Elliptical Craters on the Mid-Sized Saturnian Satellites and Their Relationship to the Impact Environment

The age and origin of the mid-sized Saturnian satellites are still outstanding questions. Accretion models for the formation of Mimas, Enceladus, Tethys, Dione, and Rhea (henceforth referred to as the MIMs) have explored formation from the proto-Saturn disk (refs) or from the rings themselves, migrating to their current orbital positions over the age of the Solar System (Canup 2010, Charnoz et al., 2010; 2011). Models of the post-accretion impact and orbital dynamics have suggested that the MIMs may be as young as 100 Myr old (Ćuk et al., 2016). Hence, there are two distinct endmember scenarios where either the current MIMs formed billions of years ago or formed in the most recent epoch of Solar System history. Geologic observations provide one approach to solving this dichotomy.

            Through an impact cratering lens, previous work on the cratering histories of the Saturnian system have found that the satellites have been heavily cratered and may be more in line with a formation age of ~4 Gyr ago (Kirchoff et al., 2010; 2015). However, many of the previous studies of the cratering histories have relied on the production functions of Zahnle et al., (2003), which assumed heliocentric impacts for both their Case A and Case B scenarios. While these production functions, particularly Case A, have appeared to better fit the data of the Jovian satellites and the Pluto/Charon system (Singer et al., 2019), the fits to the data of the Saturnian satellites and Triton aren’t as well matched (Ferguson et al., 2020; 2022b; 2024, Kirchoff et al., (2022).  For the Uranian system, craters with D > 10 km tend to follow the heliocentric shapes of the Jovian and Pluto systems. But at smaller diameters (D < 10 km), terrains on Miranda and Ariel differ in shape, suggesting a planetocentric origin (Kirchoff et al., 2022). This has led to the interpretation that the Saturnian and Uranian moons have likely been cratered by a combination of heliocentric and planetocentric material, further complicating the question of their ages and origins. Invoking planetocentric material as an impactor source adds in additional challenges towards building a chronology.

            Examining the planetocentric population of impactors at Saturn has proven to be a challenge because there are fewer constraints on the mass and size-frequency distribution of these objects. Our approach to characterizing this impactor source is to look at the elliptical craters present on the surfaces of Mimas, Tethys, Dione, and Rhea. Elliptical craters form in a low velocity and low impact angle collision with the surface. By looking at the distributions of elliptical craters and the orientations of their major axes, we can examine trends in the impact environment across the system. Due to the conditions in which elliptical craters are formed, we assume that these craters formed from planetocentric material. 

In our prior work (Ferguson et al., 2022a, 2024), we documented evidence of the elliptical crater populations on Mimas, Tethys, and Dione. We found that there is a concentration of elliptical craters oriented in the mid latitudes (30° S – 30° N) and across all longitudes that are oriented in an East/West direction on Mimas, Tethys, and Dione. We found an additional component of the elliptical craters on Mimas are oriented in a North/South direction closer to its north pole (i.e., oriented radially from the pole). This is in contrast to a more isotropic distribution of orientations in the polar areas of Tethys and Dione.

            Here, we present new results of the elliptical crater survey, extended to the largest of the MIMs, Rhea. Rhea’s location on the edge of the mid-sized moon system suggests the satellite may be best poised to reveal how this impactor source varied over time and/or distance to Saturn. Across the four satellites, we find that the densities peak at Tethys and drop off with increasing distance from Saturn for the mid-equatorial spatial densities or using the un-adjusted area for the total mapped terrain. When looking at the polar (60°-30°S, 30°- 60°N) regions, we find a slightly higher spatial density of elliptical craters on Mimas, a similarly high value on Tethys, and then increasingly lower spatial densities on Dione and Rhea.

Looking at the orientations of the elliptical craters (Figure 1), we observe a similar East/West trend on Rhea that we observe on the other satellites. Peculiarly, Rhea’s craters also show a Northwest/Southeast orientation in the polar latitudes. While unobserved at the other satellites, we have narrowed the location of this signal down to the leading Northern hemisphere of Rhea. On the other three satellites, the elliptical crater orientations maintain the same patterns seen in Figure 1. We will present additional findings, including comparisons between our Rhea data set and more regional scale mapping of Rhea’s surface, and discuss the implications of this work for the larger picture questions of the age and origin of the satellites.

Figure 1 Caption: Rose diagrams of elliptical craters across all four moons. 30° S - 30° N is referred to as the mid-equatorial regions. The other legend entry is considered our more polar terains and is shown in a contrasting color to the mid-equatorial data. 

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