Leveraging the Herschel Impact Basin to Probe the Evolution of a Young Ocean on Mimas

Introduction: Mimas is the smallest (r = 198 km) and innermost of Saturn’s mid-sized moons. Its heavily cratered surface, including the large impact basin Herschel (D = 139 km, Figure 1), has been interpreted as evidence for a largely inert geologic history. However, measurements of Mimas’ libration and pericenter precession favor the presence of an ocean, a possibility which can be reconciled with Mimas’ surficial geology if the ocean is young (~10 million years). Here, we revisit the timeline for the development of Mimas’s putative young ocean by reconsidering the conditions necessary to produce Herschel.

Figure 1. Mimas possesses a heavily cratered surface (left, PIA20523), which appears at odds with current hypotheses for its internal evolution, which include internal heating and a relatively young ocean. The formation of Herschel, Mimas’s large impact basin (right, N1644784749), could have occurred during the recent growth of the ocean and may provide further insight.

Methods: We simulate the formation of Herschel using the iSALE-2D shock physics code (Amsden et al., 1980; Collins et al., 2004; Wunnemann et al., 2006), approximating Mimas as a flat, two-layer target with water ice of variable thicknesses overlying a liquid water ocean. While previous work determined that a Herschel-like basin could be produced in a thick (³ 55 km) conductive ice shell overlying an ocean (Denton & Rhoden, 2022), thermal-orbital modeling of the growth of Mimas’ ocean from tidal heating suggest that ocean expansion is so rapid that the ice shell is only ³ 55 km for ~1 Myr (Rhoden et al., 2024). To assess the plausibility of Herschel’s formation under a broader range of pre-impact conditions for Mimas, and thus expand the window of time in which the basin may have formed, we consider two possibilities:

• Mimas’ ice shell was entirely frozen at the time of impact, but was actively warming. Such a scenario encompasses the period in Mimas’ history when its eccentricity underwent pumping through resonance with another Saturnian satellite (Baillie et al., 2019; Noyelles et al., 2019), creating tidal heating that transitioned the bottom portion of the ice shell from conductive to isothermal. This period is thought to last up to ten million years.
• Mimas was not entirely frozen at the time of impact, and instead possessed an emerging ocean with a thick (45-65-km) overlying ice shell. We consider the influence of a depressed melting point due to the presence of volatiles, which could reduce the basal temperature of Mimas’ ice shell to between 240-260 K. The influence of a colder ice shell on tidal dissipation in Mimas has not been explicitly modeled; however, we expect that the higher viscosity of colder ice would reduce tidal dissipation, prolonging ice shell thinning in an ocean-bearing Mimas.

Expanded conditions for the formation of Herschel: In our simulations, we seek to identify the suite of ice shell thicknesses and thermal structures that result in an impact basin whose morphology is consistent with that observed in the present day, including Herschel’s diameter (~139 km), depth (~10 km), and central peak (Moore et al., 2004; Schenk et al., 2018). We use these criteria to estimate “best fits” to Herschel, finding that a Herschel-like basin can indeed form in a fully frozen Mimas, provided that the ice shell is warm enough to possess an isothermal layer close to the melting point (260-270 K). Colder ice shells produce craters with poor fits to the basin, including diminished central peaks and depths in excess of 15 km. We find that it is also possible to reproduce a Herschel-like basin in a much colder ocean-bearing Mimas than assumed in previous work (Denton & Rhoden, 2022), provided that the ice shell is still at least 55 km thick. Producing a Herschel-like basin with thinner ice shells may be feasible, but the ice shell and ocean would have to be much colder than currently predicted (200 K rather than 260-270 K). The success of both fully frozen and ocean-bearing scenarios illustrates that Herschel may have formed either before or after the initiation of an ocean within Mimas, but the events are still connected. Herschel either forms during the warming phase of tidal heating that precedes melting and runaway growth of the ocean, or during the early stages of ocean evolution, when the ice shell is more than 75% of the total thickness of Mimas’ hydrosphere.

Conclusions: Our simulations indicate that, to adequately reproduce Herschel’s present-day morphology, either Mimas’ hydrosphere was fully frozen but close to its melting point or included a liquid ocean under an ice shell at least 55 km thick. These findings encompass a range of potential thermal structures, which depend on the ice shell’s composition and pre-impact thermal history. Further reconciliation of the surface geology of Mimas with the growth of its ocean can be more fully addressed through measurements by future spacecraft, as well as additional detailed modeling of the moon’s thermal-orbital evolution