Does Saturn's "Death Star" Moon Harbor an Ocean?

Mimas, the moon that orbits just clear of Saturn’s rings, is one of the more notable moons of the solar system — if only because it looks suspiciously like the “Death Star” from Star Wars. Fittingly, this Death Star moon also appears geologically dead. Unlike Saturn’s other icy moons, whose surfaces are slashed with cracks and fissures that spew evidence of subsurface oceans, Mimas is covered in craters.

Now, in the February 8th Nature, Valery Lainey (Paris Observatory) and colleagues report evidence that Mimas does have a subsurface ocean after all. It’s just that it’s so new, having formed only between 2 and 25 million years ago, that it hasn’t had time yet to impact the surface.

However, not everyone’s convinced by measurements, which were challenging to make. “While the authors present an interesting new piece of evidence in support of the ocean hypothesis, I don't think it's a slam dunk,” says Luke Dones (Southwest Research Institute).

Moons and Tides

The evidence for a subsurface ocean comes from NASA’s Cassini mission, which ranged around Saturn for 13 years before it plunged into the gaseous atmosphere in 2017. While Cassini was operating, it photographed not only the ringed planet but its moons, tracking their positions and orientation over time and thereby revealing clues to their interiors.

As each moon orbits Saturn, its gravity tugs on and deforms the planet’s gases. That deformation changes over time. Mimas, for example, circles Saturn in 22 hours, while the planet itself spins every 10.7 hours, so gas within the planet must constantly reflow to reshape the planet as it spins. The effects echo back to the moons, changing their positions and orbits.

Like most of Saturn’s inner moons, Mimas always shows the same face toward Saturn. However, tides cause the moon to rock its Saturn-facing side back and forth very slightly as it orbits, an effect called libration. (The Moon “rocks” in this way, too; Earthbound viewers can actually use lunar librations to take a peek at the Moon’s farside.)

In 2014, the same team of researchers measured Mimas’s spin, which allowed them to calculate the moon’s libration. The rocking motion was larger than expected, indicating one of two possible interiors — both of them unexpected. Either the icy moon has an elongated core, shaped like a silicate, oval pancake, or, more surprisingly, it hosts a subsurface ocean in between an icy crust and a more spherical core.

Now, Lainey and his team zero in on those scenarios by examining another aspect governed by tides: Mimas’s orbital precession, in which Mimas’s oval-shape trajectory around Saturn traces out a rosette shape over time.

“We monitored extremely cautiously the orientation of Mimas’s orbit over time,” Lainey explains. “Its drift had to be constrained to a precision of better than few kilometers to constrain Mimas’s interior.”

The team’s analysis of Cassini measurements shows that Mimas’s farthest position from Saturn changed by 0.7 kilometer (0.4 mile) per year. While this drift is incompatible with an elongated core, it is compatible with a subsurface ocean, the team claims. (Dones notes, however, that the drift they measure is “tiny” and thus requires some assumptions to measure.)

Based on measurements of both Mimas’s orbital precession and the libration, Lainey and colleagues calculate that the putative ocean is between 2 million and 25 million years old and hides beneath 20 to 30 kilometers (10 to 20 miles) of rigid ice.

Short-lived Ocean

Why would a geologically dead moon suddenly sprout a subsurface ocean a few million years ago? Whatever the cause, it must have temporarily put the moon in a more elongated, or eccentric, orbit. The stresses of Saturn’s changing gravitational pull throughout the moon’s orbit would then have heated its interior, melting ice into ocean.

Maybe Mimas briefly danced with the moons Dione and Titan as all three orbits evolved naturally. Or, maybe the whole system lost an icy moon to Saturn’s gravity some 100 million years ago in an event that also created the planet’s bright rings. Either hypothetical event could have recently changed Mimas’s orbit.

Interestingly, if Mimas does harbor a subsurface ocean, then tides within those waters would have made the orbit more circular over time. But Mimas currently swings around Saturn on an eccentric trajectory. The team therefore calculates that its orbit must have been twice as elongated in the past. That timescale fits the scenario in which some recent event upended the Saturnian system.

It also implies the subsurface ocean won’t last long. “Mimas’s current eccentricity is about 2%,” says Lainey. “We are currently close to the phase where the eccentricity is getting low enough for Mimas’ icy crust to get thicker again.” As the icy crust thickens, the subsurface ocean will once again freeze solid.

“The fact that Mimas does not have any evidence of former internal heating episodes suggests that it is probably much younger than the age of the solar system,” Lainey adds.

But what Lainey sees as evidence of a recent event makes Dones cautious of adopting this scenario. He thinks it’s a bit of a coincidence that the ocean formed so recently that no signs of it are visible on the surface. “The timing seems almost too good to be true.”

That said, if the discovery of a subsurface ocean on Mimas pans out, it has wide-ranging implications not just for the Saturnian system but for icy moons across the solar system.

“Adding Mimas to the catalogue of ocean worlds changes the general picture of what these moons can look like,” write Matija Ćuk (SETI Institute) and Alyssa Rose Rhoden (Southwest Research Institute) in an accompanying perspective piece. For example, a suite of icy moons around Uranus might hide oceans as well — tempting targets for NASA’s Uranus Orbiter and Probe mission concept, which could launch as soon as 2031.