Callisto

Picture
Callisto is the second largest moon of Jupiter, the third largest in the solar system, and is about the same size as Mercury. It has a diameter of 2, 985 Miles (4,804 kilometers)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.

                                A Heavily Cratered Surface

Picture
Callisto is without a doubt the most heavily cratered object in the known solar system. This suggests that the surface is very old and inactive. The largest impact crater on Callisto is known as Valhalla. It is 2,485 miles (4,000 km) in diameter. Another large crater, Asgard, has a diameter of 994 miles (1,600 km). It appears that Callisto's surface has undergone very little change since its formation over four billion years ago. Because its surface is so geologically inactive, astronomers believe that Callisto may represent what many of the moons in the Solar System may have looked like at a much earlier stage in their development.


                                     Features of Callisto

Picture
Callisto is very different from other rocky moons in the Solar System. There are no mountains on Callisto. Astronomers believe this to be a result of movement in its icy surface. The entire surface of Callisto is covered with craters. The largest craters have concentric rings that look like huge cracks in the surface. These rings may have formed when water beneath the crust leaked through to the surface. Like Ganymede, Callisto's craters have been smoothed out by the flow of its icy crust. The craters lack the ringed mountains and central depressions that characterize craters on Mercury and the Moon. One very interesting feature is known as Gipul Catena. This is a long series of craters arranged in a nearly straight line. This was likely caused by an object that was broken into several pieces by tidal forces before it impacted the surface. Callisto has an extremely thin atmosphere of carbon dioxide. No evidence of a magnetic field has been detected. Callisto has the lowest density of the four Galilean satellites. Astronomers believe it has a crust about 124 miles (200 km) thick. They also believe that beneath this crust may exist a salty ocean. Callisto's core is believed to be composed of compressed rock and ice.

                          Statistics for Callisto

Discovered by                        -    Galileo Galilei & Simon Marius
Year of Discovery                   -    1610
Diameter                              -    2,996 miles (4,821 km)
Mean Distance from Jupiter     -    1, 169,855 miles (1,882,700 km)
Rotational Period                   -    16.689 days
Orbital Period                        -    16.689 days
Orbital Eccentricity                 -    0.007
Orbital Inclination                   -    0.281 degrees
Mean Surface Temperature      -    -279 F (-172 C)
Main Atmospheric Component   -    None
Apparent Magnitude                -    5.65