Decline of giant impacts on Mars by 4.48 billion years ago and an early opportunity for habitability

A first step to understanding the initial conditions for habitability pathways in planetary systems is to determine when heavy meteorite bombardments waned and the earliest crust remained below the known thermal and shock pressure limits on microbiota survival (121°C, 78 GPa). We have determined this timing on Mars by documenting the metamorphic histories of its oldest known, 4.476 Ga to 4.430 Ga, grains of the highly resilient minerals zircon and baddeleyite in the Rabt Sbayta polymict breccia meteorites; crustal fragments of the southern highlands. Here we show using electron and atom probe microscopy that the Mars grains (n=121) have all remained beneath 78 GPa conditions, with 97% exhibiting weak to no shock metamorphic features, or thermal overprints due to shock-induced melting and magmatism. This is opposite to bombarded crust on Earth and Moon wherein ~80% of grains show such features. The nearly pristine state of the Mars minerals thus establishes a lower age bracket of 4.48 Ga for the planet-scale impact that created the hemispheric dichotomy, and obviates any later cataclysmic bombardments. Considering existing thermal habitability models, portions of early Mars crust reached habitable conditions by at least 4.2 Ga, the onset of the martian ‘wet’ period, as much as ~500 million years earlier than the earliest record of life on Earth. An early giant impact period on Mars, broadly coeval with Moon formation, may have heralded early abiogenesis on both planets.