 Callisto
("ka LIS toh") is the eighth of Jupiter's
known satellites and the second largest. It is the outermost of the Galilean
moons.
orbit: 1,883,000 km from Jupiter
diameter: 4800 km
mass: 1.08e23 kg
Discovered by Galileo
and Marius in 1610.
Callisto is only slightly smaller than Mercury
but only a third of its mass.
 Callisto
is composed of ice, and iron-rich rock.
Callisto's surface is covered entirely with craters. The surface is very old,
like the highlands of the Moon
and Mars. Callisto has the
oldest, most cratered surface of any body yet observed in the solar system;
having undergone little change other than the occasional impact for 4 billion
years.
The largest craters are surrounded by a series of concentric rings which look
like huge cracks but which have been smoothed out by eons of slow movement of
the ice. The largest of these has been named Valhalla (right). Nearly
3000 km in diameter, Valhalla is a dramatic example of a multi-ring basin,
the result of a massive impact. Other examples are Callisto's Asgard (left),
Mare Orientale on the Moon
and Caloris Basin on Mercury.
Like Ganymede, Callisto's ancient craters have collapsed. They lack the high
ring mountains, radial rays and central depressions common to craters on the Moon
and Mercury. Detailed
images from Galileo
(left) show that, in some areas at least, small craters have mostly been
obliterated. This suggests that some processes have been at work more recently,
even if its just slumping.
Callisto has a very tenuous atmosphere composed of carbon dioxide.
Galileo
has detected evidence of a weak magnetic field which may indicate some sort of
salty fluid below the surface.
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