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

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

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

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

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

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

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

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

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

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! 2003 VB12 (Sedna?) is not a binary despite very slow rotation (Brown, priv. Comm.) 2004 May 3 HST Symposium

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

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

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

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