In what could be called a “eureka” moment for Dawn researchers and planetary scientists alike, hydrogen has been found on the surface of Vesta, a 550-km (340-mile) -wide protoplanet and the second most massive world in our Solar System’s main asteroid belt. The elemental discovery was made with the Gamma Ray and Neutron Detector (GRaND) instrument on board NASA’s Dawn spacecraft, which just completed its 13-plus-month-long mission orbiting Vesta and is now heading for Ceres.
Likely deposited on the protoplanet by countless impact events over the course of billions of years, the hydrogen is located in a large swatch circling Vesta’s equator — a region where water ice isn’t stable.
Carbon-rich meteorites colliding with Vesta’s surface at relatively slow speeds are thought to be the source of the hydrogen, as low-velocity impacts would have preserved the volatile materials within the meteorites long enough to implant them into Vesta’s crust.
Eventually any high-velocity impacts that occurred would have heated Vesta’s surface, evaporating any underground water ice and releasing the hydrogen during explosive outgassing events. Evidence for this scenario comes in the form of clusters of deep “potholes” observed within some of Vesta’s younger craters. Referred to as “pitted terrain” by Dawn researchers, some of these individual potholes have been found to be as much as 700 feet (200 meters) deep and over half a mile (1 km) across.
The potholes even have analogies on other much larger worlds, like Mars.
“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 Brett Denevi, a Dawn participating scientist based at the Johns Hopkins University Applied Physics Laboratory and lead author of a complementary study. “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.”
The video below shows a color-coded map of Vesta’s hydrogen “hot spots”, created by overlaying GRaND data onto a model of Vesta created with images taken by Dawn’s Framing Camera.
In addition to hydrogen, researchers compared the ratios of iron to oxygen and iron to silicon detected by GRaND in Vesta’s crust. The results support the hypothesis that certain meteorites — called howardite, eucrite and diogenite, or HED, meteorites — found on Earth originated on Vesta, since the ratios of these elements were found to be identical.