As the Dawn spacecraft approaches Ceres, an astonishing claim suggests its biggest discovery may be behind it, with a new paper suggesting Vesta once had surface water.

Vesta is the second most massive asteroid in the inner solar system. Dawn orbited it from July 2011 to September 2012 in what was considered a prelude to its study of Ceres. As UCLA's Dr. Jennifer Scully put it, “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.”

Nevertheless, this is exactly what Scully is claiming to have found, although she admits the water was far from pure.

During its exploration, Dawn provided outstanding images of Vesta's heavy cratering, and detected signs of an ancient magnetic core. At the American Geophysical Union meeting in December, Scully presented evidence that had been missed at the time: The curved gullies around at least eight of Vesta's impact craters look like channels carved by water bodies on Earth. "They form kind of complex networks, similar to what we see in [Arizona's] Meteor Crater," Scully told Space.com.

Surprisingly, a claim of such huge significance didn't attract public attention at the time, but it has now been published in Earth and Planetary Science Letters, and is making up for lost time.

Scully describes both linear and curvilinear gullies on Vesta. She proposes that the sinuous rills are formed by transient water released from subsurface ice deposits. The ice may have been left beneath the surface from previous encounters with comets and then released by asteroid impacts. Dawn's instruments indicate that some of Vesta's rocks contain ice or chemically bound water. 

Aware of the adage about extraordinary claims, Scully and colleagues have supported their claims with computer modeling and physical experiments. They also point to the “pitted terrain” in which the curved gullies exist, which others have attributed to escaping gasses.

"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 in a media release. The team estimate that the proportion of water in the flows would have been less than 30%.

Vesta is roughly as old as the solar system, having been prevented from growing into a planet because Jupiter got there first, disrupting anything with an inconveniently located orbit. However, the impact craters are probably a few hundred million years old at most, and the gullies must be the same age or younger.

The gullies average 30 meters (100 feet) wide and 900 meters (3,000 feet) long, but some, most notably in Cornelia Crater, interconnect and end in fan-shaped deposits.

^{Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Cornelia's gullies drawn in for clarity.}

Confirmation of the theory will be hard. "If present today, the ice would be buried too deeply to be detected by any of Dawn's instruments," Scully said, but Ceres will be closely examined for similar hints.