Saturn's Other Icy Moons: Geologically Complex Worlds in Their Own Right
Abstract
Global mapping of the mid-sized icy moons of Saturn (besides Enceladus) at multiple wavelengths by the Cassini orbiter has revealed geologically complex worlds. These bodies exhibit diverse geological histories, much of which was not recognized by the Voyager reconnaissance. All the moons, except perhaps Iapetus, have been tectonically deformed and all but Iapetus reveal indications of higher heat flow in the past by virtue of viscously relaxed impact and tectonic features, or by relatively low-amplitude spheroidal global shapes that contrast sharply with cold and lumpy Iapetus. The tectonic signatures of these moons include relatively pristine and so presumably late-forming or reactivated rift systems on Tethys, Dione, and Rhea. These moons, as well as Mimas, also show more degraded and so likely older tectonic systems, indicating complex and protracted thermal histories. Impact craters dominate and are characterized by bowl shapes, hummocky floors, or prominent central peaks. Central pits are not observed (except probably at Odysseus on Tethys), and extensive ponded impact melt deposits are not evident. Giant Odysseus might have been responsible for Tethys' Ithaca Chasma rift system, although independent origins are not yet precluded. A cryptic red-stained fracture system on Tethys appears to be part of a long history of tectonism on that ice-rich moon. Putative volcanic resurfacing in the form of (relatively) smooth plains is limited in scope but appears to have affected nearly half of Dione and parts of Tethys. Any such terrains on the other moons have been erased by impact cratering. The origins of these terrains are as yet uncertain, but the erasure of older craters, flat-lying topography, and formation of two large irregular scarp-enclosed depressions in the center of smooth terrains on Dione tend to support a volcanic origin. An alternative explanation is that these terrains are remnants of an old apex-antapex cratering asymmetry that have been rotated 180° of longitude. The extent of smooth plains, relaxed craters, and tectonic patterns of various ages indicate that Dione has the most complex and perhaps most protracted thermal evolution of any of these moons, after Enceladus. The factor of 5 lower topographic amplitudes of the five inner moons indicates that a major heating event erased the ancient large basins and deep relief evident on outer moon Iapetus. Two satellites, Rhea and Iapetus, bear equatorial scars of the reaccretion of circumsatellite debris rings, in the form of discolorations on promontories and a large discontinuous equatorial ridge. Whether Saturn's satellite system is very young relative to the age of the solar system, as has been speculated, remains unclear, but the geologic record of these mid-sized bodies does not rule out this possibility.
- Publication:
-
Enceladus and the Icy Moons of Saturn
- Pub Date:
- 2018
- DOI:
- 10.2458/azu_uapress_9780816537075-ch012
- Bibcode:
- 2018eims.book..237S