Saturn's Moons

Epimetheus (Saturn XI)

Epimetheus ( "ep eh MEE thee us" ) is the fifth of Saturn's known satellites.

orbit:151,422 km from Saturn
diameter:115 km (144 x 108 x 98)
mass:5.6e17 kg
Observed by:Walker in 1966. But the situation was confused since Janus is in a very similar orbit. So Walker officially shares the discovery of Epimetheus with Fountain and Larson who showed in 1977 that there were two satellites involved. The situation was clarified in 1980 by Voyager 1. Epimetheus and Janus are "co-orbital".

There are several craters larger than 30 km in diameter as well as both large and small ridges and grooves. The extensive cratering indicates that Epimetheus must be quite old.

Janus (Saturn X)

Janus ( "JAY nus" ) is the sixth of Saturn's known satellites.

Orbit:151,472 km from Saturn
Diameter:178 km (196 x 192 x)
Mass:2.01e18 kg
Discovered by:the French astronomer Audouin Dollfus in 1966.

Dollfus is credited with the discovery of Janus but it's not really certain whether the object he saw was Janus or Epimetheus and his observations led to a spurious orbit. (Walker discovered it independently but his telegram arrived a few hours after Dollfus').

Larson and Fountain determined in 1978 that there are in fact two moons at about 151000 km from Saturn. This was confirmed in 1980 by Voyager 1.

Janus and Epimetheus are "co-orbital". The orbital radii of Janus and Epimetheus differ by only 50 km, less than the diameter of either. Their orbital velocities are thus very nearly equal and the lower, faster one slowly overtakes the other. As they approach each other they exchange a bit of momentum the end result of which is to boost the lower one into a higher orbit and to drop the higher one to a lower orbit. They thus exchange places.

The exchange takes place about once every four years. The orbital data given here is as of the time of the Voyager encounters. Janus is extensively cratered with several craters larger than 30 km but few linear features. Its surface appears to be older than Prometheus' but younger than Pandora's.

Mimas (Saturn I)

Mimas ("MY mas") is the seventh of Saturn's known satellites.

orbit:185,520 km from Saturn
diameter:392 km
mass:3.80e19 kg
Discovered in :1789 by Herschel.

Mimas' low density (1.17) indicates that it is composed mostly of water ice with only a small amount of rock.
The surface of Mimas is dominated by an impact crater 130 km across, known as Herschel; it's almost 1/3 of the diameter of the entire moon. Herschel's walls are approximately 5 km high, parts of its floor measure 10 km deep, and its central peak rises 6 km above the crater floor. The impact that made this crater must have nearly disrupted Mimas. Fractures can be seen on the opposite side of Mimas that may be due to the same impact.

The surface is saturated with impact craters. But no others are nearly as large as Herschel. This suggests that early in its history, Mimas was probably impacted by even larger bodies than the one that created Herschel which completely disrupted the new moon (wiping out the evidence of earlier large impacts) but that the impact debris then coalesced again to form present-day Mimas.

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Last Modified : 5 Sep. 2001
Created By#C0115361 Team