Evidence of past water flows discovered on Vesta

Chuck Bednar for redOrbit.com – Your Universe Online

Vesta, the propoplanet visited by NASA’s Dawn spacecraft from 2011 through 2013, may have once been home to short-lived flows of water-mobilized material – a discovery which could have tremendous implications for the field of planetary science, according to the US space agency.

Experts previously believed Vesta, the second largest asteroid in the solar system behind Ceres, was unable to retain surface water due to low temperature and pressure. But new data from Dawn has led to the discovery of geological features indicating that it may not have always been completely anhydrous.

“Nobody expected to find evidence of water on Vesta. The surface is very cold and there is no atmosphere, so any water on the surface evaporates,” said Jennifer Scully, one of the authors of the study and a postgraduate researcher at the University of California, Los Angeles. “However, Vesta is proving to be a very interesting and complex planetary body.”

Specifically, she and her colleagues identified a handful of young craters on the protoplanet that had curved gullies and fan-shaped (lobate) deposits. Writing in the February issue of the  journal Earth and Planetary Science Letters, they proposed that transient water flowed on its surface in a debris flow-like process that left the distinctive features on its surface.

“We’re not suggesting that there was a river-like flow of water. We’re suggesting a process similar to debris flows, where a small amount of water mobilizes the sandy and rocky particles into a flow,” Scully said. “These features on Vesta share many characteristics with those formed by debris flows on Earth and Mars.”

She added that these curved gullies differ significantly from those formed by the flow of dry material. Combined with the recent discovery of water evaporation on Ceres and water activity on smaller asteroids, the authors report that their findings “support the new paradigm that water is widespread in the asteroid belt.”

The gullies are described as being fairly narrow, averaging just 100 feet (30 meters wide), and they are a little over one-half mile (900 meters) in length, the researchers said. Some of the best example of these curved gullies and fan-shaped deposits were found in Vesta’s Cornelia Crater, which is nine miles (15 kilometers) wide.

The other gullies

So what was the source of these curved gullies? Scully and her colleagues believe they came from small, localized ice patches in the subsurface of Vesta. The origins of this ice is unknown, but it is possible that comets or other ice-rich bodies impacted onto the asteroid and left some of their ice buried deep in the subsurface.

This would then be followed by another impact, which forms a crater, heats up some of the ice patches, and releases liquid water onto the crater walls. The H2O then caused sandy and rocky particles to flow down the crater walls, carving out the gullies and leaving the lobate deposits in their wake.

“If present today, the ice would be buried too deeply to be detected by any of Dawn’s instruments. However, the craters with curved gullies are associated with pitted terrain, which has been independently suggested as evidence for loss of volatile gases from Vesta,” Scully said. Also, evidence from Dawn’s visible and infrared mapping spectrometer and other instruments indicates that some rocks on Vesta’s surface contain hydrated material.

The craters with curvy gullies appear to be less than a few hundred million years old, while Vesta itself is roughly 4.6 billion years old. Laboratory experiments performed at NASA’s Jet Propulsion Laboratory in California indicated that there could have been enough time for the curved gullies to form on the asteroid before all of the water evaporated.

“These results, and many others from the Dawn mission, show that Vesta is home to many processes that were previously thought to be exclusive to planets,” said UCLA’s Christopher Russell, principal investigator for the Dawn mission. “We look forward to uncovering even more insights and mysteries when Dawn studies Ceres.”

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