Mimas, a small moon of Saturn, is heavily cratered and lacks the typical features of an ocean-bearing moon.

Colleagues began exploring Mimas’s surface to understand how its interior evolved after a Southwest Research Institute scientist found stunning evidence that Saturn’s smallest, the innermost moon, can generate enough heat to sustain an internal liquid ocean. hold.

The structure of Mimas’ Herschel impact basin, the most prominent feature of its highly cratered surface, and the absence of tectonics are consistent with a thinning ice cover and a geologically young ocean, according to numerical simulations of the moon.

SwRI’s Dr. Alyssa Rhoden, a specialist in the geophysics of icy satellites, said: “Towards the end of NASA’s Cassini mission to Saturn, the spacecraft identified a curious libration or oscillation in Mimas’s rotation, often indicating a geologically active body that may support an internal ocean.”

“Mimas seemed like an unlikely candidate, with its icy, heavily cratered surface marked by a giant impact crater making the tiny moon look a lot like the Death Star from Star Wars. If Mimas has an ocean, it represents a new class of small, ‘stealth’ ocean worlds with surfaces that do not betray the existence of the ocean.”

Rhoden teamed up with Purdue doctoral student Adeene Denton to better understand how a moon with as many craters as Mimas could have an inland sea. Denton used the simulation program iSALE-2D to model the formation of the Hershel impact basin.

According to the models, Mimas’s ice sheet must have been at least 34 miles (55 km) thick at the time of Herschel’s impact. On the other hand, if Mimas is still home to an ocean, observations of the planet and simulations of internal heating limit the thickness of the current ice cover to less than 19 miles (30 km).

Denton, who is now a postdoctoral researcher at the University of Arizona, said: “We found that Herschel could not have formed in an ice sheet of its current thickness without erasing the ice shell at the impact site. If Mimas has an ocean today, the ice sheet has thinned since Herschel’s formation, which could also explain the lack of fractures on Mimas.”

“If Mimas is an emerging ocean world, that imposes important constraints on the formation, evolution and habitability of all of Saturn’s medium-sized moons.”

Rhoden said, “While our results support a current ocean within Mimas, reconciling the moon’s orbital and geological features with our current understanding of thermal-orbital evolution is challenging.”

“Evaluating the status of Mimas as an oceanic moon would provide a benchmark for the formation and evolution models. This would help us better understand Saturn’s rings and medium-sized moons and the prevalence of potentially habitable oceanic moons, particularly near Uranus. Mimas is a tempting target for further research.”

Magazine reference:

  1. C.A. Denton, A.R. Rhoden. Tracking the evolution of an ocean within Mimas using the Herschel Impact Basin. Geophysical Survey Letters. DOI: 10.1029/2022GL100516