"At the very beginning of the solar system, Earth and Vesta may have been almost identical bodies. Earth kept growing to its much larger size and complicated evolution of crust, plate tectonics, etc.," Massachusetts Institute of Technology planetary scientist Richard Binzel writes in an email to Discovery News.
"Vesta had its growth stunted because Jupiter stirred up the asteroid belt by virtue of its massive gravity and being next door to the asteroid belt. So Vesta is a snapshot of what Earth looked like at a very early stage," Binzel said.
The discovery of hydrogen is not the only evidence that Vesta, which is about one-sixth the size of Earth's moon, was battered by water-rich bodies crashing into its surface.
Related research shows Vesta is pocked with irregular shaped, rimless depressions in and around many of its impact craters. Scientists believe they were formed when materials containing volatile gases were heated up during the impacts and released, forming the holes. Mars bears similar marks.
"The water finding is a surprise at Vesta," Binzel said. "We know water-rich asteroids are further out in the asteroid belt and that sometimes their orbits will cross Vesta and make craters when they hit. The surprise is that the net result is water being left behind on Vesta - nominally we might think the energy (heat) that makes the crater would vaporize all the water."
Dawn orbited Vesta from July 2011 until this month. The probe is now on its way to the dwarf planet Ceres, the largest object in the asteroid belt and the only body bigger than Vesta.