Jovian Planet Moons and Rings Each planet has multiple moons and seem like “mini-solar systems” Moons are mostly made of ice, so quite soft and can be more active than similar sized rocky planets/moons Rings are made of individual small particles, orbiting like “micromoons”

SATURN'S MOONS Saturn has at least 18 moons bigger than 10 km across and Cassini is discovering more, plus plenty of smaller ones. One giant moon, Titan Six pretty big moons (400 to 1530 km diameters): Mimas, Enceladous, Tethys, Dione, Rhea & Iapetus (a=186,000 km to 3,560,000 km) The rest (61 in total) are fragments or captured asteroids

TITAN: the biggest by far a = 1,220,000 km; D = 5150 km (> Mercury!); M = 1.83 MMoon  = 1.9 g/cm3  very thick ice layer OR more likely, a mixture of rock and ice An atmosphere thicker and denser than Earth's, w/ P = 1.6 bar; Atm over 400 km thick; several haze layers Tsurf= 94K Methane and ethane rain or snow. Possible hydrocarbon lakes found by Huygens. Slim chance that some very different form of life has evolved in Titan's putative oceans.

Titan’s Atmosphere

ESA’s Huygens Probe to Titan A large crater and water ice + methane springs Huygens detached from Cassini; landed Jan 14, 2005

Medium Moons 400 < D < 1530 km; 1.0 < < 1.4 g cm-3 Most of them have high albedo's; along w/ low density  icy exteriors Mimas: smallest of this lot:  Cassini division, via a 2:1 orbital resonance. Enceladus: very shiny with albedo ~ 1: a continuous outflow of water was detected by Cassini: volcanoes or geysers? Dione: bright streaks and younger “maria” -- one Lagrangian moon: Helene Iapetus: outermost of these; very dark leading side; very bright trailing side: due to dark, tilted rings

Tethys and Rhea Tethys: cratered and cracked surface -- also, two Lagrangian moons off by 60 degrees in orbit: Telesto and Calypso. Rhea: biggest of this group -- very hard ice surface with lots of craters in bright ice (polar cap)

A few of the little moons worth mentioning 200 < D < 370 km Just outside outermost ring are Janus & Epimetheus: co-orbital satellites, which lap each other every 4 years and swap nearly identical orbits. Hyperion: Titan's gravity prevents it from having a circular orbit, so not synchronous: apparently tumbles fully CHAOTICALLY. Phoebe: much farther than any other satellite, a=13,000,000 km; its retrograde orbit implies capture.

Moons of Uranus (+ Neptune’s Proteus)

Moons of Uranus 5 moderate sized ones ( 480 < D < 1580 km) 1.1 < Density < 1.7 g cm-3 , so ice/rock mixtures All very dark: organic molecules formed from solar wind and UV bombardment? MIRANDA: a = 130,000 km; smallest, weirdest: cratered part; ridges; oval faults. Blown apart and reassembled?

More Uranian Moons ARIEL: a = 191, 000 km and UMBRIEL: a = 266,000 km are very similar in size but Ariel shows some evidence for ancient activity, which Umbriel lacks; Umbriel is the darkest TITANIA: a = 436,000 km is the biggest, but OBERON, a = 583,000 km is only slightly smaller; both are heavily cratered. 10 smaller moons (inc. Ophelia, Desdemona, Juliet & Portia) are mostly within the ring system and perform sheparding duties. 4 very distant moons have inclined, retrograde orbits; captured asteroids.

Neptune’s Moons: Some Odd Orbits TRITON was discovered in 1846 by Wm. Lassell a = 355,000 km; D = 2710 km; M = 0.292 M ; Density = 2.1 g cm-3 Pretty big: about 1/2 of Europa's mass Few craters, implies activity: nitrogen geysers. Has a thin nitrogen atmosphere; freezes out at T = 37 K. Retrograde period of 5.88 days; inclined about 20 degrees to plane of Neptune's equator. Spiraling TOWARD Neptune; inside Roche limit in 108yr: so Neptune will then lose a moon and (very probably) gain a spectacular ring system!

Triton S polar area: black streaks are liquid N geysers Frozen lake from ice volcano?

Other Moons of Neptune 2nd biggest is Proteus, D = 440 km, a=118,000 km NEREID: discovered by G. Kuiper in 1949: a = 5,510,000 km; D = 340 km; e = 0.76 Some powerful interactions drove Triton retrograde and Nereid so eccentric; perhaps Pluto?? 5 little moons known inside the Roche limit; affect rings.