1. Solar System Frontiers
Michael F. A’Hearn
2004 May 3
HST Symposium

2. NRC Decadal Survey - SSE
Nat. Acad. Press 2003
Context was SSE missions but key scientific questions are widely applicable
Overall themes
Are we alone, where did we come from, what is our destiny
Six Continuing Mysteries
Diversity of bodies - is this common to all planetary systems
Contrast between Earth & Venus
Habitability of Mars
Asteroids & Comets & effects on Earth
Activity on satellites in outer solar system
Nature of Kuiper belt and its objects
2004 May 3
HST Symposium

3. HST: Unique Capabilities
Ultraviolet
Next great observatory has no uv
GALEX probes low spatial resolution
Limiting magnitude for point sources
Especially at optical wavelengths (& uv)
High spatial resolution with stable PSF
Temporal variability with P ~ 1 day
Not truly unique but very difficult from the ground
2004 May 3
HST Symposium

4. Where Has HST Been? UV Spectroscopy High-resolution imaging
Satellite atmospheres
Satellite surfaces
Comet chemistry
High-resolution imaging
Discovery of satellites (binaries)
Size distributions
TNOs
Cometary nuclei
2004 May 3
HST Symposium

5. Satellite Atmospheres
Oxygen emission
Only HST result in recent Physics Today issue dedicated to planets
Ganymede
Strong variability
Correlated with Jovian mag field connection to Ganymede’s poles, thus auroral
Europa
Variability but prob. associated with bright surface areas, not auroral
e- impact on O2 from line ratios
Ganymede - Feldman et al. 2000
2004 May 3
HST Symposium

6. Satellite Atmospheres
SO2 Outgassing from Pele (on Io)
Also see other species in Pele’s plume, like S2 (only other place seen is in comets & SL9 impacts)
Do Pele & other volcanos sustain the SO2 atmosphere?
Io - McGrath et al. 2000
2004 May 3
HST Symposium

7. Satellite Surfaces SO2 frost is Widespread
Certainly on Io
Apparently on parts of Europa and Callisto as well
Other species identified in a variety of places
Is Io’s atmosphere in sublimation equilibrium with SO2 frost?
Noll et al. 1997
2004 May 3
HST Symposium

8. C/Ikeya-Zhang HO H O CO C S C2 CS CO Cameron S2 Weaver et al. 2002
2004 May 3
HST Symposium

9. Cameron Bands High Spatial Resolution + Bright comet
Only way to measure CO2 (among other things)
Results
Q(CO) ~ 8% Q(H2O)
Q(CO) >~ 2 Q(CO2)
Weaver et al. 2002
2004 May 3
HST Symposium

10. Binary TNOs Binaries (size ratio <~ 2:1) are not rare
1997 CQ29 Noll et al. 2002
Binaries (size ratio <~ 2:1) are not rare
Separations small compared to Hill radius
How make them? Are Plutinos different from classical KBOs?
Only way to estimate mass
But! VB12 (Sedna?) is not a binary despite very slow rotation (Brown, priv. Comm.)
2004 May 3
HST Symposium

11. TNO Size Distribution Deep Search didn’t find enough Problem for
Expected collisional evolution in KB
Source of Jupiter-family comets
Mass of CKB (<0.01 M⊕)
Bernstein et al. 2004
2004 May 3
HST Symposium

12. Comet Nucleus Size Distribution
Size distribution based largely on HST data
Lamy et al. find cum. Slope -1.87
Binned data yield slope -2.5 suggesting inadequate statistics
Slope -2.5 would be surprising since implies self-similar collisions but outgassing erosion is not self-similar; need better models for size distributions under physical assumptions
Virtually no data for non-Jupiter-family comets - need more
2004 May 3
HST Symposium

13. What’s missing
Much interesting work goes on under “How Planets Work” that I skipped over for lack of time.
Saturn & Cassini (listen to Dick French)
time variable phenomena (uv auroral studies, etc.)
asteroids
planetary atmospheres, etc.
2004 May 3
HST Symposium

14. Whither HST?
Emphasize UV and selective use for optical imaging/spectroscopy
Consider completing surveys but not starting new ones
Consider more DD time and more ToO time for special events (bright comets, new discoveries elsewhere, etc.)
Go deep - high SNR spectra and imaging
Consider issues of temporal variability and need for larger programs
2004 May 3
HST Symposium