Vesta is the only remaining example of the original objects that came together to form the rocky planets, like Earth and Mars, some 4.6 billion years ago. This assessment is based on data from the Dawn probe which has been orbiting the second largest body in the asteroid belt for the past 10 months. The findings from the NASA mission are reported in Science magazine. They confirm that Vesta has a layered interior with a metal-rich core, just as Earth, Mars, Venus and Mercury do. Using information about the shape of the asteroid and its gravity field, scientists can even say something about the likely size of this core. The Dawn team calculates it to be about 220km (135 miles) across, representing about 40% of the radius of Vesta, or roughly 18% of its total mass. “This mission at Vesta has been a spectacular success,” said Carol Raymond, Dawn's deputy principal investigator from NASA's Jet Propulsion Laboratory in Pasadena, California. “It's transformed Vesta from a fuzzy orb into a planetary body, which has exceeded our expectations in many ways.” Dawn has studied in detail the pattern of minerals exposed in Vesta's surface by innumerable impacts through the eons. It has also mapped the diverse geological features that shape its surface. These observations have enabled scientists to elucidate a history for the colossal rock, which is second in size only to Ceres in the band of material moving between Mars and Jupiter. The researchers believe Vesta formed within two million years of the first solids coming together in the Solar System, before the planets we know today were assembled. Short-lived radioactive materials would have generated enough heat to melt Vesta's insides, producing a subsurface ocean of magma. This melting would have led to differentiation, to denser materials like iron falling to the asteroid's centre. Other such bodies in the infant Solar System with magma oceans ended up becoming parts of Earth and the other planets. Somehow, Vesta did not; somehow, it survived obliteration in the cascade of impacts that would have marked those early times. Nothing telescopes see today in the asteroid belt quite matches what Dawn has seen at Vesta, suggesting the mighty rock is unique hangover. “Vesta is special because it has survived the intense collisional environment of the asteroid belt for billions of years, allowing us to interrogate a key witness to the events at the very beginning of the Solar System,” said Dr Raymond.
