2. Galileans to Scale

4. Interiors of the Galileans

5. Overview of Surfaces

6. Outer Galilean Moons Callisto: Moderately dark surface Heavily cratered No magnetic field ~1/4 of interior is ice (by mass) Ganymede: Moderately bright surface Weak magnetic field and evidence for a denser core Perhaps melted in past to allow for differentiation

7. Callisto Terrain

8. Ganymede Terrain

9. Inner Galilean Moons Europa: Bright surface Absence of craters – young surface Possible mag. Field and denser core Surface ice is a thin layer (100’s of meters) floating on H 2 O ocean (!) – requires a source of heating Io: Rocky interior Orbits inside Jupiter’s magnetosphere Io is tidally heated and volcanically active (predicted in 1979 and discovered by Voyager 1 in same year) Surrounded by a yellow Na-cloud

10. Europa Terrain

11. Crater on Europa

12. Close-up on Europa

13. Subsurface Model

14. IO: Example Volcanic Activity

15. Volcanic Eruption at Io: Before and After

16. Io’s Sodium Cloud

17. Sulphur Ring of Io

18. Titan 1944 – Kuiper discovers a thick atmosphere Atmosphere is a reddish, featureless haze; mainly N 2 with 10% CH 4 and smog: P surf =1.6P E and T air =93 K (-292 o F) Only satellite with a thick atm.: –CH 4 rain, snow, and ice? –Pools of liquid N 2 and CH 4 2004 – Cassini probe called Huygens parachuted into Titan’s atmosphere

19. Titan to Scale

20. Impression of View of Saturn from above Titan

21. Surface of Titan

23. Earth-Titan Comparison

24. The Active Atmosphere of Titan

25. Huygens Probe at Titan

27. Cold Geysers at Enceladus

28. An Ocean Below Enceladus’ Icy Crust? NASA’s Cassini spacecraft has observed plumes of material escaping from Saturn’s small icy moon, Enceladus The plume is mostly water vapor, with tiny ice particles and other gaseous molecules mixed in (e.g. CO 2, CH 4, C 2 H 6 ) The plume supplies ice particles to one of Saturn’s rings Some ice particles contain salt, which may indicate they originate in an ocean deep below the icy crust Image mosaic of Enceladus taken by Cassini, showing individual plumes of gas and ice escaping from the surface. The plumes extend 100’s of km into space from the ~500 km diameter moon.

29. Plumes may be material escaping through surface cracks from an internal salty ocean or lake Alternatively, ice along cracks may sublime or melt, followed by escape of water vapor and icy particles Many scientists find the salty ocean model most convincing, but others favor combinations of alternative explanations What Process Creates the Plume? Left: Enceladus may have a salty subsurface ocean that releases material to space through cracks in the moon’s icy shell. Right: The walls of icy cracks in the surface may melt or sublime, venting gas and icy particles to space.

30. The Big Picture Enceladus is surprisingly active for such a small body - likely a consequence of tidal heating Future flybys of Enceladus by Cassini may help to resolve whether Enceladus joins the growing “club” of solar system bodies believed to have oceans If Enceladus has an ocean, then it contains all of the ‘‘ingredients’’ known to be important for life: liquid water, molecular building blocks, and energy Image of Enceladus showing the ‘tiger stripes’ region in the southern hemisphere, where the plumes originate Tiger stripes

31. Mimas

32. The Impact at Mimas Diameter ~ 390 km Huge crater from impact, almost big enough to shatter this moon Estimate: Binding Energy of Mimas Kinetic Energy of Collider Mass and Size of Collider mv M D  H2O

33. Moons of Uranus Umbriel and Oberon Heavily cratered Large cracks Ariel and Titania Lighter cratering Resurfacing by flows of volcanic water (!) Some regions of Ariel are featureless - very recent! System of deep cracks (tidal heating effects?) Miranda Rolling and cratered terrain Some fractures 20 km cliff - takes 10 min to fall (in contrast to 1 min as at Earth)

34. Moons of Uranus

35. Miranda

36. Triton: Geyser Plumes

37. Triton Triton is the only large moon with a retrograde orbit (possibly a captured body?) Tidal bulges on Neptune lag instead of lead Triton  Orbit is degrading  Will eventually shatter as nears Roche Limit (in ~10 9 years to make a ring to rival Saturn’s) Composition: 75% rock, 25% ice Atm.: thin N 2 Polar Capts: N and CH 4 ices Evidence for resurfacing and internal activity, even geyser eruptions of N gas, leading to sooty plumes

38. The Tides at Triton