Jupiter's Moons

Jupiter has 16 - no, now 61!! - moons that may be subdivided into three groups

The innermost four with orbits closer to Jupiter than Io's. These moons are small and irregular in shape
Four large "Galilean" Satellites
Small outer moons that appear to form two groups; the moons in each group have nearly the same average distance from Jupiter suggesting that they are broken apart asteroids captured by Jupiter. More of this type are being discovered all the time, causing the total known number to climb rapidly

A picture of Amalthea, the best-known inner moon, is to the right, and a movie of how it would look from a spaceship is here (from Galileo Project)

Galilean Moons

Galilean moons montage

In order of distance from Jupiter:

Io:

rocky interior

tidal heating due to its proximity to Jupiter

active sulfur volcanoes powered by the tidal heating

From Galileo mission, http://www.jpl.nasa.gov/galileo/images/images.html

Artist's concept of being on the surface of Io

Artist's impression of Io (William Hartmann)

Europa:

outer layers of ice

smooth, icy surface due to modest tidal heating (Galileo orbiter discovered what must be a very recently formed crater)

cracks in the ice suggest that Europa's interior may be liquid which has led to speculation that it could harbor life

recent Galileo results show existence of a thin atmosphere

More European surface features

(From Galileo Mission, http://www.jpl.nasa.gov/galileo/images/images.html)

Close up showing "freeze marks" on Europa

A closeup (smallest features about 10km across).

From Galileo mission, http://www.jpl.nasa.gov/galileo/images/images.html

Artist's concept on the surface of Europa

Artist's concept on the surface of Europe (version 2)

Two artist's impressions of the surface of Europa (Don Dixon)

Ganymede:

largest moon in the solar system; Ganymede is larger than Mercury

surface shows many more craters and is darker than Europa's implying that its surface is much older. The dark color comes from meteorite dust.
Recent Galileo orbiter results suggest that the surface may be divided in plates much like the Earth's surface

	Left: different terrains on Ganymede. Can you tell which area is younger? (hint: count craters)

Artist's impression from the surface of Ganymede

Artist's impression of the surface of Ganymede (Don Dixon)

Callisto:

mostly ice
surface very heavily cratered suggesting that it has not been refreshed

Artist's impression from the surface of Callisto

Artist's impression of the surface of Callisto (to right) (Don Dixon)

The interiors of Io, Europa, Callisto, and Ganymede, clockwise from upper left. All except Callisto have metallic (iron/nickel) cores, shown in gray, surrounded by rock in brown. The rock shells in Europa and Ganymede are surrounded by liquid water (blue) and ice (white). Callisto is shown as an undifferentiated mixture of ice and rock.

Comparison of interiors of Gallilean moons

Notice the pattern in properties of the Galilean moons with increasing distance from Jupiter:

less active surface

lower average density

greater proportion of water

less differentiated core

This trend mimics the trend in properties of planets with distance from the Sun. The properties of the moons indicate that they formed as small objects around a large one, with similar consequences to the formation of the (relatively) small planets around the much larger sun. A combination of heating by the central object and the larger energy of collisions due to the high orbital velocities close to it can account for these trends in both cases.

Shani, Vedic god of Saturn, http://www.payer.de/kommkulturen/kultur123.htm

Huygen's drawing explaining the various aspects of the rings of Saturn. http://www.surveyor.in-berlin.de/himmel/astro/Huygens-e.html

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