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Saturn Moon has Active Volcanoes Spewing Ice - 12.21.2008

Article from SF Chronicle
David Perlman, Chronicle Science Editor

The frozen mixture of those chemicals, plus heat from somewhere in the moon's interior that scientists believe must be driving what they have termed the "cryovolcanoes," could conceivably mean that Titan - like many other moons circling other planets - is a source for unknown alien life forms.

The scientists, attending the American Geophysical Union's annual meeting in San Francisco, presented reports this week on recent observations of Titan gathered by instruments aboard the Cassini spacecraft, which has been orbiting Saturn and flying past its 18 moons for more than four years.

For more than a year now, NASA teams viewing Titan's cryptic surface from information received from far-off Cassini have puzzled over strangely alternating bright and dark spots along its equator, and what appeared to be sandy stretches and dunes - but without signs of a sea.

Detected by an instrument aboard the NASA spacecraft called a visual imaging mass spectrometer, or VIMS, during four out of a dozen flybys, Robert Nelson, a physicist at NASA's Jet Propulsion Laboratory in Pasadena and expert on spectroscopic observations, suggested that the episodes of brightness and darkness - and the sandy stretches - might well be violent upwellings from Titan's interior.

"Looking down at Titan, the atmosphere is totally opaque," Nelson said in an interview during this week's meeting. "After the very first flyby, I looked at the surface and in spots along Titan's equatorial region, it obviously had gotten brighter; it was the brightest thing we could see there."
On and off bright spots
The scientists, he said, realized that something was turning two of those bright spots off during some of the flybys, and then turning them on again later.

"Our VIMS spectrometer, gathering images in the infra-red, told us it was ammonia being deposited over water ice," Nelson said. "Ammonia was thought to exist on Titan only beneath the surface, and here it was: Ice thrusting up through the surface along with water, and probably methane too."

The science of the ultra-cold is called cryogenics, and at the temperature of Saturn's moons - about 93 degrees Kelvin, or nearly 300 degrees below zero Fahrenheit - "ice makes the best rocks you can have," Nelson said. "So here (the ice) was behaving just like magma does in volcanoes on Earth - a real cryovolcano."

The first time the scientists detected a cryovolcanic eruption, some believed the heat inside Titan required to cause such an eruption must have been caused by tidal influences from Saturn's gravity.

Then Giuseppe Mitri, another NASA scientist at the Jet Propulsion Laboratory, proposed a better answer: Titan's solid rocky core, he argued, contains elements like uranium and radioactive potassium - and warmth from their radioactivity would be exactly what's needed to thrust the solid ice and water upward to the surface.

"You don't need tidal heating at all," Nelson said. "Radiogenic heating can do the job very well."
Radar removes doubt
The VIMS instrument aboard Cassini can't take really clear images of Titan's surface, but the spacecraft's radar can, and Stephen Wall at the Jet Propulsion Laboratory used the radar at very high resolution along successive narrow paths on the moon's surface - "like observing the surface through a drinking straw," said Nelson.

"The radar evidence pushed the whole concept of cryovolcanism over the top," he said.

They revealed very distinct signs of steep-sided hills topped with craters -"true volcanic morphology," as Nelson put it - rising above large areas of sand dunes made from varied hydrocarbons.

And what of the chemicals in those cryovolcanoes and the speculation about the possible existence of life forms?

"There's water, and methane and ammonia and heat," Nelson said.

Hydrogen and oxygen come from the water; carbon and hydrogen from the methane; nitrogen and hydrogen from the ammonia - all that plus heat from Titan's deep core.
'The stuff of life'
"And those are the very components that make up the stuff of life," Nelson said.

To Jonathan Lunine, a physicist at the University of Arizona's Lunar and Planetary Laboratory and also a Cassini team member, the most recent radar images from the spacecraft are the most convincing evidence yet for the erupting cryovolcanoes.

Stereo images show the absence of impact craters in the volcanic regions and reveal the topography of the volcanoes themselves, he said in an interview.

The dune field in that region, he said, is very much like the dunes of Africa's Nab desert, although their sands are not silicates but hydrocarbons like methane and ethane.

"They're organics," he said, "and relevant, perhaps, to the origin of life."

NASA, in fact, is now planning two possible life-seeking interplanetary missions, Lunine said: One to Titan, specifically to look for precursors to microbial life, and the other to Europa, the ice-crusted moon of Jupiter, whose deeply cracked surface is believed to cover a deep ocean where organic molecules may already have evolved into life.

Those NASA missions are only in the planning stage, and competition for space funds is heavy, but maybe - some day.

E-mail David Perlman at dperlman@sfchronicle.com.

This article appeared on page A - 2 of the San Francisco Chronicle

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