Cassini Regio (Iapetus)

Italian-French astronomer who discovered Iapetus in 1671, Rhea in 1672, Tethys and Dione in 1684 (1625-1712).

Cassini Regio is the enigmatic dark area that covers the leading half of Saturn's moon Iapetus. The nature of the material that caused the extensive discoloration is not known, but it appears to be quite thin. It may be the result of cryo-vulcanism, a spattering of material from outer moons, or residue left by the sublimation of brighter ice. (wikipedia entry)

A regio is a large area marked by reflectivity or color distinctions from adjacent areas, or a broad geographic region.