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Title: A colossal impact enriched Mars' mantle with noble metals
Author: R. Brasser, S. J. Mojzsis

Once the terrestrial planets had mostly completed their assembly, bombardment continued by planetesimals left-over from accretion. Highly siderophile element (HSE) abundances in Mars' mantle imply its late accretion supplement was 0.8 wt.%; Earth and the Moon obtained an additional 0.7 wt.% and 0.02 wt.%, respectively. The disproportionately high Earth/Moon accretion ratio is explicable by stochastic addition of a few remaining Ceres-sized bodies that preferentially targeted Earth. Here we show that Mars' late accretion budget also requires a colossal impact, a plausible visible remnant of which is the hemispheric dichotomy. The addition of sufficient HSEs to the martian mantle entails an impactor of at least 1200 km in diameter to have struck Mars before ca. 4430 Ma, by which time crust formation was well underway. Thus, the dichotomy could be one of the oldest geophysical features of the martian crust. Ejected debris could be the source material for its satellites.

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  Meteorite impacts brought precious metals to early asteroids

Planets and moons weren't the only cosmic objects to get splattered with gold early on in the solar system's history - small asteroids got some precious metals too.
Gold, platinum and other iron-loving, or "siderophile", elements get dragged to the core of a planet if they are present when it is still forming. So siderophiles found in planets' mantles must have been delivered later, by meteorites.

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Some of the worlds rarest and most precious metals, including platinum and iridium, could owe their presence in the Earths crust to iron and stony-iron meteorites, fragments of a large number of asteroids that underwent significant geological processing in the early Solar System.
Dr Gerhard Schmidt from the University of Mainz, Germany, has calculated that about 160 metallic asteroids of about 20 kilometres in diameter would be sufficient to provide the concentrations of these metals, known as Highly Siderophile Elements (HSE), found in the Earths crust. Dr Schmidt will be presenting his findings at the European Planetary Science Congress in Münster on Monday 22nd September.

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