Geophysical Research Letters

Active volcanism on Venus in the Ganiki Chasma rift zoneE. V. Shalygin1, W. J. Markiewicz1, A. T. Basilevsky1,2,3, D. V. Titov4, N. I. Ignatiev5, and J. W. Head3

1Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany, 2Vernadsky Institute, Moscow, Russian Federation,3Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, Rhode Island, USA,4ESA-ESTEC, Noordwijk, Netherlands, 5Space Research Institute, Moscow, Russian Federation

Abstract Venus is known to have been volcanically resurfaced in the last third of solar system historyand to have undergone a significant decrease in volcanic activity a few hundred million years ago.However, fundamental questions remain: Is Venus still volcanically active today, and if so, where and inwhat geological and geodynamic environment? Here we show evidence from the Venus Express VenusMonitoring Camera for transient bright spots that are consistent with the extrusion of lava flows thatlocally cause significantly elevated surface temperatures. The very strong spatial correlation of the transientbright spots with the extremely young Ganiki Chasma, their similarity to locations of rift-associatedvolcanism on Earth, provide strong evidence for their volcanic origin and suggests that Venus is currentlygeodynamically active.

The current surface geology, geodynamics, and atmospheric characteristics of Venus, as well as its history,differ significantly from those of Earth [Phillips and Hansen, 1998; Smrekar and Phillips, 1991; Bullock andGrinspoon, 2001, 2013; Baines et al., 2013]. In contrast to global plate tectonics that dominates Earth geody-namics (geologically young seafloor, ancient continents, and tectonism and volcanism concentrated at plateboundaries), Venus is characterized by a single global lithospheric plate, like the Moon, Mars, and Mercury, butthe age of its surface is anomalously young and Earth-like [Solomon and Head, 1982; McKinnon et al., 1997].Furthermore, the atmosphere of Venus is radically different from that of Earth (the pressure at the surfacelevel is almost 100 times higher, and this pressure is created almost entirely by CO2). The question of why theEarth and Venus display such sharp divergence is one of the most fundamental problems in planetary science[Phillips and Hansen, 1998; Smrekar and Phillips, 1991; Baines et al., 2013].

Analysis and geologic mapping of the surface of Venus over the course of the space age has revealed thatgeological units representing the first 80% of the history of Venus are no longer exposed at the surface andthat, unlike the Earth, Venus may have undergone a geologically rapid global resurfacing within the last billionyears [Phillips and Hansen, 1998; Smrekar and Phillips, 1991]. This global resurfacing included tectonic defor-mation [Solomon et al., 1992], creating the deformed highlands (tesserae), followed by near-global volcanicresurfacing [Head et al., 1992], creating the regional plains that cover more than 70% of the surface. Strati-graphic relationships and the density of superposed craters provide strong evidence that the rates and stylesof tectonism and volcanism changed significantly a few hundred million years ago [Ivanov and Head, 2011,2013]. Broad upwellings, shield volcanoes, and a system of regional intersecting linear rift zones replacedtesserae formation and global flood volcanism. Stratigraphic relationships show this transition clearly [Ivanovand Head, 2011], but the exact time of its occurrence, the rates of tectonism and volcanism, and whether Venusis still active today, are uncertain due to the small number of superposed impact craters on young terrain.The formation and degradation of radar-dark parabolas associated with the most recent impact craters[Izenberg et al., 1994] provides evidence that rifting has been active in the last tens of millions of years[Basilevsky, 1993], but unknown is the presence, level, and location of any current activity. The extreme youthof these craters was deduced from observation that only a very small fraction of the population is embayedby volcanic lavas or fractured by tectonic faults, while for craters having no parabolas, such volcanic andtectonic superpositions are less rare. This was demonstrated through analysis of practically complete popu-lation of Venusian craters [Izenberg et al., 1994] and geologic analysis, first, on a regional basis Basilevsky andHead [1995] and then globally [Ivanov and Head, 2011, 2013].

Observations of changes in the composition of the atmosphere over the course of the space age have beencited as possible evidence for surface volcanic activity [Esposito et al., 1988], with volcanic effusions anderuptions as candidates for transient anomalies of SO2 detected in the atmosphere. Emissivity anomalies

RESEARCH LETTER10.1002/2015GL064088

Key Points:• VMC was able to sound Venus surface

through the atmosphere transparencywindow

• Transient bright phenomena wereobserved in the Ganiki Chasma zone

• They are consistent with hypothesis oflava lakes on the surface

Supporting Information:• Readme• Texts S1 and S2, and Table S1• Figure S1• Figure S2• Figure S3

Correspondence to:E. V. Shalygin,shalygin@mps.mpg.de

Citation:Shalygin, E. V., W. J. Markiewicz,A. T. Basilevsky, D. V. Titov,N. I. Ignatiev, and J. W. Head (2015),Active volcanism on Venus in theGaniki Chasma rift zone, Geophys. Res.Lett., 42, doi:10.1002/2015GL064088.

Received 2 APR 2015

Accepted 15 MAY 2015

Accepted article online 23 MAY 2015

©2015. American Geophysical Union.All Rights Reserved.

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Figure 1. Northern Atla Regio and examples of geologically recent volcanic activity: (a) Topographic map (blue is low, red is high); red contours outline areaswhere transient bright spots were identified by VMC measurements. (b) Magellan Venus Radar Mapping Mission (MGN) synthetic aperture radar (SAR) image ofthe same area; radar-dark parabola associated with the crater Sitwell is seen in the center-right (black arrows indicate locations of lava flow/rift interactionsshown in Figures S1a–S1d). (c) Portion of the global geologic map of Venus by Ivanov and Head [2011] showing the study area and its surroundings. Thestratigraphically youngest units (rift zones, purple; lobate plains, lava flows, red) are contemporaneous. VMC transient bright spots (A–D, white ovals) are closelyassociated with the rift zone. Inset shows global MGN SAR map with study area location.

associated with several lava flow complexes have been cited as evidence for volcanism in the last 250,000 years[Smrekar et al., 2010], and the radar properties of one lava flow complex (which displays a significant apparentmicrowave thermal emission excess, suggesting increased subsurface temperature [Bondarenko et al., 2010])and the summit of Maat Mons (which shows the partial absence of high-reflectivity material above a criticalaltitude, suggesting a shortage of time to complete typical surface alterations [Klose et al., 1992]) are consistentwith relatively recent volcanism.

The Venus Monitoring Camera (VMC) [Markiewicz et al., 2007] onboard the European Space Agency VenusExpress (VEx) spacecraft [Svedhem et al., 2007] provides the opportunity to observe changes in the thermal

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Figure 2. Maps of relative brightness in the VMC IR2 channel. Each panel shows the ratio of the mosaic composed from images obtained in the given to theaveraged VMC mosaic of the region. Orbital mosaics were obtained for specified dates in orbits: (a) 793, (b) 795, and (c) 906. Ganiki Chasma and object “A” inorbit 795 are outlined with white lines in each panel. The grid size is 5∘ by 5∘, that is ≈528 km at the equator.

emission [Basilevsky et al., 2012] of the surface of Venus that might be associated with current ongoing vol-canic eruptions. Recent analysis of VMC images reveals several regions whose brightness changed during aseries of successive observations (Figures 1, 2, and Figure S3 in the supporting information). Careful analyseswere undertaken to demonstrate that the transient brightening events were not due to instrument artifacts(see Text S1). The variations of the apparent brightness of these spots are interpreted to correspond to volcaniceruptions and related changes in surface temperature due to eruption of lavas.

VMC obtains images in four spectral channels; one of these, centered at 1.01μm (IR2), registers the night-side thermal emission from the surface of Venus [Markiewicz et al., 2007] inside an atmosphere transparencywindow [Allen and Crawford, 1984]. Variations of the brightness registered by the camera can be caused byeither variations of the atmospheric attenuation or by variations of the flux from the surface, which, in turn,can be due to variations of the surface emissivity and surface temperature [Basilevsky et al., 2012]. Variations ofemissivity can change registered brightness to a limited degree (it cannot exceed that of the ideal black bodywith the temperature equal to the temperature of the surface), and anomalous brightness detected abovethese limits must therefore be attributed to temperature or attenuation changes.

At this wavelength, and with a mean temperature of the surface of ≈740 K [Seiff et al., 1985], thethermal near-infrared (NIR) flux from the surface strongly depends on the surface temperature, providing theopportunity to detect higher surface temperatures associated with volcanic eruptions. The appearance anddisappearance of such thermal anomalies (“bright spots”) in the VMC data would be strong evidence for tran-sient volcanic events (see estimations of their visibility by VMC in Shalygin et al. [2012]). These measurementsare at the limit of VMC capability. Even at the maximum exposure of 30 s, the faintness of the surface emissionand low efficiency of the CCD detector (≈2%) at 1.01μm result in the measured signal not exceeding 200 DN(digital numbers, which are ≈3% of the CCD full well) and the average signal-to-noise ratio (SNR) for an indi-vidual image is ∼4. However, since VMC takes many overlapping surface images (usually ∼10) the value ofSNR in mosaics is appropriately higher. To remove uncertainties of the VMC radiometric calibration [Shalyginaet al., 2014, section 3] we did not rely on radiometric brightness of the observed bright spots but instead uti-lized relative measurements that is possible, because in all surface observations, VMC uses the same exposuretime (30 s) and the temperature of the camera does not vary significantly during one observational session(orbit). Therefore, the radiometric sensitivity can be assumed to be stable during an observation session.If so then every VMC orbital mosaic is radiometrically consistent. We divide mosaics by values at some point(or mean value in a region(s)) and use these normalized mosaics, which do not bear information aboutabsolute value of fluxes but contain correct contrasts.

Measurements performed by landers showed that the temperature near the surface is a stable function ofaltitude. Therefore, to detect only the presence of a transient bright spot on the surface, one can comparebrightness maps obtained at different times provided that the expected brightness variations are significantlyhigher than those caused by the variations of the atmospheric attenuation and emissivity. Such a detectionwas made in VMC observations of a 1.44 × 106 km2 area of northern Atla Regio (5∘N–25∘N, 180∘E–200∘E),

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in a region of geologically recent volcanoes and rift zones in the western part of the geologically youngBeta-Atla-Themis (BAT) region (Figure 1).

VMC performed 316 observational sessions (37 among them are of relatively good quality) and took 2463images in this area (among them 899 images within the location of object “A,” 770 within “B,” 726 within “C,”and 911 within object “D”). From these data we constructed orbital mosaics, which are maps of registeredbrightness in Mercator projection. During the systematic analysis of these mosaics we identified a bright fea-ture that is present at the same location in several consequent orbits (well seen in orbits 793 and 795), butthat disappeared afterward (the next images of this location were taken in orbit 906, see Figure 2). The brightanomaly discovered is much brighter than typical brightness variations due to changes in the atmosphere:averaged VMC brightness in the surrounding region is 13.5 mW/m2μm sr), which correspond to averagebrightness of 1 in Figure 2 (but see Shalygina et al. [2014, section 3.2]) and its standard deviation betweenorbits is ∼2 mW/m2μm sr). This bright spot is detectable without any assumptions about the surface or theatmosphere properties. Its key difference from other bright spots, caused by clouds (for example, ones likethe spot in the SE corner of the Figures 2a and 2b) is that it does not move in surface coordinates. Detection ofother possibly existing brightness anomalies that are fainter than the limit implied by clouds is not possiblein this way. Several assumptions and radiative transfer modeling are needed to reveal possible not so brightobjects. The basic logic remains the same: transient bright anomalies that are brighter than the limit imposedby emissivity variations and that do not change their geographic locations from orbit to orbit are very likelyto be caused by a process on the surface, since the typical wind speeds at the level of the main cloud deck(where attenuation of most of the surface thermal flux occurs) is ∼ 102 m/s and the VEx orbital period is 24 h.Assuming horizontal optical homogeneity of the atmosphere on a scale of ∼ 102 km (the size of point spreadfunction (PSF)), we can model emission intensity at a point with horizontal coordinates (x, y) at the top of theatmosphere:

I(x, y) =t(x, y)𝜀(x, y)

1 − (r(x, y)(1 − 𝜀(x, y)))⋅ ∫ ∫ B

[TS(x′, y′)

]⋅ F(x − x′, y − y′)dx′dy′

where t(x, y) is the atmospheric transmittance, r(x, y) is the atmospheric reflectance of surface radiation inbackward direction (both depend on surface altitude), 𝜀(x, y) is the emissivity distribution of Lambertian sur-face, B(TS) is the Planck function of the surface temperature TS, and F is the point spread function (PSF).Comparing such model images I0 with the VMC one I, we compute (the method is described in our previousworks [Basilevsky et al., 2012; Shalygin et al., 2012; Shalygin, 2013]) maps of local relative surface brightness(𝜀B)∕(𝜀0B0) (index means model as before), i.e., the relative brightness that would be detected by a hypotheti-cal observer near the surface. In these maps we found three more events that are likely to be caused by surfaceprocesses. Examining VMC observations of the NW and SW parts of this rift system (outside of the region inFigure 1a), we have not yet found any evidence of similar transient phenomena.

The bright spots are located at the edges of the stratigraphically recent tectonic rift zone, Ganiki Chasma(Figures 1 and S3). The most prominent feature (“A”) is seen in mosaics from VEx orbits 793 (22 June 2008) and795 (24 June 2008). The next good observation here was obtained from orbit 906 (13 October 2008, 111 Earthdays afterward) and showed no anomalous brightness. Bright spots “B” and “C” behave in a similar manner:they are bright in images obtained from two and three subsequent orbits (in the second week of June 2009)and are not visible in orbits prior to or after these detections. Object “D” was imaged under conditions thatdo not permit a certain identification of change (see Figure S3 for observation dates and orbit numbers).

Among these four objects, “A” and “B” show distinct differences from the other ones and a clear differencefrom the regular pattern of surface images that VMC obtains. For these two objects, VMC has obtained obser-vational sequences that show how the objects become brighter on the time scale of days. For all four objectswe computed temperature excess out of brightness excess (brightness for all of them are above the emis-sivity variation limits). These results are presented as maps of temperature excess (Figure 3). We reject thehypothesis that changes in brightness for the objects “A” and “B” might be caused by global changes in theatmospheric transparency, because such changes should change the mean level of brightness in the VMCmosaics, but this is not observed (and see also the supporting information).

The obtained excess temperature could be produced by areally extended sources or due to strong scat-tering (blurring) in the atmosphere, by smaller and much hotter sources (see Text S2 in the supportinginformation) as well as by any configuration in between. Estimations were made for both extreme cases

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Figure 3. Maps of excess temperature. Each subfigure shows a map of surface temperature derived from MGN data (black and white) with and without theretrieved excess temperature overlain (color). The grid size is 5∘ by 5∘.

(Figures S2 and 4). VMC data do not allow to give a precedence to one of the configurations, except thefollowing hint: the smallest dimensions of all registered bright spots and their spatial profiles are very similarto those of the atmosphere point spread function (PSF). Such a coincidence seems to be unlikely and givesus reason to believe that their true dimensions are much smaller (otherwise the profiles would differ from thepoint spread function (PSF) shape). We found (see Text S2, Table S1, and Figure 4) that a few hot spots withan area of 1 km2 each and temperatures up to 1100 K can explain the brightening in VEx orbits 793 and 795(object “A,” Figure 4); small spots with temperatures up to 950 K together with larger areas up to 200 km2 at800 K can explain features observed in orbits 1147 and 1148 (objects “B” and “D”).

Rift zones such as Ganiki Chasma are typical of the latest stage in the history of Venus (network rift-ing/volcanism regime) [Ivanov and Head, 2011, 2013] and are characterized by extensive crustal and litho-spheric extension and thinning, mantle upwelling, tectonic rifting, and the extrusion of numerous long lavaflows from the rift faults and fractures. The location of the transient bright spots is typically near the flankingfaults of the rift (Figure 1), that are often the sites of active eruptions in terrestrial rift zones [Franke, 2013].These associations strengthen the interpretation that the transient bright spots represent the sites of activevolcanic eruptions.

Active lava flows and flow fields on Earth commonly display broad thermal anomalies associated with sourceregions and distribution systems (source ponds and lakes, multiple channelized flows with continuouslyexposed lava, pahoehoe breakouts, partly roofed lava tubes, etc. [Flynn et al., 1994]). These anomalies persistthroughout the eruption period and the subsequent cooling of the lava, periods often measured in years.

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Figure 4. Pairs of VMC to synthetic mosaics ratios showing the excess brightness modeled (top) without and (bottom)with hot material on the surface. In the top panels the model image is calculated assuming a constant surface emissivityand the adiabatic temperature lapse rate of −8.1 K/km. In the bottom panel of each pair, artificial hot spots are added.The locations of hot material are marked by crosses and lines. The grid size is 1∘ by 1∘ . The hot spot parameters aregiven in Table S1.

The estimated dimensions of the hot spots on Venus are similar to those of a wide variety of common activeeruption phenomena on Earth (lava flows, lava channels, ponded parts of lava flows, and lava lakes) [Pyle,1999] and thus can readily explain the bright spot magnitudes above the ambient surface background andtheir duration. Relatively short (comparing to Earth) duration of the Ganiki temperature anomalies may bedue to more effective cooling by very dense Venusian atmosphere (65 kg/m3) [Head III and Wilson, 1986].Besides, posteruption thermal anomalies on Earth are often supported by circulation of ground waters whichare absent on Venus. Similar configurations are well known in areas of active volcanism on Earth [Flynn et al.,1994; Pyle, 1999] and are observed elsewhere on Venus in older deposits [Ivanov and Head, 2013]. We con-sidered the possibility that the bright spots might represent explosive eruptions but favor effusion because(1) the very high atmospheric pressure significantly inhibits explosive activity [Head III and Wilson, 1986], (2)explosive eruptions are favored from edifices, rather than rifts [Glaze, 1999], (3) candidate examples of explo-sive volcanic deposits are very rare in the geologic record of Venus [Ivanov and Head, 2013], and (4) the linear

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alignment with the rift is more consistent with lava flows. In summary, the characteristics and behavior ofthese bright spots suggest that they represent the volcanic eruption of lava onto the surface of Venus, causingtransient thermal anomalies.

Further evidence for the extreme youth of the tectonic and volcanic activity in this area comes from rela-tionships with the radar-dark parabola impact crater Sitwell (Figure 1). Dark parabolas are associated withthe youngest impact craters mapped on Venus and craters are known to lose their parabolas through eolianmodification over the course of a few tens of millions of years [Izenberg et al., 1994]. Parabolas have beensuccessfully used as a stratigraphic indicator of the youngest end member of geologic activity on Venus[Ivanov and Head, 2011]. Thus, any geologic activity that superposes or cuts these parabolas must be amongthe absolutely youngest activity on Venus [Basilevsky, 1993]. Magellan Venus Radar Mapping Mission (MGN)synthetic aperture radar (SAR) images of the flanks and interior of Ganiki Chasma and the Sitwell Crater darkparabola (Figure S1) in the areas of the transient bright spots show clear evidence of (1) lava flows super-posed on rift fractures and faults, and flooding them, (2) fresh faults cutting very young lava flows, and (3)lava flows superposed on the Sitwell Crater dark parabola. Together, these observations strongly support theinterpretation that the transient bright spots represent the sites of currently active volcanic eruptions.

The detection of current volcanic eruptions in VEx VMC images indicates that the Atla Regio rise area ispresently geologically and geodynamically active and that historically observed variations in atmosphericchemistry [Esposito et al., 1988; Marcq et al., 2011] could be due to active volcanic eruptions. Atla Regio shouldreceive priority in terms of future Venus exploration and change detection experiments.

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AcknowledgmentsWe acknowledge teams at the Institutfür Planetenforschung of DeutschesZentrum für Luft- und Raumfahrtand the Institut für Datentechnikund Kommunikationsnetze derTechnische Universitt Braunschweigfor their efforts in supporting the VMCexperiment. The authors are gratefulto Deutsches Zentrum für Luft- undRaumfahrt, who provided the VMCdata processing (data are availablefrom ESA PSA), especially to T. Roatschand K. D. Matz.

The Editor thanks Robin Fergason andLionel Wilson for their assistance inevaluating this paper.

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GEOPHYSICAL RESEARCH LETTERS

Supportive information for “Active Volcanism on Venus in the

Ganiki Chasma Rift Zone”

E.V. Shalygin,1W.J. Markiewicz,

1A.T. Basilevsky,

1,2,3D.V. Titov,

4N.I. Ignatiev,

5J.W. Head

3

Contents of this file

1. Text S1 to S2

2. Figures S1 to S3

3. Table S1

Text S1: Possible explanations by artificial causes

We considered the following possible explanations due to observa-

tional or data processing artifacts but rejected them after analysis.

Bright spots caused by the camera The spots are present in several

Venus Monitoring Camera (VMC) images in each orbit. The spacecraft

rotates/moves during the imaging, thus in every other image a given

point at the surface is pictured in different pixels. Therefore it is un-

likely that any brightening in the camera will follow this scheme.

Incorrect pointing information in the region with large altitude gradients

In this case, every artificially bright spot should be coupled with an arti-

ficial dark “ghost”, and this is not observed. Also, there are other places

along the rift with similar altitude gradients where no bright spots were

seen.

“Holes” in the clouds In the super-rotating atmosphere of Venus it

is unlikely that a hole would not move for several tens of hours, but just

change shape and transparency. The hole could remain in the same po-

sition, however, if the cause of the “hole” is some active process at the

surface. Also dense clouds would lower the signal registered by VMC,

but we selected only orbits with “background” value of the VMC signal

far higher (> 1.5 times) than that in “dark” orbits. These considerations

suggest that even if the phenomena are formed in the atmosphere they

are driven by some process at these specific locations on the surface.

Text S2: Estimations of parameters associated with the scale of vol-

canic activity

Under the interpretation that the bright spots are caused by hot spots

on the surface of Venus, we can use the VMC data to estimate their size

and temperature for a variety of plausible scenarios. In order to inter-

pret quantitatively the VMC night side images we need to account for

the low signal/noise ratio (u�u�u� ≈ 4) and the availability of a single

spectral channel. The second VMC near infra-red (NIR) channel, cen-

tered at 0.965 μm (inside the H2O absorption band) does not always

perform co-aligned observations. In the case of the orbits analyzed here,

the 0.965 μm channel was imaging only Maat Mons and its vicinity, to

the south-east of the bright spots in Ganiki Chasma (fig. 1). The other

instrument on-board Venus Express (VEx) that can perform surface ob-

servations in the infra-red, the Visible and Infra-red Thermal Imaging

Spectrometer (VIRTIS) [Drossart et al., 2007], did not observe these

locations at these times.

The low u�u�u�makes image restoration challenging and therefore we

undertook the following steps. Comparing VMC mosaics with model

images [Basilevsky et al., 2012] we determined: 1) the total excess ther-

mal flux in the area of the bright spots (assuming the value of surface

1Max-Planck-Institut für Sonnensystemforschung, 37077 Göttingen,

Germany2Vernadsky Institute, 119991 Moscow, Russian Federation3Brown University, Providence, Rhode Island 02912, USA4ESA-ESTEC, 2200 AG Noordwijk, The Netherlands5Space Research Institute, 117997 Moscow, Russian Federation

Copyright 2015 by the American Geophysical Union.

0094-8276/15/$5.00

emissivity u� = 0.5 and surface temperature, lapse rate and atmospheric

absorption the same as those used by Basilevsky et al. [2012]), 2) the

maximum contrast, and 3) the size of those areas. Strong scattering in

the venussian atmosphere [Tomasko et al., 1985; Seiff et al., 1985] leads

to blurring of the surface images at ≈ 1 μm with half-width of the at-

mosphere point spread function (PSF) of∼ 50 km. Because of blurring

in the atmosphere, a given bright spot can be modeled with equally good

fits using different combinations of size and temperature. The same is

true for the resolution of the camera: it is not possible to decide uniquely

if we are observing several sub-resolution size spots or, alternatively, a

single spot with lower temperature. In surface observations the spatial

resolution of VMC is ∼ 1 km. Total flux excess from a bright spot as

comparing to the same area without bright spot allows to estimate total

energy flux excess from a hot area, and thus possible combination of size

and temperature of the hot spot. Peak brightness in a bright spot is de-

termined by the maximal temperature of the hot spot and transmittance

of the atmosphere. The latter can fluctuate, of course (but see section

S1).

On the basis of the maximal contrast, size and total excess brightness

in the bright spots, we estimated possible combinations of temperature

and size that can produce the observed flux excess (fig. 5). These es-

timations are based on the assumption that the entire additional flux is

generated by a single hot spot. In this case, we do not account for ex-

tremely elongated (with aspect ratios ≫ 100) lava lakes or flows andtheir possible influence on the observed contrast [Shalygin et al., 2012].

It is also possible that we are observing a set of small and hot spots

[Drossart et al., 2007]. Therefore using the method developed earlier

[Basilevsky et al., 2012; Shalygin et al., 2012], we performed direct

modeling of the observed bright regions. We modeled events in orbits

793 and 795 (object “A”); orbits 1147 and 1148 (objects “B” and “D”).

Object “C” was imaged close to the edges of VMC frames where the

number of overlapping images is small and therefore noise is high, mak-

ing modeling results uncertain, and thus we did not model this region.

In themodeling we used the following criteria: 1. The smallest possi-

ble size of the hot spot is 1 km×1 km, a value that is very slightly larger

than the resolution of the VMC. 2. To model the observed contrasts, we

used as small a number of small spots and elongated rectangles as possi-

ble. Precedence was given to spots, but if a single elongated rectangular

spot gave the same results as 4 or more spots, it was used instead. 3. If

the given bright spot was imaged from two subsequent orbits, we give

precedence to configurations where hot spots are located at the same

places but possibly change temperature and/or size.

The goal of the modeling was to use the values of excesses in the

VMC images to model ratios in the bright spot regions. The baseline

level for the ratio was chosen as the mean value in the very close vicinity

of the bright spots, but not the mean level in images as a whole, because

we are unable to estimate cloud optical thickness variations and pos-

sible brightness variations caused by these variations, and because the

radiometric calibration of the VMC is still uncertain [Shalygina et al.,

2014].

Results of the modeling are presented in figs. S2 and 5 and shown in

table S1. These results show that the bright spots can be accounted for

by plausible ranges of hot spot configurations. Variations in the temper-

ature (or size) of the candidate hot spots can explain the range of values

in the ratio images. The results show general agreement with the possi-

bility of lava flows, lava channels, ponded parts of lava flows, and lava

lakes, configurations that are well known on the Earth.

1

X - 2 SHALYGIN ET AL.: ACTIVE VOLCANISM ON VENUS

Table S1. Parameters of the hot spots

Latitude Longitude Area [km2] Aspect ratio Azimuth Temperature [K]

Object “A”, orbit # 0793

12°38′39″ 197°29′4″ 1 1 N/A 900

12°38′6″ 197°29′4″ 1 1 N/A 1000

12°10′55″ 197°26′48″ 1 1 N/A 1000

12°58′35″ 197°46′7″ 1 1 N/A 900

12°29′47″ 197°54′38″ 1 1 N/A 1000

12°7′1″ 197°22′49″ 1 1 N/A 1000

Object “A”, orbit # 0795

12°38′39″ 197°29′4″ 1 1 N/A 1000

12°38′6″ 197°29′4″ 1 1 N/A 900

12°10′55″ 197°26′48″ 1 1 N/A 1100

12°58′35″ 197°46′7″ 1 1 N/A 1100

12°29′47″ 197°54′38″ 1 1 N/A 1000

Object “B”, orbit # 1147

16°30′3″ 197°35′53″ 4 1 N/A 800

16°14′15″ 197°47′15″ 4 1 N/A 800

16°38′46″ 197°17′9″ 200 200 50° 800

16°48′1″ 198°3′43″ 1 1 N/A 950

Object “B”, orbit # 1148

16°30′3″ 197°35′53″ 4 1 N/A 800

16°10′59″ 197°44′59″ 16 1 N/A 850

16°38′46″ 197°17′9″ 200 200 50° 800

16°48′1″ 198°3′43″ 1 1 N/A 950

Object “D”, orbit # 1148

10°55′13″ 199°27′48″ 200 50 10° 800

10°10′33″ 199°30′38″ 16 1 N/A 900

12°37′33″ 199°7′55″ 16 1 N/A 900

11°40′54″ 199°19′16″ 16 1 N/A 900

1. *

ReferencesBasilevsky, A. T., et al. (2012), Geologic interpretation of the near-infrared im-

ages of the surface taken by the Venus Monitoring Camera, Venus Express,

Icarus, 217(2), 434–450, doi:10.1016/j.icarus.2011.11.003.

Drossart, P., et al. (2007), Scientific goals for the observation of Venus by VIR-

TIS on ESA/Venus Express mission, Planet. Space Sci., 55(12), 1653–1672,

doi:10.1016/j.pss.2007.01.003.

Seiff, A., et al. (1985), Models of the structure of the atmosphere of Venus from

the surface to 100 kilometers altitude, Adv. Space Res., 5(11), 3–58, doi:

10.1016/0273-1177(85)90197-8.

Shalygin, E. V., A. T. Basilevsky, W. J. Markiewicz, D. V. Titov, M. A.

Kreslavsky, and T. Roatsch (2012), Search for ongoing volcanic activity on

Venus: Case study of Maat Mons, Sapas Mons and Ozza Mons volcanoes,

Planet. Space Sci., 73(1), 294 – 301, doi:10.1016/j.pss.2012.08.018.

Shalygina, O. S., E. V. Petrova, W. J. Markiewicz, N. I. Ignatiev, and E. V. Sha-

lygin (2014), Optical properties of the Venus upper clouds from the data ob-

tained by Venus Monitoring Camera on-board the Venus Express, Planet.

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Tomasko, M. G., L. R. Doose, and P. H. Smith (1985), The absorption of solar

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Corresponding author: E. V. Shalygin, Max-Planck-Institut für Sonnensys-

temforschung, 37077 Göttingen, Germany (shalygin@mps.mpg.de)

SHALYGIN ET AL.: ACTIVE VOLCANISM ON VENUS X - 3

Figure S1. Examples of geologically recent volcanic activity: a)

Lava area (white arrows) with central pit (black-and-white arrow);

b) Lava flows cut by young rift-associated faults (white arrows) and

covering older faults (black-and-white arrows); c) Radar-bright lava

flows (white arrows) seen among the materials of the radar-dark

parabola of the crater Sitwell and 0.5 – 1.5 km pits (interpreted as

volcanic vents) with surrounding bright lava (black-and-white ar-

rows); d) Field of geologically recent lava flows (white arrows) in a

local depression outlined by rift-associated faults.

X - 4 SHALYGIN ET AL.: ACTIVE VOLCANISM ON VENUS

700

800

900

1000

1100

1200

1300

100 101 102 103 104

Tem

pera

ture

[K]

Area [km2]

793, 'A'795, 'A'

1142, 'C'1147, 'B'1148, 'B'1148, 'D'

Figure S2. Combinations of temperature and size of the hot spots that produce observed excess of the brightness.

SHALYGIN ET AL.: ACTIVE VOLCANISM ON VENUS X - 5

Figure S3. Retrieved maps of relative surface brightness at 1 μmunder the assumption of horizontally homogeneous atmosphere.

Each panel shows map obtained for the specific date calculated

from data obtained in orbits: 793 (a), 795 (b), 906 (c), 1142 (d),

1146 (e), 1147 (f), 1148 (g), 1149 (h). Brightness variations are

caused by various surface temperatures and emissivities, as well as

by inhomogeneity in the real atmosphere (see discussion in text). In

each panel the grid size is 5° by 5°.