The protoplanet（原行星） Vesta has been witness to an eventful past: images taken by the framing camera onboard NASA's space probe Dawn show two enormous craters¹ in the southern hemisphere. The images were obtained during Dawn's year-long visit to Vesta that ended in September 2012. These huge impacts not only altered Vesta's shape, but also its surface composition. Scientists under the lead of the Max Planck Institute for Solar System Research in Katlenburg-Lindau in Germany have shown that impacting small asteroids²（小行星） delivered dark, carbonaceous material to the protoplanet. In the early days of our solar system, similar events may have provided the inner planets such as Earth with carbon, an essential building block for organic molecules³. These results were published in the November-December issue of the journal Icarus.

 

Vesta is remarkable⁴ in many respects. With a diameter of approximately 530 kilometres, Vesta is the one of the few protoplanets in our solar system still intact today. Like other protoplanets, Vesta underwent complete melting approximately 4.5 billion years ago. However, most of the volcanic⁵ activity on Vesta is thought to have ceased within a few million years making it a time capsule from the early solar system. Dawn observations of Vesta have shown a surface with diverse brightness variations and surface composition. There is bright material on Vesta that is as white as snow and dark material on Vesta as black as coal.

 

The enigmatic（神秘的） dark material holds the key to understanding the impact environment around Vesta early in its evolution. Research led by scientists at the Max Planck Institute in Katlenburg-Lindau has shown that this dark material is not native to Vesta but was delivered by impacting asteroids. "The evidence suggests that the dark material on Vesta is rich in carbonaceous material and was brought there by collisions with smaller asteroids," explains. Vishnu Reddy from the Max Planck Institute for Solar System Research and the University of North Dakota, the lead author of the paper. In the journal Icarus, he and his colleagues now present the most comprehensive analysis of this material so far. Compositional analysis, mapping, and modelling of dark material distribution on Vesta suggest that it was delivered during the formation of giant impact basins on Vesta.

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