Mercury may have formed as the result of one or more 'hit-and-run' collisions between the many protoplanets in the early Solar System.
Mercury, the closest planet to the Sun, is unusual because its large metallic core lacks a massive rocky mantle like the ones that cover Earth, Venus and Mars. Erik Asphaug and Andreas Reufer at Arizona State University in Tempe modelled impacts between planetary embryos in the early Solar System. They show that Mercury may be a survivor of one or multiple early Solar System collisions with a larger planet that stripped off the mantle of the smaller body, leaving only the iron-rich core.
Although the chance of the smaller body surviving each such collision is unlikely, the authors show that if the planet did survive, it is statistically probable that it would have suffered multiple hit and runs.
Nature Geosci. http://dx.doi.org/10.1038/ngeo2189 (2014)
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Hit-and-run origin for Mercury. Nature 511, 129 (2014). https://doi.org/10.1038/511129c