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.