A colossal impact enriched Mars' mantle with noble metals

Abstract

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 that 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 emispheric 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 ~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.

Jupiter's Influence on the Building Blocks of Mars and Earth

“…For Earth we set k ic = 0.4 (Dauphas, 2017), while for Mars we use k ic = 1 because its accretion most probably involved smaller impactors that were more likely to equilibrate with its mantle (Brasser & Mojzsis, 2017;Dauphas & Pourmand, 2011;Kobayashi & Dauphas, 2013;Kobayashi & Tanaka, 2010;Mezger et al, 2013). For Earth we set k ic = 0.4 (Dauphas, 2017), while for Mars we use k ic = 1 because its accretion most probably involved smaller impactors that were more likely to equilibrate with its mantle (Brasser & Mojzsis, 2017;Dauphas & Pourmand, 2011;Kobayashi & Dauphas, 2013;Kobayashi & Tanaka, 2010;Mezger et al, 2013).…”

“…where x is the mass fraction accreted by the planet. For Earth we set k ic = 0.4 (Dauphas, 2017), while for Mars we use k ic = 1 because its accretion most probably involved smaller impactors that were more likely to equilibrate with its mantle (Brasser & Mojzsis, 2017;Dauphas & Pourmand, 2011;Kobayashi & Dauphas, 2013;Kobayashi & Tanaka, 2010;Mezger et al, 2013). The Earth's core mass fraction is 0.33, while for Mars it is approximately 0.25 (Rivoldini et al, 2011).…”

Hemispheric Dichotomy in Lithosphere Thickness on Mars Caused by Differences in Crustal Structure and Composition

“…The origin of this surface dichotomy remains debated and various formation mechanisms have been proposed such as a heterogeneous fractionation of an early magma ocean (Solomon et al, 2005), a mantle overturn (Elkins-Tanton et al, 2005), a degree 1 mantle convection pattern (Roberts & Zhong, 2006;Yoshida & Kageyama, 2006), or an impact origin either in the north (Andrews-Hanna et al, 2008;Marinova et al, 2008) or in the south (Golabek et al, 2011;Leone et al, 2014;Reese et al, 2010). In any case, the dichotomy formation would probably date back to the early stages of Mars evolution, between 4.5 and 4 Gyr (Bottke & Andrews-Hanna, 2017;Brasser & Mojzsis, 2017;Frey, 2008;Nimmo & Tanaka, 2005;Nyquist et al, 2001). The dichotomy in topography is likely compensated by a difference in crustal thickness and/or density.…”

Mars Colonization: Beyond Getting There

“…Moreover, promising results obtained by rovers and a low‐frequency radar installed on the Mars Express spacecraft have long sustained the belief that it might be possible to find undersurface and subglacial liquid water . Furthermore, similar to Earth, Mars is expected to have substantial mineral resource at and under its surface layer, with a recently confirmed evidence of metal ores and other vital mineral substances . Although no one has seriously demonstrated a practical means for the extraction and refining of these resources into useful products on Mars, a distant possibility of doing so is considered a principal point in favor of colonization.…”

“…[2] Furthermore, similar to Earth, Mars is expected to have substantial mineral resource at and under its surface layer, with a recently confirmed evidence of metal ores and other vital mineral substances. [3] Although no one has seriously demonstrated a practical means for the extraction and refining of these resources into useful products on Mars, a distant possibility of doing so is considered a principal point in favor of colonization. These features of the Red Planet have firmly cemented its status as an ultimate space colonization destination for near future, [4] despite the obvious immediate challenges such as a dusty carbon dioxide-rich atmosphere, the pressure of which is reaching only 0.09 atm.…”