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 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...

's moon

Iapetus In Greek mythology, Iapetus (; ; ), also Japetus, is a Titan, the son of Uranus and Gaia and father of Atlas, Prometheus, Epimetheus, and Menoetius. He was also called the father of Buphagus and Anchiale in other sources. Iapetus was linked ...

. It is named after

Giovanni Cassini Giovanni Domenico Cassini (8 June 1625 – 14 September 1712) was an Italian-French mathematician, astronomer, astrologer and engineer. Cassini was born in Perinaldo, near Imperia, at that time in the County of Nice, part of the Savoyard sta ...

, 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 celestial bodies such as planets and their moons, asteroids, comets, and meteorites. Although the geo- prefix t ...

for a large area that is strongly differentiated in colour or

albedo Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...

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

. 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 referred to as an ice volcano) is a type of volcano that erupts gases and volatile material such as liquid water, ammonia, and hydrocarbons. The erupted material is collectively referred to as ''cryolava''; ...

, 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 agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

''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 {{Saturn-stub