Callisto is the outermost Galilean satellite of Jupiter but still twice as bright as Earth's Moon.

Callisto is the outermost Galilean satellite of Jupiter and the darkest of the four, but still twice as bright as Earth's Moon. Mottled appearance from bright and dark patches; bright ones look like rayed or brite craters on our Moon. This face of Callisto is always turned toward Jupiter. Photo taken through violet filter.