Solar System:Satellites & Summary

Melissa A. McGrathSpace Telescope Science Institute

n Determine the evolutionary processes that led to the diversity of Solar System bodies and the uniqueness of Earth

n Use the other objects of our Solar System as natural science laboratories

Broad Goals (COMPLEX, NASA strategic plan)

Solar System science is different because s/c exploration makes many of our targets more observationally mature.

[We are doing a lot more “weather” than the rest of you…]

Io

Europa

Ice rafting

Karkoschka 1998

Satellites science with HST: Greatest Hits

Leading Anti-Saturn

Saturn facing Trailing

Smith et al. 1996

Detection of tenuous oxygen atmospheres on Europa & Ganymede

1356/1304 ratio ~ 1-2à O2 gas

Hall et al. 1994Hall et al. 1998

GHRS spectroscopy

Aurora on Ganymede confirmed by HST

STIS imaging spectroscopyFeldman et al. 2000

OI] 1356A emission

Galileo discovery of a magnetic fieldon Ganymede

Gurnett et al. 1996Kivelson et al. 1996

Detection of solid state absorbers on many icy satellites

n SO2 in ice on Europa and Calliston O3 in ice on Ganymede, Rhea, Dionen O2 (solid state) on Ganymede

FOS SpectroscopyNoll et al. 1995, 1996, 1997;Calvin and Spencer 1997

Saturn Ring Plane crossing

Triton stellar occultationFGS scan – Elliot et al. 1998

Model-derived T and P

Global warming on Triton

Pluto – methane ice

Charon – water ice

Figure courtesy M. Buie, W. GrundyLowell Observatory

Io: The most observed satellite

Given its small size, andlocation deep within thegravitational well andmagnetic cavity, it has a huge impact on the Jovian system…

Io & the Jovian magnetosphere

Diagram courtesy John Spencer, Lowell Observatory

Io, 6Rj1028 S,O/sec

Io plasma torus

Relative motion of plasma & satellite:

induces corotational E fieldEi = -vrel x B

57km/sec x 2000nT ~500kV potential across Io drives currents of few x 106 Amps

Satellite signatures in the Jovian aurora

HST/STIS image courtesy John Clarke

Io

EuropaGanymede

Sodium CloudMendillo et al.

Meaningful studies of the Io volcanoes from Earth vicinity

HST/WFPC2 - Pele

Spencer et al. 1997

Detection of SO2, SOPlume spectroscopy

FOS 0.3” aperture Pele volcano

McGrath et al. 2000

atomic sulfur emission

Plume spectroscopy

STIS long-slitspectroscopy ofPele plume

Specner et al. 2000

Detection of S2 in the Pele plume

HI Lyman-αα images (1215.67A) of Io

Dark = more SO2 gas

1998 observationsFeldman et al. 2000

A “picture” of theSO2 atmosphere

1999 observationsMcGrath et al. 2001

Ø Surface T is not axisymmetric, it’s colder at poles

Ø Atmosphere is not global, it falls off w/ latitude

Ø Atmosphere is obviously variable

Motion of “spots” changes with B field orientation

OI] 1356 emissionRoesler et al. 1999

“Aurora” on Io

n Cassini at Saturn: 2004-2008 (nominal)

n Outer planets mission priorities beyond Cassini:« Pluto: earliest possible arrival 2015, KBO

arrival ~2025« Europa orbiter: date unclear at this time

Upcoming missions of relevance for satellites

n Galilean satellites will continue to be primary targets – we are still (almost always) photon starved in the UV

Satellite science w/ next UV/opt telescope(s)

n Enhanced capabilities will:« Allow detailed follow-up to Cassini w/ “Io-like”

science for Titan and mid-size Saturn satellites« Open up the distant solar system (Uranus, Neptune,

Pluto/Charon, etc.), which remains largely unexploredn In future there will be a bigger focus on nitrogen

(N2, NH3) in the outer solar system (Titan, Triton, Pluto, Charon atmospheres and surfaces). Cryovolcanism may be important on more distant icy satellites (and there are lots of them!).

n Another Saturn ring-plane crossing in 2010, then 2024

Possible for Triton (& others?)

Spencer et al. 1997

n Serendipity – SL9 was one of the greatest events of all time w/ HST

n Synergy with other NASA missions(HST-Galileo; HST-Cassini; HST-Lunar Prospector; HST-Chandra; HST-MGS)

n It sells well in Peoria (=Capitol Hill)Planetary science is very popular, and is PR’d disproportionate to the actual amount of observing time (~5% per cycle with HST)

Other compelling reasons to support solar system science w/ NHST

n Ground-state/resonance transitions of many atoms, ions, and molecules. « H2, H, O, C, S, N, SO2, S2, N2, CO, CO2, …« Io and its plasma torus have perhaps the richest S,O

emission line spectra known

n Many UV absorbers important for planetary atmospheres: hydrocarbons (CH4, C2H2, C2H4, etc.); NH3; SO2, SO, S2, …

n Their solar type spectrum, combined with very low UV reflectivities combine to give very UV continuum compared to the visible, making energetic emission line processes such as aurorae and dayglow detectable.« Jovian aurora not detectable in (visible) Hα from Earth

Why the UV?

n First: don’t preclude planetary observations in the early stages. (E.g., level 2 requirements do not preclude moving target tracking for NGST.)

Summary & Discussion lead-in

Then: at minimum do the simple things that enable planetary observations. « Example: many missions (IUE, HST, HUT) have solar

avoidance limit that (barely) precludes Venus. Is this technically necessary, or “historical”?

n Moving target tracking « Include requirement for capability from the beginning or it

probably won’t happen.« It can be relatively simple and low-cost.

n More “modes” is not always the answer« Example: planetary slits on STIS (not being used)

n FOV/spatial resolution« High resolution imaging channel large enough for

Jupiter (=large enough for Saturn+rings), ~50”

HST/WFPC2 HST/STIS

« As many resolution elements as possible for small targets: Pluto (<0.09”), Triton (<0.13”)Fiducial: 8m @ 0.25µm = 15 (λ/D)s across Pluto

Stern et al. 1997

Pluto/Charon FOC imaging:Pluto ~7.5 pxs diameter (~3 λ/D),0.92” separation

Albrect et al. 1994

n FOV/spatial resolution

« Large FOV (degrees) desirable for KBOs and comets. But, ~400 KBOs now known, many more expected by 2010 from g-b searches. Like HST & NGST, HST IIs niche will be the small, faint end of the distribution, which does not require large FOV.

n For almost all the science we’ve shown today we are still photon starved in the UV. The very factors that make the UV emissions detectable also makes them hard to detect…

n How much more? As much as we can possibly get, both via the aperture and the detector technology improvements.

n For example, the UV emission N2/N spectrum of Titan has not been detected (at all) since 1980 by the Voyager UVS, and it too should have an interesting interaction with the Saturn magnetosphere

Sensitivity

n Solar blind detectors are criticaln Lyα is important (à above geocorona)n UV below Lyα is importantn Near-UV is important

Spectral coverage

Spectral resolution/Slitsn R = 10,000 adequate for most problems; 30,000

would give Doppler shifts and winds on, e.g., Ion Slits: long & narrow like STIS

« Long for throughput and spatial information« Narrow for spectral resolution on large, extended targets

[slit width = resolution element]

n Slitless spectroscopy useful (e.g., STIS on Galilean satellites)

HST/STIS imaging spectroscopy

McGrath et al. 2001

geocoronal emission

Europa

spat

ial

Distribution of dense-phase (sub-surface) O2 on Ganymede

FOS spectroscopyCalvin and Spencer 1997

n Time-tag/rapid read out (~0.1s resolution) is important (e.g., occultations)

n Low Earth Orbit gives no CVZ for solar system targets

n Many problems could be addressed w/ relatively small (HST class) aperture with continuous time coverage (planetary weather/climate, aurorae, satellite-magnetosphere interactions)

n Smaller solar avoidance to allow Venus, comets in inner solar system (HST limit is 50o)

Temporal coverage (synoptic monitoring)

n Cassini at Saturn: 2004-2008 (nominal)n Outer planets mission priorities beyond Cassini:

« Pluto: earliest possible arrival 2015« Europa orbiter: date unclear at this

n Upcoming NASA & ESA« On-going Mars at most 24-month opportunities« Contour & Rosetta (comets)« Deep Impact (comet penetrator)« Messenger & Beppo-Colombo (Mercury)« DAWN (asteroids)

n New Frontiers line ($600M cap) in President’s 2003 budget

Upcoming missions of relevance for Solar System

n H2 and H emission (aurorae)n Desire to get above geocoronan Desire to go below Lyαn Imaging spectroscopyn Synoptic monitorinn 1 mas in the UVn “weather” ☺

There is a lot of commonality with needs/interests already discussed