New data from NASA's Dawn mission bears out the theory that the dark color of the protoplanet Vesta is caused by the impact of carbon-rich asteroids.
In November, scientists suggested that this was the case, rather than the dark patches having originated through geological processes. Now,
compositional analysis, mapping, and modelling of the dark material's distribution suggest that it was delivered during the formation of Vesta's giant impact basins.
"The evidence suggests that the dark material on Vesta is rich in carbonaceous material and was brought there by collisions with smaller asteroids," says lead author Vishnu Reddy.
The dark material was found to a much greater degree around the edges of the giant impact basins in the southern hemisphere of Vesta. A closer examination showed that it was most probably delivered during the formation of the older Veneneia basin, when a slow-impacting asteroid hit two to three billion years ago.
Dark material from this basin was later covered up by the impact that created the Rheasilvia basin.
There's more evidence for dark material in the form of the HED meteorites that come from Vesta, some of which show dark inclusions that are carbon-rich. And the color spectra of dark material on Vesta are identical to these inclusions.
"Our analysis of the dark material on Vesta and comparisons with laboratory studies of HED meteorites for the first time proves directly that these meteorites are fragments from Vesta," says Lucille Le Corre from the Max Planck Institute.
In the early days of our solar system, it's believed that similar collisions provided our own planet with carbon - an essential building block for organic molecules.