Saturday, June 11, 2005
Ice volcanoes, they are out of this world!
Titan, one fo Saturn's moons, has ice volcanoes.
Saturn's moon Titan does not possess the exotic methane oceans that many experts thought it might have, but the Cassini space probe may have spotted something just as strange - volcanoes that spout a brew of icy, tar-like goo.
An international team of researchers reached those dual conclusions in a study of Cassini images to be released Thursday. Although they found none of the vast, flat expanses that would mark an ocean, the scientists spotted one spiral formation about 19 miles across that may be an ice volcano.
Such "cryovolcanoes" could explain the methane in Titan's thick atmosphere of orange haze. The methane would have disappeared ages ago if something were not replenishing it.
Experts say it's possible that a mix of water, methane and ammonia flows beneath Titan's surface, like molten rock within the Earth. Eruptions at the surface could release the methane into the atmosphere.
Although the temperature is thought to be minus 150 degrees Fahrenheit even inside Titan, ammonia would keep the water semi-liquid. The "lava" that emerged at the surface would look like nothing normally seen on Earth.
"If you touched it, your hand would be frozen immediately," said Bob Brown, a professor of planetary science at the University of Arizona and co-author of the paper in the journal Nature.
"Water with ammonia at these temperatures is gooey and sticky, with the consistency of slightly warm tar," Brown said. Another expert likened the substance to cold waffle syrup.
Titan, which is larger than the planet Mercury, is the only moon in the solar system with a dense atmosphere, made mostly of nitrogen and small amounts of methane.
The Cassini mission is the first chance experts have had to pierce the thick cloud cover with detailed infrared imaging and radar mapping.
The absence of methane oceans on Titan came as something of a surprise to planetary scientists. Before Cassini arrived at the planet last year, such oceans were "the most widely accepted explanation" for the methane in Titan's atmosphere, study leader Christophe Sotin said in a statement from NASA's Jet Propulsion Laboratory.
Instruments that measure the sloshing of ocean waves were part of the Huygens probe, which was released from Cassini and landed on Titan earlier this year. That mission's failure to see large bodies of liquid methane, combined with the new Cassini results, means any bodies of liquid would be no bigger than lakes or puddles, Brown said.
"If there were an ocean or something the size of the Gulf of Mexico, we would have seen it," he said.
Yet Huygens did spot winding channels that may have formed from the flow of liquid methane and could be linked to volcanic eruptions.
Methane from the eruptions may collect in the atmosphere and rain periodically onto the surface in what Brown called "methane thunderstorms." That could help create small, short-lived channels like the ones Huygens saw.
"It's obvious that this landscape has been scoured by floods," said study co-author Larry Soderblom, a geophysicist with the U.S. Geological Survey in Flagstaff, Ariz.
The bright, circular feature that resembles a volcano popped out in images because it looked like a snail's shell, team members said. A black spot in the middle would be the caldera, or basin, and the rings of the "shell" may be layer upon layer of frozen material ejected from the volcano.
That conclusion is speculative, but it's a reasonable interpretation of the feature, said Dave Stevenson, a professor of planetary science at the California Institute of Technology who was not involved in the study.
"It's difficult to explain in any other way," said Stevenson, who was among the first scientists 20 years ago to propose that Titan had oceans composed of methane and ethane. Stevenson said the paper did not address a key point - whether volcanism is enough to explain the methane in Titan's atmosphere.
The apparent volcano could still turn out to be an ordinary impact crater from a meteor, according to researcher Louise Prockter of Johns Hopkins University, who wrote a commentary on the study for Nature. Experts hope to study the feature more closely in the coming years as Cassini, which is in orbit around Saturn, makes at least 40 more passes by Titan.
"We are only at the beginning of this fantastic journey," Prockter wrote.