* Most of the satellites in the solar system fall into one of three categories: regular satellites that formed about their parent planets, collisional fragments of regular satellites, and captured asteroids. The Moon, Triton, and Charon do not fit into any of these three categories.
* Nearly all of the satellites of the outer planets have densities
less than 2000 kg/m3. This is much lower than the density of rocky
material, 3500 kg/m3, so ices must make up a large fraction of
the material of most satellites. Most of the ice in the outer
solar system is probably frozen water.
* There are three sources of internal heat for satellites: the
conversion of gravitational energy into heat during formation,
radioactive heating, and tidal heating. The thermal history of
a satellite depends not only on these sources of energy, but also
on how easily heat is transported to its surface by convection
and conduction.
* Phobos and Deimos, the satellites of Mars, are believed to be
captured asteroids. Their surfaces are pocked by craters. Cracks
on Phobos radiate from the crater Stickney, which was produced
by an impact nearly violent enough to destroy Phobos.
* The intense tidal heating of Io makes it the most volcanically
active body in the solar system. Volcanic material is deposited
so rapidly on Io's surface that all evidence of impact cratering
has been covered up. Sulfur and its compounds provide the yellow
and red colors of Io's surface and may also power the plumes that
propel material hundreds of kilometers above Io's surface.
* Europa has a very smooth, icy surface even though it is made
mostly of rock. Europa's surface has been smoothed by glacier-like
flows and, probably, by flows of water from the interior. A thick
ocean of water probably exists below its icy crust.
* It is hard to understand how Ganymede and Callisto, so similar
in mass and diameter, could have such different appearances. Ganymede's
surface shows evidence of past internal activity and crustal movement,
while Callisto's heavily cratered surface shows no evidence of
any internal activity. Convection within Callisto may be so efficient
that it cooled too rapidly for internal activity to take place.
* Saturn's icy satellites show evidence of a wide range of internal
activity. At one extreme is Rhea, which has a heavily cratered
surface, while at the other extreme is Enceladus, which has been
extensively resurfaced. Tidal heating my explain why Rhea seems
to have experienced little or no internal activity while Enceladus
seems to have experienced so much.
* Saturn's largest satellite is Titan. Its atmosphere contains
mostly nitrogen and is thicker than the Earth's. Titan is cold
enough that atmospheric ethane can condense to liquid form. In
fact, Titan may have ethane rain and lakes or oceans of ethane.
* All of the larger satellites of Uranus have been more active
than the Saturnian satellites of comparable size. Tidal heating
may have powered this internal activity.
* Uranus's satellite, Miranda, is very unusual in appearance.
Its surface is a jumbled mixture of different types of terrain.
The way in which Miranda evolved to its present form is not known.
* Triton orbits Neptune in a retrograde direction and is slowly
spiraling inward. Triton probably formed elsewhere and was captured
into a retrograde orbit. The surface of Triton shows evidence
of considerable resurfacing and internal activity. Plumes of dark
material erupt from beneath the surface of Triton and are carried
far downwind by its thin atmosphere.
collision fragments, corona, plume, regular satellite, tidal heating, volatile
1. Why are no impact craters seen on Io? it has extensive volcanic activity
2. The heat that keeps the interior of Io molten is due to tides
3. Suppose a planet is orbited by a number of satellites. Which
of the satellites will feel the strongest tidal forces due to
the planet? a large satellite in a close orbit
4. The surface of Europa (Jupiter's satellite) is very smooth.
Why? it is covered by frozen water
5. What is remarkable about the surface of Ganymede? it shows
evidence of ancient tectonic activity
6. What are the bright patches on the surfaces of Callisto and
some of the satellites of Saturn? regions of freshly exposed ice
7. Why are the inner Galilean satellites denser than the outer
ones? the outer ones contain more icy material
8. The satellites of Saturn, Uranus, and Neptune are made mostly
of ice
9. Which of the following correctly describes the atmosphere of
Saturn's satellite, Titan? ,thick, made of nitrogen, cold
10.What is remarkable about Uranus' satellite Miranda? it has
a bizarre appearance