Solar System: Satellites & Summary. Melissa A. McGrath Space Telescope Science Institute

Transcription

1 Solar System: Satellites & Summary Melissa A. McGrath Space Telescope Science Institute

2 Broad Goals (COMPLEX, NASA strategic plan) 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 ]

3 Europa Io Ice rafting

4 Karkoschka 1998

5 Satellites science with HST: Greatest Hits

6 Leading Anti-Saturn Saturn facing Trailing Smith et al. 1996

7 Detection of tenuous oxygen atmospheres on Europa & Ganymede 1356/1304 ratio ~ 1-2 O 2 gas Hall et al Hall et al GHRS spectroscopy

8 Galileo discovery of a magnetic field on Ganymede Aurora on Ganymede confirmed by HST Gurnett et al Kivelson et al OI] 1356A emission STIS imaging spectroscopy Feldman et al. 2000

9 Detection of solid state absorbers on many icy satellites SO 2 in ice on Europa and Callisto O 3 in ice on Ganymede, Rhea, Dione O 2 (solid state) on Ganymede FOS Spectroscopy Noll et al. 1995, 1996, 1997; Calvin and Spencer 1997

10 Saturn Ring Plane crossing

11 Triton stellar occultation FGS scan Elliot et al Global warming on Triton Model-derived T and P

12 Pluto methane ice Charon water ice Figure courtesy M. Buie, W. Grundy Lowell Observatory

13 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

14 Io & the Jovian magnetosphere Io plasma torus Io, 6R j S,O/sec Diagram courtesy John Spencer, Lowell Observatory

15 Relative motion of plasma & satellite: induces corotational E field E i = -v rel x B 57km/sec x 2000nT ~500kV potential across Io drives currents of few x 10 6 Amps

16 Satellite signatures in the Jovian aurora Io Ganymede Europa HST/STIS image courtesy John Clarke

17 Sodium Cloud Mendillo et al.

18 Meaningful studies of the Io volcanoes from Earth vicinity HST/WFPC2 - Pele Spencer et al. 1997

19 Plume spectroscopy Detection of SO 2, SO FOS 0.3 aperture Pele volcano McGrath et al atomic sulfur emission

20 Plume spectroscopy Detection of S 2 in the Pele plume STIS long-slit spectroscopy of Pele plume Specner et al. 2000

21 HI Lyman-a images ( A) of Io Dark = more SO 2 gas

22 A picture of the SO 2 atmosphere 1998 observations Feldman et al Surface T is not axisymmetric, it s colder at poles Atmosphere is not global, it falls off w/ latitude Atmosphere is obviously variable 1999 observations McGrath et al. 2001

23 Aurora on Io OI] 1356 emission Roesler et al Motion of spots changes with B field orientation

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

25 Satellite science w/ next UV/opt telescope(s) Galilean satellites will continue to be primary targets we are still (almost always) photon starved in the UV 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 unexplored In future there will be a bigger focus on nitrogen (N 2, NH 3 ) 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

26 Possible for Triton (& others?) Spencer et al. 1997

27 Other compelling reasons to support solar system science w/ NHST 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)

28 Why the UV? Ground-state/resonance transitions of many atoms, ions, and molecules. H 2, H, O, C, S, N, SO 2, S 2, N 2, CO, CO 2, 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. Jovian aurora not detectable in (visible) Hα from Earth

29 Summary & Discussion lead-in 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? 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)

30 FOV/spatial resolution High resolution imaging channel large enough for Jupiter (=large enough for Saturn+rings), ~50 HST/WFPC2 HST/STIS

31 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. As many resolution elements as possible for small targets: Pluto (<0.09 ), Triton (<0.13 ) Fiducial: 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 Stern et al. 1997

32 Sensitivity 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 N 2 /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

33 Spectral coverage Solar blind detectors are critical Lyα is important ( above geocorona) UV below Lyα is important Near-UV is important Spectral resolution/slits R = 10,000 adequate for most problems; 30,000 would give Doppler shifts and winds on, e.g., Io Slits: long & narrow like STIS 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)

34 spatial Europa geocoronal emission HST/STIS imaging spectroscopy McGrath et al. 2001

35 Distribution of dense-phase (sub-surface) O 2 on Ganymede FOS spectroscopy Calvin and Spencer 1997

36 Temporal coverage (synoptic monitoring) 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 50 o )

37 Upcoming missions of relevance for Solar System Cassini at Saturn: (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

38 There is a lot of commonality with needs/interests already discussed H 2 and H emission (aurorae) Desire to get above geocorona Desire to go below Lyα Imaging spectroscopy Synoptic monitorin 1 mas in the UV weather