Although surprising, water on Vesta is not as much a revelation as it would have been a decade ago. In the interim researchers have found evidence for water ice on the moon and Mars. High-resolution observations of other small bodies such as Eros and Ida could reveal an even moister solar system, says Andy Rivkin, a planetary scientist at APL who was not involved in either paper. "If water was brought into Vesta via external impacts," he adds, "we would expect everything in the Asteroid Belt to have some water."
Richard Binzel, a professor of planetary science at Massachusetts Institute of Technology, says the two findings are "the highlight of the Dawn mission" so far. He was not involved in the work for either paper but wrote a commentary accompanying them.
The Prettyman team's analysis of GRaND data delivered another key bit of knowledge about Vesta: The findings conclusively matched its composition to a class of meteorites on Earth called HED meteorites (composed of howardite, eucrite and diogenite). Researchers in the 1970s had matched the colors and reflective properties of Vesta's surface to the meteorites. Now, the Prettyman analysis of the asteroid's chemical composition confirms their source. "We are now fully confident that the [HED] meteorites are from Vesta," Binzel says.
As a result, researchers can prod, scrape and peer at the chemical and physical properties of the HED meteorites here on Earth and know they hold a record of the chemistry and history of Vesta. Because Vesta formed by the same process as Earth, called differentiation, Binzel says, the rocks are "almost a model of what the very early Earth would be like chemically."