By Jet Propulsion Laboratory, Pasadena, California — Published: April 11, 2011
Four moons huddle near Saturn's multi-hued disk. NASA/JPL-Caltech/Space Science Institute
Have the surface and belly of Saturn's smog-shrouded moon, Titan, recently simmered like a chilly, bubbling cauldron with ice volcanoes, or has this distant moon gone cold? Data collected by the Cassini spacecraft suggest Titan may be much less geologically active than some scientists have thought.
In a paper, scientists conclude Titan's interior may be cool and dormant and incapable of causing active ice volcanoes.
"It would be fantastic to find strong evidence that clearly shows Titan has an internal heat source that causes ice volcanoes and lava flows to form," said Jeff Moore from NASA's Ames Research Center in Moffett Field, California. "But we find that the evidence presented to date is unconvincing, and recent studies of Titan's interior conducted by geophysicists and gravity experts also weaken the possibility of volcanoes there."
Scientists agree that Titan shows evidence of having lakes of liquid methane and ethane, valleys carved by these exotic liquids, and impact craters. However, a debate continues to brew about how to interpret the Cassini data on Titan. Some scientists theorize ice volcanoes exist and suggest energy from an internal heat source may have caused ice to rise and release methane vapors as it reached Titan's surface.
The scientists conclude that the only features on Titan's surface that have been unambiguously identified were created by external forces — such as objects hitting the surface and creating craters, wind and rain pummeling its surface, and the formation of rivers and lakes.
In December 2010, a group of Cassini scientists presented new topographic data on an area of Titan called Sotra Facula, which they think makes the best case yet for a possible volcanic mountain that once erupted ice on Titan. Although Moore and Pappalardo do not explicitly consider this recent topographic analysis, they do not find the recent analysis of Sotra Facula to be convincing so far. It remains to be seen whether ongoing analyses of Sotra Facula can change minds.
Titan, Saturn's largest moon, is the only known moon to have a dense atmosphere, composed primarily of nitrogen, with 2 to 3 percent methane. One goal of the Cassini mission is to find an explanation for what, if anything, might be maintaining this atmosphere.
Titan's dense atmosphere makes its surface difficult to study with visible-light cameras, but infrared instruments and radar signals can peer through the haze and provide information about both the composition and shape of the surface.
"Titan is most akin to Jupiter's moon Callisto, if Callisto had weather," Moore said. "Every feature we have seen on Titan can be explained by wind, rain, and meteorite impacts, rather than from internal heating."
Callisto is almost the exact same size as Titan. It has a cratered appearance, and because of its cool interior, internal forces do not affect its surface features. Moore and Pappalardo conclude that Titan also might have a cool interior, with only external processes like wind, rain, and impacts shaping its surface.
The Cassini spacecraft, currently orbiting Saturn, continues to make flybys of Titan. Scientists will continue to explore Titan's mysteries, including investigations of the changes in the landscapes.