Icy volcanism likely on Titan
ASTRONOMY NOW

Posted: 18 December, 2008

According to the latest research presented at the American Geophysical Union meeting in San Francisco this week, cyrovolcanoes could be spewing a super-chilled liquid into Titan’s atmosphere.

"Cryovolcanoes are some of the most intriguing features in the Solar System," says Rosaly Lopes, a Cassini radar team investigation scientist. "To put them in perspective, if Mount Vesuvius had been a cryovolcano, its lava would have frozen the residents of Pompeii."

Two regions identified by Cassini that could show evidence for cryovolcanism. In region 1, lobate, flow-like features have been observed. Image: NASA/JPL.

Instead of spewing molten rock in a terrestrial style volcanic eruption, Titan volcanoes are thought to emit volatiles such as water, ammonia and methane. Scientists have long suspected that cryovolcanoes might be at work on Saturn’s moon, and the Cassini mission has collected data that adds fuel to this icy fire. One piece of evidence comes from imagery that reveals a suspect haze hovering over flow-like surface formations.

"Cassini data have raised the possibility that Titan's surface is
active," says Cassini scientists Jonathan Lunine. "This is based on evidence that changes have occurred on the surface of Titan, between flybys of Cassini, in regions where radar images suggest a kind of volcanism has taken place."

Another giveaway was the observed changes in brightness and reflectance - the ratio of light that radiates onto a surface to the amount reflected back - detected at two separate and distinct regions of Titan over a period of two years. In one of the two regions, the reflectance of the surface surged upward and remained higher than expected. In the other region, the reflectance also shot up but then trended downward. Evidence also comes from ammonia frost observed at one of the two sites only at times when the region was inferred to be active.

"Ammonia is widely believed to be present only beneath the surface of Titan," says Robert Nelson, Cassini Visual and Infrared Mapping Spectrometer team member. "The fact that we found it appearing at times when the surface brightened strongly suggests that material was being transported from Titan's interior to its surface."

Titan's surface is hidden by a thick atmosphere. Could cyrovolcanism be responsible for replenishing the methane content of the atmosphere? Image: NASA/JPL/Space Science Institute.

Some Cassini scientists speculate that such volcanism could release methane from Titan's interior, which explains Titan's seemingly continuous supply of fresh methane. If the methane was not replenished - by any mechanism - then Titan would not be expected to still maintain a thick methane atmosphere.

But other scientists argue that the identification of ammonia is not certain, and that the purported brightness changes might not be associated with changes on Titan's surface. Instead it is suggested that they could be attributed to the transient appearances of ground ‘fogs’ of ethane droplets very near Titan's surface, driven by atmospheric rather than geophysical processes. "There remains the possibility that the effect is caused by a local fog, but if so, we would expect it to change in size over time due to wind activity, which is not what we see," comments Nelson.

Yet another alternative hypothesis suggests that Titan could be taking tips from Jupiter’s dead world Callisto. "Like Callisto, Titan may have formed as a relatively cold body, and may have never undergone enough tidal heating for volcanism to occur," says Jeffrey Moore, a planetary geologist at the NASA Ames Research Center. "The flow-like features we see on the surface may just be icy debris that has been lubricated by methane rain and transported downslope into sinuous piles like mudflows."

The plot thickens, but more clues will undoubtedly be uncovered in future flybys of the curious moon, the next of which will occur just a few days before Christmas.