Jupiter and its moons. Computer artwork of a view across the surface of Themisto towards Jupiter (upper centre) and some of its other satellites. From left to right: Ganymede, Europa, Io, and on the far right Callisto. Little is known about Themisto. With a mean diameter of only 5 miles and an albedo (surface brightness) about half that of the Moon, no earthbound telescope or interplanetary probe has yet revealed any details of its surface. It is far too small to host an atmosphere, and too small even for its own gravity to pull it into a spherical shape.