Lone Volcano

“Imagine if there was just one volcano on all of Earth,” posits Michael Sori of the University of Arizona. “That would be puzzling.” That’s why scientists were curious when NASA’s Dawn spacecraft discovered dwarf planet Ceres’s 4-kilometer (2.5-mile) tall Ahuna Mons cryovolcano in 2015. Other icy worlds in our solar system (Pluto, Europa, Triton, Charon, and Titan) very possibly possess cryovolcanoes—where molten ice (water, usually mixed with salts or ammonia) replaces the molten silicate rock erupted by terrestrial volcanoes—but Ahuna Mons is conspicuously alone on Ceres. Could Ceres’s proximity to the Sun (closer than the above worlds anyway, with an orbit between Mars and Jupiter) have something to do with its lonely nature? Or is it the mountain’s geologic youth?

Sori and colleagues decided to find out. They started with two possibilities: Ahuna Mons is just as it appears, inexplicably alone after forming relatively recently on an otherwise inactive world. Or, option two, there is some process on Ceres that has destroyed its predecessors and left the young Ahuna Mons as the solitary cryovolcano on the dwarf planet.

Ceres has no atmosphere, so the processes that wear down volcanoes on Earth—wind, rain, and ice—aren’t possible on the dwarf planet. But the research team hypothesized that another process, called viscous relaxation, could be at work.

Viscous relaxation is the idea that just about any solid will flow, given enough time. On Earth, viscous relaxation is what makes glaciers flow. The process doesn’t affect volcanoes on Earth because they are made of rock, but Ceres’s volcanoes contain ice—making viscous relaxation possible. On Ceres, viscous relaxation could be causing older cryovolcanoes to flatten out over millions of years so they are hard to discern. Ceres’s proximity to the Sun could make the process more pronounced.

The scientists ran models and determined that Ahuna Mons would need to be composed of more than 40 percent water ice to be affected by viscous relaxation. At this composition, Sori estimates that Ahuna Mons should be flattening out at a rate of 10 to 50 meters (30 to 160 feet) per million years. That doesn’t sound like much but it’s enough to render cryovolcanoes unrecognizable in hundreds of millions to billions of years, suggesting there could have been other cryovolcanoes on Ceres, which have disappeared over the dwarf planet’s 4.57 billion year lifetime.

“We think we have a very good case that there have been lots of cryovolcanoes on Ceres but they have deformed,” Sori says. And as for the lone cryovolcano? “Ahuna Mons is at most 200 million years old. It just hasn’t had time to deform.”

The next step for Sori and his team will be to try and identify the flattened remnants of older cryovolcanoes on Ceres. The findings could help scientists better decipher the history of how the dwarf planet formed, Sori adds. Their findings were published recently in Geophysical Research Letters.