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Transcript
Tides, Moons, Rings, and Pluto [Oct 27, 2016] As with all course material (including homework, exams), these lecture notes are not be reproduced, redistributed, or sold in any form. Ocean Tides high low Ocean Tides water closer to Moon feels stronger gravitational pull. Moon Earth Gm1 m2 F = d2 Ocean Tides similarly the Earth is pulled by gravity, but less so than the ocean water closest to the Moon Moon Gm1 m2 F = d2 Ocean Tides and finally, what about the rest of the water?... Moon Gm1 m2 F = d2 Ocean Tides what would this look like from the perspective of the Earth? Moon Gm1 m2 F = d2 Ocean Tides whatwill would ocean be this look like from the perspective of the Earth? shallower ocean will be deeper Gm1 m2 F = d2 Moon Ocean Tides ocean will be shallower (low tide) ocean will be deeper (high tide) Earth ocean will be shallower (low tide) ocean will be deeper (high tide) Moon Tides are the result of the gravitational force from the Moon and the Sun Effect of the Moon on high/low tides High tide Low tide Solar System more than just the planets Image not to scale — solar system is much less dense than this! The Galileo Mission joint US + Germany mission Launched on Oct 18, 1989 from the Space Shuttle Atlantis Goal: study Jupiter The Galileo Mission Flight path included gravity assists from Venus and Earth Passed by two asteroids on its way to Jupiter Went into orbit around Jupiter in 1995; spent 8 years (35 orbits) there The Galileo Mission In July, 1995, Galileo dropped this probe into Jupiter’s atmosphere. After jettisoning its heat shield and deploying a parachute, it sent back ~1 hour of data before succumbing to the pressure and heat of Jupiter’s atmosphere. probe is ~1.3 meters in size Why name this mission after Galileo? Galileo Galilei (1564-1642) Jupiter’s moons 4 largest: the Galilean satellites Io, Europa, Ganymede, Callisto 67 moons now known: some are only a few km across, and are probably captured asteroids Jupiter’s Moons 1. Metis 2. Adrastea 3. Amalthea 4. Thebe 5. Io 6. Europa 7. Ganymede 8. Callisto 9. Themisto 10. Leda 11. Himalia 12. Lysithea 13. Elara 14. S/2000 J11 15. Iocaste 16. Praxidike 17. Harpalyke 18. Ananke 19. Isonoe 20. Erinome 21. Taygete 22. Chaldene 23. Carme 24. Pasiphae 25. S/2002 J1 26. Kalyke 27. Magaclite 28. Sinope 29. Callirrhoe 30. Euporie 31. Kale 32. Orthosie 33. Thyone 34. Euanthe 35. Hermippe 36. Pasithee 37. Eurydome 38. Aitne 39. Sponde 40. Autonoe 41. S/2003 J1 42. S/2003 J2 43. S/2003 J3 44. S/2003 J4 45. S/2003 J5 46. S/2003 J6 47. S/2003 J7 48. S/2003 J8 49. S/2003 J9 50. S/2003 J10 51. S/2003 J11 52. S/2003 J12 53. S/2003 J13 54. S/2003 J14 55. S/2003 J15 56. S/2003 J16 57. S/2003 J17 58. S/2003 J18 59. S/2003 J19 60. S/2003 J20 61. S/2003 J21 62. S/2003 J22 63. S/2003 J23 The Galilean Moons of Jupiter Io Europa Ganymede Callisto Semimajor axis of orbit around Jupiter: 421,000 km 671,000 km 1,070,400 km 1,882,700 km Io Io’s volcanos Voyager Galileo Io’s surface is very volcanically active- it’s the youngest surface of any solar system object Numerous volcanos erupting silicate lava, with geysers erupting sulfur and sulfur dioxide Galileo orbiter found more than 100 volcanoes erupting simultaneously Changes in Io’s surface Voyager, 1979 Galileo, 1996 Interior heating in Io Io’s orbit is slightly elliptical, due to gravitational pull from Europa Jupiter 1/2 degree wobble Io Note- diagram not to scale! The orbital ellipticity is greatly exaggerated here. Tidal forces from Jupiter heat Io’s interior Io radiates away 100 trillion watts of power from this tidal heating Europa’s structure Ganymede The largest moon in the solar system: radius 2634 km (larger than Mercury) Differentiated structure: iron & rocky core, mantle of ice and silicates, and crust of mostly water ice Possible liquid ocean about 200 km below the surface? Complex terrain with mountains, valleys, craters, lava flows Differentiated structure of Ganymede Callisto Almost as large as Mercury: radius 2403 km Very heavily cratered: one of the oldest surfaces in the Solar System possible ocean of liquid water about 100 km below the surface? The Galileo Mission Mission finished in 2003, spacecraft was crashed into Jupiter’s atmosphere at a speed of ~50 km/s. Cassini-Huygens — launched in 1997 — mission: to study Saturn and its moons — flyby of Earth, Venus, & Jupiter — 4th probe to visit Saturn, 1st to orbit it — has been orbiting since 2004 — Huygens landed on Titan in 2005 — mission will end in 2017 Saturn’s Moons Titan The 2nd-largest moon in the solar system, with radius 2575 km Thick, hazy atmosphere: about 1.5 times Earth’s atmospheric pressure Atmosphere is primarily nitrogen, with some methane, ethane, other compounds liquid hydrocarbon lakes on the surface (methane, ethane), and liquid methane rain in the atmosphere The Huygens Probe Cassini images of Saturn’s moons: Tethys Iapetus Dione Rhea Mimas Epimetheus ra Telesto Hyperion Enceladus Recent discovery from Cassini: geysers of water ice erupting from Enceladus Probably from sub-surface pockets of liquid water These plumes are most prominent at the southern pole of Enceladus. Likely physical origin of plumes on Enceladus Cassini in the news… On Oct 28, 2015, Cassini passed by Enceladus at a distance of ~50 km (or 30 miles), which allowed it to directly study (i.e. sample) the material in the plume. One of the goals of this close flyby is to determine what molecules may be in the plume. Enceladus is particularly interesting as it may be one of the best places to search for life — e.g. if it has water, energy, and complex molecules. Close-up of Enceladus 1 km Planetary rings Gm1 m2 F = d2 1 r2 log(r) Planetary Rings and the Roche Limit Consider a small moon made of dust and pebbles that’s held together by its own self-gravity: a “rubble pile” rather than a solid boulder Ro ch e Lim it If the moon is far away from the planet, it remains stable. What happens if we move the moon closer to the planet? Planetary Rings and the Roche Limit Ro ch e Lim it As the moon gets closer to the Roche limit, it becomes distorted by the tidal pull of the planet Planetary Rings and the Roche Limit Ro ch e Lim it As the moon gets closer to the Roche limit, it becomes distorted by the tidal pull of the planet Planetary Rings and the Roche Limit Ro ch e Lim it At the Roche limit, the tidal forces on the moon are so strong that it begins to break up Planetary Rings and the Roche Limit Ro ch e Lim it Material closer in to the planet will orbit faster than material farther out This makes the material gradually spread out into a ring Planetary Rings and the Roche Limit Ro ch e Lim it Material closer in to the planet will orbit faster than material farther out This makes the material gradually spread out into a ring as it orbits the planet Planetary Rings and the Roche Limit Ro ch e Lim it Eventually, we’re left with a ring of small particles orbiting the planet The Roche Limit Inside the Roche limit, material in a ring or disk can’t clump together to form a single satellite, because the planet’s tidal forces are too strong In this region, material ejected from impacts or collisions on moons can settle into rings orbiting the planet Rings are temporary features: over time rings will dissipate due to collisions, and particles will drift away from the rings So why haven’t Saturn’s rings dissipated? [rings are fed by material ejected from moons — e.g. plumes on Enceladus] Saturn’s rings Cassini’s division Rings are only about 20 meters thick Composition: mostly water ice, some organic compounds and carbon Sizes from small pebbles up to large boulders Saturn’s rings Cassini’s division is produced by the gravitational force of the moon Mimas This is due to an orbital resonance: a particle in Cassini’s division would orbit twice for each one orbit of Mimas Saturn’s rings Huge numbers of gaps and ringlets- the rings aren’t smooth! These are caused by the gravitational interaction of the ring particles with Saturn’s moons and with many tiny moonlets within the ring system Some very thin rings are kept in place by “shepherd” satellites that prevent the ring from spreading out further Shepherd moons Pandora Prometheus Jupiter’s ring Discovered by Voyager Ring radius is about 1.8 times Jupiter’s radius Not very reflective- composted of sooty dust particles Main ring is made of dust originating from moons Adrastea and Metis; gossamer rings are dust that originated from Amalthea and Thebe Uranus Uranus’s rings were originally discovered because they blocked the light of background stars as Uranus moved across the sky Shepherd moons and Uranus’s rings Neptune’s rings Solar System more than just the planets What about poor Pluto? It’s very small, its moon is very large, it has a highly elliptical and inclined orbit Definition of a planet Then in 2005, an object orbiting beyond Pluto but 27% more massive than Pluto was discovered: this object was named Eris So should we make Eris the “tenth planet”? In 2006, the International Astronomical Union convened in Prague to set the definition of a planet and settle the question A planet within the solar system is a celestial body that 1. Orbits the Sun 2. Is massive enough for its self- gravity to give it a nearly round shape 3. Has cleared the neighborhood around its orbit Objects that meet the first two criteria but not the third are dwarf planets – so Pluto and Eris are both dwarf planets Deciding Pluto’s fate