The dwarf planet Ceres may actually host many "hidden" ice volcanoes, besides the only one discovered recently, a study says.
NASA's Dawn spacecraft discovered Ceres's four-kilometre tall Ahuna Mons cryovolcano in 2015.
Other icy worlds in our solar system, like Pluto, Europa, Triton, Charon and Titan, may also have such mountains of icy rock — called cryovolcanoes — but Ahuna Mons is conspicuously alone on Ceres.
Now, in a paper accepted for publication in the journal Geophysical Research Letters, scientists showed there might have been cryovolcanoes other than Ahuna Mons on Ceres millions or billions of years ago, but these cryovolcanoes might have flattened out over time and become indistinguishable from the planet's surface.
"We think we have a very good case that there have been lots of cryovolcanoes on Ceres but they have deformed," said lead study author Michael Sori of Lunar and Planetary Laboratory at University of Arizona in Tucson, US.
Ahuna Mons is a prominent feature on Ceres, rising to about half the height of Mount Everest. Its solitary existence has puzzled scientists since they spied it.
Adding to the puzzle are the steep sides and well-defined features of Ahuna Mons — usually signs of geologic youth, Sori said.
That leads to two possibilities. Ahuna Mons is just as it appears, inexplicably alone after forming relatively recently on an otherwise inactive world. Or, the cryovolcano is not alone or unusual, and 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, according to Sori.
Ceres has no atmosphere, so the processes that wear down volcanoes on Earth — wind, rain and ice — are not possible on the dwarf planet.
Sori and his colleagues hypothesised 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. For example, a cold block of honey appears to be solid. But if given enough time, the block will flatten out until there is no sign left of the original block structure.
On Earth, viscous relaxation is what makes glaciers flow, Sori explained.
"Ahuna Mons is at most 200 million years old. It just has not had time to deform," Sori said.
The researchers said they would next 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 added.