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Saturn moons star in dark drama

NASA / JPL-Caltech / SSI

A plume of water ice is backlit as it spews from the south polar region of Enceladus, one of Saturn's moons. The image was taken in visible light with the Cassini spacecraft's narrow-angle camera on Feb. 20, from a distance of about 83,000 miles (134,000 kilometers).

NASA's Cassini mission has delivered a dark but dramatically backlit view of "Spaceship Enceladus": an icy moon of Saturn with geysers of water ice spewing from its south polar region, as if it were turning on its thrusters.

Enceladus isn't going anywhere, of course, but the geysers have launched a lot of speculation about what might be giving rise to the spray. Is water from a hidden ocean welling up through the cracks known as "tiger stripes"? If so, what creatures might lurk in that subsurface sea?

The picture released today by Cassini's imaging team fires up the imagination, even for team leader Carolyn Porco. "Now try to tell me Enceladus isn't the coolest, most fascinating moon there is!" she said in a Twitter update touting the view.

Really? But what about Europa, an icy moon of Jupiter that is also thought to harbor a subsurface ocean, and perhaps life. A probe to study Europa is high on NASA's list of future big-ticket missions, even though such missions are currently on hold due to budgetary constraints. (There's also some recent research suggesting that Europa's hidden ocean might be too acidic for life as we know it.)

Porco made her preference plain in a volley of tweets: "Enceladus, with the most accessible habitable zone beyond Earth, is far better for discovering anything about life than Europa."

NASA is said to be planning a concept study for an eventual Enceladus mission, although tight budgets may force a change of plan. The German Aerospace Center recently unveiled a study project known as Enceladus Explorer, or EnEx, which is looking at the possibility of putting a base station on the moon's surface and drilling down into the ice. The concept calls for a type of probe known as an IceMole to melt its way down to a water crevasse, retrieve a sample of liquid water and analyze it for the presence of microbes.

EnEx's collaborators have been testing a prototype IceMole on Switzerland's Morteratsch Glacier, and they're planning to try it out on glaciers in Alaska and Antarctica, leading up to the sampling of a subglacial lake in Antarctica in 2014. If those tests are successful, the team will propose sending IceMoles to Mars and eventually to Enceladus.

Enceladus isn't the only star of this week's Saturnian show. Fresh pictures of Saturn's second-largest moon, Rhea, were released as well. These pictures were captured over the weekend during a flyby that brought Cassini within 26,000 miles (42,000 kilometers) of the heavily cratered moon. NASA says the flyby was "relatively distant" but well-suited for global geologic mapping.

NASA / JPL-Caltech / SSI

This raw, unprocessed image of Rhea was acquired by the Cassini spacecraft on Saturday and received on Earth on Sunday. The camera was pointing toward Rhea from a distance of about 26,000 miles (41,873 kilometers).

NASA / JPL-Caltech / SSI

This raw image of Rhea was taken on Saturday from a distance of about 26,000 miles (42,096 kilometers). The pattern of lines on the right side of the image is the result of data loss during transmission.

The next big flyby is scheduled for March 27, when Cassini is due to come within 46 miles (74 kilometers) of Enceladus. That's close enough to sample those plumes of ice directly — and perhaps take one more step toward unraveling the mystery of Enceladus' hidden seas.

More about Enceladus, Rhea and Cassini:

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding Cosmic Log's Google+ page to your circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.