Paul Drye's reaction on Facebook to the news--reported by the BBC's Jonathan Amos
--that the surface of asteroid/dwarf planet Vesta
might be marked by running water was "!!!". I share the sentiment: the idea that a dwarf planet 525 kilometres in diameter, with gravity 2.5% that of Earth and no atmosphere whatsoever, could have supported running water is counterintuitive.
[P]ictures 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 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.
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.