Jupiter is an amazing planet with sixty seven (as of 2014) confirmed moons. Callisto is one of its larger natural satellites, in fact bigger in size than the planet Mercury. NASA’s Voyager probe was sent to explore Jupiter and some of its moons and took this picture of Callisto in 1979.  The bright portion is believed to be an impact crater about 600 km across. Callisto was discovered by Galileo Galilei in 1610 and over time it has been determined that Callisto is mostly composed of rock and ices (water and ammonia frozen in ice form due to extreme temperatures of about 120 degrees Kelvin or -243.67 degrees Fahrenheit at its equator).