Saturn's moon mysteries multiply

Dan Vergano USA TODAY @dvergano | USATODAY

Surely real estate moguls in the future will have only one real choice when they decide they want to drive real estate prices up to astronomical levels.

That would be Saturn's moons, offering ringside seats to the most sublime views in the solar system, the rings of the second-largest planet.

It won't happen anytime soon, but NASA may have boosted the chances of folks enjoying these views up close someday by awarding a $17.8 million contract this week to Bigelow Aerospace to install an inflatable "space habitat" aboard the International Space Station in 2015. Bigelow Aerospace is owned by a hotel magnate, billionaire Robert Bigelow, who has long harbored hopes of hosting honeymooners in space.

And the vista around Saturn would be worth the trip. With the international Cassini spacecraft mission now eight years into its tour of the ringed planet and its moons, the views of Saturn those moguls may enjoy someday have only gotten better. And some of the mysteries about Saturn's satellites, from the spouting geysers of Enceladus to the frozen lakes of Titan, have only grown deeper.

The latest puzzler comes from Dione, Saturn's fourth-largest moon, some 700 miles wide, an ice-crusted rock first spotted by the original Cassini, astronomer Giovanni Cassini of the Paris Observatory, in 1672. His namesake probe first flew over the moon in 2004 and revealed that "wispy terrain" long observed on the frozen moon was actually a series of ridges and cliffs, hundreds of feet high, cutting across one side of the moon.

How did Dione's eggshell end up cracked? A team led by Noah Hammond of the SETI Institute in Mountain View, Calif., looks at this riddle in an upcoming study in the journal Icarus.

One clue comes from the craters on Dione. Planetary scientists use craters to estimate the ages of moons and asteroids throughout the solar system. The leading face of Dione as it circles Saturn is pockmarked with impact divots, for example, some 6 miles or so deep. On the trailing side, however, the surface is smoother, with fewer of these craters. Estimates are that the heavily cratered side's surface represents changes that date to 4 billion years ago, based on the number of divots, while the cliff-riven surface got its facelift about 2 billion years ago, based on its smoother appearance.

So, what happened there? The study team looked at one prominent ridge on Dione called Janiculum Dorsa (dorsa means ridge in Latin). The name is drawn from Virgil's ancient poemThe Aeneid, from which all the names for features on the moon are drawn. (Just imagine how pleased those future real estate folks will be to have such classic names for their Saturnian apartment blocks at hand.) Janiculum Dorsa stretches more than 300 miles across Dione. A scar some 18 miles wide, it stands nearly a mile tall and has deeply bent the moon's crust.

That bending implies the surface was once warm and weak, Hammond says. But how weak? Crunching the numbers with Cassini data that looked carefully at the ridge, Janiculum Dorsa, the team concludes that the energy necessary to bend Dione's crust under the weight of a ridge that size must have been substantial. Too substantial to be explained easily by radioactive elements decaying in the 400-mile-wide rocky core of the moon, the team concludes, even throwing in added heating generated by stress from the tidal pull of Saturn's other moons.

The one thing that would make it easy would be a 30-mile-thick ocean hidden under the frozen crust of the moon, the study concludes. Higher heating from a subsurface ocean in existence at the time the ridge formed might do the trick. That same heat could explain "highly relaxed" craters, nicely rounded ones, on the younger face of Dione, as well. The refreezing of the ocean would have strained the surface of the moon to create the cliffs seen elsewhere across its younger face. "These observations, in combination with our results, suggest it is likely Dione had a subsurface ocean at the time Janiculum Dorsum formed," the study concludes.

Now, all we have to do is get those moguls up there to start enjoying the skiing on Dione and the views of Saturn's glorious rings. There will be plenty of ice for their drinks.