22 February 2005AST 2010: Chapter 11 1 Moons, Pluto, and Rings

22 February 2005AST 2010: Chapter 11 2 Ring and Satellite Systems (1) The rings and moons in the outer solar system are different in composition from objects in the inner solar system Most contain dark, organic compounds mixed with ice and rock The presence of dark material implies that they reflect very little light

22 February 2005AST 2010: Chapter 11 3 Ring and Satellite Systems (2) Most satellites in the outer solar system are in direct or regular orbits They revolve about their planet in a west-to- east direction and in the plane of the planet’s equator A number of them have irregular orbits They orbit in a retrograde (east-to-west) direction, or else have orbits with high eccentricity or high inclination These satellites are usually smaller, located relatively far from their planet, probably formed far away and subsequently captured by the planet they now orbit

22 February 2005AST 2010: Chapter 11 4 Jupiter's Moons To date, Jupiter has 63 known satellites The largest four are Callisto, Ganymede, Europa, and Io Europa and Io are the size of our Moon Ganymede and Callisto are bigger than Mercury The rest are much smaller More moons will likely be found in the future

22 February 2005AST 2010: Chapter 11 5 The Galilean Moons Callisto, Ganymede, Europa, Io They were first seen by Galileo (the astronomer) Studied by the Galileo space probe and by Hubble Space Telescope The combined data about the moons has shown important similarities to the terrestrial planets The differences between the moons seem to be mostly due to distance from Jupiter Io Europa Ganymede Callisto

22 February 2005AST 2010: Chapter 11 6 Callisto: Cratered World (1) This is the outermost of the Galilean satellites 2 million km from Jupiter Noon-time surface temperature: 130 K (140°C below freezing!) Its diameter: 4820 km, about the same as Mercury’s Its mass: about 1/3 of Mercury’s mass So it’s 1/3 as dense as Mercury It has far less rocky and metallic materials than do the terrestrial planets It is composed largely of ice

22 February 2005AST 2010: Chapter 11 7 Callisto: Cratered World (2) Callisto seems not to have fully differentiated (separated into layers of different density materials) The details of gravitational pull on the Galileo spacecraft suggest that Callisto lacks a dense core This is surprising to astronomers! All big icy moons are expected to be differentiated (they should have differentiated more easily than rocky ones) Yet Callisto appears to have been frozen solid before the differentiation was complete Callisto is covered with impact craters Although they look similar to craters on the Moon, they formed very differently Callisto lacks the interior forces to drive geological change — it’s geologically dead

22 February 2005AST 2010: Chapter 11 8 Ganymede: Largest Moon (1) It is the largest satellite in the solar system It is cratered, but less so than Callisto Crater counting suggests that ¾ of the surface may have formed more recently than the lunar maria The lighter areas are younger than the darker ones Ganymede is geologically very different from Callisto It is a differentiated world, like terrestrial planets a core about the size of our Moon a mantle and crust of ice “floating” above the core a magnetic field, a signature of a partly molten interior It’s geologically active, powered by internal heat

22 February 2005AST 2010: Chapter 11 9 Ganymede: Largest Moon (2) It has a diameter of 5262 km Slightly larger than Mercury Why is Ganymede very different from Callisto? Possible explanation Their small difference in size and internal heating may have led to the significant differences between the two moons now The gravity of Jupiter may be responsible for Ganymede’s continuing geological activity Ganymede is close enough to Jupiter that tidal forces from Jupiter may have occasionally heated Ganymede’s interior This could have triggered major convulsions on its crust

22 February 2005AST 2010: Chapter Ganymede: Old Dark Terrain Old: it is covered with craters Dark: ice covered with dust from meteoroid impacts

22 February 2005AST 2010: Chapter Ganymede: New Bright Terrain New: suggested by fewer craters Bright: due to fracturing of the icy surface

22 February 2005AST 2010: Chapter Europa: Moon with Ocean (1) Its surface is covered with a thick layer of water ice For the most part, it is smooth, but is crisscrossed with cracks and low ridges often stretching for thousands of kilometers across icy plains Under the ice, there may be liquid water or slush If so, tidal heating may be responsible for keeping the water in liquid form Actual color Enhanced color

22 February 2005AST 2010: Chapter Evidence for Warm Oceans on Europa Galileo images appear to confirm the existence of a global ocean on Europa It has ridges and multiple-line features that may have formed when thick layers of ice were broken up into giant icebergs and ice flows and then refrozen in place It also has smooth areas where water may have flowed up and refrozen If it has liquid water, could life exist on Europa? Enhanced color

22 February 2005AST 2010: Chapter Io Io: Volcanic Moon It is the most volcanically active world in the solar system Io has an elliptical orbit, causing it to twist back and forth relative to Jupiter and experience tidal forces This twisting and flexing is the likely source of Io’s internal heating that drives its massive volcanism Io’s interior heat may also have produced a differentiated interior Io probably has an iron core surrounded by a molten rocky mantle Jupiter

22 February 2005AST 2010: Chapter Volcanism on Io Io has no impact craters They must have been eradicated by its volcanic activity Volcanoes produce plumes of material that extend up to 280 km above the surface The colors on Io come from sulfur (yellow, black, red) and from sulfur dioxide (SO 2, white) Io’s volcanoes can be very long lived Some have been observed for 20 years Loki volcano erupts Lava fountains

22 February 2005AST 2010: Chapter Pele Volcano

22 February 2005AST 2010: Chapter Saturn’s Moons To date, Saturn has 33 known satellites The largest is Titan Almost as big as Ganymede The only satellite with a substantial atmosphere

22 February 2005AST 2010: Chapter Some of Saturn’s Moons

22 February 2005AST 2010: Chapter Titan: Moon with Atmosphere (1) It’s the second largest moon in the solar system It’s the only moon in the solar system with a substantial atmosphere The thick atmosphere makes its surface impossible to see Why does Titan have an atmosphere? Possible reasons: Titan is large enough to have a strong gravitational field Titan is cold enough so that the gas in the atmosphere is slow moving

22 February 2005AST 2010: Chapter Titan: Moon with Atmosphere (2) Its atmosphere has a pressure 1.6 times Earth's is comprised of mostly nitrogen, plus 6% argon and a few percent methane has trace amounts of organic compounds (such as carbon monoxide, ethane, and hydrogen cyanide) and water has multiple layers of clouds the bottom layer is probably composed of methane the top layer includes a dark reddish haze or smog, which hides Titan’s surface from our view Its surface has a temperature of about 90 K This means that on Titan’s surface methane may exist in liquid or solid form, and there may even be seas or lakes of methane, as well as methane ice

22 February 2005AST 2010: Chapter 11 21

22 February 2005AST 2010: Chapter 11 22

22 February 2005AST 2010: Chapter After separating from Cassini, the Huygens probe landed on Titan in Jan. 2005

22 February 2005AST 2010: Chapter Some of Huygens’ Images

22 February 2005AST 2010: Chapter Uranus System To date, it has 27 known satellites none are really large Its rings and satellites are tilted at 98 o just like the planet itself It has 11 rings composed of very dark particles discovered 1977 consist of narrow ribbons of material with broad gaps very different from the rings of Saturn

22 February 2005AST 2010: Chapter Neptune’s Moons To date, it has 13 known satellites 6 are regular, close to the planet 2 are irregular, farther out Its largest moon is Triton (in mythology, Triton is Neptune’s son) the only large moon in the solar system that circles its planet in a direction opposite to the planet's rotation (a retrograde orbit) may once have been an independent object that Neptune captured has an atmosphere and active volcanism bears some resemblance to Pluto

22 February 2005AST 2010: Chapter Triton: Icy World Its surface has the coldest temperature (between 35 and 40K) of any of the worlds our robot spacecrafts have visited Its surface material is made of frozen water, nitrogen, methane, and carbon monoxide Triton has a very thin nitrogen atmosphere, with a pressure 16 millionths of our atmospheric pressure

22 February 2005AST 2010: Chapter Ice volcanoes on Triton Ice volcanoes on Triton: plume rising 8 km above the surface and extending 140 km "downwind" On Triton: eruptions of volatile gasses like nitrogen or methane driven by seasonal heating from the Sun On Earth, Venus, Mars: rocky magma driven by internal heat On Io: sulfur compounds driven by tidal interactions with Jupiter plume

22 February 2005AST 2010: Chapter Pluto (1) Discovered through systematic search at P. Lowell observatory in 1930 Highest inclination to the ecliptic (17°) Largest eccentricity ~ Average distance from the Sun ~40 AU Perihelion closer than Neptune Orbital period ~248.6 earth years Rotation: ~6.4 days on its side Pluto's diameter 2240 km Only 1 known satellite: Charon Charon’s orbit is locked to Pluto, revolving and rotating at the same time as Pluto rotates HST Picture Pluto Charon

22 February 2005AST 2010: Chapter Pluto (2) The only planet not yet visited by spacecraft Very faint from Earth observation requires the best telescopes Diameter is ~2190 km (60% of the Moon) Density is ~2.1 g/cm 3 Mixture of rocky material and water ice Similar to Neptune’s moon Triton Has a highly reflective surface frozen methane, carbon monoxide, & nitrogen Surface temperature between 50 and 60K Has a tenuous atmosphere

22 February 2005AST 2010: Chapter Quaoar – New planet? Its orbit is more circular than Pluto's It is closer to the ecliptic 7.9° inclinatiion compared to Pluto's 17° Its diameter is 1280 km compared to Pluto's 2240 km Possibly Pluto and Quaoar are both Kuiper-belt objects

22 February 2005AST 2010: Chapter Pluto (3) Pluto is not like the terrestrial or jovian planets Pluto, Quaoar, Charon, and possibly Triton, are examples of Kuiper-belt objects The Kuiper belt is a disk-shaped region of space beyond Neptune’s orbit

22 February 2005AST 2010: Chapter Planetary Rings (1) All four giant planets have rings Each ring is a system of billions of small particles (moonlets) Each ring displays complicated structure related to the interaction between the rings and satellites The four ring systems are very different from each other in mass, structure, and composition

22 February 2005AST 2010: Chapter Planetary Rings (2) Saturn’s rings made up of icy particles spread out into several vast, flat rings, with a great deal of fine structure Neptune’s & Uranus’ rings made up of dark particles, confined to a few narrow rings, with broad empty gaps Jupiter’s rings transient dust bands, constantly renewed by erosion of dust grains from small satellites

22 February 2005AST 2010: Chapter Planetary Rings (3)

22 February 2005AST 2010: Chapter What causes Rings? Each ring is a collection of vast numbers of objects and particles Each particle obeys Kepler’s laws Inner particles revolve faster Ring does not rotate as a solid body Better to consider the revolution of individual moonlets Particles within the ring are close to one another Exert mutual gravitational influence, even collide in low speed collisions Gives rise to waves that move across the rings Two basic theories of how the rings came to be Breakup hypothesis: remains of a shattered satellite Another hypothesis: particles that were unable to fuse into a single body

22 February 2005AST 2010: Chapter Ring Causes Continued In either theory, the gravitation of the planet plays an important role Tidal forces for orbits close to the planet can tear bodies apart, or inhibit loose particles to come together The rings of Saturn and Uranus are close to the planet In the breakup theory, a satellite, or a passing comet, may have come too close and been torn apart under tidal forces, or through some collision Some scientists believe that some of the rings are young and must therefore be the result of a breakup

22 February 2005AST 2010: Chapter Rings of Saturn Consists of many rings and subrings: A, B, C B Ring: brightest, most closely packed particles A and C rings: translucent Total mass of B estimated to be similar to that of an icy satellite 250 km in a diameter A & B separated by a wide gap called Cassini division Rings are broad and very thin Width of main ring ~70000 km, thickness ~20 m Ring particles composed mainly of water ice Particles range from grains the size of sand up to house-sized boulders A handful of narrow rings ~100-km wide, in addition to the main rings

22 February 2005AST 2010: Chapter Rings of Saturn

22 February 2005AST 2010: Chapter Narrow and black Almost invisible from Earth Nine rings discovered (1977) during observation of a star – occultation First seen by Voyager (1986) Outermost and most massive called Epsilon 100 km wide, ~ 100 m thick km from the planet Other rings much smaller: 10 km wide Particles are very dark black carbon and hydrocarbon compounds Rings of Neptune are similar but even more tenuous Rings of Uranus and Neptune

22 February 2005AST 2010: Chapter Satellite-Ring Interactions Each ring has an intricate structure, as discovered by Voyager Structures due to mainly gravitational effects of satellites Without satellites, the rings would be flat and featureless There could even be no rings at all… Gaps in Saturn’s A-ring result from gravitational resonances with smaller inner satellites, especially Mimas