Herschel impact basin probes the onset of a young ocean on mimas

Abstract

While observations of Mimas’ cratered surface suggest a largely inactive geologic history, measurements of its libration and pericenter precession indicate the presence of an ocean under an ice shell that is 20–30 km thick. Recent thermal-orbital models can reconcile the surface geology and interior constraints as long as Mimas’ ocean is young, most likely formed in response to a recent spike in eccentricity in the last ∼10 million years. We explore whether this timeline for ocean development is consistent with Mimas’ surface geology by revisiting the formation of its largest impact basin, Herschel, using iSALE-2D Our goal is to determine whether Herschel’s formation requires the presence of an ocean, as previous models suggest, or whether the impact instead occurred prior to melting of the ice shell, which could expand the time window for its formation. We find that Herschel’s morphology is consistent with a range of possible internal structures, including scenarios in which the ice shell is fully frozen, though in these cases the ice shell must be warm enough to be near its melting point. These formation conditions are consistent with inferences from crater age dating that Herschel is young, and thus, formed in close proximity to the geologically-recent growth of Mimas’ ocean.