NASA probe sees possible water-cut gullies on Vesta

Pictures of Vesta taken by NASA’s Dawn probe show complex gullies running down the walls of some craters.

London: Pictures of Vesta taken by NASA’s Dawn probe show complex gullies running down the walls of some craters.

But researchers say the possibility of liquid erosion needs to be considered because any free water on the surface of the airless body would ordinarily boil rapidly and vaporise.

“We want to hear what other people’s opinions are. We’re just putting it out there to the community; we’re not suggesting anything hard and fast at this stage,” Jennifer Scully, from the University of California, Los Angeles (UCLA), told BBC News.

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.

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,” she said.

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

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,” he stated.


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