Early oxidation of Martian crust triggered by impacts

The mechanisms of formation of terrestrial planets, and in particular the earliest planetary differentiation processes following accretion, remain highly debated. Due to the presence of plate tectonics, rocks older than 3.5 billion years are scarce at the surface of the Earth, and it has significantly hampered the utilization of terrestrial rocks to study the Earth's earliest differentiation. In contrast, Mars has preserved very early geologic records that can be dated back to as early as 4.4-4.5 billion years ago, and recently Bouvier et al. (2018) shows that a primordial crust of Mars has been reworked very early. To clarify the mechanisms controlling early crustal differentiation of Mars, we integrate stable isotopic studies on a group of Martian meteorites (including shergottites, nakhlites and a brecciated meteorite NWA 7034), and melting experiments. It shows that Martian crust has been oxidized at least locally 4.4-4.5 billion years ago, leading to 4- to 5-order increases in the oxygen fugacity of the crust compared to that of shergottites. The combination of isotopic and trace element data supports that the re-melting of a reduced Martian crust triggered by impacts is likely responsible for its early oxidation. This shows that impacting has played a crucial role in shaping the ancient crust of Mars and probably of all terrestrial planets. Through investigating these impact-modified rock records, we can further trace the meteoritic or volatile delivery into the early inner solar system.