NOT JUST A DEATH STAR: THE SURPRISINGLY COMPLEX HISTORY OF SATURN’S MOON MIMAS

On first glance, Mimas appears to be nothing more than another heavily cratered world. However, new constraints on its interior and orbital evolution reveal a complex history. Measurements of Mimas’ libration by [1] suggest that either Mimas had to have an irregularly shaped core or that it needed to have an ocean underneath an ice shell. Estimates of Mimas’ formation age range widely from ~ 4 Gyr [2] to potentially as young as 100 Myr [3]. Mimas’ proximity to Saturn and relatively high eccentricity suggest that it should be pervasively fractured and tidally active like Enceladus [4]. Recent modeling by [5] shows that, the current orbital eccentricity of Mimas would result in tidal heating that could sustain a thick ice shell over an ocean.

Utilizing images from the Cassini-ISS dataset for Mimas, we re-examine the crater record of this moon, with a focus on re-assessing the relative surface age of the Herschel basin and surveying the potential elliptical craters (EC’s) on the surface. Our preliminary results of the trailing hemisphere terrain show a similarly sloped SFD when compared to Dione, which suggests a similar impactor population for both moons. Our analysis of crater counts in and around the Herschel impact basin is ongoing; we will present the results of this analysis.

For ECs, we map 159 craters, which is roughly an order of magnitude fewer number of craters than what was found by [6] on Tethys and Dione. Additionally, the ECs present on Mimas are not oriented in an east/west direction like what we've observed on Tethys & Dione [6]. These differences suggest that either whatever source was responsible for ECs on those moons had dissipated by the time Mimas was fully formed (assuming a young Mimas) or that the source responsible for ECs on Tethys & Dione did not extend inwards to Mimas. Modeling of the timescale and distribution of impactors from different sources will be useful in determining whether a difference in age or bombardment history led Mimas to express such a different cratering history.

References: [1] Tajeddine et al., (2014), Science, 346(6207), 322–324. [2] Neveu & Rhoden (2019), Nature Astronomy, 3, 543–552. [3] Ćuk et al., (2016), AJ, 820(2), 97. [4] Rhoden et al., (2017) JGR: Planets, 122(2), 400–410. [5] Rhoden & Walker (2022), Icarus, 376. [6] Ferguson et al., (2022b), Earth and Planetary Science Letters, 593, 117652.