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Solar System:
Satellites & Summary

Melissa A. McGrath

Space Telescope Science Institute


Broad Goals (COMPLEX, NASA strategic plan)
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Determine the evolutionary processes that led to the diversity of Solar System bodies and the uniqueness of Earth Use the other objects of our Solar System as natural science laboratories

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...]


Europa

Io

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 ~ 1-2 Ю O2 gas

Hall et al. 1994 Hall et al. 1998

GHRS spectroscopy


Galileo discovery of a magnetic field

on Ganymede Aurora on Ganymede confirmed by HST

Gurnett et al. 1996 Kivelson et al. 1996

OI] 1356A emission
STIS imaging spectroscopy Feldman et al. 2000


Detection of solid state absorbers on many icy satellites
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SO2 in ice on Europa and Callisto O3 in ice on Ganymede, Rhea, Dione O2 (solid state) on Ganymede

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


Saturn Ring Plane crossing


Triton stellar occultation FGS scan ­ Elliot et al. 1998

Global warming on Triton

Model-derived T and P


Pluto ­ methane ice

Charon ­ water ice

Figure courtesy M. Buie, W. Grundy Lowell Observatory


Io: The most observed satellite
Given its small size, and location deep within the gravitational well and magnetic cavity, it has a huge impact on the Jovian system...


Io & the Jovian magnetosphere

Io plasma torus

Io, 6Rj 1028 S,O/sec
Diagram courtesy John Spencer, Lowell Observatory


Relative motion of plasma & satellite: induces corotational E field Ei = -vrel x B 57km/sec x 2000nT ~500kV potential across Io drives currents of few x 106 Amps


Satellite signatures in the Jovian aurora
Io

Ganymede

Europa

HST/STIS image courtesy John Clarke


Sodium Cloud
Mendillo et al.


Meaningful studies of the Io volcanoes from Earth vicinity

HST/WFPC2 - Pele Spencer et al. 1997


Plume spectroscopy

Detection of SO2, SO

FOS 0.3" aperture Pele volcano McGrath et al. 2000

atomic sulfur emission


Plume spectroscopy
Detection of S2 in the Pele plume

STIS long-slit spectroscopy of Pele plume

Specner et al. 2000


HI Lyman- images (1215.67A) of Io Dark = more SO2 gas


A "picture" of the SO2 atmosphere

1998 observations Feldman et al. 2000

ь Surface T is not axisymmetric, it's colder at poles ь Atmosphere is not global, it falls off w/ latitude ь Atmosphere is obviously variable

1999 observations McGrath et al. 2001


"Aurora" on Io

OI] 1356 emission
Roesler et al. 1999

Motion of "spots" changes with B field orientation


Upcoming missions of relevance for satellites
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Cassini at Saturn: 2004-2008 (nominal) Outer planets mission priorities beyond Cassini:
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Pluto: earliest possible arrival 2015, KBO arrival ~2025 Europa orbiter: date unclear at this time


Satellite science w/ next UV/opt telescope(s)
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Galilean satellites will continue to be primary targets ­ we are still (almost always) photon starved in the UV Enhanced capabilities will:
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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!). Another Saturn ring-plane crossing in 2010, then 2024

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 unexplored

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Possible for Triton (& others?)

Spencer et al. 1997


Other compelling reasons to support solar system science w/ NHST
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Serendipity ­ SL9 was one of the greatest events of all time w/ HST Synergy with other NASA missions (HST-Galileo; HST-Cassini; HST-Lunar Prospector; HST-Chandra; HST-MGS) 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)

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Why the UV?
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Ground-state/resonance transitions of many atoms, ions, and molecules.
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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

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Many UV absorbers important for planetary atmospheres: hydrocarbons (CH4, C2H2, C2H4, etc.); NH3; SO2, SO, S2, ... 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.
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Jovian aurora not detectable in (visible) H from Earth


Summary & Discussion lead-in
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First: don't preclude planetary observations in the early stages. (E.g., level 2 requirements do not preclude moving target tracking for NGST.) 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"?

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Moving target tracking « Include requirement for capability from the beginning or it probably won't happen. « It can be relatively simple and low-cost. More "modes" is not always the answer « Example: planetary slits on STIS (not being used)


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FOV/spatial resolution
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High resolution imaging channel large enough for Jupiter (=large enough for Saturn+rings), ~50"

HST/WFPC2

HST/STIS

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

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

Albrect et al. 1994

Stern et al. 1997

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

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Spectral coverage
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Solar blind detectors are critical Ly is important (Ю above geocorona) UV below Ly is important Near-UV is important

Spectral resolution/Slits
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R = 10,000 adequate for most problems; 30,000 would give Doppler shifts and winds on, e.g., Io Slits: long & narrow like STIS
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Long for throughput and spatial information Narrow for spectral resolution on large, extended targets [slit width = resolution element]

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


spatial

Europa geocoronal emission

HST/STIS imaging spectroscopy

McGrath et al. 2001


Distribution of dense-phase (sub-surface) O2 on Ganymede
FOS spectroscopy Calvin and Spencer 1997


Temporal coverage (synoptic monitoring)
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Time-tag/rapid read out (~0.1s resolution) is important (e.g., occultations) Low Earth Orbit gives no CVZ for solar system targets Many problems could be addressed w/ relatively small (HST class) aperture with continuous time coverage (planetary weather/climate, aurorae, satellite -magnetosphere interactions) Smaller solar avoidance to allow Venus, comets in inner solar system (HST limit is 50o)

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Upcoming missions of relevance for Solar System
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Cassini at Saturn: 2004-2008 (nominal) Outer planets mission priorities beyond Cassini: « Pluto: earliest possible arrival 2015 « Europa orbiter: date unclear at this Upcoming NASA & ESA « On-going Mars at most 24-month opportunities « Contour & Rosetta (comets) « Deep Impact (comet penetrator ) « Messenger & Beppo-Colombo (Mercury) « DAWN (asteroids) New Frontiers line ($600M cap) in President's 2003 budget

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There is a lot of commonality with needs/interests already discussed
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H2 and H emission (aurorae) Desire to get above geocorona Desire to go below Ly Imaging spectroscopy Synoptic monitorin 1 mas in the UV "weather" J