Linking Shergottites to the Martian Crust: A New Martian Cratering Chronology

Absolute ages for planetary surfaces are often inferred by crater densities, and are only indirectly constrained by ages of meteorites. Cratering chronology models are calibrated based on lunar samples, and are transferred towards other planets using crater scaling laws and cratering rate ratios. This transfer often relies on extrapolations. We have shown that the <5 Myrs old and 55-km wide Mojave Crater is the ejection source for the meteorites classified as shergottites [1], and thus allows the calibration of the chronology model of Mars. Shergottites and this crater are linked by their coinciding meteorite ejection ages and the crater formation age, and mineralogical constraints [1]. Thus, the cratering-based age determination method for Mars is now calibrated.

Our new Martian cratering chronology model is tested against the in situ determined radiometric and exposure ages for a mudstone in Gale crater [3], and the ages found with both methods are identical. Because Mojave formed on 4.3 Gyrs old terrain, the original crystallization ages of shergottites are old, as inferred by Pb-Pb isotope ratios [4-6], and the much-quoted <600 Myrs shergottite ages [e.g., 7] are due to resetting. Martian surface features dated with cratering statistics shifts the absolute age of the oldest terrains of Mars backwards by 200 Myrs.

References: [1] Werner et al. 2014, Science 343, 1343-1346. [2] Herzog & Caffee, Treatise on Geochemistry, 1, 419–454. [3] Farley et al. 2014 Science 343, 1247166-1-5. [4] Bouvier et al.2009, Earth Planet. Sci. Lett. 280, 285–295. [5] Bouvier et al. 2005, Earth Planet. Sci. Lett. 240, 221–233. [6] Bouvier et al. 2013, Lunar Planet. Sci. Conf. XLIV, abstr. 2421. [7] Nyquist et al.,2001, Space Sci. Rev. 96, 105–164.