Surprises lurk inside a Saturn moon

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.