NASA's Dawn spacecraft has discovered evidence of water on the surface of the asteroid Vesta - but not much of it.
The data shows that the evaporation of volatile materials has colored the protoplanet's surface in a broad swath around its equator. And these volatiles were released from minerals likely containing water, with pothole-like features marking areas where the volatiles boiled off.
While Dawn didn't discover actual water ice at Vesta, it found evidence of hydrated minerals delivered by meteorites and dust - signatures of hydrogen, likely in the form of hydroxyl or water bound to minerals in Vesta's surface.
"The source of the hydrogen within Vesta's surface appears to be hydrated minerals delivered by carbon-rich space rocks that collided with Vesta at speeds slow enough to preserve their volatile content," says Thomas Prettyman, the lead scientist for Dawn's gamma ray and neutron detector (GRaND).
Scientists thought it might be possible for water ice to survive near the surface around the giant asteroid's poles. However, Vesta has no permanently shadowed polar regions where ice might survive, and the strongest signature for hydrogen in the latest data came from regions near the equator, where water ice is not stable.
In some cases, space rocks crashed into these deposits at high speed. The heat from the collisions converted the hydrogen bound to the minerals into water, which then evaporated. Escaping water left holes as much as six-tenths of a mile wide and as deep as 700 feet.
"The pits look just like features seen on Mars, and while water was common on Mars, it was totally unexpected on Vesta in these high abundances," says Brett Denevi of the Johns Hopkins University Applied Physics Laboratory.
"These results provide evidence that not only were hydrated materials present, but they played an important role in shaping the asteroid's geology and the surface we see today."
The new findings solidly confirm the connection between Vesta and a class of meteorites found on Earth called the Howardite, Eucrite and Diogenite meteorites, which have the same ratios of iron to oxygen and iron to silicon.