 Callisto is the eighth of Jupiter's moons, and its second largest, with a diameter of 4800 km. Callisto's surface is completely covered with craters, and is very old, like the highlands of the Moon, and parts of Mars. In fact, Callisto has the oldest, most cratered surface of any body yet discovered in the solar system, having changed little, except for scattered meteorite impacts, in the 4 billion years since the solar system was formed. Callisto orbits just beyond Jupiter's main radiation belt.  Callisto is the outermost of the 'Galilean moons', so named because they were the original four moons of Jupiter discovered by Galileo (and independently by the scientist Marius) in 1610. The Galilean moons are also Jupiter's four largest moons, and can be seen, top to bottom, in the order of their distance from Jupiter, against the backdrop of the giant planet, in the picture at the right. They are Io, Europa, Ganymede and the furthest, Callisto. These moons are very large. The smallest of these, Europa, is the size of Earth's moon. Ganymede is the largest moon in the solar system. In fact, Ganymede with a diameter of almost 5000 kilometres is larger than the planets Mercury and Pluto! Jupiter's Great Red Spot appears in the background. This huge storm system (two to three Earths could fit inside it) has been raging in Jupiter's atmosphere for over 300 years. These photos were taken by the Voyager spacecraft in 1979 and by Galileo, which began exploring the Jovian system in 1995. 
Callisto is slightly smaller than Mercury, but has only a third of its mass, indicating that it is mostly rock and ice. Callisto has no large mountains, probably due to the fact that its surface is so icy. Craters and the fractures caused by meteorite impact are about the only features to be seen on Callisto. Some of the larger craters have been erased by movement of the crust over the past few billion years.
There is little evidence of tectonic activity on Callisto. This has puzzled scientists, since Callisto is very similar in makeup to Ganymede, but the latter moon seems to have a lot of activity under its surface. However, like Ganymede, Callisto's oldest craters have disappeared, or collapsed. There are no high crater-ring mountains or deep central depressions as there are in craters on the Moon and Mercury. Callisto may have a very thin atmosphere, perhaps composed of carbon dioxide. The spacecraft Galileo has also detected evidence of a weak magnetic field, which seems to indicate some sort of 'salty fluid' below the surface. 
Callisto has a low density (1.86 gm/cm3). It appears to be composed of a crust about 200 kilometers thick. Beneath the crust it is suggested that there may be a salty ocean layer that is more than 10 kilometers thick. Beneath that underground ocean layer, the interior seems to be composed of compressed rock and ice, with the proportion of rock increasing towards the centre. Callisto is about Meteorites have punctured holes in Callisto's crust, causing water to spread over the surface, and forming bright rays and rings around the craters. The idea that there may be water below Callisto's crust is explained by Dr. Margaret Kivelson, space physics professor at the University of California at Los Angeles. "The premise of an ocean came from studying the magnetic fields around Jupiter and its moons. It was found that Callisto's magnetic field varies (flows in various directions at different times) in response to the background magnetic field generated by Jupiter. The new Galileo data certainly suggest that something is hidden below Callisto's surface, and that something may very well be a salty ocean". 
The largest craters on Callisto are surrounded by concentric rings which look like huge cracks, and which have been smoothed out by billions of years of slow movement of ice. The largest of these has been named Valhalla (left). It has a bright central region that is 600 kilometers in diameter, and the entire crater is nearly 3000 km across. Valhalla is the result of a massive impact by a meteorite or small asteroid. (Another example is Caloris Basin on Mercury). Photos and some information from NASA Picture of the Day, as well as the excellent Views of the Solar System |