NASA's Dawn spacecraft is currently orbiting Ceres, the solar system's innermost dwarf planet. Many weird and wonderful things have been discovered, but some of these findings have created new and perplexing mysteries. One such mystery is centered around Ceres' ice volcanoes - or lack thereof.
Ice volcanoes, or cryovolcanoes, are features that are thought to be common on the surfaces of cold bodies in the outer solar system, creating ice analogs of the volcanoes we have on Earth. Terrestrial volcanoes are driven by tectonic activity forcing molten rock, or magma, from the Earth's interior to the surface, forming volcanic features. However, in the outer solar system, volatile ices are slowly forced to the surface, not magma, forming cryovolcanic features of ice mixed with rock.
When Dawn went into orbit about Ceres, scientists have been wanting to confirm whether or not the small world is really composed of large quantities of water ice. Indeed, Ceres does have a plentiful supply of ice as it turns out, but the only candidate cryovolcano it has found to date is the beautifully-formed Ahuna Mons (pictured above) - a neat-looking, steep-sided mountain about half the height of Mount Everest. There are few geological processes that could have formed Ahuna Mons, so cryovolcanism is the leading candidate. But if Ceres is active, it's strange that it should have only one cryovolcano.
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"Imagine if there was just one volcano on all of Earth," said Michael Sori, of the Lunar and Planetary Laboratory at the University of Arizona in Tucson. "That would be puzzling."
So, in new research accepted for publication in the journal Geophysical Research Letters, Sori and his team believe that Ceres actually did have more cryovolcanoes, but Ahuna Mons' siblings have quite literally morphed into the landscape and no longer possess their cryovolcanic shape.
As Ceres has no atmosphere, landscape features aren't simply eroded away by weathering and though asteroid impacts would no doubt have their own erosional effect on the landscape, the researchers realized that a far more subtle process is likely at work. "Viscous relaxation" is a process by which a an apparently solid material will slowly flow. For example, viscous relaxation is what causes glaciers to flow into the ocean.
By running computer simulations of cryovolcanoes that are composed of between 40-100 percent ice, the researchers realized that viscous relaxation would have an effect on mountains on Ceres' surface, and, if Ahuna Mons is at least 40 percent ice, it too will be succumbing to the process, shrinking 10 to 50 meters (30 to 160 feet) per million years.
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"Ahuna Mons is at most 200 million years old. It just hasn't had time to deform," said Sori in a statement.
The next step is seek out landforms that might reveal the flattened remains of ancient cryovolcanoes that have completely succumbed to viscous relaxation, in turn revealing a fascinating insight to how Ceres evolved, how much ice it contains and how cryovolcanoes elswhere in the solar system might be formed.
"It would be fun to check some of the other features that are potentially older domes on Ceres to see if they fit in with the theory of how the shapes should viscously evolve over time," said Kelsi Singer, a postdoctoral researcher at the Southwest Research Institute in Boulder, Colo., who wasn't involved in this study. "Because all of the putative cryovolcanic features on other worlds are different, I think this helps to expand our inventory of what is possible."
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