Exam 3 Study Guide

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EAPS 105, The Planets Exam 3 Study Guide Know the following: Unit 7: Planetary Atmospheres 1. The main components of a primary atmosphere. All planets would have had a primary atmosphere that consisted of hydrogen and helium gas, the most abundant elements in the solar nebula. While gas giants kept their hydrogen and helium atmosphere, the rocky planets lost theirs and replaced with new ones 2. The factors that influence the escape velocity of a planet A planet’s escape velocity ( ? e) is a key factor in determining what kind of atmosphere a planet can hold onto. where: M is the mass of the planet R is the radius of the planet (for surfacene) G is the universal gravitational constant • The greater the mass of the planet, the higher the velocity needed to escape its gravity. • The greater the radius of the planet, the lower the velocity needed to escape its gravity (because the center of the planet is farther away). The differences in the escape velocities is an important factor for why planets have different atmospheres Mercury: no atmosphere Earth: Nitrogen/Oxygen Jupiter: Hydrogen/Helium 3. The factors that influence gas molecule velocities. The temperature and atomic weight of a gas molecule determines whether it is likely to achieve enough velocity to exceed the escape velocity of the planet and depart. •A gas is a volume of molecules moving around and bumping into each other. • The higher the temperature, the faster gas molecules move. • The lighter its atomic weight, the faster gas molecules move. 4 The gases that should be retained by Mars’ atmosphere. Oxygen, nitrogen 5. How the Earth obtained most of the oxygen in its atmosphere. Hydrogen (H2) and Helium (He) Could not be held due to too small an escape velocity. Water vapor (H20), Carbon Dioxide (CO2), and Ammonia (NH3) Volcanism brought Sunlight broke down these up from Earth’s interior. Water vapor condensed to form the oceans. Nitrogen (N2), Oxygen(O2) and a little carbon dioxide (CO2) Sunlight broke down NH3 into N2 and H2 (H2 then escaped). Life (microbes) converted most of the CO2 into O2. The rise of oxygen producing bacteria that feasted on carbon dioxide, leading to our present atmosphere 6. The major differences between the atmospheres of Earth and Venus. Venus, our deadly sister •Atmospheric pressure is 90 times greater than on Earth. 1

•CO2 traps so much heat from the Sun that Venus has the hottest surface of any planet, 465°C (860°F) — sufficient to melt lead. •Sulfuric acid clouds. But billions of years ago, when the Sun was 25% cooler, Venus is thought to have had a much thinner atmosphere, similar in pressure to Earth's today, and thus was much cooler; perhaps sufficient to retain water on its surface and have been lush and hospitable to life. 7. The major differences between the atmospheres of Venus and Mars. Mars and Venus have similar atmospheric chemistry (almost all CO2), but Venus’ atmosphere is15,000 times denser than Mars' 8. The ability of wind on Mars to move stuff. No, the atmospheric density on Mars is too low to provide sufficient force to move heavy objects. At only 0.6% of Earth’s atmospheric density, Mars winds carry little force, even at 100 km/hr (maximum for Mars). Because Mars’ atmosphere is so thin, even high winds on Mars would not pick up objects bigger than grains of sand. Though Mars' winds can only pick up very small grains of sand, there are dust storms and dust devils. This is a reasonable depiction of dust devils on Mars and lightning induced by their electrical activity. 9. The major reason why there is no liquid water on the surface of Mars today. In addition to being warm enough, the existence of liquid water on a planetary surface requires sufficient atmospheric pressure. Mars' atmosphere is currently too thin to enable water on its surface regardless of temperature. 10. The history of water on the surface of Mars. Though there is no water on the surface of Mars today, billions of years ago there was a thicker atmosphere and a lot water, inferred from the extensive number of dry river channels and lake beds. When the atmospheric pressure dropped, where did the water go? Detected by ground penetrating radar, Mars has an ocean’s worth of water trapped as ice beneath the surface (permafrost). 11. The duration of water on Mars in its past. We do not know 12. Where all the water that used to be on the surface of Mars disappear to. Detected by ground penetrating radar, Mars has an ocean’s worth of water trapped as ice beneath the surface (permafrost). An asteroid impact that created a 150 m crater ejected boulders of ice from the subsurface The impact was detected by the Insight lander seismometer (Dec. 24, 2021) and later found by the Reconnaissance Orbiter imager. Evidence of ice just below the surface has also been detected when warm temperatures enabled some of the permafrost to melt, bringing dampness to the surface. This can only happen if the water is very salty, which lowers its freezing temperature. Because of low pressure, ice can only exist where the temperature remains below freezing for most of the Martian year — its north and south poles. • 70% water ice, 30% carbon dioxide ice • If melted the volume of ice is sufficient to cover the surface to a depth of ~20 meters 13. The primary cause of Earth’s seasons. Tilt of earth, and how that influences to each hemisphere The length of each season (winter, spring, summer, fall) on a planet is about one-quarter of the time it takes to orbit the Sun. Thus, the length of seasons is proportional to the distance a planet is from the Sun. 14. Consequences of Uranus spinning on its side regarding its seasons and lengths of days. Because of its extreme tilt, Uranus has extreme seasons — 21-year-long summers are one long day and 21- year- long winters are one long night. During spring and fall, days and nights shorten to only 17 hours long. 2

15. How the layers of terrestrial planetary atmospheres are defined. Atmospheric layers are defined by where they switch from cooling with altitude to heating. 16. The atmospheric layer weather and clouds exist. 17. Why the temperature of the troposphere decreases with altitude. The troposphere is heated from the ground, causing it to be warmest at the surface. Temperatures then decrease with increasing height due to decreasing atmospheric pressure, which causes the air to expand and cool (same energy distributed in a larger volume). 18. The atmospheric layer not found on Venus or Mars. 19. What happens when warm, moist air rises. Warm surface water evaporates (becomes invisible water vapor), then rises. As the warm moist air rises, it expands and cools due to lowering pressure with altitude. Cool air cannot hold water vapor, so it condenses into water droplets and a cloud forms. 20. What happens when warm, moist air cools. Clouds appear when invisible water vapor rises, cools, and condenses to become visible liquid droplets or ice crystals. A drop in pressure causes air to expand and cool, causing invisible water vapor to condense into visible ice crystals (pressure would be too low for water droplets) — a cloud will form. 21. Atmospheric conditions at the top of Venus’ clouds. At the top of Venus’ troposphere the temperature and pressure are just right – but bring your own oxygen. 22. Differences between Jupiter’s belts and zones. Jupiter is perhaps most notable for its beautiful clouds, which are divided into colorful bands. Dark colored bands are called belts Light colored band are called zones. The temperate differences between higher altitude (and thus cooler) zones and lower altitude (and thus warmer) belts, can be observed in an infrared view of Jupiter. Darker regions cooler (higher altitude), lighter regions are warmer(lower altitude). The color differences is due to different ices condensing at different temperatures 23. Why Saturn’s colors are muted compared to Jupiter’s. Saturn’s atmosphere is also banded. The change in temperature in Saturn’s atmosphere is more gradual Saturn’s colors are muted because its clouds are thicker due to a more gradual temperature change with altitude, and because sunlight at Saturn is dimmer. 24. The causes a planetary atmospheric banding. Atmospheric banding on Jupiter is caused by clouds at different altitudes having different temperatures, compositions, and colors. The temperate differences between higher altitude (and thus cooler) zones and lower altitude (and thus warmer) belts, can be observed in an infrared view of Jupiter. 25. Why Uranus’ and Neptune’s atmosphere is blue. Uranus and Neptune are blue because their methane clouds absorb reds and reflect blue light — the opposite of Jupiter and Saturn’s ammonia clouds. 26. The main factor influencing global temperatures throughout Earth’s history. The main factor in determining the temperature of a body's surface or atmosphere (if it has one) is its orbital parameters, which can vary with time. Changes in Earth's orbital parameters have led to significant changes in temperature throughout Earth’s history– there are noticeable cycles on the order of 25,000 (tilt), 40,000 (procession), and 100,000 (eccentricity) years. On Earth changes in orbital parameters are known as the Milankovitch Cycles 27. How greenhouse gases cause warming of planetary atmospheres. The Greenhouse Effect works by converted short-wavelength radiation (visible light) to longer wavelength radiation (infrared light, heat) and trapping it. 1. Visible light (shortwave radiation) from the Sun passes through the glass. 3

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2. When shortwave radiation hits a surface, it excites the atoms, which converts it to heat, which is re-emitted as long-wave radiation (infrared light). 3. Glass traps most of the heat (longwave radiation/infrared light), causing the greenhouse to heat up In our atmosphere, greenhouse gases allow visible light from the Sun to pass to the surface, but traps some of the re- admitted infrared light (heat), causing the planet to warm. With greenhouse gases the average temperature on Earth is 61°F (16°C). Without greenhouse gases the Earth would average only 0°F and become completely ice 28. Consequences if Earth had no greenhouse gases. Without greenhouse gases the Earth would average only 0°F and become complete 29. How day and night temperatures vary on Venus and Mars. Because Venus’ thick atmosphere efficiently traps and circulates the Sun’s energy around the entire planet, the nights on Venus are as warm as the days (872 F, 467 C), even though its days are 243 Earth days long. Because Mars' atmosphere is so thin and does not retain much heat from the Sun, its day and nightime temperatures vary greatly. +37 -131 30. What albedo is. the capacity of a surface to reflect light, which determines how much visible light gets converted into heat (infrared light). 31. The influence of ice on Earth’s global temperatures. High albedo: Ice reflects 90% of sunlight and converts only 10% to heat. On Earth there is a positive feedback effect: Melting of ice leads to more surface water, which leads to warmer temperatures, and thus more melting of ice 32. General observations regarding Jupiter’s Great Red Spot. The Great Red Spot is about twice the diameter of the Earth. The Great Red Spot has been observed since 1830 and maybe as early as 1665 (Giovanni Cassini). • On Earth cyclones die when they 2014 hit land and they lose their energy. Jupiter has no land and almost unlimited heat to fuel its cyclones, so they are very long lived. The Great Red Spot is shrinking . Storms on Jupiter produce lightning 33. Why we might find diamonds within the interiors of Uranus and Neptune. It might be snowing diamonds inside Uranus and Neptune High pressures can strip the hydrogen from methane (CH4), leading to free carbon which can compress into diamonds and snow down onto the planet’s core. 34. The strange feature that Saturn has at its north pole. One of the most unusual atmospheric features in the Solar System is Saturn’s unique hexagonal cloud pattern around its north pole. • A stable feature still visible 20 years after first detected. • A massive cyclone tightly centered on the pole, with an eye 50 times larger than the average hurricane eye on Earth. • We are still not sure why it formed. Unit 8: Minor Bodies 35. What most asteroids look like. Asteroids are remnants left over from the early formation of our solar system 4.6 billion years ago. They are the building blocks of the planets, and thus inform us planet formation. Have smooth surface 36. What regolith is. regolith — a layer of dust and small particles that rain down on their surface after impacts. 4

37. How asteroid Bennu looks different compared to larger asteroids. 1. The asteroid Bennu (D = 1.7 km) does not display the regolith smoothed surface we have come to expect of asteroids. Instead, it is very rubble. It is too small. Bennu is so small that most impact ejecta escapes its gravitational pull and does not land back on the asteroid 38. Where the main asteroid belt is located. Most (not all) asteroids can be found in the main asteroid belt located between Mars and Jupiter 39. How crowded the asteroid belt is. 2. There are an estimated 1 million asteroids 1 km in diameter or greater. Av distance 500000 km. There are an estimated 10 in 14 asteroids >1 m. The average distance between them is 5000 km. • We have sent a dozen spacecraft through the asteroid belt without incident. 40. Why asteroid belt could never accrete into a planet. The asteroid belt could never accrete into a planet because its total mass only adds up to 3% of the mass of the Moon and tidal forces from Jupiter keep asteroids from accreting (like a ring around a planet). 41. Characteristics of C-type asteroids. Most common type of asteroid (75%): Very dark from rocks with carbon. They have similar chemistry to the Sun (minus the hydrogen and helium) and are thus amongst the most ancient. 253 Mathilde 42. Why many asteroids contain water ice. there is actually a lot of ice as well because of mixing due to the Grand Tack (when Jupiter traveled into the inner Solar System). 43. What the bright patches on the surface of asteroid Ceres are. False color based on brightness variations illuminates differences in the mineralogy of Ceres' surface, a mixture of water ice and water-rich rocks (clay), and salty deposits (very bright regions). 44. Basic characteristics of the asteroid Vesta. Vesta is the only known remaining rocky protoplanet (with a differentiated interior) of the kind that formed the terrestrial planets (all the others have significant ice). The 525-km-diameter asteroid Vesta is not quite big enough to pull itself into a sphere, and thus is not considered a dwarf planet Vesta's mineralogy is consistent with a family of meteorites that fell to Earth — these would have been ejected from impacts. Rheasilvia and Veneneia basins are the largest impact craters on Vesta and probably the source of the Vesta- like meteorites found on Earth. 45. Why comets are referred to as dirty ice balls. Comets are a mixture of ice and rock/metal (often referred to as dirty ice balls). They represent some of the earliest building blocks of the outer planets (beyond the ice line) 46. Why comets have tails. The release of gas and dust leads to the formation of gas and dust tails — telltale sign of a comet. 47. Why the average density of comets is so low. Density measurements from spacecraft flybys and orbits suggest that comets have average densities much less than that of ice, implying lots of pore (empty) space inside. 48. Why comets are referred to as rubble piles. The abundant pore (empty) space within comets implies they are rubble piles — collections of smaller objects loosely held together by their small gravitational attractions. 49. Characteristic of comet gas tails. The gas tail emits blue light as the gas is ionized (electrically charged) by ultraviolet light from the Sun. • It is swept directly away from the Sun by the solar wind (charges particles streaming from the Sun) that interacts with the ions. 50. Characteristic of comet dust tails. The dust tail is swept away from the comet by pressure from sunlight, then rotates away from direction the comet is traveling, separating it from the gas tail. Objects in more distant orbits, orbit slower (Kepler's 3rd law). Thus, as dust moves away from the 5

Sun (more distant orbits), it slows down causing the dust tail to rotate away from the direction the comet is traveling 51. How dust jets do not initiate at sunrise and cease at sunset. Dust jets do not turn on and off with sunrise and sunset. They turn on sporadically and last for short time periods (10s of minutes). We are not sure what initiates dust jets, maybe landslides push debris into gas jets. Comet 67P’s dust jets turning on and off in 10s of minutes though the rotation time is 12 hours Cometary jets may turn turn on due to landslides that push debris into invisible gas jets. 52. Where orbits of ultra-short-period comets extend to. Ultra-short-period comets orbit inside Jupiter's orbit 53. Where orbits of short-period comets extend to. Short-period comets orbits extend to the Kuiper Belt beyond Neptune's orbit 54. Where orbits of long-period comets extend to. Long-period comet orbits extend to the Oort Cloud, way beyond the visible solar system 55. Why Haley’s comet is the most famous of all comets. Halley’s Comet is the most famous comet because it is a short-period comet that comes into view about once in a lifetime (~75 years) 56. Where the Kuiper Belt located. which lies beyond Neptune’s orbit. It is a donut- shaped region 20 times wider and 100 times more massive than the asteroid belt, and with many larger bodies (including Pluto). 57. The differences between the asteroid belt and the Kuiper belt. 20 times wider and 100 times more massive 58. How we know the Oort cloud extends several light-years from the Sun. . By the elongated shape of the orbits of long-period comets 59. How we know the Oort cloud is a sphere as opposed to a disc. Because long-period comets come at us from all directions 60. What meteor showers arise from. Meteor showers are created when the Earth passes through the remnants of a broken-up comet 61. How Pluto was discovered. In 1930 Clyde Tombaugh discovered Pluto by comparing pictures of the night sky taken 6 days apart using a blink comparator and looking for anything that moved relative to the fixed background stars. 62. Observations made by New Horizons of Pluto. impact basin When New Horizons spacecraft arrived at Pluto, it found a tectonically active planet with young surfaces and recent faulting — not the cold dead world we were expecting. The lack of impact craters suggests that the water-ice mountains are only 100 million years old, while the surface of the nitrogen ice that fills Sputnik Planum is only a million years old. Nitrogen ice polygons are a natural consequence of shallow convection within a 10-km-thick volume of nitrogen-ice. Relatively young faults normal faults on Pluto suggest the existence of a sub-surface ocean that is in the process of freezing. As the water turns to ice it expands, causing extension at the surface. Cold top Hot bottom Boiling a thick liquid demonstrates the surface expression of convective cells 63. In general, how Pluto compares in size to other Kuiper Belt objects. Many of the Kuiper belt objects found are of similar size to Pluto (big enough to be spherical), creating a little controversy... 64. The evidence for the possible existence of another major planet in the Kuiper Belt 6

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. Dwarf planets in the Kuiper Belt all have somewhat elliptical orbits tilted from the ecliptic, mostly due to resonances with Neptune and maybe another big body out there... The elliptical orbits of several Kuiper Belt objects are all to one side of the Sun. This might be explained by the gravitational influence of a Neptune-size planet somewhere in the Kuiper Belt. 65. Why the IAU felt the need to come up with a criteria to demote Pluto. If you let Pluto stay, you logically have to let the number of planets rise to 24 or 25, with the possibility of 50 or 100 within the next decade. Do we want schoolchildren to have to remember so many? No, we want to keep the numbers low.” In 2006, the International Astronomy Union (IAU) voted to demote Pluto from a planet to a dwarf planet — and consider dwarf planets to not be planets. The vote involved just 424 astronomers — less than 5% of the world’s astronomers — and no planetary scientists. 66. The condition for a planet that the IAU come up with to demote Pluto. • There are 118 elements, yet most people are content to learn only a few — no dwarf elements needed! 67. Consequences of IAU definition on the status of current planets. planet that kicked Pluto out. 1. Must orbit the Sun: Designed to rule out moons 2. Nearly-round in shape: Designed to rule out objects smaller than ~700 km in diameter 3. Clear the neighborhood: Designed to rule out everything in the asteroid and Kuiper belts 1. Must orbit the Sun: Designed to rule out moons Thus, all 4000 and counting exoplanets are not planets since they do not orbit the Sun. 3. Clear the neighborhood: Designed to rule out everything in the asteroid and Kuiper belts But Jupiter has not cleared its neighborhood of the Trojans asteroids. Thus, it is not a plane Clear the neighborhood: Designed to rule out everything in the asteroid and Kuiper belts Neptune has not cleared its orbit of Pluto and 4 other dwarf planets. So, it is not a planet either. 68. The group besides the IAU that also categorization celestial objects based on their orbits. A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape regardless of its orbital parameters.” • Considers the intrinsic physical properties of planetary bodies, not the nature of their orbits (like astrologists!). • Excludes stars or stellar objects such as white dwarfs, neutron stars, and black holes. • As with every other animal, vegetable, or mineral that is defined as dwarf but is still of that species, terrestrial planets, giant planets, dwarf planets, and moon planets, should all simply be categories of planets. • In practice, planetary scientists consider Pluto a planet. Unit 9: Moons and Rings 69. Which major planets have no moons. Mercury and Venus 70. The indications that a moon was created by a giant impact. A prograde orbit (same direction as the planet's spin) — the planet's angular momentum is much larger than an impactors. • A circular orbit aligned with the planet’s equator — due to gravitational tidal forces that quickly circularize orbits. 7

• A close-in orbit only a few planet diameters away — the maximum range of ejecta without reaching escape velocity). 71. The observations that support the idea that our Moon formed from a giant impact. The direction of its orbit Other evidence that the Moon resulted from a giant impact comes from the chemistry of the Moon's anorthosite crust, which only arises from the slow cooling of a magma ocean, which can only arise from the heat of a giant impact. Evidence that the Moon resulted from a giant impact also lies in the Moon's small core, consistent with only part of Theia’s and none of the Earth’s core being ejected into orbit. Further evidence of an impact origin lies in the similar chemistry of the Moon and Earth due to the mixing of the crust and mantle of the impactor and Earth. 72. What the lunar dichotomy is. The farside of the Moon has relatively high topography compared to the nearside. This is known as the lunar dichotomy. 73. The uniqueness of the South Pole-Aitken basin on the Moon. The South Pole-Aitken basin in the southern hemisphere is the largest and oldest (4.3 billion years) recognized impact basin. 74. How Mars acquired its moons Phobos and Deimos. Based on their orbits, the moons of Mars, Phobos and Deimos, likely formed from a large asteroid impact. Being much smaller than our Moon implies that the impactor was much smaller. Instead of Mars-sized, perhaps only that of a large asteroid. • Prograde orbits • Equatorial orbits • Circular orbits • Close-in orbits 75. How Jupiter acquired its Galilean moons. The accreted from a ring of material while Jupiter formed 76. What controlled the percentage of ice to rock in the mineralogy Jupiter’s major moons. In the same way the ice line influenced the composition of the planets forming around the Sun, there was an iceline around a warm Jupiter that influenced the composition of its major moons — from no ice to lots of ice. 77. The indications that a moon was captured. Jupiter has many small moons with orbits that are retrograde (opposite the spin of the planet), not aligned with Jupiter’s equatorial plane, elliptical, and are extremely far from the planet. These orbital characteristics suggest these moons were captured by Jupiter's strong gravity. If celestial bodies pass close to a planet, they can be captured by the planet's gravity and guided into orbit. These passing objects must approach at just the right distance and velocity to be captured. If they venture too close, they will be pulled in and crash; too far and their trajectories will be altered, but they will continue on their way. Captured moons are identifiable by having orbits that are not aligned with the planet’s equator, highly elliptical, and often retrograde. 78. How Jupiter acquired its many small moons that orbit far from the planet. Jupiter has many small moons with orbits that are retrograde (opposite the spin of the planet), not aligned with Jupiter’s equatorial plane, elliptical, and are extremely far from the planet. These orbital characteristics suggest these moons were captured by Jupiter's strong gravity. 79. The actual discoverer of the Galilean moons. Galileo Galilei in 1610 — the first moons discovered outside our own. 80. The way in which Jupiter and the Galilean moons system is like the Sun and planets. In the same way the ice line influenced the composition of the planets forming around the Sun, there was an iceline around a warm Jupiter that influenced the compositi 8

81. What is unique about Jupiter’s moon Io compared to all other moons in the solar system. With over 400 active volcanoes, Io is the most volcanically active body in the solar system. • Io gets its splotchy colors from volcanic plumes that rise to 200 km, showering the terrain with orange sulfur and rocky ash. • Due to its low gravity, Io has mountains that rise as high as Mount Everest. 82. What is unique about Jupiter’s moon Ganymede compared to all other moons in the solar system. Ganymede, the largest moon in the Solar System, has a liquid iron outer core that produces the only substantial magnetic field generated by a moon in the Solar System. The surface is a mix of very old, highly cratered, dark regions and slightly younger, lighter regions marked with an extensive array of ridges. 83. What is unique about Jupiter’s moon Callisto compared to all other large moons in the solar system. Despite being the size of Mercury, Callisto's internal structure might be unique in being the only major moon to not have completely differentiated. The ancient surface of Callisto is one of the most heavily cratered in the Solar System — on par with our Moon. There are no other features to be seen 84. What is happening at the Tiger Stripes on Saturn’s moon Enceladus. Geysers within the Tiger Stripes shoot jets of ice particles from a subsurface ocean into space. 85. Why the Odysseus impact crater on Saturn’s moon Tethys is interesting. The 450-km- diameter Odysseus impact crater (40% of Tethys' diameter) is the largest impact crater relative to the parent body in the Solar System. This impact surely almost destroyed this moon. 86. What is unique about Saturn’s moon Titan compared to all other moons in the solar system. Titan is a planet. It is bigger than Mercury, more active than Mars, the only moon in the Solar System with a thick atmosphere, and the only body besides Earth to have liquid lakes (methane, not water) on its surface. False color Cassini radar view of Titan showing dark (very smooth) radar returns of its liquid methane lakes — making Titan the only other body besides Earth in the solar system with liquid on its surface Like Earth, Titan’s hydrological cycle includes rivers, clouds, rain, and snow, but it is a methane cycle, as Titans surface is much too cold (-270°F) for a water cycle. Titan’s methane is constantly replenished by volcanism (which is destroyed by sunlight) – so it is a very active moon 87. What is unique about Neptune’s moon Triton compared to all other large moons in the solar system. d 7 irregular moons that orbit farther out with highly irregular orbits (captured), including the largest of them all, Triton. Neptune's moon Triton is the only large moon in the solar system to have been captured Neptune Triton Triton has a highly elliptical, retrograde orbit (opposite Neptune's spin), tilted 23° from Neptune’s equatorial plane, and has a mineralogy similar to other Kuiper Belt objects (half ice/half rock). 88. What is unique about Pluto’s moon Charon. Large canyons, cryovolcanism, and few craters suggests that Pluto's moon Charon is geologically active and has a subsurface ocean. Charon's surface is dominated by water ice, as opposed to Pluto's nitrogen and methane ices. Pluto’s moon Charon has the following characteristics: • Prograde, circular, equatorial orbit • Orbits 8 Pluto diameters away 9

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• Comprised of similar mineralogy to Pluto Origin giant impact Because Charon is almost half the size of Pluto, they are tidally locked to each other and orbit about a point just above Pluto’s surface. This makes Pluto the only planet to be tidally locked to one of its moons. 89. Why the Moon is moving away from Earth. The Moon, which is tidally locked to the Earth, takes 27 days to complete one orbit. Thus, the Earth rotates much faster than the Moon orbits. This influences the location of Earth's tidal bulges. Earth’s faster rotation carries its tidal bulge ahead of the Moon’s orbit. This causes an offset to the gravitational force of the Earth on the Moon, which pulls the Moon into a higher (or more distant) orbit (yellow arrow). Thus, the Moon is slowly drifting away from the Earth at 4 cm/yr. 90. Why Phobos is moving toward Mars. Mars’ rotation is slower than Phobos’ orbit, causes the tidal bulge to lag behind. As a result, Phobos’ orbit is slowing down causing Phobos to slowly orbit closer to Mars. It will eventually crash into Mars. 91. The most likely reason why Mercury and Venus do not have any moons. Mercury and Venus spin too slowly to sustain an orbiting moon — tidal forces would be such that any moon would crash onto their surface in a fairly short time span. 92. The major reason why our Moon does not experience as much tidal heating as Io. A surprisingly accurate scene that portrays the Moon breaking up when it crosses the Roche limit (actually 9,500 km) and correctly predicts that the Earth's spin rate will increase (shorter months — because of a transfer of angular momentum as the Moon approaches). 93. The likely result of tidal heating of an icy moon. 94. The thickness range of Saturn’s rings. This depiction of Saturn’s Rings being so thin you could fly up and reach out to touch them is accurate. Another excellent depiction of icy rings; note the distributed chunks of ice — that is also accurate as electrostatic attraction (static cling) causes ice particles to coalesce while tidal forces are ripping them apart. Due to tidal forces the rings align exactly with Saturn's equator and are only on average 10-100 m thick. Saturn’s rings are made mostly of pure water ice particles, centimeters to 10 meters in size. 95. The major planets that have rings. Saturn, Jupiter, uranus, Neptune 96. What the Roche limit is. Roch e limit is the distance from a planet where tidal forces are equal to the moon’s own gravitational forces that work to keep it spherical. 97. What happens to a moon if it migrates inside the Roche limit. If a moon migrates across the Roche limit, it is pulled apart by tidal forces and the particles orbits as ring Crossing the Roche limit, the moon disintegrates and a ring forms as particles are stripped away 98. Which of the following does not cause gaps in planetary rings. Mineralogical differences in ring particles 99. Why Saturn’s moon Iapetus has a bulge around its equator. Saturn's icy moon Iapetus has a 20 km high equatorial ridge that is thought to be the result of a ring crashing down onto its surface. • The fate of all rings is to eventually crash into their parent bodies because of gravitational forces. • Saturn's rings are thought to be less than 100 million years old and may be gone in the next 300 million years. 100. The fate of Neptune’s moon Triton. Tidal forces are causing Triton to migrate toward Neptune. Eventually, it will cross the Roche limit and be ripped apart to form a spectacular ring around Neptune. 10