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Orbit: 1,883,000 km from Jupiter
Orbital period: 17 days
Gravity: 1.236 m/s²
Diameter:  4800 km
Mass: 1.08e23 kg

Callisto is a moon of the planet Jupiter. It was discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede.

Callisto was a  nymph, beloved of Zeus and hated by Hera.       Hera changed her into a bear  and Zeus then placed her in       the sky as the constellation Ursa Major.

Discovered by Galileo    and Marius in 1610.

Callisto is only slightly smaller than Mercury    but only a third of its mass.

Callisto surface

Unlike Ganymede, Callisto seems to  have little  internal structure; however there are signs from recent Galileo data that the interior materials have settled partially, with the percentage of  rock increasing toward the center. Callisto is about 40% ice and 60% rock/iron.         Titan and Triton     are probably similar.

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.

Callisto surface

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 (above).  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, Mare Orientale       on the Moon  and Caloris Basin on Mercury.




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