Comparative Planetology of Jupiter and Saturn

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0 Comparative Planetology of Jupiter and Saturn Chapter 23

0 Guidepost • As we begin this chapter, we leave behind the psychological security of planetary surfaces. You can imagine standing on the moon, on Mars, or even on Venus, but Jupiter and Saturn have no surfaces. Here you face a new challenge—to use comparative planetology to study worlds so unearthly we cannot imagine being there. As you study these worlds you will find answers to five essential questions: • How do the outer planets compare with the inner planets? • How is Jupiter different from Earth? • How did Jupiter and its system of moons and rings form and evolve? • How is Saturn different from Jupiter? • How did Saturn and its system of moons and rings form and evolve?

0 Guidepost (continued) • Your study of Jupiter and Saturn will give you a chance to answer two important questions about science. • How Do We Know? How is science different from technology? • How Do We Know? Who pays to gather scientific knowledge? • There’s no place to stand on Jupiter or Saturn, but be sure to bring your spacesuit. Both planets have big systems of moons, and when you visit them you will be able to watch erupting volcanoes, stroll through a methane rain storm, and swim in Saturn’s rings. It will be interesting, but it is no place like home.

0 Outline • A Travel Guide to the Outer Planets • The Outer Planets • Atmospheres and Interiors • Satellite Systems • Jupiter • A. Surveying Jupiter • B. Jupiter's Magnetic Fields • C. Jupiter's Atmosphere • D. Jupiter's Ring • E. Comet Impact on Jupiter • F. The History of Jupiter

0 Outline (continued) III. Jupiter's Family of Moons A. Callisto: The Ancient Face B. Ganymede: A Hidden Past C. Europa: A Hidden Ocean D. Io: Bursting Energy E. The History of the Galilean Moons IV. Saturn A. Planet Saturn B. Saturn's Rings C. The History of Saturn V. Saturn's Moons A. Titan B. The Smaller Moons C. The Origin of Saturn's Moons

0 A Travel Guide to the Outer Planets All are much larger than the terrestrial planets, have thick atmospheres, but are very inhospitable: Visual image • No solid surfaces • Cloud belt patterns • Mostly Hydrogen and Helium • Rings • Strong atmospheric circulation (winds/storms) • Multiple moons

0 Jupiter Largest and most massive planet in the solar system: Contains almost 3/4 of all planetary matter in the solar system. Most striking features visible from Earth: Multi-colored cloud belts Explored in detail by several space probes: Visual image Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Galileo Infrared false-color image

0 The Mass of Jupiter Mass can be inferred from the orbit of Io, the innermost of the 4 Galilean Moons: Using Kepler’s third law MJupiter = 318 MEarth

0 Jupiter’s Interior From radius and mass Average density of Jupiter ≈ 1.34 g/cm3  Jupiter can not be made mostly of rock, like earthlike planets. Jupiter consists mostly of hydrogen and helium. T ~ 30,000 K Due to the high pressure, hydrogen is compressed into a liquid, and even metallic state.

0 The Chemical Composition of Jupiter and Saturn

0 Jupiter’s Rotation Jupiter is the most rapidly rotating planet in the solar system: Rotation period slightly less than 10 hr. Centrifugal forces stretch Jupiter into a markedly oblate shape.

0 Jupiter’s Magnetic Field Discovered through observations of decimeter (radio) radiation Magnetic field at least 10 times stronger than Earth’s magnetic field. Magnetosphere over 100 times larger than Earth’s. Extremely intense radiation belts: Very high energy particles can be trapped; radiation doses corresponding to ~ 100 times lethal doses for humans!

0 Aurorae on Jupiter Just like on Earth, Jupiter’s magnetosphere produces aurorae concentrated in rings around the magnetic poles. ~ 1000 times more powerful than aurorae on Earth.

0 The Io Plasma Torus Some of the heavier ions originate from Jupiter’s moon Io. Io flux tube Io flux tube Visible UV Inclination of Jupiter’s magnetic field against rotation axis leads to wobbling field structure passing over Io Acceleration of particles to high energies.

0 Jupiter’s Atmosphere Jupiter’s liquid hydrogen ocean has no surface: Gradual transition from gaseous to liquid phases as temperature and pressure combine to exceed the critical point. Jupiter shows limb darkening hydrogen atmosphere above cloud layers. Only very thin atmosphere above cloud layers; transition to liquid hydrogen zone ~ 1000 km below clouds.

0 Jupiter’s Atmosphere (2): Clouds Three layers of clouds: 1. Ammonia (NH3) crystals 2. Ammonia hydrosulfide 3. Water crystals

0 The Cloud Belts on Jupiter Dark belts and bright zones. Zones higher and cooler than belts; high-pressure regions of rising gas.

0 The Cloud Belts on Jupiter (2) Just like on Earth, high-and low-pressure zones are bounded by high-pressure winds. Jupiter’s Cloud belt structure has remained unchanged since humans began mapping them.

0 The Great Red Spot Several bright and dark spots mixed in with cloud structure. Largest and most prominent: The Great Red Spot. Has been visible for over 330 years. Formed by rising gas carrying heat from below the clouds, creating a vast, rotating storm. ~ 2 DEarth

0 The Great Red Spot (2) Structure of Great Red Spot may be determined by circulation patterns in the liquid interior

0 Jupiter’s Ring Not only Saturn, but all four gas giants have rings. Galileo spacecraft image of Jupiter’s ring, illuminated from behind Jupiter’s ring: dark and reddish; only discovered by Voyager 1 spacecraft. Composed of microscopic particles of rocky material Location: Inside Roche limit, where larger bodies (moons) would be destroyed by tidal forces. Ring material can’t be old because radiation pressure and Jupiter’s magnetic field force dust particles to spiral down into the planet. Rings must be constantly re-supplied with new dust.

0 Comet Impact on Jupiter Impact of 21 fragments of comet SL-9 in 1994 Impacts occurred just behind the horizon as seen from Earth, but came into view about 15 min. later. Impact sites appeared very bright in the infrared. Visual: Impacts seen for many days as dark spots Impacts released energies equivalent to a few megatons of TNT (Hiroshima bomb: ~ 0.15 megaton)!

0 The History of Jupiter • Formed from cold gas in the outer solar nebula, where ices could condense. • In the interior, hydrogen becomes metallic (very good electrical conductor) • Rapid rotation strong magnetic field • Rapid growth • Soon able to trap gas directly through gravity • Rapid rotation and large size belt-zone cloud pattern • Heavy materials sink to the center • Dust from meteorite impacts onto inner moons trapped to form ring

0 Jupiter’s Family of Moons Over two dozen moons known now; new ones are still being discovered. Four largest moons already discovered by Galileo: The Galilean moons Io Europa Ganymede Callisto Interesting and diverse individual geologies.

0 Callisto: The Ancient Face Tidally locked to Jupiter, like all of Jupiter’s moons. Av. density: 1.79 g/cm3 composition: mixture of ice and rocks Dark surface, heavily pocked with craters. No metallic core: Callisto never differentiated to form core and mantle. No magnetic field. Layer of liquid water, ~ 10 km thick, ~ 100 km below surface, probably heated by radioactive decay.

0 Ganymede: A Hidden Past Largest of the 4 Galilean moons. • Av. density = 1.9 g/cm3 • Rocky core • Ice-rich mantle • Crust of ice 1/3 of surface old, dark, cratered; rest: bright, young, grooved terrain Bright terrain probably formed through flooding when surface broke

0 Jupiter’s Influence on its Moons Presence of Jupiter has at least two effects on the geology of its moons: 2. Focusing of meteoroids, exposing nearby satellites to more impacts than those further out. 1. Tidal effects: possible source of heat for interior of Gany-mede

0 Europa: A Hidden Ocean Av. density: 3 g/cm3 composition: mostly rock and metal; icy surface. Close to Jupiter should be hit by many meteoroid impacts; but few craters visible. Active surface; impact craters rapidly erased.

0 The Surface of Europa Cracked surface and high albedo (reflectivity) provide further evidence for geological activity.

0 The Interior of Europa Europa is too small to retain its internal heat Heating mostly from tidal interaction with Jupiter. Core not molten No magnetic field. Europa has a liquid water ocean ~ 15 km below the icy surface.

0 Io: Bursting Energy Most active of all Galilean moons; no impact craters visible at all. Over 100 active volcanoes! Activity powered by tidal interactions with Jupiter. Av. density = 3.55 g/cm3Interior is mostly rock.

0 Interaction with Jupiter’s Magnetosphere Io’s volcanoes blow out sulfur-rich gasses tenuous atmosphere, but gasses can not be retained by Io’s gravity gasses escape from Io and form an ion torus in Jupiter’s magnetosphere

0 The History of the Galilean Moons • Minor moons are probably captured asteroids • Galilean moons probably formed together with Jupiter. • Densities decreasing outward Probably formed in a disk around Jupiter, similar to planets around the sun. Earliest generation of moons around Jupiter may have been lost and spiraled into Jupiter; Galilean moons are probably a second generation of moons.

0 Saturn Mass: ~ 1/3 of mass of Jupiter Radius: ~ 16 % smaller than Jupiter Av. density: 0.69 g/cm3Would float in water! Rotates about as fast as Jupiter, but is twice as oblate No large core of heavy elements. Mostly hydrogen and helium; liquid hydrogen core. Saturn radiates ~ 1.8 times the energy received from the sun. Probably heated by liquid helium droplets falling towards center.

0 Saturn’s Magnetosphere Magnetic field ~ 20 times weaker than Jupiter’s weaker radiation belts Magnetic field not inclined against rotation axis Aurorae centered around poles of rotation

0 Saturn’s Atmosphere Cloud-belt structure, formed through the same processes as on Jupiter, but not as distinct as on Jupiter; colder than on Jupiter.

0 Saturn’s Atmosphere (2) Three-layered cloud structure, just like on Jupiter Main difference to Jupiter: Fewer wind zones, but much stronger winds than on Jupiter: Winds up to ~ 500 m/s near the equator!

0 Saturn’s Rings A Ring Ring consists of 3 main segments: A, B, and C Ring B Ring C Ring separated by empty regions: divisions Cassini Division Rings can’t have been formed together with Saturn because material would have been blown away by particle stream from hot Saturn at time of formation. Rings must be replenished by fragments of passing comets & meteoroids.

0 Composition of Saturn’s Rings Rings are composed of ice particles moving at large velocities around Saturn, but small relative velocities (all moving in the same direction).

0 Shepherd Moons Some moons on orbits close to the rings focus the ring material, keeping the rings confined.

0 Divisions and Resonances Moons do not only serve as “Shepherds”. Where the orbital period of a moon is a small-number fractional multiple (e.g., 2:3) of the orbital period of material in the disk (“resonance”), the material is cleared out Divisions

0 Titan • About the size of Jupiter’s moon Ganymede. • Rocky core, but also large amount of ice. • Thick atmosphere, hiding the surface from direct view.

0 Titan’s Atmosphere Explored in detail by the Cassini spacecraft and the Huygens probe, which landed on Titan’s surface in 2005. • Atmosphere consists mostly of nitrogen, methane and ethane • Surface pressure: 50% greater than air pressure on Earth • Surface temperature: 94 K (-290 oF) methane and ethane can condense and lead to rain of methane and ethane Methane is gradually converted to ethane in the Atmosphere Methane must be constantly replenished, probably through breakdown of ammonia (NH3).

0 Titan’s Surface Before Huygens, Titan’s surface could only be probed with infrared cameras. • Revealed bright and dark regions on the surface • Bright regions possibly a result of methane and ethane ice Huygens discovered outflow channels, possibly of liquid methane and grapefruit sized rocks on the surface

0 Saturn’s Smaller Moons Hyperion: Too small to pull itself into spherical shape. All other known moons are large enough to attain a spherical shape.

0 Saturn’s Smaller Moons (2) Saturn’s smaller moons are formed of rock and ice; heavily cratered and appear geologically dead. Iapetus: Leading (upper right) side darker than rest of surface because of dark deposits. Phoebe and Tethys: Heavily cratered, ancient surfaces Enceladus: Possibly active; regions with fewer craters, containing parallel grooves, possibly filled with frozen water.

0 The Origin of Saturn’s Satellites • No evidence of common origin, as for Jupiter’s moons. • Probably captured icy planetesimals. • Moons interact gravitationally, mutually affecting each other’s orbits. • Co-orbital moons (orbits separated by only 100 km) periodically exchange orbits! • Small moons are also trapped in Lagrange points of larger moons Dione and Tethys.

Comparative Planetology of Jupiter and Saturn