Jupiter and Saturn Chapter 23

As we begin this chapter, we leave behind the psychological security of planetary surfaces. We can imagine standing on the moon, on Venus, or on Mars, but Jupiter and Saturn have no surfaces. Thus, we face a new challenge—to use comparative planetology to study worlds so unearthly we cannot imagine being there. One reason we find the moon and Mars of interest is that we might go there someday. Humans may become the first Martians. But the outer solar system seems much less useful, and that gives us a chance to think about the cultural value of science. This chapter begins our journey into the outer solar system. In the next chapter, we will visit worlds out in the twilight at the edge of the sun’s family. Guidepost

I. 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 II. 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 Outline

III. Saturn A. Planet Saturn B. Saturn's Rings C. The History of Saturn IV. Saturn's Moons A. Titan B. The Smaller Moons C. The Origin of Saturn's Satellites Outline (continued)

Jupiter Largest and most massive planet in the solar system: Contains almost 3/4 of all planetary matter in the solar system. Explored in detail by several space probes: Pioneer 10Pioneer 10 (1973), Pioneer 11 (1974), Voyager 1 (1979), Voyager 2 (1979), Galileo ( ) Pioneer 11 Voyager 1 Voyager 2 Galileo Most striking features visible from Earth are the multi-colored cloud belts Visual image Infrared false-color image

The Mass of Jupiter Mass can be determined from the orbit of any of the innermost four Galilean moons Earth Jupiter Moon Io Relative sizes and distances are to scale Using Kepler’s third law: M Jupiter = 318 M Earth 27.3 day orbital period 1.8 day orbital period click for orbital animation

Jupiter’s Interior From volume and mass, average density of Jupiter is 1.34 g/cm 3 [compare to Mercury (5.44), Venus (5.24), Earth (5.50), Mars (3.94)] Therefore, Jupiter cannot be made mostly of rock, like earthlike planets, but consists mostly of hydrogen and helium. Due to the high pressure, hydrogen is compressed into a liquid, and even metallic state. T = 30,000 K (hotter than sun’s surface!)

The Chemical Composition of Jupiter and Saturn Hydrogen gas Helium gas Water/ice Methane Ammonia

Jupiter’s Rotation Jupiter is the most rapidly rotating planet in the solar system: Rotation period slightly less than 10 hours. Centrifugal forces stretch Jupiter into a oblate shape (like an M&M).

Jupiter’s Magnetic Field Discovered by observations of radio waves and microwaves Magnetic field at least 10 times stronger than Earth’s magnetic field. Magnetosphere over 100 times larger than Earth’s. Intense radiation belts trap very high energy particles (electrons and protons). Radiation doses are 100 times lethal amount for humans!

Auroras on Jupiter Just like on Earth, Jupiter’s magnetosphere produces auroras concentrated in rings around the magnetic poles. They are 1000 times more powerful than auroras on Earth!

Explorable Jupiter (SLIDESHOW MODE ONLY)

The Io Plasma Torus Some of the heavier ions originate from Jupiter’s moon Io. Jupiter’s magnetic field sweeps past Io, creating a donut-shaped plasma torus (donut-shaped ring of ions). Electrical current (over 1 million amps!) flows along the flux tube, creating bright spots in the aurora. plasma torus flux tube

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, so hydrogen atmosphere exists above cloud layers. Only very thin atmosphere above cloud layers. Transition to liquid hydrogen zone about 1000 km below clouds.

Jupiter’s Atmosphere (2): Clouds Three layers of clouds: 1. Ammonia (NH 3 ) crystals 3. Water crystals 2. Ammonia hydrosulfide (NH 4 SH)

Planetary Atmospheres (SLIDESHOW MODE ONLY)

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

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.cloud belt

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

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

Jupiter’s Ring Not only Saturn, but all four gas giants have rings. Jupiter’s ring: dark and reddish; only discovered by Voyager 1 spacecraft. Galileo spacecraft image of Jupiter’s ring, illuminated from behind 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.

Roche Limit (SLIDESHOW MODE ONLY)

Comet Impact on Jupiter Impacts released energies equivalent to a few megatons of TNT (Hiroshima bomb was 0.15 megaton)! Visual: Impacts seen for many days as dark spots 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. video clip

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

Jupiter’s Family of Moons Over five dozen moons known now and new ones are still being discovered! Four largest moons discovered by Galileo in 1610 are called the Galilean moons IoEuropaGanymedeCallisto Each moon has interesting and diverse individual geologies.

Callisto: The Ancient Face Tidally locked to Jupiter, like all of Jupiter’s moons. Density is 1.8 g/cm 3 Composition is mixture of ice and rocks Dark surface, heavily pocked with craters. No metallic core because it never differentiated to form core and mantle. No magnetic field. Layer of liquid water, about 10 km thick, about 100 km below surface, probably heated by radioactive decay.

Ganymede: A Hidden Past Largest of the all moons in the solar system. Density is 1.9 g/cm 3 Rocky core Ice-rich mantle Crust of ice 1/3 of surface old, dark, cratered. 2/3 is bright, young, grooved terrain. Bright terrain probably formed through flooding when surface broke

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

Europa: A Hidden Ocean Density is 3,0 g/cm 3 Composition is mostly rock and metal with icy surface. Close to Jupiter so should be hit by many meteoroid impacts, but few craters visible. Why? It has an active surface, so impact craters rapidly erased.

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

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

Io: Bursting Energy Most active of all Galilean moons, with no impact craters visible. Over 100 active volcanoes! Geologic activity powered by tidal interactions (heating) with Jupiter. Density is 3.6 g/cm 3, so interior is mostly rock.

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

The History of the Galilean Moons Minor moons are probably captured asteroids Galilean moons probably formed together with Jupiter. Moon densities decreasing outward – moons probably formed in a “mini solar nebular disk around Jupiter, similar to how the planets formed around the sun. Galilean moons are probably a second generation of moons (earlier moons spiraled into Jupiter. Io, Europa, and Ganymede are in orbital resonance with 1:2:4 ratio of periods orbit animation

Saturn Mass is 1/3 of mass of Jupiter Radius is 16 % smaller than Jupiter Density: 0.69 g/cm 3 So low it would float in water! Rotates about as fast as Jupiter, in 10 hr 40 min, but is twice as oblate since it has no large core of heavy elements. Mostly hydrogen and helium with liquid hydrogen core. Saturn radiates 1.8 times the energy received from the sun. Probably heated by liquid helium droplets falling towards center, similar to how sun heats while it contracts.

Saturn’s Magnetosphere Saturn’s magnetic field: has weaker radiation belts not inclined (tilted) to rotation axis Auroras are centered around poles of rotation is 20 times weaker than Jupiter’s driven by dynamo effect

Saturn’s Atmosphere Has zone-belt structure, formed through the same processes as on Jupiter, but not as distinct and colder than on Jupiter since Saturn is twice as far from the Sun.

Saturn’s Atmosphere (2) Three-layered cloud structure, just like on Jupiter Main difference to Jupiter is fewer wind zones, but much stronger winds than on Jupiter. Winds up to 1100 mph near the equator!

Saturn’s Rings Ring consists of 3 main segments: A, B, and C ring separated by empty regions called divisions. A Ring B Ring C Ring Cassini Division Rings must be replenished by fragments of passing comets & meteoroids. Rings did not form with Saturn because ice material would have been heated at the time of formation.

Composition of Saturn’s Rings Rings are composed of ice particles moving at large but equal speeds around Saturn, so the astronaut shown here could “swim” through the ring.

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

Divisions and Resonances Some moons act as “shepherds” that “herd” material into rings with gravitational pull. Moons can also create divisions (gaps) when the orbital period of a moon is a small ratio of the orbital period of material in the disk (for example “2:3 resonance”).

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. video clip

Titan’s Atmosphere Because of the thick, hazy atmosphere, surface features are only visible in infrared images. Many of the organic compounds in Titan’s atmosphere may have been precursors of life on Earth. Surface pressure is 50% greater than air pressure on Earth Surface temperature is -290 o F Methane and ethane are liquid! Methane is gradually converted to ethane in the atmosphere Methane must be constantly replenished, probably through breakdown of ammonia (NH 3 ).

Saturn’s Smaller Moons Saturn’s smaller moons, formed of rock and ice, are heavily cratered and appear geologically dead. Tethys Heavily cratered and marked by 3 km deep, 1500 km long crack. Iapetus Leading (upper right) side darker than rest of surface because of dark deposits. Enceladus Possibly active with regions of fewer craters, containing parallel grooves, possibly filled with frozen water.

Saturn’s Smaller Moons (2) Hyperion is too small to pull itself into spherical shape. All other known moons are large enough to attain a spherical shape. video clip

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.

Coorbital Moons (SLIDESHOW MODE ONLY)

oblateness liquid metallic hydrogen decameter radiation decimeter radiation current sheet Io plasma torus Io flux tube critical point belt zone forward scattering Roche limit gossamer rings grooved terrain tidal heating shepherd satellite spoke New Terms

1. Some astronomers argue that Jupiter and Saturn are unusual, while other astronomers argue that all solar systems should contain one or two such giant planets. What do you think? Support your argument with evidence. 2. Why don’t the terrestrial planets have rings? Discussion Questions