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Ceres May Have Vanishing Cryovolcanoes

Ahuna Mons, a 2.5-mile- (4 km) tall mountain on Ceres interpreted as a geologically young cryovolcano, may have some hidden older siblings, according to a team of planetary researchers led by Dr. Michael Sori of the University of Arizona.

Ahuna Mons, Ceres’ lonely mountain, is seen in this simulated perspective view. The elevation has been exaggerated by a factor of two. The view was made using enhanced-color images from NASA’s Dawn orbiter. Images taken using blue (440 nm), green (750 nm) and infrared (960 nm) spectral filters were combined to create the view. The spacecraft’s framing camera took the images from Dawn’s low-altitude mapping orbit, from an altitude of 240 miles (385 km) in August 2016. Image credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA / PSI.

Ahuna Mons, Ceres’ lonely mountain, is seen in this simulated perspective view. The elevation has been exaggerated by a factor of two. The view was made using enhanced-color images from NASA’s Dawn orbiter. Images taken using blue (440 nm), green (750 nm) and infrared (960 nm) spectral filters were combined to create the view. The spacecraft’s framing camera took the images from Dawn’s low-altitude mapping orbit, from an altitude of 240 miles (385 km) in August 2016. Image credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA / PSI.

Ahuna Mons cryovolcano was spotted in 2015 by NASA’s Dawn spacecraft.

Other icy worlds in the Solar System — like Pluto, Europa, Triton, Charon and Titan — may also have cryovolcanoes, but Ahuna Mons is conspicuously alone on Ceres.

Now, Dr. Sori and co-authors 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.

“Imagine if there was just one volcano on all of Earth. That would be puzzling,” Dr. Sori said.

Adding to the puzzle are the steep sides and well-defined features of Ahuna Mons – usually signs of geologic youth.

That leads to two possibilities: (i) Ahuna Mons is just as it appears, inexplicably alone after forming relatively recently on an otherwise inactive world; (ii) 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.

Ceres has no atmosphere, so the processes that wear down volcanoes on Earth — wind, rain and ice – aren’t possible on the dwarf planet.

The researchers 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. 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,” Dr. Sori said.

“The process doesn’t affect volcanoes on Earth because they are made of rock, but Ceres’ 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’ location close to the Sun could make the process more pronounced.”

To test the idea that viscous relaxation had caused cryovolcanoes to flatten out on Ceres, Dr. Sori and his colleagues from NASA’s Jet Propulsion Laboratory and Goddard Space Flight Center, USGS Astrogeology Science Center, German Aerospace Center and the University of California, Los Angeles, 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% water ice to 40% water ice.

Ahuna Mons would need to be composed of more than 40% water ice to be affected by viscous relaxation, the authors found.

At this composition, they estimate that Ahuna Mons should be flattening out at a rate of 30-160 (10-50 m) feet per million years. That is enough to render cryovolcanoes unrecognizable in hundreds of millions to billions of years, suggesting there could have been other cryovolcanoes on Ceres.

“Ahuna Mons is at most 200 million years old. It just hasn’t had time to deform,” Dr. Sori said.

The findings were published online January 24, 2017 in the journal Geophysical Research Letters.

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Michael M. Sori et al. The vanishing cryovolcanoes of Ceres. Geophysical Research Letters, published online January 24, 2017; doi: 10.1002/2016GL072319