Dawn probe spies possible water-cut gullies on Vesta

Vesta gullies Water suspect: The Type B gullies are longer and narrower

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Scientists say they have seen features on Asteroid Vesta that look as though they could have been cut by some sort of fluid flow - possibly liquid water.

If correct, it is an extraordinary observation because any free water on the surface of the airless body would ordinarily boil rapidly and vaporise.

But pictures of Vesta taken by Nasa's Dawn probe show complex gullies running down the walls of some craters.

The possibility of liquid erosion needs to be considered, say the researchers.

"We want to hear what other people's opinions are," Jennifer Scully, from the University of California, Los Angeles (UCLA), told BBC News.

"We're just putting it out there to the community; we're not suggesting anything hard and fast at this stage."

Ms Scully was speaking here at the American Geophysical Union (AGU) Fall Meeting, the largest annual gathering of Earth and planetary scientists.

The Dawn satellite spent more than a year investigating Vesta, the second largest member in the asteroid belt between Mars and Jupiter.

The probe departed the body this past September, but not before it had mapped most of the surface from an altitude of just 210km.

This allowed the Nasa mission to pick out surface features in fine detail.

Ms Scully examined all of the craters on Vesta that measured about 10km and wider, cataloguing the shapes of the gullies that etched their walls.

In the majority of cases (about 50 examples), the troughs trace simple descent lines and are presumably the consequence of loose rock or soil falling down the slope. But in a second, smaller group (11 examples), the pattern the gullies cut in the surface is quite different. They are complex; they are interlaced.

"The first group we call Type A. They're very typical of dry-mass wasting; the sort of thing you would get on Earth's Moon and on other, smaller asteroids. But the Type B gullies are the ones we think may have this liquid water origin; they have quite distinct morphologies. They are longer and narrower. They also interconnect, branching off one another."

If it was liquid water that carved these features, the question then arises as to its source.

Cornelia Cornelia Crater has some of the best examples of the more complex gullies

Vesta is recognised generally to be a very dry body. Geological processes in its early history are thought to have driven off the vast majority of its volatile materials.

And in any case, with no pressure from an atmosphere, the asteroid cannot sustain liquid water at its surface for very long. Any such fluid would be lost to space in short order.

This means any reserve of water must be retained beneath the surface.

"[It] would be cool enough just a few metres or even some centimetres beneath the surface that water could be preserved for a long time," said Prof Chris Russell, the principal investigator on the Dawn mission. "So we have some mechanisms like comets that might bring water to the surface - then it could be stored for some period of time."

Perhaps that buried ice later melted and burst out; may be it was water bound up in the rock, in hydrated minerals, that was released by the heating associated with an impact at the surface.

In the near-equatorial Cornelia depression, for example, the Type B gullies tend to form in the dark material that dominates some areas of the crater.

Vesta The Dawn probe studied Vesta for over a year

It has been suggested this material is very similar to carbonaceous chondrite meteorites, which can contain a lot of water and the related hydroxyl (OH) mineral.

Could the impact that created Cornelia have temporarily increased local temperatures and pressures sufficiently to allow liquid water briefly to flow and cut the gullies before spontaneously boiling off?

The observations mirror those for Mars back in 2000 when satellite pictures also revealed complex gully systems running down the walls of craters on the Red Planet.

A fierce debate then ensued between those who argued they were water-cut and those who proposed other explanations that did not require water. One alternative proposed that the channels were cut by a tumbling mass of dry material suspended in a flow of carbon dioxide.

The controversy of the Martian gullies prompts Prof Russell to be cautious in the interpretation of the Vesta gullies.

"We want to be very, very sure on any statement or pronouncement we make about the gullies or water or anything like that, because it turns out that there are a lot of different interpretations - we have to work our way through them," he said.

"That's exactly how the scientific method works - we have an idea; we have to test it against the available evidence, and a lot of people are shareholders in that evidence and they will talk to us about what their evidence is," he told BBC News.

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos

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