New evidence suggests that frozen water lurks in the dusty, pitted surface of our solar system's second most-massive asteroid. The discovery at Vesta is helping researchers understand how a once-molten protoplanet—a category that includes Earth's embryo—could gather water early in its history as it cooled and spun through space. Vesta's regolith, or rocky soil, is estimated to hold only 5 percent water by weight, however; hardly enough to get future astronauts wet or even offer them much of a drink. Space travelers would have better luck mining water on other, wetter asteroids.

The water conclusion was drawn from two teams' independent analyses of data from NASA's Dawn mission. Before embarking for dwarf planet Ceres September 5, the spacecraft orbited Vesta for more than a year, passing over the asteroid's poles (at an average altitude of just 210 kilometers for some of that time) as the protoplanet rotated below. From this orbital vantage point, all of Vesta's surface was eventually exposed to Dawn's instruments.

The results, published online September 20 in two papers in Science, reveal abundant hydrogen on Vesta and pitted craters that betray the presence of volatiles, or chemicals with low boiling points such as nitrogen, carbon and water. Taken together, the findings point toward water on the asteroid. Its small size and lack of an atmosphere, however, means the water could not exist there in a liquid state but might linger as a solid beneath its surface or disperse as a vapor into space.

Thomas Prettyman, a researcher at the Planetary Science Institute in Tucson, Ariz., led a team that looked closely at data gathered by Dawn's Gamma-Ray and Neutron detector (GRaND), which can distinguish compounds by recording the spray of particles produced when cosmic rays routinely slam into the asteroid's surface. The instrument detected an abundance of hydrogen in much of the asteroid's regolith.

The second team, led by Brett Denevi, a planetary scientist from the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Md., has investigated some unique telltale pits on the asteroid's face. The pits are thought to result from space rock impacts that heat surface volatiles until they vaporize, leaving pockmarks that are "small and closely spaced, with overlapping, almost polygonal boundaries," she says.

Combine that with the hydrogen finding, and the scientists suspect that the volatiles are hydrogen-bearing compounds—hydroxyl ions and water, Denevi and her colleagues wrote. The water is probably locked within the lattice of carbonaceous chondrites, a class of dark-colored meteorites known to have struck Vesta in the past and mixed into its surface, Denevi says.

Early in the solar system's history asteroids and terrestrial planets were formed from molten material. The sizzling temperatures of young Vesta would have instantly evaporated any volatile material such as water. Therefore, the water must have arrived on Vesta after it cooled.

A plethora of low-velocity impacts over a billion or more years likely transported the volatiles to the protoplanet, Denevi says. Prettyman and colleagues also conclude that fragments from water-rich bodies—perhaps other asteroids—sprinkled Vesta's surface with the hydrogen volatiles. The impacts would have ploughed into the rocky asteroid at slower speeds than impacts at Earth or the moon, Denevi explains. That gentler pummeling would have preserved the volatiles and mixed them into the dusty regolith, in a process known as gardening.

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."