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Dawn Probes Role of Cryovolcanism on Ceres

Ceres Ahuna Mons.jpgAhuna Mons is a towering mound on Ceres likely created when a supercold mixture of water and other compounds erupted from below. In this simulated perspective view, assembled from enhanced color images taken by NASA's Dawn spacecraft, elevations are exaggerated by a factor or two.

NASA's Dawn mission has opened up the brave new world of 1 Ceres, our solar system's largest asteroid. Now a recent study, published today inNature Astronomy, has revealed something of the fascinating history of cryovolcanic activity on this dwarf planet, leading up to the surface we see today.

Cryovolcanisminvolves eruptions of slurries of water mixed with some kind of "antifreeze" - perhaps liquid ammonia (NH3) or methane (CH4) - at temperatures far below water's usual freezing point. The resulting flows are the supercold equivalents of the familiar rock magma involved in volcanic activity here on Earth.

Because Ceres is believed to harbor a deep, briny mix of fluid and solids beneath a rigid crust, geologists expect to find evidence of cryovolcanism on its surface. The large, isolated mound called Ahuna Mons is a fine example. The big question, though, is why Ceres isn't dotted with many more of these icy piles.

The key appears to lie in the subtleties ofrheology,the physics of how material (in this case, ice) flows over time in response to gravity. The study used a combination of flow modeling coupled with topographic analysis to piece together the cryovolcanic history of Ceres.

Led by Michael Sori (University of Arizona), researchers carefully analyzed the appearance of 22 domes on Ceres, their aspect ratios (that is, how wide they are versus how high they rise above the surrounding plain), and their distribution by latitude. For the purposes of the study,tholi(the IAU-approved moniker for smaller, eruptive domes) andmontes(larger mountains) were grouped together generically as "domes".

The team used the unique feature called Ahuna Mons as their reference standard. It towers 2.4 miles (4 km) above the surrounding landscape, and (based on the paucity of impact craters nearby) it appears to be no older than 240 million years - and it could be much younger. Ahuna Mons has a relatively high aspect ratio of near 0.2 (its base is five times wider than its height).

Then Sori and his colleagues estimated the age of 21 domes (all at least 10 km across) by assuming that they started out as tall piles of cryovolcanic material, much like Ahuna Mons, that slumped into shorter, broader mounds at a predictable rate over time. (The technical term for this isviscous relaxation.) The resulting age estimates range from hundreds of millions to 2 billion years old.

The study also found that the higher a dome's latitude on Ceres, the higher its aspect ratio. For example, domes near the equator of Ceres have low aspect ratios of around 0.06, while a large dome named Yamor Mons (at 85½° north) nearly matches Ahuna's 0.2 aspect ratio. Since the poles of Ceres are much colder, the rate of slumping there should be glacially slow.

Mori and his team used the global distribution of the domes and their estimated ages to deduce how often cryogenic activity should occur on Ceres. They deduced that an eruption might happen once per 50,000 years on average. Over time, these eruptions could disgorge about 10,000 cubic meters of briny slush onto the surface per year. That's equivalent to the volume of a large movie theater or four Olympic-size swimming pools - but it's a tiny fraction of the rate here on Earth, whose volcanoes spew more than 1 billion cubic meters of molten rock each year.

"We're excited that we were able to come up with a cryovolcanic rate to compare with the rates of 'regular' volcanism on terrestrial planets, something that hadn't been done before from observations," Sori toldSky & Telescope.

What powers cryovolcanism on Ceres? Shouldn't such a small body have cooled internally by now? The most likely internal source powering cryovolcanic processes on Ceres is radioactive decay of certain isotopes in the rocky parts of the tiny world, as happens on other planets. Another, more controversial hypothesis is that large impacts on the surface of Ceres jump-start the process locally.

  "We've shown Ceres may have been cryovolcanically active throughout its entire history, and there's no need to try to invoke a weird thermal/volcanic history that would have caused only a single cryovolcano (Ahuna Mons) to form very recently, with no volcanism before that," says Sori. "Cryovolcanism is important on Ceres, but not nearly as dominant or important as rocky volcanism has been on Earth or Mars."

Source: Sky and Telescope

























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