Callisto is the third largest moon in the solar system, narrowly beaten out only by Ganymede (at Jupiter) and Titan (at Saturn). It is the outermost of the 4 Galilean satellites of Jupiter, with a mean distance of 26.6 Jupiter Radii, and it orbits Jupiter once every 16.7 days. Despite its larger size, its low density (1.86 g/cm^3, the lowest of the 4 large moons) gives its surface a gravity of 0.127g, smaller even than little Europa! (Surface gravity scales as density times radius.)

(Original Caption with Image): Jupiter's icy moon Callisto is shown in approximate natural color (left) and in false color to enhance subtle color variations (right). This image of Callisto's Jupiter-facing hemisphere shows the ancient, multi-ring impact structure Valhalla just above the center of the image. Valhalla, possibly created by a large asteroid or comet which impacted Callisto, is the largest surface feature on this icy moon. Valhalla consists of a bright inner region, about 600 kilometers (360 miles) in diameter surrounded by concentric rings 3000 to 4000 kilometers (1800-2500 miles) in diameter. The bright central plains were possibly created by the excavation and ejection of 'cleaner' ice from beneath the surface, with a fluid-like mass (impact melt) filling the crater bowl after impact. The concentric rings are fractures in the crust resulting from the impact.

The false color in the right image shows new information, including ejecta from relatively recent craters, which are often not apparent in the natural color image. The color also reveals a gradual variation across the moon's hemisphere, perhaps due to implantation of materials onto the surface from space.

More on Callisto can be found here.