110: SURVEY OF ASTRONOMY

6. Gas Giants & Worlds

1. The Jovian

2. Satellites and Rings The contains two planetary behemoths and two planets which are merely enormous by our standards. & have hundreds of times the 's ; both radiate more energy than they receive from the , and this outflow of energy powers dramatic activity in their interiors and . & , 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 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 . Two Types of Jovian Planets

“Gas Giants” “Water Worlds” 1. THE JOVIAN PLANETS

a. Internal Structure

b. Outward Appearance Internal Structure:

Anywhere inside a (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 , 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 H + He (due to extreme pressure)

Metallic H (liquid) with dissolved He; good 4

Core (, ); ~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 . 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 , 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 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. — in metallic H! — local dynamo in “ocean”? Internal Structure: Magnetic Fields Outward Appearance

12 hours on Jupiter Outward Appearance: 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 gas forms white NH4SH ; NH3 clouds; circulates faster. circulates slower. Outward Appearance: Circulation Patterns

Jupiter Unpeeled Outward Appearance:

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 , 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 & rock • formed in disks around parent planets (not ) • orbit in same direction as planets spin (not Triton) Small

• too small to be spherical • orbit both directions! The Jupiter Satellite Page Activity: Galilean Satellites

IO Wide range of geological activity: — is most volcanic object in solar system! — Callisto exhibits very little activity. Io: Volcanic

Loki Patera: active center

Pele: Io’s largest volcanic pluume

Topography and on Io Io: Volcanic Moon

Ongoing Volcanic Eruption at Tvashtar Catena Tvashtar’s Plume

Why so active? Tides Tides of Io

Jupiter

Io is alternately stretched and squeezed in its elliptical orbit around Jupiter, creating heat through . Over time, this friction should make Io’s orbit more and more circular; what keeps Io’s orbit elliptical? The Role of

2:1 resonance The 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 -Covered Ocean?

Fractures in

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 , and hints heavily cratered terrain, with of tectonic activity. a possible subsurface ocean? : a Moon With Atmosphere

Thick smog of N2 and hydrocarbons (CH4, C2H6, etc)! Hydrocarbon on Titan

Reflection of Sunlight off Titan Radar Shows Evidence of Seas Meandering Riverbeds on Titan

Xanadu's Meandering Rivers Titan’s Climate

• Surface : 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 : 2 PMimas = 2 PCassini ⇒ aMimas = ∛2 aCassini Other moons and create other structures. Origin of Planetary Rings

Ring formation may be ongoing or catastrophic. — moonlet fragmentation — tidal disruption of moon Tidal Disruption