As measured from Saturn's centre, the outer edge of the main ring system is 2.3 planetary radii, the slender 'G' ring lies at 2.9 and Mimas is just beyond, at 3.1, which is sufficiently far outside the Roche radius to be safe from tidal disruption. At 390 kilometres in diameter, it is the smallest of the historically known satellites. Its rotation is synchronous. One study of gravitational focusing suggested that Mimas should be the most heavily cratered member of Saturn's retinue, with a preference for strikes on its leading hemisphere.22

Voyager 1 was able to view most of the side that faces away from the planet and

The Voyagers provided our first views of the major Saturnian satellites. In order of distance from the planet, and to scale, they are Mimas (top left), Enceladus, Tethys, Dione, Rhea, Hyperion, Iapetus and Phoebe. Notice that Mimas would fit into the giant crater Odysseus on Tethys.

Two views of Mimas, showing (right) the crater Herschel on the leading hemisphere and (left) the south polar region with the pole on the terminator. North is towards the top in each case. Notice the curvilinear feature crossing the southern hemisphere.

much of the southern part of the near side with a resolution of a few kilometres per pixel. It certainly lived up to the stereotype of an ancient battered icy moon, in that it is heavily cratered. The cratering is not uniform, though. Most of the craters form deep cavities. The larger craters tend to have central peaks, and their ejecta blankets are indistinct, which suggests that on a small body with weak gravity the material was distributed far and wide. The most prominent crater, appropriately named Herschel, is on the leading hemisphere. At about 130 kilometres across, it spans fully one-third of Mimas's diameter. Its wall rises about 5 kilometres from the surrounding terrain, its floor is depressed 10 kilometres, and its well-defined central peak rises 6 kilometres. It is remarkable, therefore, that the moon survived the impact. Mimas's cratered surface has been transected by several prominent chasms up to 10 kilometres across, several kilometres in depth and 100 kilometres in length,





Saturn's moonlets as seen by Voyager 2. They are shown (working left to right) in order of distance from the planet and to scale. Ranging from a few dozen to a few hundred kilometres in size, the irregularly-shaped moonlets are either loose accretions of ice or fragments of bodies which were shattered by collisions.

whose origin may be related to the Herschel impact.23 In terms of the impactor populations, none of its craters, not even Herschel - which is large only in proportion - dates back to the post-accretional bombardment. This suggests that the impacts by the interlopers were devastating. E.M. Shoemaker concluded that Mimas must have been shattered and re-accreted several times.24 The inner moonlets have albedos and spectra that resemble those on Mimas, so they would seem to be water ice, and their irregular shapes suggest that they are either loose accretions of ice or fragments from shattered bodies. A few of them might even be fragments of an earlier 'Mimas' that failed to re-accrete. The presence of craters on some of these inner moonlets indicates that their surfaces have been exposed for at least several billion years.

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