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Astronomy 161
An Introduction to Solar System Astronomy
Prof. Scott Gaudi

Lecture 39:
The Moons of Jupiter

Key Ideas:

Jupiter has 63 known moons

4 Galilean Moons:

59 Small Moons:

The 63 Moons of Jupiter

4 Galilean moons:

59 Small moons:

4 Inner small moons are on regular orbits, the rest are outer moons on irregular orbits, some retrograde. Interestingly, many of the irregular outer moons come in dynamical "familes" of many moons with similar orbital properties. This suggests that some of the captured moons are smaller pieces of an originally larger "parent" body that was disrupted either by tides or collisions during the capture process.

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

Order: Io Europa Ganymede Callisto (I Eat Green Cows)

Large, mostly rocky spherical moons.

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

Orbital Periods:

The inner 3 Galilean moons are in a 1:2:4 orbital resonance and evolve together. Callisto is almost part of this, and will eventually get locked into a resonance with 8 times Io's orbit.

Volcanically Active Io

Io is the innermost Galilean moon. Io has a hot, molten interior:

Results in active volcanoes:

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

Smooth Europa

Europa is the smallest of the Galilean moons. It has an icy surface covering a large rocky core:

What is under Europa's ice? Two ideas:

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

If there is liquid water, is there life?

Grooved Ganymede

Ganymede is the largest of the Galilean moons, and the largest moon in the Solar System.

Covered with unique grooved terrain:

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

Callisto is the outermost of the Galilean moons. It has a surface consists of dirty ice pocked with numerous impact craters.

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

Moons in Comparison

Io & Europa are mostly rock:

Ganymede & Callisto are mixed ice & rock:

Interior Heat

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

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

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