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

Mimas' small size, lack of geologic activity, and high eccentricity suggest a frozen, inactive history. However, Cassini libration measurements are best explained by a present‐day liquid ocean under an ice shell 24–31 km thick, a configuration that tidal heating can support. These unexpected findings have motivated further study of Mimas' surface geology to constrain its interior evolution. Here, we model the formation of Mimas' large impact basin, Herschel, using iSALE‐2D. Our goal is to determine whether the current estimated ice shell thickness is sufficient to withstand the impact and produce the observed basin morphology. We find that, if Mimas had an ocean at the time of the Herschel‐forming impact, the ice shell had to have been 10s of km thicker than today. These findings are consistent with inferences from minimal tectonic activity that Mimas must be a young ocean world, if it indeed has an ocean today. Plain Language Summary Mimas, a small moon of Saturn, is heavily cratered and lacks the typical characteristics of an ocean‐bearing moon, such as the active surface of its neighboring moon, Enceladus. However, measurements of Mimas, made by the Cassini mission, are best explained by an ocean under a relatively thick ice shell. Here, we try to understand how this ice shell and ocean may have changed with time by modeling the formation of Mimas' largest impact basin, Herschel. When such large impacts occur, the shapes and depths of the resulting basin can be affected by the presence of the ocean. We find that Herschel could not have formed in an ice shell at the present‐day thickness without obliterating the ice shell at the impact site. Rather, we conclude that, if Mimas has an ocean today, the ice shell has been thinning since the formation of Herschel, which could also explain the lack of fractures on Mimas. Mimas as an emerging ocean world places important constraints on the formation, evolution, and habitability of all of the mid‐sized moons of Saturn. Key Points We performed numerical simulations of the formation of the Herschel impact basin to explore Mimas’ evolution The Herschel impact basin could not have formed in an ice shell of the thickness indicated by Mimas’ present‐day librations Herschel’s morphology and the lack of tectonics on Mimas are compatible with a thinning ice shell and geologically young ocean