Volcanoes on Earth are associated with eruptions of fiery rock, ash, and gases. However, another type of volcano exists in the universe—ice volcanoes.

Dr. Rosaly Lopes, a senior research scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, U.S.A., says cryovolcanoes, or ice volcanoes, are found on Enceladus, a moon of Saturn. Lopes and other scientists hypothesize that cryovolcanoes also exist on Triton, a moon of Neptune, and Titan, another moon of Saturn.

Lopes has studied volcanoes on Earth, which spew hot, molten rock known as lava. Cryovolcanoes erupt with different materials. “These satellites have ice crusts,” she says. “And under the ice crusts, there is a layer of water, or perhaps water with something else like ammonia, and if that liquid can come to the surface, that is what we call cryovolcanism. It just means cold volcanism.”

Though the material erupting from a cryovolcano is different from that of a terrestrial volcano, the action that causes the eruption is comparable, she says.

“We call it volcanism because it’s a process that brings material from the interior of the satellite to the surface,” Lopes says. “So that is similar to volcanism as we see on other planets, but the material itself is quite different. It’s an aqueous mixture rather than molten rock.”

According to Lopes, there are two necessary ingredients for cryovolcanism: “You need a heat in the interior and a liquid under the surface that can become buoyant.”

Lopes says the best examples of cryovolcanism are on Saturn’s moon Enceladus. When NASA’s Cassini spacecraft flew by Enceladus in 2005, it took images of at least 20 icy plumes spewing a mixture of ice particles, water vapor, and other materials into space.

Dr. Randy Kirk, a geophysicist at the U.S. Geological Survey’s Astrogeology Science Center in Flagstaff, Arizona, U.S.A., compares the icy plumes to a phenomenon that occurs here on Earth. “It’s like a geyser that reaches escape velocity and blasts the steam into space,” he says.

Kirk and Lopes note that there is evidence of past cryovolcanism on Triton, the largest of Neptune’s 13 moons. NASA’s Voyager 2 spacecraft helped scientists observe that Triton’s surface is composed of smooth plains, mounds, and rock pits. Many scientists hypothesize that the landscape was created in part by a cryovolcano’s icy flows.

Titan
Recently, Lopes and Kirk have turned their attention to Titan, the largest of 62 known moons orbiting Saturn. After Kirk constructed a 3-D model of an area on Titan known as Sotra Facula, Lopes began thinking the moon might be home to cryovolcanoes.

“It became apparent that the feature [Sotra Facula] looked volcanic,” Lopes says. “There was a tall mountain and a crater next to it like a pit. There were flows. Our best interpretation is that it’s a cryovolcanic region.”

The existence of cryovolcanism on Titan is still debated in the scientific community. Lopes admits scientists have yet to see evidence of significant heat—a necessary part of cryovolcanism—on the moon.

“What would excite me the most would be if we actually saw a thermal signal that indicated active cryovolcanism or some other surface change that really could confirm the idea that volcanism on Titan is still taking place,” she says.

If cryovolcanism were occurring on Titan, it would make the moon a more interesting place for scientists. “The question is whether Titan is dead or alive,” Lopes says. “Is it a world that’s still changing from its interior, or has it stopped doing anything a long time ago and now the surface just sits there being modified by what we call exogenic processes, which are processes that are external, like erosion and impact cratering.”

Kirk says that if the interior of Titan is composed of methane, cryovolcanism could help account for why the gas is so present in the moon’s atmosphere.

“It would be a puzzle piece that would help explain why we see a methane-rich atmosphere on Titan,” he says.

The confirmed existence of cryovolcanism on Titan could lead to an even greater discovery.

“If it had volcanism in the past or it still has any activity, you open up the possibility for some very interesting chemistry if you have heat and you have water,” Lopes says. “Then there is the possibility of life.”