19 July 2005AST 2010: Chapter 11 1 Moons, Pluto, & Rings

19 July 2005AST 2010: Chapter 112 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 of these rings and moons contain dark, organic compounds mixed with ice and rock The presence of dark material implies that the objects reflect very little light

19 July 2005AST 2010: Chapter 113 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 They were probably formed far away and subsequently captured by the planet they now orbit

19 July 2005AST 2010: Chapter 114 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

19 July 2005AST 2010: Chapter 115 Galilean Moons They were first seen by Galileo, using his telescopes most recently studied by the Galileo spacecraft and by the Hubble Space Telescope The combined data about the moons have 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

19 July 2005AST 2010: Chapter 116 Major Satellites in Solar System

Callisto: Cratered World (1) This is the outermost of the Galilean satellites 2 million km from Jupiter Its noon-time surface temperature is 130 K (140°C below freezing!) Its diameter is 4820 km, about the same as Mercury’s Its mass is about 1/3 of Mercury’s So it’s 1/3 as dense as Mercury This means that Callisto has far less rocky and metallic materials than do the terrestrial planets It must instead be composed largely of ice Movie Movie from a mosaic of images taken by the Voyager spacecraft

19 July 2005AST 2010: Chapter 118 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 It 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

19 July 2005AST 2010: Chapter 119 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 thought to be younger than the darker ones Ganymede is geologically very different from Callisto It is a differentiated world, like terrestrial planets, with 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 is geologically active, powered by internal heat

Ganymede: Largest Moon (2) It has a diameter of 5262 km Slightly larger than Mercury Why is Ganymede very different from Callisto? Possible reasons 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 It is close enough to Jupiter that tidal forces from Jupiter may have occasionally heated its interior This could have triggered major convulsions on its crust MovieMovie from a mosaic of images taken by the Voyager spacecraft

19 July 2005AST 2010: Chapter 1111 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

19 July 2005AST 2010: Chapter 1112 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

19 July 2005AST 2010: Chapter 1113 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 Io, the moviemovie Io: Volcanic Moon Jupiter Io

19 July 2005AST 2010: Chapter 1114 Volcanism on Io Io has no impact craters They must have been eradicated by its volcanic activity Many of its volcanic eruptions produce plumes of material that extend hundreds of km out into space The colors on Io come from sulfur (yellow, black, red) and sulfur dioxide (white) Io’s volcanoes can be very long lived Some have been observed for 20 years Loki volcano erupts Lava fountains

19 July 2005AST 2010: Chapter 1115 Infrared Images of Io Images taken by the Keck telescope plus adaptive optics Bright spots indicate hot lava erupted by volcanoes Io in infrared, the moviemovie Galileo image Without adaptive optics

19 July 2005AST 2010: Chapter 1116 Saturn’s Moons To date, Saturn has 46 known satellites The largest is Titan almost as big as Ganymede Titan is the only satellite among the jovian moons that has a substantial atmosphere

19 July 2005AST 2010: Chapter 1117 Some of Saturn’s Moons

19 July 2005AST 2010: Chapter 1118 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 massive enough to have strong gravity that can retain its atmospheric gases Titan is cold enough so that the gases in the atmosphere are slow moving and hence do not easily escape into space

19 July 2005AST 2010: Chapter 1119 Titan: Moon with Atmosphere (2) Titan’s 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 probably composed of methane the top layer including 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

19 July 2005AST 2010: Chapter 1120

19 July 2005AST 2010: Chapter 1121 Cassini Spacecraft

19 July 2005AST 2010: Chapter 1122 After separating from Cassini, the Huygens probe landed on Titan in Jan. 2005

19 July 2005AST 2010: Chapter 1123 Some of Cassini’s Images Cassini-Huygens mission’s website website

19 July 2005AST 2010: Chapter 1124 Uranus System To date, it has 27 known satellites none are really large Its rings and satellites are tilted at 98° just like the planet itself It has 11 rings composed of very dark particles discovered in 1977 consisting of narrow ribbons of material with broad gaps very different from the rings of Saturn

19 July 2005AST 2010: Chapter 1125 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) It is the only large moon in the solar system that circles its planet in a direction opposite to the planet's rotation (a retrograde orbit) It may once have been an independent object that Neptune captured Triton has an atmosphere and active volcanism bears some resemblance to Pluto Neptune’s Moons

19 July 2005AST 2010: Chapter 1126 Triton: Icy World (1) It has an atmosphere and active volcanism Its surface has the coldest temperature (between 35 and 40 K) of any of the worlds our robot spacecraft have visited Its surface material is made of frozen water, nitrogen, methane, and carbon monoxide It has a very thin nitrogen atmosphere, with a pressure of only 16 millionths of our atmospheric pressure Triton resembles Pluto in size, composition, and temperature

19 July 2005AST 2010: Chapter 1127 On Triton, volcanic eruptions produce plumes of nitrogen gas and are driven by seasonal heating from the Sun The plumes can rise 10 km above the surface On Earth, Venus, Mars, rocky magma is driven by internal heat On Io, sulfur compounds are driven by tidal interactions with Jupiter Triton moviemovie Triton: Icy World (2)

19 July 2005AST 2010: Chapter 1128 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 It has only 1 known satellite, Charon Charon’s orbit is locked to Pluto, revolving and rotating at the same time as Pluto rotates Pluto Charon

19 July 2005AST 2010: Chapter 1129 Pluto (2) It is the only planet not yet visited by spacecraft It appears very faint from Earth Its observation requires the best telescopes Its diameter is ~2190 km (60% of the Moon) Its density is ~2.1 g/cm 3 It is composed of a mixture of rocky material and water ice similar to Neptune’s moon Triton It has a highly reflective surface frozen methane, carbon monoxide, & nitrogen Its surface temperature is between 50 and 60 K It has a tenuous atmosphere

19 July 2005AST 2010: Chapter 1130 Pluto (3) Pluto is not like the terrestrial or jovian planets Pluto, Quaoar, Charon, and Triton are probably examples of Kuiper-belt objects The Kuiper belt is a disk-shaped region of space beyond Neptune’s orbit

19 July 2005AST 2010: Chapter 1131 Quaoar: New planet? Its orbit is more circular than Pluto's It is closer to the ecliptic 7.9° inclination 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

19 July 2005AST 2010: Chapter 1132 Quaoar

19 July 2005AST 2010: Chapter 1133 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

19 July 2005AST 2010: Chapter 1134 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

19 July 2005AST 2010: Chapter 1135 Planetary Rings (3) The dotted vertical line is the limit inside which the gravitational (tidal) force of a planet can break up a satellite In other words, only really small bodies survive inside the tidal stability limit

19 July 2005AST 2010: Chapter 1136 What Causes Rings? (1) Each ring is a collection of vast numbers of objects and particles Each particle obeys Kepler’s laws The inner particles revolve faster than those farther out The ring does not rotate as a solid body Particles within the ring are close to one another The particles exert mutual gravitational influence and occasionally even touch in low speed collisions Two basic theories on how the rings formed have been proposed Breakup hypothesis: they are the remains of a shattered satellite Another hypothesis: the rings consist of particles that were unable to fuse into a single body

19 July 2005AST 2010: Chapter 1137 What Causes Rings? (2) 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 planets 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

19 July 2005AST 2010: Chapter 1138 Rings of Jovian Planets Jupiter’s rings Saturn’s rings Uranus’ rings Neptune’s rings

19 July 2005AST 2010: Chapter 1139 Satellite-Ring Interactions Each ring has an intricate structure, as discovered by Voyager The rings’ structures are due mainly to the gravitational effects of the satellites Without the 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