Titan's volcanoes ooze cold slurry rather than molten lava
Titan, the haze-shrouded moon of Saturn, displays tantalising evidence of ice volcanoes.
Two regions of Titan have been seen recently, by the Cassini spacecraft, to undergo clear changes in brightness.
This activity, and radar images hinting at flow-like structures, suggest the presence of volcanoes, scientists say.
Rather than erupting molten rock, Titan's "cryovolcanoes" are thought to ooze a slurry made of water ice, ammonia and methane.
There are suggestions that these frigid lava flows could be as much as 200m thick.
"Cryovolcanism is a process that many people have modelled in theory and shown to be viable in the outer Solar System on an object of Titan's size," said Bob Nelson from the US space agency Nasa's Jet Propulsion Laboratory.
Researchers working on the Cassini mission argued their case here at the American Geophysical Union's Fall Meeting.
Early flybys of Titan by the US-European spacecraft spotted intriguing surface features that suggested the presence of cryovolcanism, but the thick atmosphere that shrouds this enigmatic world has always made definitive statements tricky.
The evidence, however, is mounting.
Cassini scientists can now point to distinct changes in brightness and reflectance at two separate locations in Titan's equatorial region.
The changes were picked up by Cassini's Visible and Infrared Mapping Spectrometer instrument on flybys from July 2004 to March 2006.
In one of the two regions, the reflectance of the surface surged upward and remained higher than expected. In the other region, the reflectance shot up, but then declined.
The Cassini spacecraft spotted the suspected volcanoes in different regions
Cassini's radar - an instrument that can pierce the thick atmosphere to map the surface, at low resolution - sees lobe-like features at the two locations. Their thickness, about 200m, is consistent with a cyrovolcanic flow interpretation.
"These flows would come out as a thick slurry," said Dr Rosaly Lopes, a Cassini radar team investigation scientist. "They can be thick because cryomagma would be viscous, similar to lava flows on Earth."
Scientists say they also have evidence that ammonia frost is sometimes present at one of the two sites. The ammonia was evident only at times when the region was inferred to be active.
"Ammonia is a material that many thought would be in Titan's interior but not found on the surface," explained Dr Nelson.
"So the finding of ammonia on the surface for temporary periods of time strongly implies materials from the interior are being transported and fused on to the surface."
Titan's thick atmosphere makes observing the planet's surface difficult
Scientists like the idea of cryovolcanism because it is one way to explain why so much methane is retained in Titan's atmosphere.
Without some means of replenishment, the moon's original methane content should have been destroyed long ago by the Sun's ultraviolet light.
Not all scientists are convinced by the latest assessment, however.
Jeffrey Moore, a Nasa planetary geologist independent of the Cassini mission, told the meeting: "The flow-like features we see on the surface may just be icy debris that has been lubricated by methane rain and transported down-slope into sinuous piles like mudflows."
Dr Nelson countered: "Scepticism is part of the evolution of a scientific finding but logic dictates that we start looking at things in certain ways when certain patterns start falling together."
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