Unexpected variety among small inner satellites of Saturn

Post by Peter Thomas, Cornell University, Ithaca, New York

 Small satellites (< 150 km mean radius) usually resemble potatoes. Their irregular shapes are formed by a history of impact cratering without the benefit of internally-driven processes of volcanism, tectonics, or atmospheric effects (Castillo-Rogez et al., 2012).  During its 9 years orbiting Saturn, the Cassini spacecraft has shown that the small satellites orbiting close to Saturn have a variety of shapes, most of which deviate from the expected familiar battered potato appearance.  These objects are likely dominated by water ice as determined from mean densities and spectroscopy (Thomas et al., 2010; Buratti et al. 2010).  Satellites within rings have equatorial ridges (Charnoz et al. 2007; Porco et al., 2007).  Others, such as Janus and Epimetheus, the “co-orbitals” are almost lunar-like in appearance, close to the expected potato variety.

Image 1: Best available view of Helene. N1687119756, UV3 filter, phase = 97°, sub-spacecraft point is 2.7°N, 124.8°W.  North is down in this presentation.  Taken June 18, 2011.

Image 1: Best available view of Helene. N1687119756, UV3 filter, phase = 97°, sub-spacecraft point is 2.7°N, 124.8°W. North is down in this presentation. Taken June 18, 2011.

Two sets of small satellites stand out: ones that reside in ring arcs or in whole rings: Methone, Aegeon, and Pallene (Hedman et al., 2009).  These are smooth, ellipsoidal shapes, unlike any other small objects so far imaged.  The smooth surface of Methone (Image 2) may have acted as a fluid on geologic times scales; if so, the mean density is indicated by the shape to be about 300 kg m-3.

Other truly distinctive satellites are Helene, Calypso and Telesto, orbiting at the same distance as Tethys and Dione, the “Trojans.”  These are distinguished by thick coverings of material that is subject to long-distance downslope transport (Image 1).  The darker, higher residual mesas in Image 2 and the smoother surface of material moving downslope indicate a history of deposition followed by (ongoing?) erosion. Other images of this body indicate this is a nearly global characteristic.

The mechanisms responsible for the smoothing and for transport of material across the surfaces of these objects are not known, though seismic shaking from impacts, electrostatic effects, or even thermal cycling might play roles.

Image 2: Methone image N1716192103, phase angle is 63°, taken May 20, 2012.

Image 2: Methone image N1716192103, phase angle is 63°, taken May 20, 2012.

References

Buratti, B.J., et al., 2010. Cassini spectra and photometry 0.25-5.1 µm of the small inner satellites of Saturn. Icarus 206, 524-536.

Castillo-Rogez, J.C., Johnson, T.V., Thomas,P. C., Choukroun, M., Matson,D.L.,  Lunine, J.I., 2012. Geophysical evolution of Saturn’s satellite Phoebe, a large planetesimal in the outer solar system. Icarus 219, 86-109.

Charnoz, S., Brahic, A., Thomas, P.C., Porco, C.C., 2007. The Equatorial Ridges of Pan and Atlas: Terminal Accretionary Ornaments? Science 318, 1622-1624.

Hedman, M.M., Murray, C.D., Cooper, N.J., Tiscareno, M.S., Beurle, K., Evans, M.W., Burns, J.A., 2009. Three tenuous rings/arcs for three tiny moons. Icarus 199, 378-386.

Porco, C.C., Thomas, P.C., Weiss, J.W., Richardson, D.C., 2007. Saturn’s Small Inner  Satellites: Clues to Their Origins. Science 318, 1602.

Thomas, P.C., 2010. Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission. Icarus 208, 395-401.

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