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6. Gas Giants & Water Worlds

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6. Gas Giants & Water Worlds Astronomy 110: SURVEY OF ASTRONOMY 6. Gas Giants & Water Worlds 1. The Jovian Planets 2. Satellites and Rings The Solar System contains two planetary behemoths and two planets which are merely enormous by our standards. Jupiter & Saturn have hundreds of times the Earth's mass; both radiate more energy than they receive from the Sun, and this outflow of energy powers dramatic activity in their interiors and atmospheres. Uranus & Neptune, each about 15 times the Earth's mass, are less active. All four planets have satellite systems and rings shaped by subtle dynamical effects over trillions of orbits. Formation of Jovian Planets 1. Outside the frost line, icy planetesimals were very common, forming planets about 10 times the mass of Earth. 2. These planets attracted nearby gas, building up giant planets composed mostly of H and He. 3. The disks around these planets produced moons. Two Types of Jovian Planets “Gas Giants” “Water Worlds” 1. THE JOVIAN PLANETS a. Internal Structure b. Outward Appearance Internal Structure: Pressure Anywhere inside a planet (or star), the pressure is the weight per unit area of the material above that point. air 100 mi On Earth, a column of air 1 inch on a side, extending from sea-level to the top of the atmosphere, weighs 14.7 lbs: P = 1 bar 14.7 lbs per sq in = 14.7 psi = 1 bar. 32 ft A column of water 1 inch on a side and 32 ft water high also weighs 14.7 lbs, so 32 ft below sea level the total pressure is 29.4 psi = 2 bar. P = 2 bar Internal Structure: Pressure Balance Imagine a column of gas. Without gravity, gas is distributed uniformly along the column. With gravity, the gas at the bottom is compressed, while the gas at the top spreads out (like a spring). The compressed gas pushes upward, resisting the weight of the gas above. If these forces cancel, the gas is in pressure balance. without gravity with gravity Internal Structure: Mass vs Radius Adding a pillow squashes Adding mass adds gravity, which those already there. squashes the “stack” even more. Internal Structure: Jupiter Gaseous H + He; 1 125 0.0002 some NH3, H2O, CH4 5×105 2000 0.5 2×106 5000 1 Liquid H + He (due to extreme pressure) Metallic H (liquid) with dissolved He; good electrical conductor 4 Core (rock, metal); ~108 2×104 25 ~10 × Earth’s mass Internal Structure: Jupiter & Saturn Saturn’s structure is similar, but proportions differ . 1. Both Jupiter and Saturn contain metallic hydrogen. However, most of Jupiter’s interior is metallic, while most of Saturn’s is not. Why? A. Jupiter contains a larger fraction of hydrogen than Saturn. B. Jupiter is colder than Saturn. C. Jupiter’s internal pressure is higher because it has more mass. D. Saturn receives less heat from the Sun. Internal Structure: Uranus & Neptune Less mass than Jupiter or Saturn, so less internal pressure; H stays gaseous. Larger fraction of C, N, O, so molecules of CH4, NH3, H2O are common. Hydrogen compounds are probably liquids, forming “oceans” deep within the planet. Internal Structure: Heat Sources J. & S. put out lots of heat — U. puts out very little heat — not due to radioactivity since — lost during giant impact? H and He isotopes are stable. — bottled up in interior? — Jupiter: “left-over” heat. N. puts out more heat — — Saturn: He differentiation? — radioactivity in core? Internal Structure: Magnetic Fields J. & S. have strong fields which U. & N. have weaker fields are well-aligned with rotation. which are highly mis-aligned. — convection in metallic H! — local dynamo in “ocean”? Internal Structure: Magnetic Fields Outward Appearance 12 hours on Jupiter Outward Appearance: Cloud Colors NH3 50 km w cloud tops NH4SH Distance belo H2O 100 km Outward Appearance: Belts Convection and rapid rotation organize atmospheric motion. Belts: sinking Zones: rising gas exposes red gas forms white NH4SH clouds; NH3 clouds; circulates faster. circulates slower. Outward Appearance: Circulation Patterns Jupiter Unpeeled Outward Appearance: Great Red Spot Approaching Jupiter Outward AppearanceAppearance: Great Red Spot H Jupiter's Great Red Spot Region Outward Appearance: Saturn Bands like Jupiter’s, but less dramatic since atmosphere is deeper due to weaker gravity. Saturn … Four Years On Outward Appearance: Uranus & Neptune Their atmospheres are deep, methane-rich, and very cold. • Cloud layers are too deep to be easily seen. • Methane (CH4) absorbs red light, transmits blue. — blue planets with subtle cloud features. Outward Appearance: Uranus & Neptune Great Dark Spot 2. SATELLITES AND RINGS a. A mess of moons b. Activity and climate c. Ring systems + 59 more A Mess of Moons • 6 large moons (D>1500 km) + 53 more — most geologically active • 12 medium (D>300 km) — some activity in past? + 22 more • 145+ small (D>1 km) — no geological activity + 11 more Large & Medium Moons • spherical due to self-gravity • composed of ices & rock • formed in disks around parent planets (not Triton) • orbit in same direction as planets spin (not Triton) Small Moons of Saturn • too small to be spherical • orbit both directions! The Jupiter Satellite Page Activity: Galilean Satellites IO EUROPA GANYMEDE CALLISTO Wide range of geological activity: — Io is most volcanic object in solar system! — Callisto exhibits very little activity. Io: Volcanic Moon Loki Patera: active center Pele: Io’s largest volcanic pluume Topography and Volcanos on Io Io: Volcanic Moon Ongoing Volcanic Eruption at Tvashtar Catena Tvashtar’s Plume Why so active? Tides Tides Tidal Heating of Io Jupiter Io is alternately stretched and squeezed in its elliptical orbit around Jupiter, creating heat through friction. Over time, this friction should make Io’s orbit more and more circular; what keeps Io’s orbit elliptical? The Role of Resonance 2:1 resonance The inner moon makes 2 The inner moon gets a orbits in the same time the tug every other time outer moon makes 1, so around, so its orbit gets this is a 2:1 resonance. more elliptical with time. Jupiter’s Moons: the Laplace Resonance 4:2:1 resonance Jupiter’s three inner moons are in a 4:2:1 resonance. — this keeps Io’s orbit moderately elliptical. — Europa’s orbit is also slightly elliptical. Europa: an Ice-Covered Ocean? Fractures in crust Pwyll: Natural and False Impact Color Views of Europa crater Exaggerated colors “Natural” colors Europa: Rafts of Ice Europa - Ice Rafting View Europa: Rafts of Ice Tidal flexing opens up cracks, creating pairs of ridges, then closes them up, grinding ice sheets together. Europa - Ice Rafting View Europa: Internal Structure Heat from tidal friction can keep Europa’s insides warm. Ganymede and Callisto Global Callisto in Color Ganymede has a mixture of Callisto has a old and very old and new terrain, and hints heavily cratered terrain, with of tectonic activity. a possible subsurface ocean? Titan: a Moon With Atmosphere Thick smog of N2 and hydrocarbons (CH4, C2H6, etc)! Hydrocarbon Lakes on Titan Reflection of Sunlight off Titan Lake Radar Shows Evidence of Seas Meandering Riverbeds on Titan Xanadu's Meandering Rivers Titan’s Climate • Surface temperature: 94° K (-179° C) • Clouds of CH4, C2H6, other hydrocarbons • CH4 falls as rain, forms rivers & lakes • Seasonal changes in CH4 lakes • CH4 replenished by cryo-volcanism Wikipedia: Titan Ring Systems All Jovian planets have rings made of tiny “moonlets”. Saturn’s Rings Saturn Only 20 m thick, the rings are aligned with the equator. Inside Saturn’s Rings Outer Solar System Moons and Rings The moonlets in Saturn’s rings are composed mostly of ice. Collisions are slowly grinding them to dust. Structure in Saturn’s Rings Mimas Cassini Division Cassini’s division is due to a 2:1 resonance with Mimas: 2 PMimas = 2 PCassini ⇒ aMimas = ∛2 aCassini Other moons and resonances create other structures. Origin of Planetary Rings Ring formation may be ongoing or catastrophic. — moonlet fragmentation — tidal disruption of moon Tidal Disruption.
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