Planetary scientists at the Institute for Astrophysics and Space Planetology in Rome, writing in the journal Nature, said the mineral olivine has been spotted, not in Vesta's deep southern impact basins -- which are thought to host excavated mantle rocks -- but mixed with a common surface rock type in the asteroid's northern hemisphere.
The researchers said the finding came from data from NASA's Dawn spacecraft, which orbited Vesta in 2011 and 2012.
"We were searching for olivine since the beginning of Dawn's orbital phase, because we know how important olivine is to understanding the differentiation processes of early bodies of the solar system," study author Maria Cristina De Sanctis told SPACE.com.
Vesta, often considered a protoplanet, retained much of its interior heat at its birth, allowing lighter molten rock to float outward while denser rock sank inward -- resulting in a layered interior like that of Earth, with an outer crust, an inner core with a mantle layer in between.
Astronomers had assumed olivine, a major ingredient of Earth's upper mantle, would also be in Vesta's mantle and thus would be found on its surface only in place where large impacts have unearthed deeply buried rocks.
The possibility olivine might exist within Vesta's crust instead of deeper within its mantle poses a puzzle for models of Vesta's evolutionary history, the researchers said.