Dwarf planet Ceres may have vanishing ice volcanoes

The dwarf planet Ceres may have lots of hidden old ice volcanoes similar to the one recently discovered by NASA's Dawn spacecraft, say scientists who have found a way such mountains of icy rock might disappear over millions of years.

The NASA spacecraft found a four kilometre tall Ahuna Mons cryovolcano in 2015 on Ceres, the dwarf planet with an orbit between Mars and Jupiter.

Scientists show there may have been cryovolcanoes other than Ahuna Mons on Ceres millions or billions of years ago, but these cryovolcanoes may 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 Michael Sori, from the University of Arizona in Tucson. 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 - signs of geologic youth, Sori said.

This means that either Ahuna Mons is a lone feature that formed recently on an otherwise inactive world, or the cryovolcano is not unusual, and there is some process on that has destroyed its predecessors, 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. Scientists 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. 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 said. On Ceres, viscous relaxation could be causing older cryovolcanoes to flatten out over millions of years so they are hard to discern. Ceres's location close to the sun could make the process more pronounced, Sori said.

To test the idea, researchers created a model using the actual dimensions of Ahuna Mons to predict how fast the mountain might be flowing. They ran the model assuming different water contents of the material that makes up the mountain - ranging from 100 to 40 per cent water ice, Sori said.

They found Ahuna Mons would need to be composed of over 40 per cent water ice to be affected by viscous relaxation. At this composition, Ahuna Mons should be flattening out at a rate of 10 to 50 meters per million years, Sori said.

That is enough to render cryovolcanoes unrecognisable in hundreds of millions to billions of years. "Ahuna Mons is at most 200 million years old. It just hasn't had time to deform," Sori said.

The study was published in the journal Geophysical Research Letters.