Surprises lurk inside a Saturn moon

Mimas may hide a subsurface sea or oddly shaped core

Saturn’s smallest major moon, Mimas

NOT A SPACE STATION  Saturn’s smallest major moon, the nearly 400-kilometer-wide Mimas, shows off a crater seemingly inspired by Star Wars in this 2010 image from the Cassini spacecraft. 

JPL-Caltech/NASA, Space Science Institute

A subsurface sea or a roughly football-shaped core might lie within Mimas, the smallest of Saturn’s major moons, new observations indicate. The finding could help researchers understand how Saturn’s diverse entourage of icy satellites formed.

Like most moons in the solar system, Mimas seesaws around its axis while keeping one side facing its planet. But Cornell University astronomer Radwan Tajeddine and colleagues noticed from photos taken by NASA’s Cassini spacecraft that the moon twists around farther than expected. The researchers suggest in the Oct. 17 Science that the large twists must be caused by an unusual distribution of mass inside Mimas, which is just under 400 kilometers across.

Tajeddine and colleagues think the most likely culprit is an elongated core — a possible remnant from the moon’s formation.  An underground ocean is also possible, such as the one inside another Saturnian moon, Enceladus (SN: 5/3/14, p. 11), but Mimas doesn’t show any other signs of a subterranean sea such as water seeping through craters.

A few flybys with Cassini, though not currently scheduled, could let scientists map the moon’s gravity and unearth what Mimas is hiding.

Christopher Crockett is an Associate News Editor. He was formerly the astronomy writer from 2014 to 2017, and he has a Ph.D. in astronomy from the University of California, Los Angeles.

More Stories from Science News on Planetary Science