Tethys

At 1,060 kilometres in diameter, Tethys is twice the size of Enceladus, but its bulk density is comparable. Its surface is uniformly bright, but it is not quite as reflective as Enceladus. It received fairly comprehensive imaging coverage, with only a section of the southern hemisphere being missed. A physiographic study identified several terrains.37 The oldest region is a hilly cratered terrain that is characterised by rugged topography. It is densely cratered, but most of the larger craters are degraded. They would appear to date from the post-accretional bombardment. As with Mimas, there is a vast impact crater on the leading hemisphere. However, while Odysseus, at 440 kilometres in diameter, is in similar proportion to Herschel, the fact that the moon is more than twice the diameter means that Mimas itself would fit comfortably within Odysseus's rim. Although Odysseus is large, it is nevertheless very shallow, and its floor actually follows the curvature of the moon. This may be the result of isostatic

On a body the size of Tethys, the 440-kilometre-diameter crater Odysseus is better described as a 'basin'. Over time, its floor has risen to restore the moon's spherical surface. Ithaca Chasma follows a 'great circle' corresponding to a 'pole' centered on Odysseus. The view (left) shows the region antipodal to that massive impact.

On a body the size of Tethys, the 440-kilometre-diameter crater Odysseus is better described as a 'basin'. Over time, its floor has risen to restore the moon's spherical surface. Ithaca Chasma follows a 'great circle' corresponding to a 'pole' centered on Odysseus. The view (left) shows the region antipodal to that massive impact.

adjustment of the icy lithosphere soon after the crater was formed. The rim has been softened by a series of terraces and the central peak has degraded into a ring-like structure. The antipodal area could reasonably be expected to have been torn up by the focusing of seismic shock waves, but it is less rugged, forming a plain displaying faint lineations and albedo variations. It is possible that the eruption of material that formed the plain was prompted by the impact.38 Any antipodal relationship is speculative, however.

Tethys's next most spectacular feature is Ithaca Chasma, a vast canyonland that winds almost all the way around the globe. This narrow branching terraced trough system is at least 1,000 kilometres long, 100 kilometres wide, and up to 3 kilometres deep.39 Sections of its rim are raised 500 metres above the adjacent terrain. While the canyon system may be related to the Odysseus impact, it does not actually encroach upon the crater. In fact, if the crater's central peak is considered to be a 'pole', the chasm tends to trace the great circle of the associated 'equator'. It has been suggested that the Odysseus impact occurred when the moon's interior was still 'soft' and the force of the impact temporarily deformed the satellite into an ellipsoidal shape, inducing transient tensional stresses sufficient to crack the surface almost all the way around.40 Because the chasm is so wide, it is possible to compare the cratering inside it with that on the adjacent terrain.41 This showed that the chasm's formation was contemporary with the post-accretional bombardment, and may indeed have been formed as a result of the Odysseus impact.

The objects in the Lagrangian points of the inner moons may be fragments from 'parent' bodies that were locked in position as their shattered parents re-accreted most of their material. If so, then Dione and Tethys, which have these companions, must have been totally disrupted. However, since some areas of these moons seem to date to the post-accretional bombardment, this would imply that the companions, if they were created in this way, must have been present since that time.42

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