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.

“Gas Giants” “Water Worlds”

Two Types of Jovian Planets

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.

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:

14.7 lbs per sq in = 14.7 psi = 1 bar.

A column of water 1 inch on a side and 32 ft high also weighs 14.7 lbs, so 32 ft below sea level the total pressure is 29.4 psi = 2 bar.

air

10

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i

P = 1 bar

wa

ter

32

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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 those already there.

Adding mass adds gravity, which squashes the “stack” even more.

Internal Structure: Jupiter

1

5×105

2×106

~108

125

2000

5000

2×104

0.0002

0.5

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4

Gaseous H + He;some NH3, H2O, CH4

Liquid H + He (due to extreme pressure)

Metallic H (liquid) with dissolved He; good electrical conductor

Core (rock, metal);~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 — not due to radioactivity since H and He isotopes are stable.

— Jupiter: “left-over” heat.

— Saturn: He differentiation?

U. puts out very little heat —— lost during giant impact?— bottled up in interior?

N. puts out more heat —— radioactivity in core?

Internal Structure: Magnetic Fields

J. & S. have strong fields which are well-aligned with rotation.

U. & N. have weaker fields which are highly mis-aligned.

— convection in metallic H! — local dynamo in “ocean”?

Internal Structure: Magnetic Fields

12 hours on Jupiter

Outward Appearance

Outward Appearance: Cloud Colors

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ops NH3

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Outward Appearance: Belts

Convection and rapid rotation organize atmospheric motion.

Belts: sinking gas exposes red NH4SH clouds; circulates faster.

Zones: rising gas forms white

NH3 clouds; circulates slower.

Jupiter Unpeeled

Outward Appearance: Circulation Patterns

Approaching Jupiter

Outward Appearance: Great Red Spot

Outward AppearanceOutward Appearance: Great Red Spot

Jupiter's Great Red Spot Region

H

Outward Appearance: Saturn

Saturn … Four Years On

Bands like Jupiter’s, but less dramatic since atmosphere is deeper due to weaker gravity.

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

+ 53 more

+ 22 more

+ 11 more

A Mess of Moons

• 6 large moons (D>1500 km) — most geologically active

• 12 medium (D>300 km) — some activity in past?

• 145+ small (D>1 km) — no geological activity

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

Wide range of geological activity:

— Io is most volcanic object in solar system!

— Callisto exhibits very little activity.

IO EUROPA GANYMEDE CALLISTO

Topography and Volcanos on Io

Pele: Io’s largest volcanic pluume

Io: Volcanic Moon

Loki Patera: active center

Ongoing Volcanic Eruption at Tvashtar Catena Tvashtar’s Plume

Io: Volcanic Moon

Why so active?

Tides

Tides

Jupiter

Tidal Heating of Io

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

The inner moon makes 2 orbits in the same time the outer moon makes 1, so this is a 2:1 resonance.

The inner moon gets a tug every other time around, so its orbit gets more elliptical with time.

2:1 resonance

Jupiter’s Moons: the Laplace 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.

4:2:1 resonance

Europa: an Ice-Covered Ocean?

Exaggerated colors “Natural” colors

Pwyll: Impact crater

Fractures in crust

Natural and False Color Views of Europa

Europa: Rafts of Ice

Europa - Ice Rafting View

Europa: Rafts of Ice

Europa - Ice Rafting View

Tidal flexing opens up cracks, creating pairs of ridges, then closes them up, grinding ice sheets together.

Europa: Internal Structure

Heat from tidal friction can keep Europa’s insides warm.

Global Callisto in Color

Ganymede and Callisto

Ganymede has a mixture of old and new terrain, and hints of tectonic activity.

Callisto has a old and very heavily cratered terrain, with a possible subsurface ocean?

Titan: a Moon With Atmosphere

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

Radar Shows Evidence of SeasReflection of Sunlight off Titan Lake

Hydrocarbon Lakes on Titan

Xanadu's Meandering Rivers

Meandering Riverbeds on Titan

Titan’s Climate

Wikipedia: Titan

• 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

Ring Systems

All Jovian planets have rings made of tiny “moonlets”.

Saturn

Saturn’s Rings

Only 20 m thick, the rings are aligned with the equator.

Outer Solar System Moons and Rings

Inside Saturn’s Rings

The moonlets in Saturn’s rings are composed mostly of ice. Collisions are slowly grinding them to dust.

Structure in Saturn’s Rings

Cassini Division

Cassini’s division is due to a 2:1 resonance with Mimas:

Mimas

PMimas = 2 PCassini ⇒ aMimas = ∛22 aCassini

Other moons and resonances create other structures.

Origin of Planetary Rings

Ring formation may be ongoing or catastrophic.

— moonlet fragmentation

— tidal disruption of moonTidal Disruption