Voyager 2 is the only spacecraft to visit all four outer planets.

Spacecraft to the Outer Solar System

Flybys: Pioneer 10, 11 Voyager 1, 2

Orbiters/ : Galileo, CassiniLanders (Jupiter) (Saturn)

Gas Giant Planets

• Our goals for learning• What are jovian planets like?• What are the clouds like? (Jupiter as a type

example)• Why are there rings systems? (Saturn as a

type example)

What are Jovian planets Like?

Similarities: Ring Systems

• All four jovian planets have ring systems, Saturn merely the most obvious.

• Others have smaller, darker ring particles than Saturn

Similarities: Interiors

• No solid surface. • Layers under high pressure and temperatures.• Strong magnetic fields => conducting materials • Cores (~10 Earth masses) made of hydrogen

compounds, metals & rock• The layers are different for the different planets

due to differences in composition and pressure.

Inside Jupiter (the best understood)

• Inside Jupiter, high pressures cause hydrogen to change phase with depth.

• Hydrogen acts like a metal at great depths because the hydrogen is squeezed until electrons move freely

Inside Jupiter

• Core is thought to be made of rock, with some metals, and hydrogen compounds

• Core is about same size as Earth but 10 times as massive

Subdividing the Gas Giants

Differences: Composition• Jupiter and Saturn

– Mostly H and He gas

• Uranus and Neptune– Mostly hydrogen compounds: water (H2O),

methane (CH4), ammonia (NH3)– Some H, He, and rock

Differences: Interiors

• Models suggest cores of jovian planets have similar composition

• Lower pressures inside Uranus and Neptune mean no metallic hydrogen

What are the Clouds Like?

Colors of The Gas Giants

high thin haze

high haze

thicker clouds .

thick heavy clouds

• Jupiter - high brown and white clouds

• Saturn- bands deeper thicker smog haze

• Uranus -light blue

• Neptune dark blue

•Colors are temperature controlled•Decreasing temp. : increasing methane clouds•methane clouds reflect blue light

Jupiter

Jupiter’svertical

atmosphericstructure

Different materials form stable ice clouds at different temperatures & pressures

=> different clouds form at different heights

Jupiterconvection

Convection cells create a pattern of white high clouds & brown low clouds

Saturn

Uranus

Early Voyager images, taken at southern summer, showed no cloud bands

This Hubble image shows the Uranus, system, with its ten rings and eight of its moons.

In this false color image, taken in infrared light, the moving bands of cloud show up well.

Uranus

Neptune

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Surprising discovery or fraud? - Saturn's core is pockmarked with impact craters and dotted

with volcanoes erupting basaltic lava.

A. Plausible. Saturn's moons also show impact craters and volcanoes.

B. Plausible. Saturn's atmosphere originated from the volatiles in impactors that were released via volcanic activity.

C. Implausible. No impactors would survive the immense pressures at the depth of Saturn's core.

D. Implausible. Saturn's high rotation would prevent an impactor from reaching its core.

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Since there are a lot of flammable gases on Gas Giants, such as methane and propane,

if you lit a match, would Jupiter burn?

A. yesB. no

What Are The Rings Like?

Saturn’s Rings: Type Example

The B ring is sandy colored, while the outer A ring is white from ice.

Rings made of thousands of small rings of ice particles like fluffy snowballs

Ring Location in Gas Giants

• Rings occur within 2-3 radii away (inside the Roche Limit)

– Region where gravity of a planet is so strong that materials can’t hold together to form a moon.

• Planet’s gravity pulls in material, moons’ gravities push and pull in opposite direction.

• Tug-o-war holds rings in place (for a while)

Some rings are held in place by the gravity of tiny moons. -They are called 'Shepherd Moons'

F ring has three shepherds.

Keeping Rings in Place 1: Shepherd Moons

close-up of the F ring and two shepherds. It is held between the gravity of these two moons.

Prometheus (moves inside the ring)

Pandora(moves outside the ring)

Keeping Rings in Place 2: Resonances

Orbital resonance with the gravity of a larger moon can push the particles, producing a ring or a gap

Ring Formation in Gas Giants• Rings aren’t leftover from planet formation

because the particles are too small to survive this long in a strong gravity field.

• There must be a continuous replacement of tiny particles.

• The most likely source is impacts with the gas giants’ moons.

Ring Formation in Gas Giants

• Impacts continually occur on the many small moons, chipping off fine material

• Fine material spreads out in orbit around the gas giant

• The gravity effects from the moons then hold the bits in place as rings.

What have we learned?• What are the clouds like?

– The gas giant planets have different color clouds at different heights due to temperature differences

– They are striped because different clouds are visible due to convection plus rapid rotation causing a strong coriolis effect.

What have we learned?• What are the rings like?

– The rings are tiny pieces of fluffy snow balls held in orbit 2-3 radii away from a gas giant world.

– They occur because gas giants have lots of moons:

• Impacts chip off material that becomes rings.• Material is nudged into resonance orbits by the gravity

of the moons, forming rings• Some rings are held by the gravity of tiny shepherd

moons.

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Why don't ring particles form a moon?

A. They collide too violently to accrete into a moon.

B. Tidal forces from moons prevent them from accreting.

C. Tidal forces from the planet prevent them from accreting.

D. Their masses are too small for them to accrete.

A Wealth of Worlds: Moons of Ice and Rock

• Our goals for learning• What kinds of moons orbit Gas Giant

planets?• Why are Jupiter’s Galilean moons so

geologically active?• What is special about Titan and other

major moons of the solar system?

What kinds of moons orbit Gas Giant planets?

• Small moons (< 300 km)– No geological activity

• Medium-sized moons (300-1,500 km)– Geological activity in past

• Large moons (> 1,500 km)– Ongoing geological activity

Medium and Large Icy Worlds

• Enough self-gravity to be spherical.• Metal/rock core, ice mantle and crust.• Some geological activity• Two main locations:

– Larger Icy moons orbiting gas giant planets– Largest objects in Kuiper belt in the outermost

solar system (Pluto, Sedna, Quoaor and others)

Medium & Large Moons

• Formed in orbit around gas giant planets.

• Circular orbits in same direction as planet rotation.

Why are Jupiter’s Galilean moons so geologically active?

Io

12th largest world

Active sulfur

volcanism

Io’s Volcanic Activity

• Io is the most volcanically active body in the solar system, but why?

Tidal Heating

Io is squished and stretched as it orbits Jupiter But what is

causing it to stretch?

Orbital ResonancesEvery 7 days, these 3 moons line up.

The tugs add up over time, making all 3 orbits elliptical, which pulls on the worlds.

Europa -13th largest world

Tidal stresses crack surface iceNo surface volcanos, but millions of faults all over the surface. A few small craters are seen.

Europa’s Ocean: Waterworld?•Chaos regions: where warm ice wells up, breaks and spreads the surface.

Inside Europa

• Secondary magnetic field (requires liquid)• Broken disrupted surfaces from flowing ice• Surface covered in faults -some areas

spread apart.

• Conclusion: • Liquid interior layer -a global ocean

Europa’s interior also warmed by tidal heating

• Largest moon in the solar system

• Surface has long fault regions, but also heavily cratered plains.

Ganymede -8th largest world

Ganymede

• Clear evidence of geological activity

• Tidal heating plus heat from radio-active decay?

• Has a magnetic Field that varies.

Callisto• cratered iceball.• No tidal heating, no

orbital resonances.

Orbital Resonances

Every 7 days, these 3 moons line up.

The tugs add up over time, making all 3 orbits elliptical.

What have we learned?• What kinds of moons orbit jovian planets?

– Moons of many sizes– Level of geological activity depends on size

• Why are Jupiter’s Galilean moons so geologically active?– Tidal heating drives activity, leading to Io’s

volcanoes and ice geology on other moons

Saturn’s Moons

What is special about Titan?

Titan’s Atmosphere• Titan is the only

moon in the solar system to have a thick atmosphere

• It consists mostly of nitrogen with ‘smog’ clouds that obscure the surface

Mosaic of Hygens Images• ~5 miles high

shows dark curving lines with branching tributaries (lower left).

• pattern is identical to desert rivers on Earth and the dry-river beds of Mars.

• conclude that Titan has flowing liquids eroding out channels.

lake

mountains

channels

Fish-eye Mosaic of Hygens Images• mountains and a shore-

line to some kind of dark-colored lake.

• However, at -290 °F at the surface, water is frozen as hard as rock.

• The lake must be made of organic liquids, probably a methane-ethane mix.

Giant North Pole Lakes• The biggest lake so far is 39,000 square miles, it is larger

than lake Superior and about the size of the Black Sea

Active Methane Cycle :(• There is a tiny amount of

methane rainfall• The lakes do not appear

to grow and shrink in size • There is probably not a

permanent methane recycling system like Earth's water cycle

TethysThey are big enough to be round, but vary in their amount of geological activity.

EnceladusDione Iapetus

Rhea

Mimas

Saturn has 6 medium-sized moons.

This is a false color image taken during Cassini’s approach to this moon.

• The south pole of Enceladus is covered in recent faults!

• This suggests some unknown, but complex, internal activity is occurring.

This colorized image shows the enormous extent of the faint plume.

Water eruption plume

• Enceladus has live volcanic ice eruptions! – The water freezes into ice particles and

rises in a plume over 300 miles high. • Only 3 other worlds have observed active

volcanism: Earth, Io and Triton.

The volcanism on Enceladus is a major source for the particles for Saturn's outer rings. Here Enceladus emits water in the middle of the E ring .

E

EnceladusVolcanic plume

Enceladus and E Ring

Neptune’s Moon Triton

Neptune’s Moon Triton• Similar to Pluto,

but larger

• Evidence for past geological activity

Icy Volcanism on Triton

Spouts of nitrogen geysering above

the surface

What have we learned?• What is special about Titan and other major

moons of the solar system?– Titan has strong active processes like Galileans– Titan is the only moon with a thick atmosphere– Titan is the only moon with liquid rivers and lakes.

– Many other major moons show signs of geological activity.

– 3 of gas giants have an actively volcanic moon.

Small Worlds

• Our goals for learning• What are asteroids like?• Why is there an asteroid belt?• Where do comets come from?

• Small Worlds are:– Not round, – Covered in craters, – Have no atmosphere, – Main surface process is mass wasting

• Asteroids are made of metals and rock • Comets and KBOs are made of rock and

volatiles

Asteroids

Asteroid Orbits• Most asteroids orbit in

a belt between Mars and Jupiter

• Trojan asteroids follow Jupiter’s orbit

• Orbits of near-Earth asteroids cross Earth’s orbit

Origin of Asteroid Belt• Rocky planetesimals

between Mars and Jupiter did not accrete into a planet.

• Jupiter’s gravity, through influence of orbital resonances, stirred up asteroid orbits and prevented their accretion into a planet.

Kuiper Belt Objects -Dirty snowballs

Two regions for small icy worlds:

• Kuiper belt, KBOs

have orderly orbits from 30-100 AU in disk of solar system

• Oort Cloud, worlds have random orbits out to >50,000 AU

Sources for comets.

Growth of Tail

What have we learned?• What are comets like?

– Comets are like dirty snowballs– Most are far from Sun and do not have tails– Tails grow when comet nears Sun and nucleus

heats up• Where do comets come from?

– Comets in plane of solar system come from Kuiper Belt

– Comets on random orbits come from Oort cloud