Saturn’s Siamese twin moons

Saturn's moon Epimetheus

Saturn's moon Epimetheus, as seen by the Cassini spacecraft narrow-angle camera from a distance of approximately 37,400 kilometres. A large crater dominates one half of the 116km-wide moon.

  • Epimetheus and Janus circle Saturn at almost the same distance
  • Every four years they swap orbits with each other
  • Both moons are probably loose piles of rocky, icy rubble

Epimetheus (pronounced ep-ee-MEE-thee-us) and the neighbouring moon Janus have been referred to as the Siamese twins of Saturn because they circle Saturn in nearly the same orbit. This co-orbital condition (also called a 1:1 resonance) confused astronomers, who at first could not believe that two moons could share nearly identical orbits without colliding.

The two moons lie amongst Saturn’s rings at a distance from Saturn of roughly 151,500 kilometres (94,100 miles). One moon orbits 50 kilometres (31 miles) higher (farther away from the planet) and consequently moves slightly slower than the other. The slight velocity difference means the inner satellite catches up to the other in approximately four Earth years.

When this happens, the gravitational interaction between the two pulls the inner moon 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, and the nearest they approach is within 15,000 kilometres (6,200 miles).

At their most recent trade in early 2010, Epimetheus’ orbital radius dropped by approximately 80 kilometres (50 miles) while Janus’ orbit increased by only approximately 20 kilometres (12.4 miles). Janus’ orbit changed only a quarter of that of Epimetheus because Janus is four times more massive than Epimetheus.

Both of the moons are “phase locked” with Saturn, which means that one side always faces toward the planet. And being so close to Saturn, they orbit around it in less than 17 hours.

Saturn's moon Epimetheus

The Cassini spacecraft snapped this image of Epimetheus from a distance of about 107,000 kilometres.

Rubble piles

Epimetheus and Janus may have formed by the break-up of one moon. If so, it would have happened early in the life of the Saturnian system because 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.

They are both thought to be composed of 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 together loosely 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 -195 degrees Celsius (-319 degrees Fahrenheit). Their reflectivity (or albedo) of 0.7 to 0.8 in the visual range again suggests a composition largely of water ice.

Epimetheus has several craters larger than 30 kilometres, including Hilaeira and Pollux.

Saturn's moon Janus

Janus, seen here, swaps orbits with Epimetheus every four Earth years.


French astronomer Audouin Dollfus spotted a moon of Saturn on December 15, 1966, for which he proposed the name “Janus.” On December 18 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 realised that the 1966 observations were best explained by two distinct objects (Janus and Epimetheus) sharing very similar orbits. Observations by the Voyager I spacecraft confirmed this in 1980, and so Larson and Fountain officially share the discovery of Epimetheus with Walker.

The Cassini spacecraft has made several close approaches and provided detailed images of the moon since it achieved orbit around Saturn in 2004.

Nineteenth-century English astronomer John Herschel suggested that the moons of Saturn be associated with mythical brothers and sisters of Kronus, known to the Romans as Saturn. (The International Astronomical Union now controls the official naming of astronomical bodies.)

The name Epimetheus comes from the Greek god (or titan) Epimetheus (or hindsight) who was the brother of Prometheus (foresight). Together, they represented humanity. The craters on Epimetheus include Hilaeira (who was a priestess of Artemis and Athena) and Pollux (who was a warrior in The Illiad and who carried off Hilaeira).

Astronomers also refer to Epimetheus as Saturn XI and as S/1980 S3, and they refer to Janus as Saturn X and as S/1980 S1.

Epimetheus data:

  • Discovered: 1966 by R. Walker
  • Distance from Saturn: 151,422 km
  • Period of orbit around Saturn: 16.7 hours
  • Diameter: 138 x 110 x 110 km
  • Mass: 5.3 x 10^17 kg

Adapted from information issued by NASA / JPL / Space Science Institute.

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