11th Largest Moon of Saturn - Epimetheus (6th Moon outwards from Saturn)

Epimetheus is an inner moon of Saturn and it is co-orbital with another moon Janus.




Epimetheus is the 11th largest moon of Saturn, and the 44th largest moon in the Solar System currently known.





Discovery

Epimetheus occupies practically the same orbit as the moon Janus. This caused some confusion for astronomers, who assumed that there was only one body in that orbit, and for a long time struggled to figure out what was going on. It was eventually realized that they were trying to reconcile observations of two distinct objects as a single object.

Audouin Charles Dollfus observed a moon on December 15, 1966, which he proposed to be named "Janus". On December 18, Richard Walker made a similar observation which is now credited as the discovery of Epimetheus. However, at the time, it was 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. This was confirmed in 1980 by Voyager 1, and so Larson and Fountain officially share the discovery of Epimetheus with Richard Walker.

Naming

Epimetheus received its name in 1983, when it is named after the mythological Epimetheus, brother of Prometheus.

In Greek mythology, Epimetheus ("hindsight", literally "afterthought,") was the brother of Prometheus ("foresight", literally "fore-thought"), a pair of Titans who "acted as representatives of mankind". They were the inseparable sons of Iapetus, who in other contexts was the father of Atlas. While Prometheus is characterized as ingenious and clever, Epimetheus is depicted as foolish.

Stats

Diameter (mean): 116 km

Semi-major axis: 151,410 km

Orbital Period: 0.694 days

Co-orbital moons

Epimetheus and Janus share their orbits, the difference in semi-major axes being less than either's mean diameter. This means the moon with the smaller semi-major axis will slowly catch up with the other. As it does this, the moons gravitationally tug at each other, increasing the semi-major axis of the moon that has caught up and decreasing that of the other.

This reverses their relative positions (proportionally to their masses) and causes this process to begin anew with the moons' roles reversed. In other words, they effectively swap orbits, ultimately oscillating both about their mass-weighted mean orbit.

Epimetheus rotates synchronously with its orbital period, keeping one face pointed toward Saturn.

Formation

Epimetheus and Janus may have formed from a disruption of a single parent to form co-orbital satellites, but if this is the case the disruption must have happened early in the history of the satellite system.

Physical characteristics

There are several Epimethean 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.

The south pole shows what might be the remains of a large impact crater covering most of this face of the moon, and which could be responsible for the somewhat flattened shape of the southern part of Epimetheus.

There appear to be two terrain types: darker, smoother areas, and brighter, slightly more yellowish, fractured terrain. One interpretation is that the darker material evidently moves down slopes, and probably has a lower ice content than the brighter material, which appears more like "bedrock". Nonetheless, materials in both terrains are likely to be rich in water ice.

From its very low density and relatively high albedo, it seems likely that Epimetheus is a very porous icy body. There is a lot of uncertainty in these values, however, and so this remains to be confirmed.

Ring

A faint dust ring is present around the region occupied by the orbits of Epimetheus and Janus, as revealed by images taken in forward-scattered light by the Cassini spacecraft in 2006.

The ring has a radial extent of about 5000 km. Its source is particles blasted off the moons' surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.