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

Abstract

Recent dynamical modeling of the formation and evolution of the Saturnian satellites suggests that the ages of the mid‐sized inner moons (Mimas, Enceladus, Tethys, Dione, and Rhea) could be as young as 100 Myr. This estimate is in contrast to most previous modeling and observational work that suggest an age more contemporaneous with the formation of Saturn 4.5 Ga ago. Given the heritage of using craters to constrain surface ages, we examine the impact craters of Dione using imagery from NASA's Cassini ISS camera and analyze their size‐frequency distributions (SFDs) to understand impactor populations. We survey four areas across different geologic terrains and compare our crater counts to standard outer solar system production functions. In addition to crater counts, we study several crater types such as elliptical and polygonal to further examine the bombardment source for the craters. We find evidence for a Saturn‐specific planetocentric impactor source, as none of the standard production functions fit the data. We compare our Dione data with our work on Tethys and find similarly shaped SFDs between the satellites. However, Dione's surface has been extensively modified to remove smaller craters (D ∼ <5 km) and has been bombarded by larger impactors, creating more D ∼ >20 km on Dione than Tethys. In contrast, Tethys more generally represents an ancient unmodified surface within the Saturn system. More complete observations and assessment of the cratering records on the satellites of Uranus and Neptune's moon Triton would enable better constraints on the bombardment history of the Saturn system.

Tracking the Evolution of an Ocean Within Mimas Using the Herschel Impact Basin

“…Hence, it is challenging to produce Herschel with a heliocentric impactor without completing destroying the moon. Second, mapping and analysis of craters across the mid‐sized moons has shown evidence of abundant planetocentric material (Ferguson, Rhoden, & Kirchoff, 2022; Ferguson, Rhoden, Kirchoff, & Salmon, 2022; Ferguson et al., 2020 and references therein), making it reasonable to consider such a source for the Herschel‐forming impactor. Third, while there is a great deal of uncertainty regarding the age of Herschel (see discussion in Kirchoff et al., 2018), due in large part to uncertainties in the size frequency distribution of planetocentric material at Saturn, it is estimated to be <1 Ga and could be substantially younger.…”

“…The age of Herschel has been estimated at <1 Ga (Kirchoff et al., 2018). However, the abundance of planetocentric material within the Saturn system makes age estimates, which are based on the inferred heliocentric impactor population (Zahnle et al., 2003), quite uncertain (see discussion in Ferguson, Rhoden, & Kirchoff, 2022; Ferguson et al., 2020). From crater superposition, we know that the terrain within Herschel is among the youngest on Mimas (Kirchoff et al., 2018).…”

A Recent Impact Origin of Saturn’s Rings and Mid-sized Moons

“…However, once the present-day moons are mostly in place, any debris that is still in orbit-including secondary and further cascade ejecta-could imprint itself in the crater populations seen today. The craters on Saturn's moons indicate a population of at least some planeto-rather than heliocentric impactors (Ferguson et al 2022) and do not match the distributions on the other outer planets' satellites. It is likely premature to consider the immediate fragment distributions from this study as a crater-forming population, since they can be expected to evolve significantly over time, but they can represent an initial approach toward future comparisons and constraints.…”

A Past Episode of Rapid Tidal Evolution of Enceladus?

“…The relatively small mass (Iess et al 2019) and rapid evolution (O'Donoghue et al 2019) of the rings suggest a relatively young age, but other authors argue that ring pollutants are lost faster than ice, implying an older age (Crida et al 2019). It appears that dominant past impactors in the Saturnian system are different from Kuiper Belt objects (Zahnle et al 2003;Singer et al 2019), and are possibly planetocentric (Ferguson et al 2020(Ferguson et al , 2022a(Ferguson et al , 2022b. This would be consistent with, but would not require, a recent origin of the system.…”