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Ahuna Mons on Ceres
Ceres’ lonely mountain, Ahuna Mons, 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 mission. (Credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA / PSI)

Scientists say a mysterious mountain on the dwarf planet Ceres was apparently once an ice volcano, spewing salty water and mud instead of lava.

They also say Ceres has patches of water ice that can be seen on or near the surface, and might have an off-and-on atmosphere that contains water vapor.

The scientists say all this and more in six research papers published in this week’s issue of the journal Science. The studies are based on more than a year’s worth of orbital observations from NASA’s Dawn spacecraft. Dawn is still circling 590-mile-wide Ceres, which is the solar system’s smallest dwarf planet as well as its biggest main-belt asteroid.

The mysterious mountain is Ahuna Mons, a 3-mile-high peak that looks like a bright space pyramid. Scientists took note of the peak’s concave top, its elliptical base, cracks at the summit, steep slopes and other features that pointed to previous volcanic activity.

 
A research team led by Ottaviano Ruesch of NASA’s Goddard Space Flight Center concluded that Ahuna Mons was built up through eruptions of ice and mud as recently as a few hundred million years ago. “This is the only known example of a cryovolcano that potentially formed from a salty mud mix, and that formed in the geologically recent past,” Ruesch said today in a news release.

“Ahuna Mons is evidence that Ceres was active in the recent past and might be still active today,” he said.

It’s conceivable that cryovolcanism played a role in creating some of the other bright spots observed on Ceres’ surface. Earlier this year, a different team of Dawn researchers reported that those patches contain high concentrations of carbonates, which might have been brought to the surface millions of years ago by upwelling water.

Researchers working on NASA’s New Horizons mission have reported ice volcanoes on Pluto, and the Cassini probe has detected water spewing from fissures in the icy crust of Enceladus, one of Saturn’s moons. Another Saturnian moon, Titan, and the Neptunian moon Triton are also cryovolcanic hot spots. But Ruesch and his colleagues say Ahuna Mons is different, due to its mix of salt, ice and mud.

Another study lays out the evidence for exposed water ice. A team led by Jean-Philippe Combe of the Bear Fight Institute in Winthrop, Wash., analyzed readings from Dawn’s visible and infrared mapping spectrometer. They concluded that water ice was mixed with surface materials in an area covering less than a square kilometer, inside Oxo Crater at Ceres’ mid-latitudes.

Oxo Crater
This perspective view shows 6-mile-wide Oxo Crater, with the arrow indicating a patch of water ice. (Credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA / PSI)

The conditions on Ceres are such that water ice should vaporize within only a few decades. That suggests that the icy material was exposed fairly recently – perhaps as the result of asteroid impacts of landslides.

Dawn’s researchers also reported the unexpected detection of energetically charged solar particles at Ceres. A team led by UCLA’s Christopher Russell, the Dawn mission’s principal investigator, proposes that the particles are interacting with a whisper-thin atmosphere that may contain water vapor.

That would be consistent with earlier findings from the Herschel Space Observatory, suggesting that wisps of water vapor emanate from Ceres. However, there could be alternate explanations for the readings from Dawn’s gamma ray and neutron detector.

“We’re very excited to follow up on this and the other discoveries about this fascinating world,” Russell said.

Dawn was launched in 2007 and has studied the asteroid Vesta as well as Ceres, at a cost of about $450 million for its primary mission. Last month, NASA extended Dawn’s mission for at least another two years. On Friday, the spacecraft is due to start raising its orbit from the current altitude of 240 miles to a fuel-saving, perspective-changing 910 miles.

Dawn 3-D view of Ceres
Get out your red-blue glasses and feast your eyes on this 3-D mosaic of Ceres’ surface, centered on the dormant ice volcano known as Ahuna Mons. Click on the image for a larger version. (Credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA / PSI)
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