22 Ice Worlds

A Wealth of Worlds: Moons of Ice and Rock

• Our goals for learning• What kinds of ice worlds are in the outer

Solar System?• 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 Ice worlds Are in the Outer Solar System?

Two main locations:– Icy moons orbiting gas giant planets– Kuiper belt just past Neptune (KBOs)

Size Scale:– Small (< 300 km)

• No geological activity– Medium-sized (300-1,500 km)

• Geological activity in past– Large (> 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• Largest Kuiper Belt Objects are counted

as “Dwarf Planets” (Pluto, Eris, Haumea, Makemake)

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?

Changing volcanoes over 2 years

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.

Channel

Delta

A closer look at the lake and shore-line region.

There appear to be drainage channels creating an ice delta or ice bar, similar to the features seen on the Nile or Mississippi deltas on Earth.

• Huygens landed with a quish!

• The probe landed in a dry-river bed with “sands” and “stones” made of ice.

• Huygens detected methane steaming off the surface from the heat of the probe.

• This saturated with liquid methane, just like wet soil on Earth would contain water.

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

This equatorial region has mountains (pale areas) and sand dunes (the black linear features).

mountains

mountains

dunes

dunes

The feature is dome-shaped

Bright from fresh ice but has a dark central pit.

Lines that seem to be flow structures.

ICE VOLCANO

Sotra Facula:A bright spot in Titan's sand sea

Etna, Italy

Meru, Tanzania

St. Helens, Washington USA

Tohil, Io

Sif, VenusLaki, Iceland

Sotra Facula, Titan

SP Crater, Arizona USASotra and similar

volcanoes in false colorSotra facula,Titan

Maat Mons,Venus

St. Helens, WA

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. • Water and carbon molecules

Inside Enceladus

Ice crustGlobal ocean

Rock core

• Enceladus has ‘forced libraton’

• (rocking back and forth toward Saturn)

• Maybe this causes enough heating to melt an internal global ocean

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

Uranus’ midsized moons

Varying amounts of geological activity, Mostly ancient tectonics

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

Why are mid-sized icy worlds more geologically active than

mid-sized rocky planets?

Rocky Worlds vs. Icy Worlds

• Rock melts at higher temperatures

• Only large rocky planets have enough heat for activity

• Ice melts at lower temperatures

• Fine forcing motions can melt internal ice, driving activity

What have we learned?

• What is special about Titan?– 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.– Has former (current??) ice volcano

What have we learned?

• Why are mid-sized icy worlds more geologically active than same-sized rocky worlds?– Many other major moons show geological activity.– 3 of gas giants have an actively volcanic moon.

– Ice melts and deforms at lower temperatures enabling tidal heating or other orbital forcing to drive activity

Pluto: Lone Dog No More

• Our goals for learning

• What are the large objects of the Kuiper belt like?

• What is Pluto like?

Kuiper Belt Objects (KBOs)• Some KBOs are very

large, much more so than asteroids.

• Pluto is one, several have been so far found.

• large, icy objects are round, but have orbits similar to the smaller objects

• So are they very large comets or very small planets?

What is Pluto like?

• Pluto is very cold (40 K) • It has 5 moons. The largest, Charon, is nearly

as large as Pluto itself (probably made by a major impact)

• Pluto has a thin nitrogen atmosphere that will refreeze onto the surface as Pluto’s orbit takes it farther from the Sun.

New Horizons Flyby June 2015

• Atmosphere with a methane haze, similar to Titan

• Thickness?

Pluto & Charon (Enhanced Color)• Taken in blue, red

and IR light• Red color is tholin

dust – Complex

hydrocarbons (solid smog!)

• Tholins are at Pluto’s equator and Charon’s pole

• Unexpected features, Mtns, ridges, glaciers, cryovolcanoes

• Mountains are similar height to Rockies!