1. © Sierra College Astronomy Department The Jovian Moons and Rings

2. © Sierra College Astronomy Department2 The Jovian Planets Jupiter’s Moons Jupiter’s Moons Jupiter’s family of 67 moons can be divided into 3 groups: 1. Outer moons, eccentric orbits, many retrograde, dark surfaces, captured asteroids. 2. 4 inner moons orbit very close to Jupiter and are probably fragmented moonlets (form and shape Jupiter’s ring). 3. 4 Galilean moons, nearly circular orbits, smallest is 5,000 times more massive than the largest of the other moons.

3. © Sierra College Astronomy Department3 Io, the Galilean moon closest to Jupiter, has active volcanic sulfuric geysers. Creates many surface layers But does not build high volcanoes (lava too fluid) Io’s heat is produced by tidal forces caused by Europa and its eccentric orbit around Jupiter. Io is surrounded by a halo of sodium atoms, which itself is embedded in a sodium torus that surrounds Jupiter. The Jovian Planets Jupiter’s Moons

4. © Sierra College Astronomy Department4 Volcanoes on Io How? Should have cooler interior than Mercury and Mars (smaller object). Io’s elliptical orbit forced by resonance with Europa and Ganymede causes differential tidal heating. Io is tidally distorted more when closer to Jupiter than farther away. This constant flexing heats the interior. Spewed material from volcanoes forms torus of sodium(?) around Jupiter (Io Torus). Io during eclipse The Jovian Planets Jupiter’s Moons Courtesy: NASA

5. © Sierra College Astronomy Department5 Resonances Previous examples: spin-orbit Moon (1:1) around Earth, Mercury (3:2) around Sun New examples: spin-orbit All other major satellites to parent planet (1:1) New examples: orbit-orbit Io-Europa (2:1) Europa-Ganymede (2:1) Later, in Saturn’s rings: Mimas-Cassini Division (2:1) The Jovian Planets Jupiter’s Moons

6. © Sierra College Astronomy Department6 Europa’s surface is ice; its moderate density indicates a rocky world covered by an ocean of frozen water. Europa also experiences some tidal heating which has resurfaced it. Ice rafts and lenticulae (100-m ice mounds) This tidal heating of Europa also suggests that an interior liquid ocean of water may exist. Europa is the smallest of the Galilean moons (and is smaller than Earth’s Moon). The Jovian Planets Jupiter’s Moons

7. © Sierra College Astronomy Department7 Ganymede has a surface that appears similar to our moon. The surface is composed mostly of ice. With fewer craters than Callisto, some resurfacing has occurred. Ganymede is the largest moon in the solar system. It also generates its own magnetic field How? There may be a layer of salty-water buried 150 km beneath the surface The Jovian Planets Jupiter’s Moons

8. © Sierra College Astronomy Department8 Callisto also has a surface that appears similar to our moon. The surface is composed mostly of ice. There may be a water ocean below the surface. Radioactive heating may contribute There is a detectable magnetic field Callisto is very heavily cratered implying that it is tectonically inactive. May be the oldest surface in the solar system Callisto’s interior is appears to be undifferentiated. The Jovian Planets Jupiter’s Moons

9. Io Europa GanymedeCallisto Size of Earth’s Moon Older Younger Closer to JupiterFurther to Jupiter DenseLess Dense Surface age determined by crater counts Galilean Moon Stats Courtesy: NASA

10. © Sierra College Astronomy Department10 The Jovian Planets Saturn’s Moons Moons of Saturn Saturn has 62 moons, second only to Jupiter in number. Major moons include (from largest to smallest): Titan (second largest moon in the Solar System), Rhea, Iapetus, Dione, Tethys, Enceladus and Mimas.

11. Rhea Mimas Enceladus Some Moons of Saturn Courtesy: NASA

12. Enceladus Enceladus has a very shiny surface (albedo = 0.9) and has just been discovered to have a “significant” atmosphere (which must be replenished) False-color image of anti-Saturn hemisphere Close-up of surface Courtesy: NASA

13. Tethys Dione Courtesy: NASA

14. Phoebe enlarged Iapetus (the two toned moon) Hyperion (next page too) Courtesy: NASA

15. Hyperion Courtesy: NASA

16. © Sierra College Astronomy Department16 The Jovian Planets Saturn’s Moons Titan Titan may be the most interesting moon in the solar system because it has an atmosphere (How?). It is composed mostly of nitrogen with 1% methane and a trace of argon. When sunlight strikes methane, it can cause the formation of organic molecules, which are a known precursor to life.

17. Titan Rhea Courtesy: NASA

18. © Sierra College Astronomy Department18 The Jovian Planets Uranus’s Moons Uranus’s Moons Five moons were known before Voyager (Miranda, Ariel, Umbriel, Titania, Oberon); now 22 more are known (total = 27). Many moons named for Shakesperian characters. All the moons appear to be low-density, icy worlds (but they appear to have had been more active than the Saturnian satellites of a similar size). The innermost, Miranda, is perhaps the strangest looking object in the solar system. It appears as if it were torn apart by a great collision and then reassembled.

19. © Sierra College Astronomy Department19 The Jovian Planets Neptune’s Moons Neptune’s Moons Before Voyager 2, Neptune was known to have 2 moons; 13 moons are now known. Triton, Neptune’s largest moon, is the only major moon to revolve around a planet in a clockwise (retrograde) direction. Causes significant enough tides on Triton. Triton is also tilted 23 deg relative to Neptune’s equator Triton has a very thin atmosphere of N 2 and CH 4.

20. © Sierra College Astronomy Department20 The Jovian Planets Neptune’s Moons Triton has a light-colored surface composed of water ice with some nitrogen and methane frost. Its surface appears young, with few craters and active geyser-type volcanoes observed (nitrogen ice and carbon compounds). Triton’s active volcanism is probably due to internal heating from tides, heating from the Sun or internal residual heat.

21. © Sierra College Astronomy Department21 The Jovian Planets Planetary Rings - Saturn Planetary Rings Saturn’s rings are very thin, in some cases less than 100 meters thick. The rings are not solid sheets but are made up of small particles of water ice or water-ice mixed with dust. Three distinct rings are visible from Earth, and were named (outer to inner) A, B, and C.

22. Saturn from Earth Voyager leaving Saturn Voyager approaching Saturn Courtesy: NASA

23. © Sierra College Astronomy Department23 The Jovian Planets Planetary Rings - Saturn The largest division between rings is known as the Cassini division. This space is caused largely by the gravity of Mimas acting synchronously (2:1 resonance) on the orbital path of nearby ring particles. Some other ring features are explained by the presence of small shepherd moons. Mimas

24. Close-up of Main Rings Cassini Division B Ring A Ring C Ring Courtesy: NASA

25. The F ring: Confined by Shepherd Satellites Pandora and Prometheus The A ring Cassini Voyager

26. © Sierra College Astronomy Department26 The Jovian Planets Planetary Rings - Saturn The Origin of Rings Saturn’s rings are probably about 100 million years old. The origin of Saturn’s rings is not well understood, but is thought to be the result of: A close-orbiting, icy moon that shattered in a collision with an asteroid. A large comet which got too close to Saturn (much like Shoemaker-Levy 9 did at Jupiter in 1994). Rings around the Jovian planets are not billions of years old and must be replaced or renewed on a much smaller time scale. Tidal forces are greater on a moon in orbit close to a planet than they are on a moon in an orbit farther out.

27. © Sierra College Astronomy Department27 The Jovian Planets Saturn’s Rings Roche limit is the minimum radius at which a satellite (held together by gravitational forces) may orbit without being broken apart by tidal forces. Saturn’s rings are inside Saturn’s Roche limit, so no moons can form from the particles.

28. © Sierra College Astronomy Department28 Jupiter’s Ring Voyager I discovered a thin ring (system) around Jupiter. The ring is close to Jupiter, extending to only about 1.8 planetary radii. The ring is thought to be replenished from the small moonlets within or near it. The Jovian Planets Planetary Rings - Jupiter Voyager from “behind” Jupiter Courtesy: NASA

29. © Sierra College Astronomy Department29 The Jovian Planets Planetary Rings - Uranus and Neptune The rings of Uranus and Neptune and are made of particles which are darker and smaller than that of Saturn. The Uranian rings are narrow, a few of which are clearly confined by shepherding moons. The Neptunian rings vary in width and are confined by resonances of some of the moons.

30. © Sierra College Astronomy Department30 The End