Cassini Regio (adjective ''Cassinian'' ) is the enigmatic dark area that covers the leading half of

Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...

's moon

Iapetus In Greek mythology, Iapetus (; ; grc, Ἰαπετός, Iapetós), also Japetus, is a Titan, the son of Uranus and Gaia and father of Atlas (mythology), Atlas, Prometheus, Epimetheus (mythology), Epimetheus, and Menoetius (mythology), Menoetius. ...

. It is named after Giovanni Cassini, the discoverer of Iapetus; ' Regio' is a term used in

planetary geology Planetary geology, alternatively known as astrogeology or exogeology, is a planetary science discipline concerned with the geology of the celestial bodies such as the planets and their moons, asteroids, comets, and meteorites. Although the ge ...

for a large area that is strongly differentiated in colour or

albedo Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refle ...

from its surroundings. The brighter half of Iapetus is composed of the

Roncevaux Terra Roncevaux Terra is the name given to the northern part of the highly reflective side of Saturn's moon Iapetus. The southern half of this side is named '' Saragossa Terra''. The other half of Iapetus, named Cassini Regio, is extremely dark. It is ...

and the

Saragossa Terra Saragossa Terra is the name given to the southern part of the highly reflective half of Saturn's moon Iapetus. It is bordered on the north by Roncevaux Terra, and on both the east and west sides by Cassini Regio. The largest named crater in Sar ...

. The nature of the material that caused the extensive discoloration of Cassini Regio is not known, but it appears to be quite thin. It may be the result of

cryovolcanism A cryovolcano (sometimes informally called an ice volcano) is a type of volcano that erupts volatiles such as water, ammonia or methane into an extremely cold environment that is at or below their freezing point. The process of formation is known ...

, a spattering of material from outer moons, or residue left by the sublimation of brighter ice. In December 2004 an image taken by the

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedin ...

''Cassini'' probe revealed that a long ridge of high mountains runs down the centre of Cassini Regio, almost perfectly following Iapetus's equator. The origin of this extremely unusual feature is as yet unknown. ''Cassini'' completed a flyby of Iapetus in September 2007, and the images it has transmitted are helping to clarify the nature of Cassini Regio. For more details, see the main article on

References

Surface features of Iapetus (moon) Regio {{Space-stub