The spacecraft revealed that Vesta has its own version of a ring around the collar and, according to two new papers based on observations from the low-altitude mapping orbit of the Dawn mission, volatile or easily evaporated materials have colored Vesta's surface in a broad swath around its equator.
As per the findings that appeared Thursday in the journal Science, pothole-like features mark some of the asteroid's surface where the volatiles, likely water, released from hydrated minerals boiled off. Although 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," said Thomas Prettyman, the lead scientist for Dawn's gamma ray and neutron detector (GRaND) at the Planetary Science Institute in Tucson, Ariz.
While the paper, led by Prettyman, described how the instrument found signatures of hydrogen, likely in the form of hydroxyl or water bound to minerals in Vesta's surface, another complementary paper led by Brett Denevi, a Dawn participating scientist based at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., described the presence of pitted terrain created by the release of the volatiles.
Scientists say it might be possible for water ice to survive near the surface around the giant asteroid's poles. However, unlike the Moon, 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 formed as the water escaped a stretch were as much as 0.6 miles (1 kilometer) across and went down as deep as 700 feet (200 meters).
"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 Denevi. "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."
Vesta is the second most massive member of the main asteroid belt. The orbit at which these data were obtained averaged about 130 miles (210 kilometers) above the surface. Dawn left Vesta earlier this month on Sept. 4 PDT and is now on its way to the dwarf planet Ceres.