SAN FRANCISCO—Radar surveys of the surface of Titan, Saturn's largest moon, have found the most compelling evidence yet for "cold" volcanoes on a celestial body other than Earth. The discovery may help solve a long-standing mystery concerning the presence of methane in that body's atmosphere, and suggests that future missions may have a shot at sampling any life-forms that could exist in its depths.
In a flyby of Titan, NASA's Cassini spacecraft scanned a region called Sotra Facula and spotted at least three geologic features that are in all likelihood volcanoes spewing liquid water through the moon's icy surface. "I was shocked when I made the video," said Randolph Kirk, a geophysicist at the U.S. Geological Survey in Flagstaff, Ariz., who presented the results December 14 here at a meeting of the American Geophysical Union.
Icy volcanoes, or "cryovolcanoes," can grow when water filters through an icy surface and then flows along the sides of the mount until it solidifies, the way lava does on earthly volcanoes.
Planetary scientists have long suspected that cryovolcanoes exist on several bodies in the solar system, notably on Jupiter's moon Europa. In 2005 Cassini observed a plume of icy particles ejected by Titan's smaller sibling Enceladus, but that satellite's weak gravity and lack of atmosphere cause the plumes to disperse in space rather than forming a volcanic cone. The new Cassini findings are "features that we think you can't make in any other way" than by volcanism, Kirk said at a press conference at the meeting.
Previous sightings of cryovolcanoes have been questioned, but this one seems legit. "I think here they have all their ducks in a row," commented Jeffrey Kargel, a planetary scientist at the University of Arizona in Tucson. Kargel spoke at the press conference but was not involved in the study.
Cassini's video showed a total of three volcanoes in the region—two conical features 1,000 and 1,500 meters in height, respectively, and a pit that resembles a large crater, but which suggests that crustal material has been removed by a violent explosion. "Topographic mapping tells us much more than just the images," Kirk said.
Titan is thought to have a thick ice crust floating atop a layer of water. How liquid water, which is denser than ice, can rise to the surface is a mystery, although Kargel says other substances such as ammonia, mixed in the water, can substantially lower density as well as make the solution thicker. If density is still higher than that of ice, some other process must either increase the water's buoyancy (for example the formation of methane bubbles) or squeeze the water inside a reservoir, perhaps as a result of tectonic pressure.
It is unclear what mix is spewing from Titan's volcanoes, but it may be an ammonia-rich solution. Other possibilities exist, such as polyethylene or paraffin waxes, Kargel said.
The cold lava also likely contains methane, which would solve a puzzle. Titan's atmosphere is rich in methane and even has methane clouds, rain and lakes. But sunlight in the upper atmosphere is continuously breaking down the gas by photolysis, so some process must be slowly replacing it. The nature of this process has long been a mystery, which has fueled speculation that some microorganism may be producing methane the way bacteria do on Earth. Volcanism, however, could keep replenishing the gas.
Still, scientists have not given up hope that there could be perhaps some microorganisms living in Titan’s vast underground ocean. If so, volcanoes could make it easier for future missions to discover any life "very conveniently brought to the surface" in the lava, rather than having to drill through miles of ice, Kirk said. "If there is life in the interior, this could be one way to sample it."
"Cold" volcanoes exist even on Earth—for example, a small bitumen (hydrocarbon) volcano off the coast of Santa Barbara, Calif. In fact, "volcanism is very general," Kargel says. It seems to exist throughout the solar system on bodies with various compositions. And super-Earths, the large Earth-like planets that seem to be abundant in our solar system, are likely to have volcanism of some sort, he says.