INTVASHT0observation (see observation details in Lopes et al., 20(0.85± 0.1) × 109 W µm−1. Power output between I31 anI32 remained steady (Fig. 4 and Table 4), but the areamained active.

It is not known when Tvashtar was active prior to I2NIMS data showed no activity in this area prior to the Ifly-by, but it is possible that faint activity would not havbeen detected from distant observations. SSI low-resoluobservations prior to I25 show low albedo areas, particulinside the largest patera (“flow site”), suggesting that acity had occurred there in recent times. Milazzo et al. (20studied SSI images of the region and proposed that theTvashtar patera may contain a lava lake similar to Lokwhich perhaps overturns from time to time, which woumean that Tvashtar also presents lokian-style activity. Therelationship between the postulated lava lake and the oactive sites is not understood. Tvashtar Catena is a parlarly complex volcanic region to interpret, given the possimigration of activity and the presence of diverse expressof volcanism: lava fountains, flows, possibly a lava lake,one of the largest plume deposits seen on Io.

4.2.2. Observations of volcanic centers showingpromethean-type activity: Prometheus and Amirani

“Promethean” eruptions are characterized by long-lilava flows that can be hundreds of kilometers long, with gerally smaller plumes (< 200 km high) associated with lavflow fronts, probably created by the interaction betweenlava and the underlying SO2 snowfield (Kieffer et al., 2000)As the name implies, the hot spot Prometheus is the prtypical locale for this eruption style (Keszthelyi et al., 200McEwen et al., 2003). Other ionian centers displayingeruptive behavior include Amirani, Zamama, and CulaThe morphology of the flows is suggestive of pahoe(McEwen et al., 1997; Keszthelyi et al., 2001), althougis important to note that the available image resolution dnot allow for unequivocal identification of specific lava flotypes. Furthermore, given the ionian ambient conditionsthe potentially exotic lava compositions (e.g., Williamsal., 2000; Kargel et al., 2003a, 2003b) it is not possiblepredict whether Hawaiian-like pahoehoe and a’a are likto be created on Io, or if ionian lava flows would have thown distinct morphologies.

The main sites of promethean type eruptions identifiedIo are Prometheus itself and Amirani. NIMS obtained hspatial resolution observations of Prometheus in I24 andand of Amirani in I27 and I31. The thermal characterist


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derived from NIMS high spatial resolution data from I24 aI27 show that Prometheus and Amirani have thermal pfiles that are consistent with the interpretation of insulaflows fed by tubes, forming compound flow fields (Lopesal., 2001). Below we examine in detail a NIMS observatshowing the whole Amirani flow field, and observationsboth volcanic centers at the regional scale (Figs. 1–3), wshow temporal changes.

Prometheus. The power output of Prometheus at 4.7 µwas obtained for all fly-by orbits and shows that Promethhas remained consistent in its power output from I24 throI32 (∼ 1010 W µm−1). The data from this time period indcate that the flow activity was mostly steady between 1and 2001 (Table 4 and Fig. 4). Steady effusion of lavaalso indicated by the very similar thermal profiles alongPrometheus flow between I24 and I27 (Lopes et al., 200

Power output levels from I24 through I32 remainedlevels considerably below some of the values reportedorbits prior to I24, which suggest periodicity in the actity, with the greater power output levels observed priorI24 perhaps being due to greater areal extent of active l(Davies, 2003). However, the possibility that activity aoccurred in the Prometheus Patera at various times cabe discounted, as the NIMS low spatial resolution obsetions prior to the Io fly-bys could not distinguish betweenpatera and the lava flow. Whatever the cause for the peathe activity of Prometheus reported from earlier data, aorbit I24 Prometheus either remained at a steady level otivity or else peaks in activity occurred during the gapstime between Galileo observations.

Amirani. The Amirani–Maui flow field (Fig. 7) is abou300 km in length and includes the longest active lava floknown in the Solar System, interpreted by Keszthelyi et(2001) as a complex, compound flow field. A high sparesolution NIMS observation obtained in I27 (Lopes et2001) shows thermal emission from the main Amirani flthat runs north–south, but no thermal emission was detefrom a secondary flow field that runs east–west fromAmirani main flow towards Maui. However, the NIMS I2observation did not cover the whole western flow field,Maui Patera itself. A hot spot previously detected from dtant NIMS observations was interpreted to be Maui Pa(Lopes-Gautier et al., 1997), but the location remainedcertain because of the large errors associated with locatinhot spots in low spatial resolution observations. So, qutions remained whether the hot spot was from Maui Paor from the east-west flow field, and whether the plumeserved by Voyager (and attributed to Maui) might be simto that of Prometheus, created by the interaction of the ewest lava flow field with the underlying SO2 snowfield. TheNIMS high spatial resolution observation of the AmiranMaui region in I31 was designed to investigate the relatiship between the Amirani flow, Maui Patera, and the locaof the Voyager-era plume.

t

Fig. 7. NIMS observed the Amirani–Maui region in I31. The SSI ima(lower left) shows (in white) the outline of the whole NIMS observatioA thermal map made from the NIMS observation (lower right) showsactive areas in this region (whites,reds, and pinks are the hottest arethe observed intensity is rendered in pseudocolor according to thebar shown). A profile (top, labeled 1through 8) is shown along the maAmirani flow. Number 1 of the profile is at northern end of the flow anumber 8 at southern end. The highest temperatures (∼ 550 K) are locatednear the inferred vent area (#8). The flow profile is consistent withexpected for an insulated flow and similar to those of Prometheus obtain orbits I25 and I27 (Lopes et al., 2001).

The NIMS thermal map made from the I31 obsertion (Fig. 7) clearly shows that Maui Patera is activealso shows that the east–west Amirani flow field has mocooled, but some active areas remain. One active areclose to the main Amirani flow and was not detected in IA second active area is close to the distal regions of the flThese areas coincide with the lowest albedo region of thflow as seen in visible wavelengths (Fig. 7).

The remnant thermal activity of the east–west flow fiinfluences the pattern of SO2 deposition in the area, as thcoincides with a noticeable decrease in the SO2 coverage(Douté et al., 2004). The low level of activity at this flofield also supports the interpretation of McEwen et al. (20


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that the Maui plume was related to that flow field. The cooling of the flow field most likely led to the cessation ofassociated plume, as may happen to Prometheus evenaccording to the model of Kieffer et al. (2000).

The NIMS thermal map shows that significant chanhappened at the main Amirani flow field (north–south)tween I27 and I31. In I27, the thermal profile alongflow field was significantly different from that of Prometheu(Lopes et al., 2001). While Prometheus showed its higtemperatures near the vent anda fairly uniform temperatureprofile along the length of the flow field, the thermal pfile of the north–south Amirani flow showed significangreater variations along its length, indicating breakoutfresh materials at several places. The highest temperatudetected by NIMS along the main flow field were at le1000 K (in the I27 observation, Lopes et al., 2001). The pence of breakouts, which corresponded to low albedo ain the SSI images, supported the conclusion of Keszlyi et al. (2001) that the Amirani north–south flow wascompound flow field, perhaps similar to flood basalt flfields on Earth, such as the Columbia River Basalts. Bthe Prometheus and the Amirani flow fields were thouto be of pahoehoe type (e.g., Keszthelyi et al. (2001)).smoother thermal profile of Prometheus was interpretean indication of an insulated flow field in which lava flowthrough tubes, keeping most of the surface temperaturesstant (Lopes et al., 2001).

The thermal profile of the Amirani main flow field madfrom the NIMS I31 observation (Fig. 7) shows that btween I27 and I31 it had become similar to the Promeththermal profile, and that the breakouts along the flow fiobserved in I27 had mostly ceased. The highest tematures detected along the flow field are lower (∼ 550 K)than in I27 (> 1000 K), and are close to its inferred sourat the southernmost end. The high temperature peaktected in I27, which were interpreted as breakouts, wnot detected in I31, indicating that the lava has cooledcrusted over at those locations. The Amirani thermal proalong the flow field in I31 is best interpreted as very silar to that of Prometheus discussed by Lopes et al. (2and Keszthelyi et al. (2001): an insulated lava flow fiepossibly pahoehoe in type, where lava is flowing throtubes.

Further insight into Amirani’s changing activity cangained from the NIMS regional scale observations inand I32. The power output at 4.7 µm (Table 4, Fig. 4)not vary significantly between I31 and I32, indicating ththe flow field activity had remained steady during this timThe power output at 4.7 µm during I27(12.3 ± 0.5) × 109

W µm−1, was higher than during I31 and I32. The decrein power output from I27 to I31 is consistent with more qescent activity taking place in the later orbit.

y,

-

-

4.2.3. Observations of patera (“lokian” type) volcanism:Io’s lava lakes?

Patera volcanism is the most prominent eruption typeIo (Radebaugh et al., 2001) and may reflect the predomimechanism driving ionian eruptions. The main characteritic of this type of activity is the presence of lava lakes andflows confined within caldera-like paterae. This type oftivity is sometimes associated with plume eruptions. Sopaterae are partially surrounded by plume deposits (seecussion on Tupan below) and, in the case of Pele, a plis often present. The paterae vary substantially in sizeeruptions within them, covering a wide range of power oput, from faint hot spots such as Emakong (which wastected as a hot spot by NIMS only during the Io fly-bys)Loki (Io’s largest patera at∼ 200 km across). Loki is thmost powerful volcano known in terms of thermal enereleased (its total energy output is of the order of 1013 W,Spencer et al., 2000) and is easily detected from grobased observations (e.g., Spencer et al., 1997). The ovations obtained by NIMS during the I31 and I32 fly-btargeted several active paterae. A major objective was totain a high spatial resolution thermal map of Loki, the “tylocation” for patera volcanism, to try to confirm the hypoesis of Rathbun et al. (2002) that Loki is a giant lava lake

Loki. Loki is the hot spot most often observed from Eaand is the suspected site of at least one volcanic outbdefined as an event that at least doubles the total 5flux from Io (Howell et al., 2001; Veeder et al., 1994Despite the high level of activity over the years, the pera’s appearance has remained almost unchanged sinVoyager observations in 1979. Therefore, if outburstscur at Loki, they must have different characteristics frpillanian-type events, which cause major surface chanVoyager images of Loki showed a dark patera floor srounding a light-colored “island” or topographically higregion near the center. Observations of Loki duringI24 and I27 Galileo fly-bys (Lopes-Gautier et al., 200Spencer et al., 2000) showed that the dark patera floorcovered by active or cooling flows, while the “island” wcold. The dark material was interpreted as either the cocrust of a lava lake or cooling flows covering the patera fland the NIMS and PPR results could not distinguish betwthese possibilities. Rathbun et al. (2002) analyzed grounbased and other temporal data on Loki’s activity obtaiover several years, which showed patterns of brighteningfading, and proposed that Loki is a lava lake that undergcatastrophic overturns. In the model of Rathbun et al. (20a wave of resurfacing by silicate magma propagates arothe patera, starting in the southwest corner. The wave swhen the crust on the lake founders, exposing hot mateResults from PPR observations in I24 and I27 (Spencer e2000) were re-interpreted as the movement of the foundefront across the floor of the patera.

Thermal maps from Galileo NIMS data have the pottial to distinguish between the lava lake and flooded pa


eas)ater

Fig. 8. NIMS observed Loki Patera in darkness during I32. A NIMS map at 2.5 µm (top right) clearly shows a hot edge (indicated by white and red arnext to the western patera wall. A temperature map from the NIMS observation (lower right) shows that the highest temperatures are found near the pawalls and against the walls of the cold “island.” The SSI image on the left, obtained earlier during the mission, illustrates the approximate outline of the areasshowing thermal emission at NIMS wavelengths.

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floor hypotheses. The NIMS I24 observation (Lopes-Gauet al., 2000) was suggestive of a lava lake because theest color temperatures on the patera floor were detectedpixels next to the patera walls (where the crust of a lavawould commonly break up, exposing underlying hot marial), but the area observed was not large enough forresults to be conclusive. An observation covering a grearea of the patera floor was obtained in I32 with the obtive of distinguishing between the flooded patera and llake hypotheses.

The NIMS I32 observation of Loki (Fig. 8) was obtained while the patera was in darkness, allowing temature determinations to be made from the data withoutneed to extract the reflected sunlight component. A sincomponent fit to the data shows that the highest temperaare found at the southwestern margin of the patera, wthe model of Rathbun et al. suggests foundering of the cstarts. A colder region is seen immediately to the easthis area that likely represents cooled crust resulting fcooling after the previous resurfacing wave (Lopes et2002). Other hot regions are located at the edges of thetera and the “island,” where the crust of a lava lake wolikely break up where it abuts the walls. Although breakonear caldera walls on Earth have also been observed inwhere lava flows within a caldera move towards the caldedges, breaking up as the lava encounter the walls (for eple, the Pago 2003 eruption, Venzke et al., 2003, also DRothery, personal communication), this is a less commscenario and, in such case, we would expect to see thepatters of several lava flows moving towards the Loki pawalls.

The NIMS thermal map (Fig. 8) is consistent with tmodel of Rathbun et al. (2002): hotter edges against thetera walls, with the hottest area observed by NIMS atlocation where their model predicts, and the coolest reg

-

s

-

s

-

l

just to the east, where their model predicts the oldest cis located. Therefore, the NIMS results are consistent wthe model of a slowly overturning lava lake (see discusson terrestrial lava lakes, Section 4). However, the timingthe last overturn before I32 is still unclear. Loki was inrelatively quiescent state during I32 and more recentsuggests it is no longer undergoing periodic overturns (Rbun et al., 2003), but it is unclear at present whether theoverturns completely stopped or are happening in a mquiescent manner.

Pele. Pele is also thought to be an active lava lake (eHowell, 1997; McEwen et al., 2000; Davies et al., 200Keszthelyi et al., 2001; Radebaugh et al., 2004, this issThe evidence for the existence of a lava lake at Pele waviewed by Radebaugh et al. (2004, this issue) and revehigh temperatures over long periods of time, no massiverounding lava flows, curving lines of very hot materials sein SSI images, and rapid variation of thermal output (scof minutes) observed from Cassini imaging data, consiswith variations observed on lava lakes on Earth (Flynn et1993).

The composition of Pele’s lavas may be mafic ortramafic, based on lava temperatures as discussed bAlthough lava temperature is an indicator of compositionwe must interpret the data with caution, as the temptures determined from the data are essentially lower limto the liquidus temperature. A high spatial resolutionservation obtained by NIMS in I27 (at 1 to 1.9 km (NIMpixel)−1) showed that high temperatures were presena broad band across the southern half of the patera,ticularly in the southeast corner (Lopes et al., 2001). Tdata, taken with NIMS gain state 2, showed considerasaturation at wavelengths longer than 1 µm. Detailedamination of the observation yielded a cluster of 15 ov


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lapping pixels near the edge of the hot area wherea few bands were saturated and a hot component cbe determined (1760± 210 K, Lopes et al., 2001). Thtemperature spans the liquidus temperature range formafic (basaltic) lavas and ultramafic lavas (high Mg-baskomatiitic basalts, and komatiites). Terrestrial basalts tcally erupt at 1300–1450 K (Morse, 1980) while terrestultramafic lavas erupt at temperatures> 1500 K (komati-ites were erupted between 1700 and 1900 K; Hess, 1Herzberg, 1992).

Pele was imaged by Cassini during that spacecraft’sby of the Jupiter system in December 1999. Radebaugal. (2004, this issue) analyzed imaging data from Cassiniobtained color temperatures most commonly of 1360 K, wa high of 1500± 80 K. Given that the lavas at the surfaceIo must have undergone some cooling from liquidus tematures, the values measured by NIMS and Cassini are lto represent ultramafic compositions.

During I31 and I32, NIMS obtained new observationsPele in darkness. A prime objective was to obtain temptures that might confirm the suggestion of ultramafic coposition. The NIMS gain state was lowered to 1, makthe instrument less sensitive. However, this was not scient to significantly reduce saturation. Initial analysisnot yielded a cluster of relatively saturation-free pixels tcould be used in an analysis similar to that done by Lopeal. (2001).

The I31 and I32 data did reveal the thermal structurea larger area of Pele (Fig. 9) than was previously obserWe now know that three main active areas were presenting I32. Their position relative to a Voyager image has bestimated and is shown in Fig. 9 (the Voyager image isthe highest resolution image of Pele in reflected sunligThe active areas seen by NIMS appear to correspondtrough or rift where a low albedo area is apparent in the Vager image. However, it is not possible to navigate the NIobservation precisely onto the Voyager image, becauspointing errors in both data sets. We must also note that tmay have been significant surface changes at Pele sinceager imaged this region in 1979. It is not clear how all thareas are related to one another in terms of style of actIt is probable that the larger of the hot regions correspondthe region of high output imaged at the same time as Nby Galileo SSI at 60 m pixel−1. The smaller NIMS hot regions to the west could be the small hot spots seen in theI32 images of Pele (see Radebaugh et al., 2004, this isDuring this observation, color temperatures were determinfrom SSI data to be as high as 1605±220 K or 1420±100 K,again within a basaltic to ultramafic temperature range.

Emakong. This heart-shaped, low albedo patera, ab66 km in diameter, is located in a region that wasserved by NIMS multiple times at low spatial resolutiprior to the Io fly-bys. However, no hot spot was detectedtil NIMS observed Emakong at spatial resolutions of ab25 km (NIMS pixel)−1 in I25 and I27. Lopes et al. (2001

;

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concluded that thermal emission in the patera most likhad stayed at a very low level throughout the Galileo mis-sion. Temperatures for this hot spot were difficult totermine from I25 and I27 observations, because the Nspectra showed the 4.1 µm SO2 absorption band, which wanot expected to be present in a volcanically active regLopes et al. (2001) combined data from the I25 and I27servations to obtainTc = 344± 60 K and postulated thaSO2 might have been deposited in colder regions insidepatera, leading to the presence of the band in the spectr

The persistently low temperatures observed at Emakare consistent with either cooled silicates or sulfur vcanism, as discussed by Lopes et al. (2001). Williamal. (2001a) suggested that the bright flows surroundEmakong might be sulfur flows. The temperatures of acsulfur flows are below about 400 K (e.g., Williams et a2001a), so if sulfur volcanism were taking place insidepatera, this would be consistent with the NIMS observatof the paterae.

The possibility of Emakong being a site of sulfur vocanism is intriguing, and therefore the patera was targetein I32 for high resolution, dedicated observations by bNIMS and SSI. Emakong was observed but a hot spotnot detected in the regional-scale observations in I31I32, therefore, we assume that it remained cool. The NIhigh spatial resolution day-side observation in I32 yielda thermal map (Fig. 10), showing that the highest temptures were 270± 90 K (the relatively low signal contributeto larger errors than usual fortemperature determinationfrom NIMS data), consistent with either sulfur or coolsilicates. The most intriguing aspect of the thermal mathat it shows very distinctly that the highest temperaturesclose to the patera walls, as would be expected if there wrigid crust on a lava lake that was breaking up againstpatera walls. The highest temperatures in the NIMS tmal map correspond very well to the darkest areas inSSI high spatial resolution image acquired in I32 (Fig.see also Turtle et al., 2004, thisissue). Originally Williamset al. (2002) interpreted the dark band seen in the I32ages to be a shadow of the inner patera wall (indicativa depth of∼ 230 m); however, additional comparison wthe lower-resolution I25 images suggests that the dark bmay extend completely around the patera and is therenot a shadow. SSI and NIMS data are therefore consiwith either a crusted-over silicate lava lake or a sulfur lak

Although we cannot determine the compositionEmakong’s material from the current data, we can examsome of the implications of a sulfur or silicate compositiThe consistently low temperatures and thermal emissioEmakong indicate that either the material has a low ming temperature (sulfur) or that a molten lake’s activity hnot been vigorous during the years of Galileo observatiand the lake has remained mostly crusted over with fewcandescent cracks. If the material is molten silicates (mafior ultramafic), then the broken crust near the patera whas cooled significantly from liquidus temperatures. Gi


erved

Fig. 9. NIMS observed Pele in darkness during orbits I32 (top right) and I31 (bottom right). A Voyager image was reprojected to the perspectives of the NIMSobservations, and the latter were navigated to the best estimate of the feature corresponding to the hot spots. The very high temperatures at Pele saturated mostNIMS wavelengths, the maps shown above are at 1 µm, the only wavelength in theNIMS range that did not show saturation in the area observed. The obsintensity is rendered in pseudocolor according to the color bar shown.

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the low levels of thermal emission observed at EmakongNIMS, it is likely that the silicate lava lake has not undgone overturning or other vigorous activity in recent yearand eruption rates have remained low enough that no lavolume breakouts of hot material have occurred.

The possibility that Emakong contains molten sulfurmains open, and we can examine some of its implicatiA sulfur composition is consistent with observed tempetures from the patera. The colors observed by SSI (darktera surrounded by white and yellow flows, see Williams

-

al., 2001a) are not inconsistent with the sulfur interpretatsince molten sulfur can be black, and it can also brightewhite or yellow as it cools and crystallizes. The interptation would then be that the patera floor deposits consimolten or partly cooled sulfur, and the brighter flows arraaround the patera represent older, completely cooled ctalline sulfur. The chief difficulty with the sulfur hypothesisis that the temperatures observed in the patera floor (1270 K) are cooler than pure, stable, molten sulfur. Sulfurseveral stable and metastable solid polymorphs, and eac


SI imagesred. T

ndith th

ownfroin thend

Fig. 10. NIMS observed Emakong during the I32 fly-by. Emakong Patera is shown in an SSI color image from orbit C21 (a) and in clear filter Sobtained during the I25 fly-by (b) and the I32 fly-bys (d). The image in (d) has a different scale from the others and its location in (b) is outlined inheNIMS thermal map (c) shows that most of the patera floor is cool at NIMS wavelengths. The highest thermal emissionis found at the edges of the patera, acoincides with darkest areas shown in the SSI high-resolution images. SSI and NIMS data are therefore consistent with a crusted over lava lake, we crustbreaking up against the patera walls.

Fig. 11. SSI imaged the Tohil region during I32 and NIMS data were acquired as a “ridealong” observation (see Table 1). A NIMS map at 4.7 µm is shhere alongside the SSI mosaic. The observed intensity is rendered in pseudocolor according to the color bar shown. NIMS detected thermal emissionma small flow in Radegast Patera, located near the center of the SSI mosaic and shown by the arrow and contour. Thermal emission shows up as whiteNIMS image. This observation was acquired near the terminator and the only other bright areas in the NIMS map correspond to sunlit mountain faces ahave no thermal emission.


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a slightly different melting point. All of these melting poinare within the range of 385–393 K, significantly warmer ththe observed patera temperatures.

The observed temperatures may be reconciled withsulfur interpretation in any of three ways.

(1) Extensive outbreaks and resurfacing may occur episcally, such that the flows at most points in time are freyoung, but thinly crusted and in the process of coolalmost everywhere.

(2) The sulfur contained in the patera may be metastcooled below its ordinary melting point but may remamolten if it contains some impurities known to enhanpolymer stability (arsenic, tellurium, or selenium, for istance) or polymer cross linking (phosphorus) at ltemperatures (Kargel et al., 1999, 2000; (MacIntyreal, 2000)). Eruption of a metastable, impure, viscosulfur-rich liquid may then occur at temperatures atupper end of the observed range, such that temperaturearound 300 K and cooler are widespread.

(3) The sulfur in the patera may be extremely impure, phaps a low-temperature eutectic. While some elemhave a very minor effect on lowering the melting poof sulfur (phosphorus and tellurium, for instance), oth(chlorine and carbon for example) produce extremlow liquidus and eutectic temperatures. A mixture35% S+ 65% carbon disulfide (CS2), for example, hasa melting point of 300 K (Touro and Wiewiorowsk1966). Dissolved sulfur monochloride (S2Cl2) has acomparable influence on the melting of elemental sfur (Hammick and Zvegintzov, 1928).

While the presence of carbon has yet to be demonstron Io, chlorine has been detected (Kuppers and Schne2000; Lellouch et al., 2003), and it is likely that someIo’s chlorine is bonded with sulfur rather than the domnant sodium chloride. In pure form S2Cl2 forms a yellow–red liquid and melts at 193 K, though when elemental sfur is in excess, higher liquidus temperatures result.lurium, selenium, and especially arsenic are also likely impurities in Io’s sulfur (Kargel et al., 1999) and may cotribute to the phase stabilities and liquid temperaturecomplex sulfurous systems.Thus, the possibility cannot beexcluded that Emakong erupts low-temperature stablmetastable sulfur-rich lavas. This possibility is consistwith the color/albedo changes seen at the Emakong volcsystem, though special pleading to a role of impuritiesquenching is required to bring the observations into csistency with a basic sulfur-dominated composition. Pcrystalline elemental sulfur in stable form is very pale yellto bright yellow at the temperatures observed in EmakoDark sulfur at those temperatures would require quencfrom over 500 K if the sulfur is pure (Meyer et al., 197Sagan, 1979), or would require the presence of impurthat are known to darken sulfur (Kargel et al., 1999).

,

We conclude that the case for sulfur at Emakongcertainly interesting, but that widespread deposits of pwell-behaved, stable liquid and solid sulfur very nearmelting point is not consistent with the observations. Tconclusion leaves open other possibilities that involvefur as a major or dominant component, however, suchcooling crusted sulfur containing traces of impuritieschromophores, metastable glassy flows of cooling suquenched from high temperatures, or molten and crussulfurous eutectics. While metastability and impuritiesto be expected, the dissatisfying aspect of these mulpermissible explanations is that the degrees of freedompractically boundless, though they are not without specobservational tests possible from some future observingform. The behavior of sulfur containing controlled amouof selected minor impurities is discussed in detail by Karand MacIntyre (2004, this issue). On the issue of the rolsulfur or silicate volcanism at Emakong, clearly further higresolution spectral and thermal observations, and/or inanalyses by future missions, are required to solve these qtions.

Radegast. SSI imaged the Tohil region near the terminator during the I32 fly-by (Williams et al., 2004, this issueTurtle et al., 2004, this issue). NIMS did not have an obsvation dedicated to this region, however, NIMS data wacquired while SSI obtained images (Table 1). Althoughquality of these NIMS “ridealong” data is not as high as tof independent observations (note the gaps in the coveraFig. 11), it was sufficient to detect thermal emission fromsmall flow inside Radegast, a 25-km diameter patera imaby SSI (Fig. 11). This feature has been interpreted as alake from its morphology (Radebaugh et al., 2002; Turtleal., 2004, this issue). Thermal emission detected by NIcomes from a small flow (about 7.5 km in diameter) tappears to be emerging from the edge of the patera, froarea where the crust of a lava lake would likely break agathe walls as observed at other paterae. The temperature esmated from the NIMS data for this active flow is 325 K, bthe small amount of overlap in NIMS fields of view resuin a lower signal-to-noise ratio than in the NIMS dedicaobservations, and the error could be as large as 50 K. Oareas on the patera floor are cold at NIMS wavelengths.power output at 4.7 µm derived from the NIMS data is ab3.8×106 W µm−1, making this the faintest hot spot detectby NIMS during the entire Galileo mission. Because ofvery limited coverage by NIMS at high spatial resoluti(this observation is 2.4 km (NIMS pixel)−1), it is likely thatmany more faint hot spots such as this one remain undeteon Io.

Gish Bar. Gish Bar is a 115-km wide irregularly shappatera that lies near the base of an 11-km high moun(Perry et al., 2003). Activity at Gish Bar was first detectedNIMS in 1996 (Lopes-Gautier et al., 1997) and has beenserved several times since then (Lopes-Gautier et al., 19


ons96)

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The power output obtained from NIMS distant observatidid not significantly change between orbits G1 (June 19and E11 (November 1997). However, the persistent actat Gish Bar may have fluctuated significantly in level durthis period and others. Gish Bar appears very bright inSSI eclipse image taken in orbit E4 (December 1996, Pet al., 2003) and is probably the site of an outburst detefrom Earth in August 1999 (Howell et al., 2001).

Gish Bar was observed by NIMS during I31 and by Sduring I32 (Fig. 12). Perry et al. (2003) compared theimage from I32 with earlier SSIimages and concluded tha new eruption took place between October 1999 andtober 2001 resulting in a new, low-albedo flow from twestern patera wall (Fig. 12). The NIMS thermal maptained in I31 sheds new light into Gish Bar’s activity. Coparison of the I32 SSI image (taken in October 2001) wthe I31 NIMS thermal map (August 2001) shows thathottest areas inside the patera do not correspond wellthe darkest areas (interpreted as the youngest lavas) oSSI image. The three hottest areas detected by NIMS correspond to:

(i) a smaller patera to the northeast of Gish Bar (“noGish Bar,” I31M) that may or may not be geneticarelated to Gish Bar,

(ii) an area in the eastern part of Gish Bar patera thatresponds well to a flow that SSI images show wasplaced between July and October, 1999, and

(iii) an area near the north and center of the patera, wstill appear dark in the SSI image from I32, indicatirecent activity.

However, the flow that appears most recent in the SSIage from I32 corresponds to a cool area in the NIMS thmal map from I31. The new flow seen in the SSI iage may have just started to be emplaced when NIMSquired its observation, as NIMS detected a warm area acent to the western patera wall, whence the flow proberupted.

An increase in activity between I31 and I32, particulafrom the western side of Gish Bar Patera, is confirmed byNIMS regional-scale observations (Fig. 1 and 2). The pooutput at 4.7 µm (Table 4) increased from(0.7± 0.2) × 109

W µm−1 to (19.3±0.3)×109 W µm−1, an increase of morthan an order of magnitude.

Previous estimates of the power output of Gish Bar frNIMS night-time distant observations were 2.6± 0.5× 109

W µm−1 (orbit G1, June 1996) and 3.5± 1.3× 109 W µm−1

(orbit E11, November 1997). Although these values arelikely to represent the power output from Gish Bar alobecause of the very large sizes of the NIMS fields of vin these distant observations, they can be consideredper limits on the power output of Gish Bar at those timTherefore we can conclude that during I31, Gish Bar wactive at similar levels to G1 and E11, and that sometbetween I31 and I32 an outburst took place. Given that

e

-

bursts are usually very short-lived (Spencer and Schne1996) it is likely that during I32 Galileo fortuitously observed Gish Bar during one of these rare events. Ofticular interest is the morphology of the patera as viewby SSI (Perry et al., 2003; Turtle et al., 2004, this issua large new flow was emplaced, but the outburst didsignificantly change the morphology of the patera, ornificantly alter the morphology around it. This appearsbe a characteristic of lokian-style volcanism, and has hpened at Loki numerous times (McEwen et al., 199Rathbun et al., 2002).

Tupan. This colorful patera, about 75 km in diameter,one of Io’s most persistent hot spots (Lopes-Gautier et1999). Tupan was a primary target for the I32 fly-by, aboth NIMS and SSI obtained high spatial resolution obvations of the patera and surrounding area. The SSI imshows a remarkably colorful patera (Fig. 13, and Turtle eal., 2004, this issue). Radebaugh et al. (2002) studiedmorphology of Tupan and proposed that it may contalava lake similar to Loki’s. Interpretation of Tupan’s actity from SSI data is discussed by Turtle et al. (2004,issue).

Comparison of the NIMS thermal map with the SSI iage (Fig. 13) shows that the areas that appear dark inSSI image are hot and that the activity is concentrated on theastern side of the patera. At NIMS wavelengths, Tupana cold “island” or topographic high similar to Loki’s. Mapping of SO2 (Douté et al., 2004) shows that the island is cenough for some SO2 to condense on it. The SO2 distribu-tion around the patera is very inhomogeneous; in fact,2deposits are concentrated on the eastern side (Douté2004). The hottest areas, shown in white in the NIMS teperature map in Fig. 13, are located near the edges opatera, consistent with a lava lake interpretation and simto other paterae described above.

Tupan and Loki show several similarities. Like Loki, Tpan is a persistent hot spot that was active throughouGalileo mission. Both Loki and Tupan have cold “islandand both show activity concentrated on one side of the is(southern in the case of Loki, eastern in the case of TupImages from Voyager (obtained in 1979) and Galileo(1996 and later) showed that no significant morphologchanges happened at either patera. Both Voyager andimages of Tupan show a light-colored island, and a lalow albedo region on the eastern side of the island.

Another similarity between Loki and Tupan is that Tpan showed significant variations in power ouput duringGalileo mission, although they have not been as dramatLoki’s. The power output at 4.7 µm (Fig. 14) shows thaI32, Tupan’s activity was at a relative high, similar to E1Activity at Tupan may be episodic, similar to Loki’s, but tpaucity of ground-based data on this hot spot precluddetail study at present. Tupan had never shown an outuntil recent ground-based observations (S. Gibbard e


he areasthe NIMSotlthis

and thepatera.e

he

Fig. 12. Gish Bar was observed at high spatial resolution by NIMS during the I31 fly-by and by SSI during the later I32 fly-by. The outlines of twhere NIMS detects temperatures higher than 350 K (from I31) are shown in white on the SSI image (from I32). Comparison of the SSI image withthermal map shows that significant changes took place between the two fly-bys. The hottest areas insidethe patera shown in the NIMS thermal map do ncorrespond to the darkest areas (interpreted as the youngest lavas) in the SSIimage. The dark flow on the western side of the patera corresponds to a cooareaon the NIMS thermal map. This indicates that the flow was emplaced between the August and October 2001 Io fly-bys. The hottest area NIMS detected inI31 observation corresponds to another patera to the northeast of Gish Bar (Estan Patera).

Fig. 13. Tupan was imaged by SSI (A, C) and NIMS (B, D) during the I32 fly-by. Comparison of the SSI image (A) with the NIMS 4.7 µm map (B)NIMS thermal map (D) shows that the areas that appear dark in the SSI image are hot and that the activity is concentrated on the eastern side of thePanel (C) shows a reprojection of the SSI image to the perspective of the NIMS observation, with overlaid contours showing areas where NIMS detects thhighest temperatures (greater than∼ 450 K). Tupan has a cold “island” or topographic high similar to Loki’s (Fig. 8). The hottest areas, shown in white in tNIMS temperature map, are located near the edges of thepatera, consistent with a lava lake interpretation.


Tu-

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Fig. 14. Power output at 4.7 µm for Tupan Patera shows that significant vari-ations have occurred during the Galileo mission. The observations obtainedin G1, C10, and E11 were at considerably lower spatial resolution than thefly-by observations (see text for discussion). The data suggest that activityat Tupan may be episodic.

personal communication) showed that an outburst frompan occurred during March of 2003.

5. Lava lakes on Io?

Patera volcanism is a prominent expression of volcanon Io (Radebaugh et al., 2001) and our observations sthat thermal emission and the distribution of temperatuare consistent with the presence of lava lakes (here defin“lokian” type eruptions) within paterae. The thermal strutures of active paterae commonly show hotter regionsthe edges. These hotter edges may be due to the cruslava lake breaking up against patera walls, or due to crresulting from subsidence. In this instance, the cracks cbe pathways for the escape of heat from the lava lakeif no molten material is exposed at the surface. The lacrelatively high temperatures away from the patera walls mimply either that the crust of the lake surface is too thickfracture in the middle, or that the deformation at the marg

llake. At

ce,centeruptedof thehe

Fig. 15. Field photographs of the Kupaianaha lava lake at Kilauea volcano, Hawaii. (A) View looking across the lava lake in January 1987, when the lakeevelwas only∼ 3 m below the rim. This view shows a level of activity that is typical of the lava lakes on Io, namely the greatest activity at the edges of thethis time, the lake surface was moving towards the left at a rate of∼ 1 m min−1. Width of lake is∼ 40 m. (B) When the drift rate was higher (in this instan∼ 3 m min−1), numerous rifts in the lake surface can occur, forming discrete “plates” on the surface. Note the small amount of newly formed incandesmaterial along the boundary of two plates in the middle foreground. Differences in the direction of formation of each plate produce the strong photomtricproperties that are seen here. Width of lake is∼ 45 m. Image taken in December 1988. (C) At higher levels of activity, the center of the lava lake is disby low-level fountaining of fresh lava and the rapid resurfacing of∼ 20% of the lake surface as new lava causes older material to founder. Drift ratesurface can be as high as 5 m min−1. Height of the wall in this image (taken in December 1988) is∼ 20 m. (D) Rare examples of frozen “islands” within tperched lava pond associated with the 1959 eruption of Kilauea Iki onthe eastern rim of Kilauea caldera, Hawaii. These islands are∼ 3–5 m high, and arepreserved remnants of a surface that predates the smooth, polygonally fractured, surface of the lake. Lighting isfrom the right in this image. (Photographs byP. Mouginis-Mark.)


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There have been several recently active lava lakesEarth that have been studied for their thermal propertiesmay offer the greatest insights into possible Io lava lakenamics, of which the Kupaianaha, Kilauea volcano, Haw(Flynn et al., 1993) and Erta Ale, Ethiopia (Oppenheimand Francis, 1997; Burgi et al., 2002), lava lakes are twthe best studied. We observed the evolution of Kupaianlava lake in the field during the period 1987–1992, and mseveral observations that may help explain the distributiotemperatures observed at Loki, Emakong, Gish Bar, andpan. First, the concentration of high temperatures aroundperimeter of the lake surface does not uniquely charactethe activity within a lava lake. More importantly it appearsbe associated either with vertical motion and shearing olake surface against the wall, or the removal of lava fromlake via a near-surface lava tube (Fig. 15a). At Kupaianvertical motion of∼ 1 − 3 m was sufficient to preserve incandescent material (observed with a spectroradiometbe in excess of 800◦C; Flynn et al., 1993) around the edof the lake. Typical horizontal movement of the Kupaianalake surface was of the order of 0.5–2.0 m min−1 during thisphase of activity.

Secondly, periods of greater activity at Kupaianaha pduced significant rifting (Fig. 15b) or fountaining (Fig. 15within the central portion of the lake. Tearing of the lasurface was also common, with the result that new hotover-ran the cooler older surface, causing large (10–2wide) segments of the surface to founder. Such activitysulted in a very different distribution of temperatures acrthe lake surface than has been observed for the examon Io, namely there were randomly distributed places othe lake surface that went from temperatures in the raof 200–300◦C to > 800◦C in a few tens of seconds. Horzontal movement of the lake surface increased at such tiwith the older surface moving at up to∼ 5 m min−1.

Finally, we saw no examples of islands created withinKupainanaha lava lake that might be analogous to thoseserved within Loki or Tupan. Although the lava lake progrsively deepened during its evolution from the lake surfbeing within 1 m of the rim in 1987 to more than 30 m belothe rim in 1991, no remnant features akin to the Loki islawere observed. Examples of topographic remnants havebeen seen in depressions referred to as “perched lava powhere lava drained into preexisting topographic low pobut failed to completely submerge remnants of the easurface. Examples of this type of feature have been seethe Kilauea Iki lava lake, Hawaii, which formed during t1959 eruption (Fig. 15d).

The temporal differences in lokian-type activity probabreflect magma transport from the interior, but the dynamof the lava lakes are likely controlled by magma recyclin a manner similar to certain terrestrial lava lakes suchErebus, Erta Ale, and Nyiragongo (Harris et al., 1999).though temporal data are limited for most of the active ion

s

,

,”

paterae, clear differences have been observed betweenand Pele (McEwen et al., 2003), the two best-studied paton Io in terms of temporal activity. Our current understaing is that the eruption styles of Pele and Loki differ substtially (McEwen et al., 2003), with resurfacing at Loki hapening quasi-periodically and dramatically (Rathbun et2002), while at Pele it appears to be more continuous, shing little change throughout the NIMS distant observatioprior to the Io fly-bys (Davies et al., 2001), yet showing vaations on timescales of minutes from Cassini observat(Radebaugh et al., 2004, this issue). Loki and Pele also sa difference in terms of temperatures detected (McEweal., 2003; Kargel et al., 2003a).

Temperatures detected at Loki have been consistentllow about 1200 K, while very high temperatures have bdetected for Pele from Galileo NIMS (1760± 210 K, Lopeset al., 2001) and SSI data (1605± 220 K, Radebaugh eal., 2004, this issue). While this temperature difference dnot necessarily imply a difference in composition (andmust emphasize that the values detected are lower limits fothe liquidus temperatures) it may indicate some differein how the hottest material is being exposed at these ltions. It is not yet known what processes control the differbehaviors of these two (possible) lava lakes, but the tperatures detected are all consistent with a range of masupply rates for mafic or ultramafic magmas.

The recent data discussed here suggest that Tupancontain a lava lake similar to Loki’s, perhaps with episoactivity. Gish Bar also shows significant variations thmay imply similar underlying mechanisms to those at LoEmakong, on the other hand, is either a quiescent sililava lake, perhaps containing a cooling body of lava acas a solid cap on top of a silicate lava lake, or else itmolten sulfur lake that is inherently at a cooler temperatu

Sites of promethean and pillanian type activity may abe associated with lava lakes. Both Prometheus and Amhave paterae associated with their lava flow fields (McEwet al., 2000; Keszthelyi et al., 2001). Amirani’s is actiand may contain a lava lake, but no activity has beentected from Prometheus Patera. Pillan Patera may havethe site of a lava lake prior to the violent 1997 erupt(McEwen et al., 1998a; Williams et al., 2001b). The largpatera at Tvashtar may also be the site of a lava lake (Milaet al., 2002). Tvashtar shows some intriguing similaritiesPele: the plume deposits are very similar in morphology,tent, and color, and the thermal maps presented herethat both locations have several active hot spots thataligned, perhaps following a fissure, and appear to benetically related.

Persistent activity within paterae has important implitions for the interior of Io, as it suggests easy accesmagma, possibly in the form of a magma ocean undernthe crust, as proposed by Keszthelyi et al. (1999, 2004,issue). Moreover, this style of activity places constraon how Io is being resurfaced. If a significant amountIo’s thermal energy is confined within paterae, then


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rate of resurfacing may be significantly lower than preously estimated by attributing the total thermal outputextensive, unconfined flows (e.g., Matson et al., 2001).resurfacing rates derived from the lack of impact cra(Johnson and Soderblom, 1982) need not be explaineeffusive eruptions. Geissler et al. (2004, this issue) sugthat plume deposits, from repeated eruptions of Promethtype (SO2 rich) plumes, are a major contributor to Io’s resfacing rate.

The apparent confinement of the majority of activitythe interiors of paterae implies a density contrast betwIo’s crust and the magma. If the crust and the magwere relatively homogeneous in composition and had silar volatile abundances or no volatiles, then there shobe extensive effusive flows driven by lava buoyancy andsinking of the colder solid crust. The fact that much ofactivity seems to be confined within paterae suggests insthat the crust has a lower density than the magma. This carise from several factors, including

(i) crust of high vesicularity,(ii) crust composed of porous pyroclastic ash,(iii) crust consisting substantially of low density volat

materials such as sulfur, or(iv) crust that is highly differentiated, so that the crus

density is lower than that of the lava currently beerupted.

Clearly, higher temporal resolution thermal observatiwould help to define the dynamics and origin of the lalakes on Io and the interaction between the lake surfacethe walls of the patera. If there is minimal new magma ining the lava lakes on Io, then shearing against the wallsbe caused by small changes in lake surface level. Howin this case we would expect to see a ring of high teperatures around most, if not all, of the lake edge, at lwhere the line-of-sight observations are not obscured bypatera wall. At Kupaianaha, Hawaii, we also observedcles of activity that lasted for 1–2 hours in a lava lake o∼ 100 m across. Phases of high fountaining and/or crurifting lasted for only a few minutes during each cycle, amay have resurfaced∼ 10–25% of the lake surface. Ththermal anomalies produced by such activity persistedwhen that segment of the lake surface had reached theof the lake. Such dynamic activity may be rare on Ioimproved spatial and spectral resolution observations whelp resolve its occurrence.

Acknowledgments

We thank the Galileo Flight Team for their dedication ahard work in making the Io flybys successful, in particlar, Frank Leader, Marcia Segura, Robert Mehlman, JaShirley, and Yanhua Anderson. Imke de Pater and FraMarchis kindly provided data for Table 2. This paper w

t-

,

e

much improved as a result of reviews by David RotheSusan Sakimoto, and Robert Howell. Part of the resedescribed in this paper was carried out at the Jet Prosion Laboratory, California Institute of Technology, undcontract with the National Aeronautics and Space Admitration. Rosaly Lopes was partly funded under grant PGG0079-0211 from NASA’s Planetary Geology Program.

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