Outer Solar System Moons

The larger moons of the outer planets are build up of icy materials. We can view them as having grown by comet impact rather than asteroid impact. Two large moons are notable -- Titan for Saturn, Triton for Neptune

Titan:

nitrogen atmosphere, trace of methane surface features barely visible through clouds Left, a picture shows nothing because of the dense cloud layer surrounding the moon completely.

Below are two paintings (Stan Richard daytime, and Don Dixon sunset) of what the surface might look like. Try an imaginary airplane ride over Titan, to the Cassini Huygen's probe on the surface (from Cassini-Huygens Project, http://saturn.jpl.nasa.gov/multimedia/videos/huygens-probe/index.cfm)

Triton:

• Surface of water ice, methane and nitrogen frost
• geysers of nitrogen (!)
• "retrograde" orbit -- will eventually fall into Neptune as tides use up orbital energy

Picture below, followed by artist's (Don Dixon) impression of the surface

Vents or geysers on Triton (arrows). (From APOD, NASA, Volager, C. J. Hamilton, http://antwrp.gsfc.nasa.gov/apod/ap950805.html)

A vent on Triton (artist's concept).(From William Hartmann, http://www.psi.edu/hartmann/planets.html)	Another version, by another artistFrom David Hardy, http://www.hardyart.demon.co.uk/html/main.html

The smaller moons of the giant planets are mainly icy – more like comets than asteroids

As an example, we show Dione and Phoebe, moons of Saturn:

Dione, to left from The Nine Planets, Bill Arnett, http://www.seds.org/nineplanets/nineplanets/ Phoebe, to right, from Cassini-Hurgens mission, http://saturn.jpl.nasa.gov/home/index.cfm

On the surface of Dione, from D. Seal http://samadhi.jpl.nasa.gov/art/surfaces.html

We use this beautiful image of Saturn's moon Mimas (center right), the rings of Saturn (bottom), and the planet itself to transition from moons to rings. It was obtained by the Cassini orbiter, http://photojournal.jpl.nasa.gov/catalog/PIA06142

Rings

• All giant planets have them --
• Jupiter, Uranus, Neptune have dark rings of rock
• Saturn has bright rings made of ice

Jupiter's Ring:

Rings around Uranus

Rings around Neptune:

What makes rings??

Rotation of Saturn rings shows subtle dark features, while the image to the right shows the complex structure imposed by gravitational tugs by Saturn's moons. (from Cassini Huygens. http://saturn.jpl.nasa.gov/home/index.cfm

In Saturn's rings (painting by William Hartmann) The rings are made almost entirely of small particles of water ice. Rings are the result of breakup of moons. We calculate that the rings should not last for the life of solar system without being replenished. Thus, they may reflect relatively recent (on the scale of the 4.5 billion year age of the solar system) events causing small moons to break up.

One way to cause a moon to break up is by having gravitational interactions with other moons to cause it to go into an orbit that penetrates inside the Roche limit. The Roche limit: distance at which tidal force on body is equal to its self-gravity Large bodies held together by gravity will be broken up by tides if they are inside the Roche limit (animation by G. Rieke)

Here is how the Roche limit works if a moon's orbit takes it too close to the planet .(Adapted by G. Rieke from J. Barnes, http://www.ifa.hawaii.edu/~barnes/ast110/

Rings can be formed at any radius if there is a collision that breaks up satellites (as in this painting by Don Dixon), but they are most likely to be within the Roche limit since self-gravity can no longer hold satellites together there.

Most of the rings in fact lie within the Roche limit, with just a few outside. (illustration by G. Rieke)

Test your understanding before going on