Janus (pronounced JAY nuss; adjective: Janian) and the neighboring moon Epimetheus have been referred to as the Siamese twins of Saturn because these two moons orbit Saturn in nearly the same orbit. This co-orbital condition (also called 1:1 resonance) confused astronomers, who at first could not believe that two moons could share nearly identical orbits without colliding.
Janus and Epimetheus may have formed by the break-up of one moon. If so, it would have happened early in the life of the Saturn system since both moons have ancient cratered surfaces, many with soft edges because of dust. They also have some grooves (similar to grooves on the Martian moon Phobos) suggesting some glancing blows from other bodies. Together, the moons trail enough particles to generate a faint ring. However, except for very powerful telescopes, the region of their common orbit appears as a gap between Saturn's prominent F and G rings.
These two moons lie amongst Saturn's rings and have orbital radial distances from Saturn of roughly 151,500 km (94,100 miles). One moon orbits 50 km (31 miles) higher and consequently moves slightly slower than the other. The slight velocity difference means the inner moon catches up to the other in approximately four Earth years. At that time, the gravity interaction between the two pulls the inner moon faster, moving it to a higher orbit. At the same time, the catching-up inner moon drags the leading outer moon backward so that it drops into a lower orbit. The result is that the two exchange places; the nearest they approach is within 15,000 km (6,200 miles).
Janus and Epimetheus are the fifth and sixth moons going out from Saturn. Both are phase locked with their parent; one side always faces toward Saturn. Being so close, they orbit in less than 17 hours. They are both thought to be composed largely of water ice, but their density of less than 0.7 is much less than that of water. Thus, they are probably "rubble piles" -- each a collection of numerous pieces held loosely together by gravity. Each moon has dark, smoother areas, along with brighter areas of terrain. One interpretation of this is that the darker material evidently moves down slopes, leaving shinier material such as water ice on the walls of fractures. Their temperature is approximately -319 degrees Fahrenheit (-195 degrees Celsius). Their reflectivity (or albedo) of 0.7 to 0.8 in the visual range, again, suggests a composition largely of water ice.
Two named features of Janus include one crater named Castor (a warrior in "The Iliad"), and one crater named Phoebe (a priestess of Artemis and Athena, who was carried off by Castor and his brother Pollux). The two craters Idas and Lynceus are named for two brothers who were rivals with Castor and Pollux for Phoebe and another priestess. (Idas was also one of the Argonauts who sailed with Jason in the quest for the Golden Fleece.)
Janus' mean diameter of 179 km (111 miles) comes from its potato-shaped dimensions of 196 x 192 x 150 km (122 x 119 x 93 miles), respectively.
Audouin Dollfus observed a moon on 15 December 1966, for which he proposed the name "Janus." On 18 December of the same year, Richard Walker made a similar observation, now credited as the discovery of Epimetheus. At the time, astronomers believed that there was only one moon, unofficially known as "Janus," in the given orbit. Twelve years later, in October 1978, Stephen M. Larson and John W. Fountain realized that the 1966 observations were best explained by two distinct objects (Janus and Epimetheus) sharing very similar orbits. Voyager I confirmed this in 1980.
The Cassini spacecraft has made several close approaches and provided detailed images of Janus since Cassini achieved orbit around Saturn in 2004.
How Janus Got its Name:
The name Janus comes from the Roman god of gates, doors, doorways, beginnings, and endings. He is usually represented as having one face to look forward and another to look back. The Romans may have partially adopted Janus from an Etruscan god, from the Greek god Hermes, or from both. His most apparent remnants in modern English are the month of January and the caretaker of doors and halls, the janitor.
Astronomers also refer to Janus as Saturn X and as S/1980 S1.