The moons of Jupiter and Saturn

1. The moons of Jupiter and Saturn

2. Jupiter’s Moons The discovery of the four Galilean satellites was important in the history of science. Galileo found objects not orbiting Earth.

4. Slightly smaller than our Moon Largest moon in the solar system

5. Orbits: Tidally locked, also note resonant orbits of first three: 1:2:4 Densities: decrease as a function of distance from Jupiter Formation of Galilean moons mimics formation of Solar System: Jupiter was hot during formation, so rockier moons closer in, icier ones further out. Rocky Io, Europa not typical of outer Solar System! Recall, a higher density implies a higher fraction of rocky material

6. Io and Europa: primarily rocky material. Ganymede and Callisto: roughly equal parts rock and water ice.

7. Jupiter's moon Io • Similar in size and density to our Moon. • The most geologically active body in the Solar System.

8. Active volcanoes 150 active volcanoes have been seen, more like geysers. Can last months or years. Ejecta speeds up to 1 km/s. Each volcano ejects about 10,000 tons/s. Entire surface covered by 1m of ejecta in 100 years. Also lava flows. Magma layer many 100’s of km thick in interior.

9. Plume on left rises to 260 km, plume on right 100 km.

10. Time between photos: a few months. Colors from different sulfur compounds (as measured by Voyager’s spectrometers), forming at different temperatures: S can be black, red, orange, depending on temperature; frozen SO2 snowflakes are white.

11. Activity causes surface to slowly change over the years: Voyager 2 (1979) Galileo (1996) Io completely lacks impact craters!

12. Heat source? • Volcanic activity requires internal heat. • Io is a small body, and should have been cooled off by now, and be geologically dead. • Tides are the answer to this question.

13. Tidal heating Io and Europa are in a "resonance orbit": Europa “pulls Io outward” here. The periodic pull on Io by Europa makes Io's orbit elliptical.

14. orbital speed faster orbital speed slower Io (top view, exaggerated ellipse) Io “tidally locked” like our Moon. Tidal bulge always wants to point to Jupiter. So bulge tries to swing around faster (with partial success) when Io is closer to Jupiter. But Io rotates on its axis at a constant rate. So bulge has some movement back and forth across surface => stresses => heat => volcanoes

15. Europa False-color visible and IR image from Galileo

16. Spectroscopic observations from Earth indicated Europa’s surface is almost pure frozen water.

17. Smooth surface • Europa is the smoothest body in the Solar system: almost no impact craters, no mountains • Young surface reprocessed by geological activity

18. Europa must have Warm Water Ocean beneath Icy Surface Fissures suggest tidal stresses. 860 km Dark deposits along cracks suggest eruptions of water with dust/rock mixed in (Europa’s density => 90% rock, 10% ice). 42 km Icebergs or "ice rafts" suggest broken and reassembled chunks.

19. Interior of Europa • Surface is water ice (about 10 km thick), below is presumably a water ocean, about 100 km deep, but most interior is rock (85%-90% of total mass). • Warm ocean – possibility of life? • Internal heat? • Too small for retained heat from formation, it must come from tidal forces (like Io, but weaker). • Resonant orbits with Ganymede and Io make Europa's orbit elliptical => varying tidal stresses from Jupiter => heat.

20. Io pulls Europa inward here. Ganymede pulls Europa outward here.

21. Ganymede • Largest satellite in solar system – larger than Mercury. • Mostly ice. • Magnetic field. Implies conducting liquid layer beneath surface – internal heat still. Calculations show orbit may have been more elliptical in past – tidal heating.

22. Terrain • Heavily cratered, but unlike the Moon and Mercury, the craters are made of ices and are flatter. • Bright vs. dark areas: dark areas are very old, more craters. • Grooves in bright areas due to tectonic stretching. Some areas appear flooded with frozen watery fluid. Age about 1Gyr only.

23. Craters are relatively flat, and circular flat features called “palimpsests” are seen (Callisto too): did icy craters just subside, or has icy material flowed and filled them in? Not clear.

24. Callisto • Also covered by frozen water layer • No evidence for geological activity. Instead, there are a number of major impact features. • Maximum cratering density • But no craters < 1km. Eroded away. Why? • Dark, dusty material covers surface. Origin not clear

25. Valhalla: huge impact basin. Crater density indicates it is 2-4 billion years old. 3000 km across.

26. Series of impact craters: probably from the impact of a comet that was tidally disrupted (like Shoemaker-Levy 9)

27. Jupiter has 67 known moons. Many very small. Most outer satellites have highly inclined, retrograde orbits, suggesting captured objects. Some in families of similar orbits. Suggests common origin in captured parent object that was shattered by a collision.

28. Inner four satellites – inside Io’s orbit Irregular shapes, orbit in Jupiter's equatorial plane. All orbit in same sense as Jupiter rotates (“prograde”). Suggests formed with Jupiter.

29. Inner satellites probably produce the ring Not visible from Earth - discovered by Voyager satellites. Faint, particles of rock. Presumably produced by meteorite impacts on inner satellites.

30. Saturn’s Moon Titan • Only Ganymede is larger. Low density  ice and rock. From Cassini-Huygens mission

31. Titan's atmosphere • Cold: Surface temp 95 K. Surface pressure 1.6 atm. • Cold and massive enough to retain atmosphere. Unique among moons in Solar System. • >95% nitrogen (possibly NH3 broken by solar UV; ~3% methane, other hydrocarbons (like ethane, propane). Cassini 2005 image

32. Surface of Titan At this cold temperature, methane and ethane can be ice, liquid or gas! So rain, rivers, lakes possible. Cassini radar image near North Pole – dark regions have no echo, suggesting smooth surface – lakes of methane, ethane? Infrared image from Cassini Cryo-volcanoes would explain methane, which would otherwise be broken by solar UV in 10 million years. One possible, inactive cryo-volcano seen in radar and infrared images.

33. The Huygens probe Taken during Huygens’ descent (Jan 14, 2005). Hills, drainage channels and a shore line.

34. Huygens on the surface • Survived for about 70 minutes • After impact, heat from probe evaporated liquid methane from surface, increase in methane measured. Methane rainfall “recent”. Estimated 5 cm/yr, but may happen in about 1000 year cycles.

35. From the landing: ice rocks, ~15cm across • Surface darker than expected: mixture of water and hydrocarbon ice • Evidence of erosion at base of rocks => fluid activity

36. Titan’s history • Should have lost its heat of formation long ago. Heat source driving cryo-volcanism may be decay of radioactive elements and some contribution from tidal heating from Saturn. • Heat enough to maintain a liquid water ocean beneath the surface. • Has right combination of gravity and temperature to trap atmosphere.

37. Besides Titan, Saturn has 6 moderate-sized moons (all in synchronous orbits) and many small ones (55 confirmed). ･ Range in diameter from 20 to 1500 km >300 km are spherical, < 300 km are irregular. ･ Mostly icy (water and ammonia), or mixes of rock & ice, with mean densities of 0.3 g/cc to 1.5 g/cc. ･ Most heavily cratered.

38. Mimas: nearly shattered by an impact? Enceladus: smooth surface < 100 million years old

39. Enceladus: Cassini discovered ice volcanoes. 2:1 orbital resonance with Dione – tidal heating like Io, Europa, ocean beneath surface These feed E-ring

40. Enceladus’ global ocean Due to elliptical orbit, it wobbles back and forth slightly as it changes speed. Wobble a bit greater than if it were solid throughout. Cassini has just flown 30 miles above S pole to determine chemistry of erupting material.

41. Hyperion – very porous structure. Origin unknown. Deep impact craters or rubble pile?

42. New moons are still being discovered! S/2004 S2 Small satellites – probably captured asteroids or collision fragments

44. Io plasma torus Charged particles in Jupiter’s magnetosphere run into plumes – they knock ions out of plumes into the magnetosphere. Forms a torus of energetic particles. As Io orbits, Jupiter’s field sweeps across it, creating 400,000 V and 5 million amps. This sets up an electric current in a cylinder of concentrated magnetic flux connecting to Jupiter ("Io flux tube"). Electrons spiraling in the field produce radio emission.

45. Tethys – region resurfaced by “lavas” of water and ammonia? Thought to be stress fractures from tectonic activity Dione Trailing hemisphere Leading hemisphere

46. Iapetus, leading black vs. trailing white hemispheres – origin not clear Rhea – heavily cratered

47. That's what's producing the 'torus' shape in the radio emission VLA image of magnetic belts around Jupiter: synchrotron emission from charged particles spiraling in magnetic fields

48. Io in Jupiter’s shadow – volcanoes glow in IR and visible light. T=1700 – 2000 K, hotter than Earth volcanoes. Implies much Mg, Fe (keeps lava from evaporating at these T’s). On Earth, such flows are billions of years old, when Earth much hotter. Suggests Io interior even hotter than Earth. Galileo

49. Dunes and craters At Earth • Large, dark regions close to equator • Prior to Cassini, these were thought to be seas of liquid hydrocarbons • Cassini instead revealed plains of “sand” dunes (some are 300 m high!). “Sand” in this case is water ice and polymers (hydrocarbon molecular chains). At Titan

50. Younger craters show light-colored rays – freshly exposed ice.