The last place you'd expect to find signs of water erosion is in the Asteroid Belt, but researchers from NASA's Jet Propulsion Laboratory (JPL) say that data collected during the Dawn spacecraft's visit to the protoplanet Vesta indicates that it not only once had water, but that it formed gullies and other erosion features on its surface.
Vesta was originally thought to be bone dry, but images and instrument readings from the unmanned Dawn probe's visit to Vesta, during which is orbited the protoplanet (or giant asteroid) from 2011 to 2013, indicate that water may once have been present on the asteroid and that it had a part to play in forming its features. In particular, Dawn sent back images of young craters with curved gullies and fan-like deposits. About 100 ft (30 m) wide and stretching half a mile (900 m), these gullies are especially prominent in the 9 mi (15 km) wide Cornelia Crater.
The JPL team is quick to point out that what happened wasn't flowing rivers running across the face of Vesta. The extreme cold, hard vacuum and extremely weak gravity wouldn't allow liquid water to exist on the surface – once exposed above a certain temperature, it would immediately sublimate into gas. Instead, the water had a more indirect role to play.
To understand the process, look at a common workshop inhabitant; graphite powder. It's an excellent lubricant that allows metal parts to slide past one another with ease. However, if you read enough old-time science fiction you're bound to come across a passage where a greenhorn astronaut jams up the radar antenna by squirting graphite powder into it, only to have it literally grind to a halt. That's because graphite is actually very gritty stuff, but it acts as a lubricant because water molecules sit between the flat carbon molecules, so they slide about like panes of glass on ball bearings. In space, the water boils away, and the graphite becomes an abrasive.
According to JPL, conditions on ancient Vesta included subsurface ice deposits; possibly left by comets striking the protoplanet. A few hundred million years ago, another impact exposed the deposits and heated them enough to melt the ice. The water trapped underground didn't boil immediately and acted like the water molecules in the graphite, allowing dust, sand, and rocks to slide along to form the gullies and deposits in the impact crater walls.
The team says that Dawn was unable to determine if water is currently present on Vesta, but readings from the spacecraft's visible and infrared mapping spectrometer and gamma ray and neutron detector indicate the presence of hydrated minerals, strengthening the case for a once-wet history.
Dawn was launched on September 27, 2007 atop a Delta II rocket from Space Launch Complex 17B at Cape Canaveral Air Force Station, Florida. After making a flyby of Mars on February 4, 2009 in a slingshot maneuver, it went into orbit around the protoplanet Vesta on July 16, 2011, where it carried out a 14-month survey of its surface.
The 1,240 kg (2,730 lb) spacecraft then used its ion thruster to send it on a three and a half year passage to Ceres. When it arrives on March 6, it will mark the first visit to a dwarf planet by any spacecraft. By visiting Vesta and Ceres, Dawn will have visited the two most massive objects in the asteroid belt, with Ceres the largest object in the asteroid belt between Mars and Jupiter, making up a third of the mass of the entire belt.
The team's results were published in Earth and Planetary Science Letters.