Published online 25 March 2009 | Nature | doi:10.1038/news.2009.190

News

Titan may boast ice-spewing volcanoes

Cassini spacecraft spots possible cryo-activity on Saturn's moon.

TitanCassini may have spotted a volcano on Titan.NASA/JPL/University of Arizona

With its winds, rains and organic molecules, the large Saturnian moon Titan already shares more qualities with Earth than any other place in the Solar System. Add yet another: volcanoes.

An area known as Hotei Arcus has been identified by scientists operating the Cassini spacecraft as the best candidate yet for a cryovolcano, which, in the cold of the outer Solar System, would spew a slurry of ice and liquid hydrocarbons, instead of lava.

"It's as if it's a sort of constant bubbling cauldron that occasionally explodes big time," says Robert Nelson, a Cassini team scientist at Jet Propulsion Laboratory in Pasadena, California. In a talk on Tuesday at the Lunar and Planetary Science Conference in Houston, Texas, Nelson presented images that showed Hotei Arcus changing during recent orbital passes by Cassini. Nelson interprets the changes as a sign that the ice volcano is erupting right now.

Lively chemistry

In addition to being yet another place in the Solar System with present-day volcanic activity (Jupiter's Io and Saturn's Enceladus being two other examples), an ice volcano on Titan would tantalize astrobiologists by putting a subterranean heat source close to liquid reservoirs where pre-biotic compounds, such as water, methane and ammonia, could mix. "That's essentially the original Miller-Urey experiment," says Nelson, referring to the famous 1952 attempt to create life in a bottle with pre-biotic chemicals and a few sparks.

“It's as if it's a sort of constant bubbling cauldron that occasionally explodes big time.”

Robert Nelson
Jet Propulsion Laboratory

The Cassini spacecraft data are by no means easy to collect — Titan's thick atmospheric shroud of methane blocks most attempts to see through to the surface, except at radar wavelengths and in specific windows in the near infrared. And the evidence for cryovolcanic activity at Hotei has not yet convinced all those on the Cassini team, let alone all planetary scientists. But the case for it is building.

Only in recent weeks has Cassini radar scientist Randy Kirk, of the US Geological Survey in Flagstaff, Arizona, been able to get the overlapping orbital passes he needs to build up a three-dimensional radar map of the area. Maps of Hotei Arcus show an area with several lobe-like fingers, each 100–200 metres high, that have a shape and thickness consistent with a highly viscous, lava-like material.

Ambiguous images

Like an advancing wall of lava, the lobes also appear to cut off several narrower channels, which are thought to be small streams carved by liquid methane. Kirk says these shapes make Hotei Arcus the best candidate for a cryovolcano yet — though he doesn't want to speculate on Nelson's more provocative assertion that Hotei is actually erupting today.

Radar picture of Hotei Arcus.Hotei Arcus: lava flows or weather?NASA/JPL/USGS

Nelson's images, from a Cassini infrared instrument, corroborate the shapes seen in the radar maps. But they also show that Hotei Arcus changes in brightness over periods of days, weeks and years, which could be evidence for an active eruption, or, simply, active weather — winds moving debris or a frost. The latest evidence for cryovolcanism, revealed in Nelson's talk on Tuesday, is that when the area brightens, a rise is seen in the spectral signature for ammonia. Ammonia, with its ability to act as an anti-freeze, is an expected ingredient of liquid cryo-eruptions.

Scientists have an extra reason to be eager for cryovolcanoes on Titan: they would explain why the satellite has methane in its atmosphere, since without them the gas would be expected to decay photochemically.

But William McKinnon, of Washington University in St. Louis, Missouri, finds the infrared images "ambiguous" and says, "It's much more logical — and prosaic — that wind or weather does this." He says the eagerness to attribute volcanism to changes in brightness is akin to a time in the 1960s when scientists studying Mars saw light and dark features and thought it could be vegetation. "It's turned out to be dust." 

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  • #60675

    Earth-like??? Well, it has no proper atmosphere, is hostile to human,life, cold, hard, icy, no decent food for miles, no green spaces, and costs loads to get there. i

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