The 3rd largest moon

Callisto [kah-LISS-toe] is the second largest moon of Jupiter, the third largest in the solar system, and is about the same size as Mercury. It orbits just beyond Jupiter's main radiation belt. Callisto is the most heavily cratered satellite in the solar system. Its crust is very ancient and dates back 4 billion years, just shortly after the solar system was formed.

Callisto lacks any large mountains. This is probably due to the icy nature of its surface. Impact craters and associated concentric rings are about the only features to be found on Callisto. The largest craters have been erased by the flow of the icy crust over geologic time. Two enormous concentric ring, impact basins are found on Callisto. The largest impact basin is Valhalla. It has a bright central region that is 600 kilometers in diameter, and its rings extend to 3000 kilometers in diameter. The second impact basin is Asgard. It measures about 1600 kilometers in diameter.

Callisto has the lowest density (1.86 gm/cm3) of the Galilean satellites. From recent observations made by the Galileo spacecraft, Callisto appears to be composed of a crust about 200 kilometers (124 miles) thick. Beneath the crust is a possible salty ocean more than 10 kilometers (6 miles) thick. Beneath the ocean, is an unusual interior that is not entirely uniform nor does it vary dramatically. Prior to Galileo, scientists believed that Callisto's interior was totally undifferentiated, but Galileo data suggests that the interior is composed of compressed rock and ice with the percentage of rock increasing as depth increases. Meteorites have punctured holes in Callisto's crust, causing water to spread over the surface and forming bright rays and rings around the crater. Callisto has no known atmosphere.

Facts about Callisto

  • Age: Callisto is about 4.5 billion years old, about the same age as Jupiter. It is the most heavily cratered object in the solar system. There is hardly any geologic activity on its surface, according to NASA. The surface has not changed since initial impacts molded its surface 4 billion years ago
  • Distance from Jupiter: It is the outermost of the Galilean moons. Because of its orbiting distance from Jupiter (about 1,168,000 miles or 1,880,000 kilometers) it takes about seven Earth-days to make one complete orbit of the planet. It also experiences fewer tidal influences than the other Galilean moons because it orbits beyond Jupiter's main radiation belt. Callisto is tidally locked, so the same side always faces Jupiter
  • Size: At 3,000 miles (4,800 km) in diameter, Callisto is roughly the same size as Mercury. It is the third largest moon in the solar system, after Ganymede and Titan
  • Temperature: The mean surface temperature of Callisto is minus 218.47 degrees Fahrenheit (minus 139.2 Celsius)

The above information was sourced from Wikipedia