Astronomy 161: An Introduction to Solar System Astronomy Prof. Richard Pogge, MTWThF 9:30

Lecture 39:
The Moons of Jupiter

Key Ideas:

4 Galilean Moons:

• Large (>3000 km), spherical & differentiated
• Io, Europa, Ganymede & Callisto.
• Io is volcanically active.
• Europa may have an ocean below the ice.

59 Small Moons:

• Small (<200 km), irregular, & undifferentiated

The 63 Moons of Jupiter

• Discovered by Galileo & Simon Marius in 1610
• Large (>3000 km across) & Spherical
• Differentiated interiors (like the Moon)

59 Small moons:

• Small (<200 km across)
• Irregular in shape (like Phobos & Deimos)
• Undifferentiated mixtures of rock & metals
• Probably captured asteroids or comet nuclei

For more information on the current census of the Jovian moon system, see Scott Sheppard's Jupiter Satellite Page at the University of Hawaii's Institute for Astronomy.

The Galilean Moons

Large, mostly rocky spherical moons.

All follow roughly circular orbits in the same direction around Jupiter:

Orbital Periods:

• Io: 1.8 days
• Europa: 3.6 days (exactly 2 times Io's period!)
• Ganymede: 7.2 days (exactly 4 times Io's period!)
• Callisto: 16.7 days

Volcanically Active Io

• Feels a strong tidal pull from Jupiter, plus an additional tug from Europa every 2 orbits.
• Heats the interior, melting silicates & sulfur.

Results in active volcanoes:

• Active eruptions & pools of molten sulfur

Io is the most volcanically active world in the Solar System.

Smooth Europa

• The surface is very smooth and young.
• Icy surface is fractured into rafts & floes a few kilometers across, like in the Arctic.
• Water geysering to the surface through the cracks has repaved it.
• Does Europa have liquid water under the ice?

What is under Europa's ice? Two ideas:

• 100-200 km of ice surrounding the rocky core.
• Thin ice crust over a 150 km thick water ocean.

Will require a lander or an orbiter with radar to probe beneath the ice.

If there is liquid water, is there life?

• It's fun to speculate, but there is no data one way or another at present.

Grooved Ganymede

• 400 km larger than Mercury, but less than half its mass (mean density is only 1.9 g/cc)
• This low density suggests it has a thick ice mantle over a rocky core.

Covered with unique grooved terrain:

• Grooves are 10 km wide and 300 m deep.
• Formed <2Gyr ago based on the number of impact craters seen on top of them.

Nobody has yet come up with an explanation for the grooved terrain, beyond speculating that they are stretch marks on the icy crust.

Cratered Callisto

• Heavy cratering indicates that Callisto is geologically inactive, and has been so ~4 Gyr
• Craters are bright - revealing clean ice below.
• Its mean density is 1.8 g/cc, suggesting that the ice is layered on a small rocky core.

Of all of the Galilean moons, Callisto shows the least signs of geological activity.

Moons in Comparison

• Mean densities of 3.5 & 3.0 g/cc respectively
• Io: rocky lithosphere, molten mantle & active volcanoes
• Europa: icy lithosphere & rocky core.

Ganymede & Callisto are mixed ice & rock:

• Mean densities of 1.9 & 1.8 g/cc, respectively
• Deep ice mantles over rocky/icy cores.
• Both are less geologically active.

Interior Heat

In the terrestrial planets, interior heat is determined by the planet's size:

• Large Earth & Venus have hot interiors
• Smaller Mercury & Mars have cold interiors

In the Galilean moons, interior heat is determined by proximity to Jupiter

• Innermost Io is hottest, outermost Callisto is coldest
• The energy source is tidal heating from Jupiter.