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Could a moon of Uranus harbour an underground ocean?

NASA/JPL

Posted on behalf of Ron Cowen.

Since 2005, astrobiologists have considered Enceladus a possible haven for life, after the Cassini mission found that the icy moon of Saturn shoots out plumes of water through fissures in its crust. But planetary scientists Elizabeth Turtle of Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, and Julie Castillo-Rogez of NASA’s Jet Propulsion Laboratory in Pasadena, California, are now turning their eyes to an even more distant Solar System locale: Ariel, a moon of Uranus that they think could also harbour an underground ocean.

Like Enceladus, Ariel’s surface appears relatively blemish free, with few large craters, as though recent activity had erased or buried older pockmarks. Flow-like features on Ariel suggest icy volcanism may have been responsible for the facelift, as it has on Enceladus. But in the frigid depths of the outer Solar System, what could maintain an ocean beneath the surface of Ariel?

In a presentation at a meeting of the Division of Planetary Sciences in Reno, Nevada, Turtle and Castillo-Rogez calculate that tidal heating — the flexing of Ariel owing to the gravitational tug of other moons — is five times greater on Ariel than Enceladus. In addition, both moons contain a relatively large amount of rock, which generates heat through the decay of radioactive elements within it. Heat from the rock would increase the internal temperature of Ariel to the point where ice would be soft enough to respond to tidal flexing, Castillo-Rogez says. Although the Uranian system is colder than of Saturn system, the Uranian moons are more likely to have captured impurities that would decrease the melting temperature of ice, Castillo-Rogez notes.

Paul Schenk of the Lunar and Planetary Institute in Houston, Texas, is intrigued. “I’ve always thought Ariel was the more interesting of the [outer] satellites,” he says. But the best current images of Ariel, from the closest approach of the Voyager spacecraft in 1986 (at right), aren’t good enough to show precisely how smooth the surface is. Better images could help planetary scientists to decide whether the ocean spills out onto the surface today or has been trapped in place for a billion years. “I can imagine an ocean is possible … but I’m not sure the surface is young enough to be active right now,” Schenk adds.

Without higher-resolution images and a spacecraft that could search for plumes on Ariel, “all we can tell is that the possibility of a deep ocean and endogenic activity in Ariel makes sense,” says Castillo-Rogez. Future missions that might tour the Uranian systems could have the capability of searching for evidence of past or ongoing activity on Ariel. For instance, the craft could search for a faint ring associated with Ariel, similar to the icy E ring of Saturn, which is continually replenished by outbursts from Enceladus.

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