Recent images snapped by NASA's Dawn spacecraft confirm that volatile, or easily evaporated materials, have colored Vesta's surface in a broad swath around its equator.
To be sure, pothole-like features mark some of the giant asteroid's surface where the volatiles, likely water, released from hydrated minerals boiled off. While Dawn did not find actual water ice at Vesta, there are signs of hydrated minerals delivered by meteorites and dust evident in the giant asteroid's chemistry and geology.
"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," explained Thomas Prettyman, the lead scientist for Dawn's gamma ray and neutron detector (GRaND) at the Planetary Science Institute in Tucson, Ariz..
According to Prettyman, Vesta is the second most massive member of the main asteroid belt. Indeed, scientists originally hypothesized that it might be possible for water ice to survive near the surface around the giant asteroid's poles. Unlike Earth's moon, however, Vesta has no permanently shadowed polar regions where ice might survive. The strongest signature for hydrogen in the latest data came from regions near the equator, where water ice is not stable.
In some cases, other space rocks crashed into these deposits later at high speed. The heat from the collisions converted the hydrogen bound to the minerals into water, which evaporated. The holes that were left as the water escaped stretch as much as 0.6 miles (1 kilometer) across and go down as deep as 700 feet (200 meters). Seen in images from Dawn's framing camera, this pitted terrain is best preserved in sections of Marcia crater.
"The pits look just like features seen on Mars, but while water was common on Mars, it was totally unexpected on Vesta in these high abundances," said Brett Denevi, a Dawn participating scientist based at the Johns Hopkins University Applied Physics Laboratory in Laurel.
"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."
Launched on Sept. 27, 2007, Dawn first entered orbit around Vesta on July 15, 2011 PDT (July 16 EDT). Over the past year, Dawn comprehensively mapped the previously uncharted asteroid, revealing an exotic and diverse planetary building block. In September, NASA's Dawn spacecraft departed Vesta, setting sail for a two-and-a-half-year journey to the dwarf planet Ceres.