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


The timing of the wane in heavy meteorite bombardment of the inner planets is debated. Its timing determines the onset of crustal conditions consistently below the thermal and shock pressure limits for microbiota survival, and so bounds the occurrence of conditions that allow planets to be habitable. Here we determine this timing for Mars by examining the metamorphic histories of the oldest known Martian minerals, 4.476–4.429-Gyr-old zircon and baddeleyite grains in meteorites derived from the southern highlands. We use electron microscopy and atom probe tomography to show that none of these grains were exposed to the life-limiting shock pressure of 78 GPa. 97% of the grains exhibit weak-to-no shock metamorphic features and no thermal overprints from shock-induced melting. By contrast, about 80% of the studied grains from bombarded crust on Earth and the Moon show such features. The giant impact proposed to have created Mars’ hemispheric dichotomy must, therefore, have taken place more than 4.48 Gyr ago, with no later cataclysmic bombardments. Considering thermal habitability models, we conclude that portions of Mars’ crust reached habitable pressures and temperatures by 4.2 Gyr ago, the onset of the Martian ‘wet’ period, about 0.5 Gyr earlier than the earliest known record of life on Earth. Early abiogenesis by 4.2 Gyr ago, is now tenable for both planets.

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The following curators and institutions are acknowledged for providing sample material; B. Hofmann (Naturhistorisches Museum Bern) for the loan of NWA 7906, J. Kashuba for the loan of the NWA 11220 thin sections and B. Hoefnagels for the loan of Rabt Sbayta 003. The following are acknowledged for their assistance with making or running the APT specimens: T. J. Prosa, I. Martin, K. P. Rice, Y. Chen, D. Olson and D. Lawrence. W. U. Reimold and R. Hewins are thanked for their thoughtful reviews. Graphics assistance from K. Vankerkoerle is acknowledged. D.E.M. acknowledges support for electron microscopy analyses, student and technical scientist funding, and facility travel costs from his NSERC Discovery Grant; J.R.D. acknowledges support from Royal Society Research Grant RG160237 and a University of Portsmouth RIEF award; F.M.M. acknowledges support from NASA’s Planetary Science Research Program; K.T.T. acknowledges NSERC Discovery Grant support.

Author information

D.E.M. was in charge of the conceptualization of the project and writing the initial draft. All the authors provided support in one or more of the roles of investigation, formal analysis, resource provision and written contributions to subsequent drafts of the manuscript. A.J.I. was the classifier of two of the specimens used in this study and provided thin and thick sections for detailed analyses.

Correspondence to D. E. Moser.

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We note that the CAMECA co-authors (D.A.R. and D.J.L.) are engaged in the manufacture and sale of atom probe instruments. The remaining authors declare no competing interests.

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Fig. 1: Example of early Mars crust and igneous zircon in polymict breccia meteorite NWA 11220.
Fig. 2: Timeline of early Mars bombardment history and habitability compared to that of Earth.