Callisto is the outermost of Jupiter's large moons. Only slightly smaller than the planet Mercury, (roughly 3000 miles in diameter) Callisto is the most heavily cratered moon in our solar system.

With the oldest surface in our solar system, Callisto's exterior still displays the elements it was born with over four billion years ago.

Though the geology of Callisto remains a mystery, there is evidence that below its ancient, icy surface may lay a narrow salty ocean.