Water and Cryovolcanism on Ceres

Most asteroids in the asteroid belt between Mars and Jupiter are little things. They’re almost cute. In the highly unlikely event that they get kicked out of the asteroid belt and enter Earth’s atmosphere, they probably wouldn’t hurt a fly.

Ceres is basically the opposite. Although it has a stable orbit around the sun and isn’t likely to be going anywhere anytime soon, it could do some serious damage to Earth if it ever hit us. This big boy is classified as a dwarf planet, basically meaning that it’s just big enough for its own gravity to make it more or less round. It is also large enough to possess a variation of volcanism known as cryovolcanism which involves large amount so fwater.

Water. Lots and lots of water.

This view of the whole Occator crater shows the brightly colored pit in its center and the cryovolcanic dome. The jagged mountains on the edge of the pit throw their shadows on parts of the pit. This image was taken from a distance of 1478 kilometers above the surface and has a resolution of 158 meters per pixel. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Ceres contains a lot of frozen water. In fact, 20 to 30 percent of the material that makes up Ceres is believed to be water ice. Even before astronomers could get a clear picture of it, they suspected that Ceres had a lot of water, most notably in one bright spot that could be water ice or just a spot that had been “polished” through encounters with other objects in the asteroid belt.

Ceres contains considerable amounts salty mineral deposits on its surface that are normally formed in seas of briny water. The more dazzling areas like the white spot that could have been frozen water may actually be a form of salt that is similar to Epsom salts known as hydrated magnesium sulfate. Ceres’ surface also has high concentrations of sodium carbonate that is usually left behind when seasonal lakes evaporate. This makes Ceres one of only three worlds known to have carbonates, the other two being Earth and Mars.

It’s possible that these salts came from water ice that transformed directly from ice to vapor, or sublimated, after an impact with another object in the asteroid belt that exposed some of Ceres’ underground ice. Scientists believe that it may have once had an ocean under its surface and could still have some liquid water as a mix of water and mud under the crust.

When the Dawn spacecraft visited Ceres, it detected a seasonal ice cycle in some parts of the dwarf planet, especially in the southern hemisphere. Ceres is close enough to the sun to heat up regions that have more water during the summer and release water vapor that can condense elsewhere on Ceres.

Cryovolcanism on Ceres

Ceres also displays signs of having geysers that spew water vapor and salts in a process known as cryovolcanism that has been observed on other worlds. The European Space Agency’s Herschel Space Observatory picked up some tentative evidence of vapor escaping from two of these geysers at a rate of 13 pounds per second as recently as 2014. Unfortunately, Dawn was not equipped to take a closer look at these possible geysers, so any geyser activity on Ceres could have been in the distant past.

The data suggests that Ceres’ cryovolcanism occurs in cycles, with an outbreak of eruptions that occurs an average of every million years. As one might expect, the estimation average annual volume of material spewed in eruptions is not very large: only about 13,000 cubic yards, or enough to fill four Olympic-sized swimming pools. Despite this rough estimate based on measurements of Ceres’ volcanic domes, predictions of future activity can prove difficult.

“What we can say is only on a speculative basis,” says ESA researcher Ottaviano Ruesch. “There is no reason to exclude events in the upcoming million years.”

Part of the difficulty is caused by the fact that no one really knows what is causing Ceres’ cryovolcanism. On most worlds, volcanism is a symptom of heat produced by radioactive elements in a world’s core or tidal friction. However, Ceres does not appear to have much of the radioactive elements that larger worlds posses and, if radioactivity is causing the cryovolcanism, then these elements may have been stirred up by an ancient impact with another world. There are also no large objects close enough to Ceres for tidal forces to be a plausible explanation.

It is possible that the presence of ammonia, methane, and salts could contribute to the cryovolcanism by lowering the melting point of water. However, the forces behind Ceres’ volcanism remain a mystery.

Just One Mountain

Considering that Ceres shows signs of ancient cryovolcanism, one might think that it has a lot of dead volcanoes made up of salts mixed with any ice that didn’t sublimate upon exposure to space. It does posess some small volcanic domes as large as 53 miles across. Temperatures are not low enough on Ceres for the ice to support very many large structures.

Because of the difficulty of building a large structure on Ceres using natural processes, Ceres only possesses one mountain, known as Ahuna Mons, a volcanic dome reaches a maximum altitude of five kilometers on its steepest side. It has features that are common in volcanic domes found on Earth, Mars, and Venus, including a characteristic pattern of cracks that are usually a sign of brittle shells at the summit that can fracture and produce rockfalls.

Ahuna Mons is believed to be a relatively young mountain because doesn’t show signs of having been hit by as many micrometeorites as the rest of Ceres. In the asteroid belt, a steady rain of micrometeorites is fairly common and has had the effect of “weathering” most of Ceres. Ahuna Mons still shows a sharp definition that shows that it hasn’t been eroded by constant impacts quite as much as the rest of Ceres. It is also relatively bright, indicating that the ice and rock that make up this mountain hasn’t been “aged” as much by solar radiation. Based on these clues, scientists estimate that Ahuna Mons could be between 70 million and 240 million years old.

Ceres is currently one of the smallest worlds known to have cryovolcanism, with others being the moons Enceladus and Europa and fellow dwarf planet Pluto. Future science missions to Ceres might be able to solve some of the mysteries related to Ceres’ cryovolcanism and single large mountain.