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Moons, Pluto, and Rings

Moons, Pluto, and Rings. 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

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Moons, Pluto, and Rings

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  1. Moons, Pluto, and Rings AST 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 AST 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 AST 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 AST 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 AST 2010: Chapter 11 Callisto

  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 AST 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 AST 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 AST 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 AST 2010: Chapter 11

  10. Ganymede: Old Dark Terrain • Old: it is covered with craters • Dark: ice covered with dust from meteoroid impacts AST 2010: Chapter 11

  11. Ganymede: New Bright Terrain • New: suggested by fewer craters • Bright: due to fracturing of the icy surface AST 2010: Chapter 11

  12. 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 AST 2010: Chapter 11 Actual color Enhanced color

  13. 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 AST 2010: Chapter 11

  14. 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 AST 2010: Chapter 11

  15. 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 (SO2, white) • Io’s volcanoes can be very long lived • Some have been observed for 20 years Lava fountains Loki volcano erupts AST 2010: Chapter 11

  16. Pele Volcano AST 2010: Chapter 11

  17. 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 AST 2010: Chapter 11

  18. Some of Saturn’s Moons AST 2010: Chapter 11

  19. 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 AST 2010: Chapter 11

  20. 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 AST 2010: Chapter 11

  21. AST 2010: Chapter 11

  22. AST 2010: Chapter 11

  23. After separating from Cassini, the Huygens probe landed on Titanin Jan. 2005 AST 2010: Chapter 11

  24. Some of Huygens’ Images AST 2010: Chapter 11

  25. Uranus System • To date, it has 27 known satellites • none are really large • Its rings and satellites are tilted at 98o 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 AST 2010: Chapter 11

  26. 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 AST 2010: Chapter 11

  27. 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 AST 2010: Chapter 11

  28. 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 AST 2010: Chapter 11

  29. Pluto (1) HST Picture • Discovered through systematic search • at P. Lowell observatory in 1930 • Highest inclination to the ecliptic (17°) • Largest eccentricity ~ 0.248 • 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 Charon Pluto AST 2010: Chapter 11

  30. 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/cm3 • 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 AST 2010: Chapter 11

  31. 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 AST 2010: Chapter 11

  32. 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 AST 2010: Chapter 11

  33. 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 AST 2010: Chapter 11

  34. 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 AST 2010: Chapter 11

  35. Planetary Rings (3) AST 2010: Chapter 11

  36. 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 AST 2010: Chapter 11

  37. 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 AST 2010: Chapter 11

  38. 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 AST 2010: Chapter 11

  39. Rings of Saturn AST 2010: Chapter 11

  40. Rings of Uranus and Neptune • 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 • 51000 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 AST 2010: Chapter 11

  41. 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 AST 2010: Chapter 11

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