Abstract We investigated the conditions under which a giant collision between a hypothetical proto-Mercury and a planet one-sixth its mass would result in the loss of most of the silicate mantle of the planet, leaving behind an iron-rich planet and thus explaining the anomalously high density of Mercury. We carried out a series of numerical simulations using our three-dimensional smoothed particle hydrocode, varying the impact parameter and the relative velocity between the planet and impactor. We demonstrate that the details of the equation of state do not play an important role. We show that a head-on collision at 20 km/sec and an off-axis (impact parameter equal to half the radius of proto-Mercury) collision at 35 km/sec are about equivalent as far as damage to proto-Mercury is concerned. Both collisions leave behind a remnant that has the required characteristics of the present Mercury. Whether this scenario is actually successful depends on the size of the condensates in the ejected cloud of debris. Preliminary estimates show that most of the ejected mass is probably removed from Mercury-crossing orbits. If this turns out to be true, a giant collision is a plausible explanation for the strange density of Mercury.
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