Small Impact Crater Populations on Saturn's Moon Tethys and Implications for Source Impactors in the System

Abstract

Current estimates place the ages of the inner Saturnian satellites (Mimas, Enceladus, Tethys, Dione, and Rhea) between 4.5 Gyr and 100 Myr. These estimates are based on impact crater measurements and dynamical simulations, both of which have uncertainties. Models of satellite evolution are inherently simplified and rely on uncertain or unknown parameters, which are often difficult to verify, whereas the interpretations of crater densities depend on the source populations of impactors, which are not well‐constrained in the outer solar system. We investigate the cratering history of Tethys, mapping the population of small impact craters, to determine the roles that planetocentric, heliocentric, or other impact debris play in its cratering record. To map the surface of Tethys, we chose five regions that were located in geographically distinct areas and had high‐resolution (~150 m/pix) image coverage by the Cassini ISS camera. We studied all craters that had at least 7 pixels across but mapped down to 5 pixels for completeness in the crater counts. We observe an abundance of small craters (D < 3 km) in the oldest region; this does not appear to be due to secondary cratering effects from the Odysseus impact basin. Fitting the production functions from Zahnle et al. (2003, https://doi.org/10.1016/S0019‐1035(03)00048‐4), we find that neither their Case A nor Case B scenarios align with the observed cratering record at Tethys. We conclude that in addition to the standard outer solar system impactor populations, there is a Saturn‐centric impactor source that is cratering Tethys.

Regional Impact Crater Mapping and Analysis on Saturn's Moon Dione and the Relation to Source Impactors

“…For Region 4, Case A roughly fits between 3 and 7 km, then Case B fits once more from 7 to ∼20 km (within error) where it then slightly changes slope in a way that the crater data does not. Similar to what was observed on Tethys (as reported in Ferguson et al., 2020), we see that neither of the described production functions fully match the SFDs of the cratering record on Dione.…”

“…Based on the CSFD's we find that the crater counts are complete down to 1.39 km on average where the roll‐off (a horizontal flattening of the SFD) is observed on the plot in Figure 7a. Craters larger than 20 km are observed more frequently in these areas than on Tethys (Figure 10 this paper and Ferguson et al., 2020), likely due to larger counting areas (average areas: 55,700 km 2 for Tethys vs. 127,900 km 2 for Dione), which allowed for more larger craters to be fully contained within an image/mosaic on Dione. Total numbers of these larger craters are relatively lower when compared with other mapped terrains within the Saturn system (Kirchoff & Schenk, 2010), likely agreeing with their finding that there is a dearth of larger craters ( D ∼80 km) across both Mimas and Dione when compared to crater distributions from Rhea and Iapetus (Kirchoff & Schenk, 2010).…”

“…It is also necessary to consider the influence of secondary craters when evaluating the cratering populations on the Saturnian satellites (Bierhaus et al., 2012, 2018). Prior examination of the cratering record on Saturnian satellites has shown that, while heliocentric impactors are likely present in the crater records, a planetocentric source must also have contributed to the observed crater size‐frequency distributions (Bell, 2020; Ferguson et al., 2020; Hirata, 2016; Kirchoff & Schenk, 2009, 2010, 2015).…”

“…Our primary investigation is how the observed size‐frequency distributions (SFDs) of craters vary with respect to surface units on Dione and what they can reveal about the most likely dominant impactor source at Dione. Additionally, we evaluate how much the crater SFDs change with increasing distance from Saturn by comparing our regional crater counts for Dione to our previously published work on Tethys (Ferguson et al., 2020). We then discuss how these analyses lead to new constraints on the overall characteristics of impactor populations in the Saturn system.…”

Crater Distributions of Uranus's Mid-sized Satellites and Implications for Outer Solar System Bombardment

“…However, even this may not completely resolve what has impacted outer solar system satellites, as this will only provide a snapshot of the current (0 to ∼3 Ga) heliocentric impactor population and not show what this or the other possible populations may have looked like in the very early solar system (see Morbidelli & Nesvorný 2020, for a review of dynamical and collisional evolution of the Kuiper Belt). Work should also continue in trying to understand bombardment in the Saturn system, in particular, due to its unusual dynamical history and configuration (e.g., Alvarellos et al 2005;Dones et al 2009;Bierhaus et al 2012;Ćuk et al 2016;Kirchoff et al 2018;Ferguson et al 2020;Morbidelli & Nesvorný 2020).…”