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
Saturn facing
Anti-Saturn
Trailing
Smith et al. 1996
Detection of tenuous oxygen atmospheres
on Europa & Ganymede
1356/1304 ratio ~ 1-2
à 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
FOS 0.3” aperture
Pele volcano
McGrath et al. 2000
Detection of SO2, SO
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-α
α images
(1215.67A) of Io
Dark = more SO2 gas
A “picture” of the
SO2 atmosphere
1998 observations
Feldman et al. 2000
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
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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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
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
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)
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
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) Hα 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.
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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
Pluto/Charon FOC imaging:
Pluto ~7.5 pxs diameter (~3 λ/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
Spectral coverage
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Solar blind detectors are critical
Lyα is important (à above geocorona)
UV below Lyα 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]
Slitless spectroscopy useful (e.g., STIS on
Galilean satellites)
spatial
Europa
HST/STIS imaging
spectroscopy
geocoronal emission
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)
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
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 Lyα
Imaging spectroscopy
Synoptic monitorin
1 mas in the UV
“weather” J