NASA scientists have spotted long, narrow gullies on the giant asteroid Vesta that they say may have been carved out by liquid water.
In images from the Dawn mission, the gullies appear on the walls of geologically young craters. Some look like straight chutes and others carve more sinuous trails that end in lobe-shaped deposits.
"The straight gullies we see on Vesta are textbook examples of flows of dry material, like sand, that we've seen on Earth's moon and we expected to see on Vesta," says Jennifer Scully, a Dawn team member at the University of California, Los Angeles. "But these sinuous gullies are an exciting, unexpected find that we are still trying to understand."
The sinuous gullies are long, narrow and curved. They tend to start from V-shaped, collapsed regions described as 'alcoves' and merge with other gullies. The team members believe that different processes formed the two types of gullies and have been looking at images of Earth and Mars for clues.
"On Earth, similar features - seen at places like Meteor Crater in Arizona - are carved by liquid water," says Christopher Russell, Dawn's principal investigator, also at UCLA.
"On Mars, there is still a debate about what has caused them. We need to analyze the Vesta gullies very carefully before definitively specifying their source."
Fresh-looking gullies were first discovered on Mars in images from NASA's Mars Global Surveyor in 2000. Some scientists suggest water as a cause, some carbon dioxide, and some neither. One study in 2010 suggested that carbon-dioxide frost was triggering fresh flows of sand.