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Lecture 11: The Solar System’s biggest (Jovian planets) and smallest (dwarf planets and smaller bodies) Astronomy 111 Wednesday October 5, 2016 Reminders • Online homework #5 due Monday at 3pm ASTR111 Lecture11 Jovian Planets ASTR111 Lecture 11 Jovian Planets and Other Solar System Constituents ASTR111 Lecture11 The Solar System: List of Ingredients Ingredient Percent of total mass Sun Jupiter other planets everything else ASTR111 Lecture 11 99.8% 0.1% 0.05% 0.05% ASTR111 Lecture 11 ASTR111 Lecture 11 Jupiter and Saturn Jupiter and Saturn consist mainly of hydrogen and helium. Jupiter: Escape speed = 60 km/sec Air temperature = 165 K (-160o F) Saturn: Escape speed = 35 km/sec Air temperature = 93 K (-290o F) Earth: Escape speed = 11 km/sec Air temperature = 290 K (60o F) ASTR111 Lecture11 Jupiter and Saturn Atmospheres of Jupiter and Saturn retain H, He. Because Jupiter and Saturn are cold and have high escape speed, they hang onto hydrogen and helium. Jupiter’s atmosphere is 75% hydrogen, 24% helium. Saturn’s atmosphere is 92% hydrogen, 6% helium. Question: Where is Saturn’s helium? Answer: Saturn is so cold, its helium condenses and rains downward. ASTR111 Lecture 11 Jupiter and Saturn Jupiter and Saturn radiate away more energy than they receive from the Sun. What is the source of the extra energy? Mostly, it is heat left over from when the planets formed. (Big objects cool more slowly). ASTR111 Lecture 11 Jupiter and Saturn Jupiter and Saturn have belts and zones of clouds, plus circular storms. Air heated from above by the Sun, from below by internal heat: Strong winds and large storms. We see clouds of ammonia (NH3), colored by complex compounds. ASTR111 Lecture 11 Jupiter’s atmosphere is divided into light-colored zones and dark-colored belts. High-speed winds blow eats or west at the boundaries between them. ASTR111 Lecture 11 Saturn has a similar pattern of zones and belts. They are less dramatic than Jupiter’s: Saturn’s clouds are buried deep in its atmosphere and are blurred by haze. ASTR111 Lecture 11 The Great Red Spot of Jupiter An enormous circular storm (up to 40,000 km across) in the southern hemisphere of Jupiter. ASTR111 Lecture 11 Clouds are usually made of different stuff from the “air” they float in. Venus: carbon dioxide atmosphere, sulfuric acid clouds. Earth: nitrogen atmosphere, water clouds. Mars: carbon dioxide atmosphere, water & carbon dioxide clouds. Jupiter & Saturn: hydrogen atmosphere, ammonia clouds. ASTR111 Lecture 11 Jupiter and Saturn are differentiated. ASTR111 Lecture 11 Moons of Jupiter and Saturn ASTR111 Lecture11 Jupiter’s moons Jupiter has ~70 known moons. ~60 are small (<300 km across) and irregular (similar to Phobos and Deimos). Many of the outer moons have retrograde orbits; they are probably captured asteroids. ASTR111 Lecture 11 Moons of Jupiter: orbits. ASTR111 Lecture11 4 of the moons of Jupiter are large (> 3000 km across) and spherical (like our Moon). These are the four Galilean moons: Io, Europa, Ganymede, Callisto ASTR111 Lecture11 Galilean Moons: A miniature analog to the Solar System Revolution counterclockwise, on nearly circular orbits, in nearly the same plane. Io and Europa: mostly rock Ganymede and Callisto: rock and ice ASTR111 Lecture11 ASTR111 Lecture 11 Terrestrial planets: Internal heat determined by size of planet. Galilean moons: Internal heat determined by proximity of Jupiter. Near Jupiter (Io): lots of tidal activity, much volcanic activity, no ice. Far from Jupiter (Callisto): little tidal heating, no volcanic activity, lots of ice. ASTR111 Lecture 11 Saturn has 56 known moons 49 are small (<300 km across) and irregular (like small moons of Jupiter). 6 are mid-sized (400--1500 km in diameter) and spherical. One of Saturn’s moons is the giant moon Titan. ASTR111 Lecture 11 Moons of Saturn: orbits. ASTR111 Lecture11 ASTR111 Lecture 11 ASTR111 Lecture 11 Uranus and Neptune • Uranus was discovered in 1781 by William Herschel • Neptune discovered in 1846 (Urbain Le Verrier using Newton’s Laws to explain perturbations of Uranus’s orbit and infer Neptune’s existence) ASTR111 Lecture11 Uranus and Neptune Uranus and Neptune are planetary ―twins‖, like Earth and Venus. Uranus is 3% larger in radius but 15% smaller in mass than Neptune. ASTR111 Lecture11 1) 2) 3) 4) Interiors of Uranus and Neptune Gaseous atmosphere: hydrogen, helium, methane Liquid outer layer: hydrogen, helium Liquid or slushy mantle: water, ammonia Solid core: rock, metal ASTR111 Lecture 11 The rotation axis of Uranus is tilted by about 90o, causing extreme seasons. Axis tilts: • Jupiter = 3o • Saturn = 27o, Neptune = 30o • Uranus = 98o The seasons of Uranus Winter solstice (AD 1985): north – perpetual dark, south – perpetual sun. Vernal equinox (2007): sun rises once every 17 hours. And so forth … ASTR111 Lecture11 Near the winter solstice (1986), the atmosphere of Uranus was boring: (left: visible light) (right: ultraviolet) No storms; very faint belts & zones. Heat flow from summery south to wintery north smeared out storms. ASTR111 Lecture 11 At the vernal equinox (in 2007), Uranus got stormier: Heating of northern hemisphere produces storms, made visible by clouds of methane. ASTR111 Lecture 11 ASTR111 Lecture 11 Uranus has 27 known moons. 22 are small (< 200 km across) and irregular. 5 are mid-sized (400 => 1600 km in diameter) and spherical. No giant moons. ASTR111 Lecture 11 Neptune Neptune has surprisingly strong storms, driven by internal heat. Temperature at Neptune’s cloud level is 55 Kelvin (-360o F). This is as warm as Uranus, although Neptune is much further from the Sun. ASTR111 Lecture 11 Neptune’s atmosphere is being warmed from below by internal heat. This extra heat drives large circular storms, like the “Great Dark Spot” seen above. ASTR111 Lecture 11 ASTR111 Lecture 11 Neptune has 13 known moons. 12 are small (< 500 km across) and irregular. One is the giant moon Triton. Proteus Triton ASTR111 Lecture 11 Nereid Dwarf planet Pluto Jan. 23, 1930 Jan 29. 1930 Discovered in 1930 by Clyde Tombaugh, while searching for Planet X. Orbital period = 248 years Rotation period = 6.4 days ASTR111 Lecture11 Pluto and Charon Pluto’s moon, CHARON, was discovered in 1978. From the ground, Charon looked like a ―lump‖ on Pluto’s side. ASTR111 Lecture 11 New Horizons At last, a mission to Pluto! (Too little, too late) Launch: January 2006 Jupiter flyby: February 2007 Pluto flyby: July 2015 Then onward into the outer solar system. ASTR111 Lecture11 Dwarf planet Eris (a.k.a. “Xena”) Oct. 21, 2003 Discovered in 2005, systematic search. Orbital period = 557 years Rotation period = 26 hours ASTR111 Lecture11 Eris (“Xena”), the troublemaker Discovered in 2005 by Mike Brown and collaborators. It has a moon. It is BIGGER than Pluto! Trouble! ASTR111 Lecture11 Orbits of Jovian planets, Pluto and Eris (Will not collide— orbits are not in the same plane) ASTR111 Lecture11 Are Pluto and Eris planets? That depends on how you define ―planet‖. As of August 2006, new category of ―dwarf planet‖. ASTR111 Lecture11 IAU: definition of “planet” The International Astronomical Union defines “planet" as a celestial body that, within the Solar System, (a) is in orbit around the Sun; (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; and (c) has cleared the neighborhood Planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune ASTR111 Lecture11 IAU: “dwarf planet” The International Astronomical Union defines a "dwarf planet" as a celestial body that, within the Solar System, (a) is in orbit around the Sun; (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape; (c) has NOT cleared the neighborhood around its orbit; and (d) is not a satellite Dwarf Planets (so far): Eris, Pluto, Haumea, Makemake, Ceres ASTR111 Lecture11 IAU: “dwarf planet” The International Astronomical Union defines a "dwarf planet" as a celestial body that, within the Solar System, (a) is in orbit around the Sun; (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape; (c) has NOT cleared the neighborhood around its orbit; and (d) is not a satellite Dwarf Planets (so far): Eris, Pluto, Haumea, Makemake, Ceres ASTR111 Lecture11 More dwarf planets? More objects like Pluto and Eris (or even bigger) might exist. They are hard to find: • Very dim • Very slow moving Perhaps more easily detected from their gravitational influence? From studies of Neptune’s orbit: No more Jovian (massive) planets within 200 A.U. of the Sun. ASTR111 Lecture11 Many smaller bodies Many rocky asteroids & icy comets populate the solar system. ASTR111 Lecture 11 Asteroids • The asteroid belt lies between Mars & Jupiter • Leftover remnants from planet formation • >90,000 named asteroids • Asteroids move in the sky relative to the stars, so they are easy to find. ASTR111 Lecture11 Asteroids https://www.youtube.com/watch?v=BKKg 4lZ_o-Y ASTR111 Lecture11 Kuiper belt ASTR111 Lecture11 Kuiper belt The Kuiper belt, beyond Neptune, contains small, icy, Pluto-like objects. The Kuiper belt lies close to the ecliptic plane, and stretches from 30 A.U. to 50 A.U. from the Sun. Named after Gerard Kuiper, who predicted it should be full of planetesimals. Over 1,000 Kuiper belt objects (a subset of trans-Neptunian objects) have been found in the belt since its discovery in 1992. ASTR111 Lecture 11 Kuiper belt The icy Kuiper Belt Objects (KBOs) are leftover planetesimals. Kuiper Belt Objects have colors (and spectra) consistent with them being icy. They are probably planetesimals that formed within the Kuiper belt. The Kuiper Belt Objects are scattered too thinly for them to have accreted into a larger body. ASTR111 Lecture11 Comets ASTR111 Lecture11 Comets: “dirty snowballs” Strip away their tails, and comets are just snowballs several km across. A comet contains: frozen water, frozen carbon dioxide, ammonia, dust & rocks, carbon, complex carbon compounds ASTR111 Lecture11 Comets are big, dusty snowballs. If a comet comes close to the Sun, the ice is vaporized, and the dust is freed. Thus, comets in the inner Solar System are surrounded by dust clouds. If the Earth passes through the dust, a meteor shower results. ASTR111 Lecture11 Kuiper belt vs. Oort cloud Most comets are in the Kuiper belt or the Oort cloud, far from the Sun. Comets with short orbital periods come from the Kuiper belt, 30-50 A.U. from the Sun. ASTR111 Lecture11 Comets with long orbital periods come from the Oort cloud, 500-50,000 A.U. from the Sun. Oort cloud is a swarm of comets that stretches one-fifth the way to the nearest neighboring star. ASTR111 Lecture11 A comet or asteroid packs a punch An object 10 kilometers across, traveling at 10 km/sec, will upon impact release as much energy as a million 100 megaton bombs. It will blast out a crater at least 100 kilometers across. ASTR111 Lecture11 A comet or asteroid impact probably caused the extinction of the dinosaurs In the Cretaceous mass extinction (65 million years ago), 70% of all species were killed off … including all dinosaurs. The extinction was rapid, geologically speaking. ASTR111 Lecture11 The Chicxulub impact crater, 180 km across, buried under thick sediment. Estimated age: 64.98 million years. The impactor was only 10 km across! ASTR111 Lecture11