A dry world with wet spots
Vesta is the second-most-massive object in the main asteroid belt, a protoplanet with a mean diameter of 525 kilometers. Unlike most asteroids, it is not a uniform chunk of rock. It is a differentiated body with a crust, mantle, and an iron-nickel core, much like a terrestrial planet. This structure indicates that Vesta formed very early in the solar system's history, likely within the first few million years, when heat from radioactive elements was sufficient to melt its interior.
Vesta's formation location presents a puzzle. It orbits within the solar system's "frost line," a region close to the young Sun where temperatures were too high for water ice to condense. This zone created "dry" bodies like Earth, Mars, and Vesta, which are composed primarily of rock and metal. Planetary scientists expected Vesta to be almost entirely free of water. Data from NASA's Dawn mission, which orbited Vesta from 2011 to 2012, challenged this assumption directly. While the spacecraft found no water ice, its instruments detected clear signatures of hydrated minerals, specifically hydroxyl (OH) bound within the surface rocks.
An imported resource
The leading theory for Vesta's unexpected water is external delivery. The primary suspects are carbonaceous chondrites, a class of primitive, water-rich meteorites from the outer asteroid belt. It is believed these objects collided with Vesta at speeds low enough to deposit their volatile contents without vaporizing them completely. Evidence for this scenario comes from patches of dark material on Vesta's surface that match the composition of these carbonaceous impactors.
Data from Dawn's Gamma Ray and Neutron Detector (GRaND) instrument revealed that hydrogen, a proxy for water, is not uniformly distributed. The strongest concentrations are found in a swath around the equator and not at the poles where ice might be stable in shadow. Further analysis by the Visible and Infrared Mapping Spectrometer (VIR) linked these hydroxyl-rich areas to specific geological features, including ancient, heavily cratered terrain.
Another curious feature is the presence of "pitted terrain." In craters like Marcia, which is about 70 kilometers in diameter, the surface is marked by pothole-like depressions. Scientists theorize that heat from later, high-speed impacts vaporized the subsurface water that had been locked in minerals. This water then degassed explosively, leaving the pitted terrain behind as evidence of Vesta's complex and surprisingly hydrated history.