Moons of the Outer Planets The outer planets have large retinues of moons, including objects as big as Mercury (Ganymede,Titan) and with an atmosphere (Titan). There are also numerous little moons, some of which are captured asteriods. The major moons formed in a disk around the planet, like a little solar system.

The Galilean Moons Galileo saw 4 major moons around Jupiter. Each of them is rather different. The surfaces of Io and Europa are rather young, Ganymede and Callisto have lots of craters. The outer 3 have icy surfaces. The moons keep one face towards Jupiter due to tides.

The Interiors of the Galilean Moons The inner moons are denser than the outer moons (like the planets).

Io – the volcanic moon Io has a 4 day period around Jupiter. Its surface is covered with volcanoes and lava flows, and changes as we watch it. The yellow color is due to sulfur, which keeps erupting. The interior of Io is heated by tidal stresses, as Europa keeps trying to disturb its tidal locking with Jupiter.

Volcanoes on Io

Europa Europa is an ice-covered rocky moon with a 3.5 day period (just twice Io’s). This “orbital resonance” keeps both Io and Europa internally heated. The stresses are apparent in the huge surface cracks. There are very few craters, indicating the surface has been continually reworked. Natural colorFalse color

An Internal Ocean on Europa? False color

Some Moons of Saturn Mimas – the “bullseye” moon. Enceladus – the bright moon. Iapetus – a dark leading surface.

Miranda – the “messed-up” moon of Uranus

The Rings of Saturn Galileo noticed these rings, although he couldn’t quite see what they were. Since they are thousands of miles across and only 20 meters thick, they are much relatively thinner than paper (and you can see through them. They are composed of billions of tiny bodies, from dust to boulder sized. We now think that rings only last for millions of years, so we are privileged to see them so nice. [Maybe 50 million years from now, Mars will be better.]

Rings and the “Roche Limit” Inside the Roche limit, the tidal stresses on a body (which grow with its size and closeness) are stronger than its internal strength (basically gravity, in the case of moons). This is generally at 2.5 planetary radii or so.

Ring Structures The rings show ephemeral “spokes”, probably due to magnetic interactions (movie). The primary structures are due to “orbital resonances with the moons, and the thousands of ringlets are “density waves”. Moonlets also play a role in sheparding rings and clearing gaps.

Jupiter’s Ring

Uranus’ Rings Very dark, originally discovered by “stellar occultations from Earth, imaged by Voyager.

Neptune’s “Arc” Rings Sheparded “braided” ring.