Naoyuki Hirata

Differential impact cratering of Saturn's satellites by heliocentric impactors

Saturnian satellites are thought to have been struck by two different types of impactors: those with heliocentric origins and those with planetocentric origins. Many of the impacts are suggested to come from planetocentric debris, while many crater count studies assume an ecliptic comet origin when determining the ages of the surfaces. To assess the contribution of planetocentric impactors, this study examines the global distribution and apex-antapex asymmetry of impact craters on Rhea and Iapetus. The results demonstrate that the craters of Rhea (more than 20 km in diameter) and Iapetus (more than 30 km in diameter) show an apex-antapex asymmetry. This suggests that most of the large craters are formed from heliocentric impacts. In contrast, the craters less than 20 km in diameter seem to show no asymmetry. Possible explanations for this are either planetocentric impactor origins or saturation with impact craters.

Particle deposition on the saturnian satellites from ephemeral cryovolcanism on Enceladus

The geologically active south pole of Enceladus generates a plume of micron-sized particles, which likely form Saturns tenuous E-ring extending from the orbit of Mimas to Titan. Interactions between these particles and satellites have been suggested, though only as very thin surficial phenomena. We scrutinize high-resolution images with a newly developed numerical shape model of Helene and find that the leading hemisphere of Helene is covered by thick deposits of E-ring particles, which occasionally collapse to form gully-like depressions. The depths of the resultant gullies and near-absence of small craters on the leading hemisphere indicate that the deposit is tens to hundreds of meters thick. The ages of the deposits are less than several tens of My, which coincides well with similar deposits found on Telesto and Calypso. Our findings as well as previous theoretical work collectively indicate that the cryovolcanic activity currently occurring on Enceladus is ephemeral.

Timing of the faulting on the Wispy Terrain of Dione based on stratigraphic relationships with impact craters

The trailing hemisphere of Dione is characterized by the Wispy Terrain, where it exhibits a hemispheric-scale network of extensional tectonic faults superposed on the moons cratered surface. The faults likely reflect past endogenic activity and Diones interior thermal history. Although fresh exposures of pristine scarps indicate that the timing of the faulting is relatively recent, the absolute age of the faulting remains uncertain. To estimate the timing of the faulting, we investigated stratigraphic relationships between impact craters and faults. Using high-resolution images obtained by ISS cameras onboard the Cassini spacecraft, we investigated craters with diameters exceeding or equal to 10 km that coincide spatially with the faults, and classified the craters as crosscut craters or superposed craters. As a result, at least 82% of the craters were interpreted as clear examples of crosscut craters and 12% of the craters were interpreted to be candidates of superposed craters, although stratigraphic relationships are often ambiguous. The paucity of superposed craters and a predicted cratering rate indicate that the faulting of the Wispy Terrain is 0.30–0.79 Ga. If 12–18% of the craters are assumed to be superposed, the timing of the faulting could be in the range 0.30–0.79 Ga. However, it is possible that the faulting of the Wispy Terrain is still ongoing.