Saturn’s Small Moons
Saturn’s Small Moons
Epimetheus is an inner satellite of Saturn. It is also known as Saturn XI.
Epimetheus was the son of Iapetus and brother of Prometheus and Atlas; husband of Pandora. “Epimetheus” is Greek for “hindsight”.
Epimetheus was first 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.
The dark line across the surface in the Voyager image to the left is actually the shadow of the Saturn’s F-ring.
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.
Discovered by Mark R. Showalter in 1990 from Voyager photos taken in 1981; reconfirmed by images from Cassini in 2005.
Pan is the innermost moon of Saturn. It is a walnut-shaped small moon about 35 kilometres across and 23 km high that orbits within the Encke Gap in Saturn’s A Ring. Small moons near the rings produce wave patterns in the rings. Prior to the discovery of Pan, an analysis of the patterns in the edge of Saturn’s A ring predicted the size and location of a small moon. Pan was discovered by reexamining the 10 year old Voyager photos at the predicted spot.
Atlas is an inner satellite of Saturn. Atlas was discovered by Richard Terrile in 1980 from Voyager photos and was designated S/1980 S 28. Atlas seems to be a shepherd satellite of the A ring.
Prometheus is an inner satellite of Saturn. It was discovered in 1980 from photos taken by the Voyager 1 probe, and was provisionally designated S/1980 S 27. In late 1985 it was officially named after Prometheus, a Titan in Greek mythology.
Prometheus is the inner shepherd satellite of the F ring.
Prometheus has a number of ridges and valleys and several craters about 20 km in diameter but appears to be less cratered than the neighboring moon Pandora, Janus and Epimetheus.
From their very low densities and relatively high albedos, it seems likely that Prometheus, Pandora, Janus and Epimetheus are very porous icy bodies. (Note, however, that there is a lot of uncertainty in these values.)
The 1995/6 Saturn Ring Plane Crossing observations found that Prometheus was lagging by 20 degrees from where it should have been based on Voyager 1981 data. This is much more than can be explained by observational error. It is possible that Prometheus’s orbit was changed by a recent encounter with the F ring, or it may have a small companion moon sharing its orbit.
Pandora is an inner satellite of Saturn. It was discovered in 1980 from photos taken by the Voyager 1 probe, and was provisionally designated S/1980 S 26. In late 1985 it was officially named after Pandora from Greek mythology. Pandora is the outer shepherd satellite of the F ring. The image at right shows both Pandora, the faint F ring, Prometheus and part of the A ring. (Click the image for a much finer image from Cassini)Craters formed on this object by impacts appear to be covered by debris, a process that probably happens rapidly in a geologic sense. The grooves and small ridges on Pandora suggest that fractures affect the overlying smooth material.
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.
It was discovered by Smith, Reitsema, Larson and Fountain in 1980 from ground-based observations, and was provisionally designated S/1980 S 13
Calypso is a moon of Saturn. It was discovered in 1980, from ground-based observations, by Dan Pascu, P. Kenneth Seidelmann, William A. Baum, and Douglas G. Currie, and was provisionally designated S/1980 S 25.
Calypso is in Tethys’ trailing Lagrange point. Calypso and Telesto are among the smallest moons in the solar system.
16 Aug, 2013
Saturn’s Small Moons
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