Establishing the absolute and relative chronology of ancient lunar events is of fundamental importance to our understanding of early Solar System history and the evolution of rocky planetary bodies. In this endeavor, the Moon has a unique quantitative role, as it is the only planetary body from which absolute and relative ages can be calibrated to one another, by way of radiometric dating of returned samples and observable surface crater densities, respectively. In contrast to the relatively young and heavily modified surfaces of some other planetary bodies, a majority of the lunar surface has been well preserved since antiquity and thus retains the most comprehensive surface cratering record presently known to exist. In this investigation, we jointly use craters with a recognizable surface expression and those inferred from quasi-circular mass anomalies (QCMAs), considered to be buried craters, preserved in the lunar gravitational field and revealed by analyses of the gravity data from the Gravity Recovery and Interior Laboratory (GRAIL) mission to re-examine the ages for the formation of lunar terranes and the chronological sequence of major impact basin formation.
We find that mare volcanism had little apparent effect on the size-frequency distribution and cumulative crater densities of the combined set of craters and QCMAs with diameters greater than 90 km, and that this crater and QCMA diameter cutoff to assess the relative ages of lunar geochemical terranes and basins.
For the lunar geochemical terranes, we find that the cumulative crater density values at diameters greater than 90 km [referred to as N(90)] of the SPA basin and Feldspathic Highlands Terrane (FHT) are indistinguishable. Additionally, we find that both of these terranes are significantly older than that of the Procellarum KREEP Terrane (PKT), which has an N(90) value of 12.1±3.2. Since the PKT could have formed as late as ~4.3 Ga, on the basis of the youngest age for urKREEP crystallization from the lunar magma ocean, it then follows that the SPA impact must have occurred prior to ~4.3 Ga. For the large inventory of lunar basins, we find that the N(90) values of those basins with diameters more than 650 km are in general agreement with the lunar chronology of Wilhelms.
Acknowledgements: This work was conducted as part of the GRAIL mission and is performed under contract to the Massachusetts Institute of Technology and the Jet Propulsion Laboratory, California Institute of Technology.
Affiliation: This work was completed while A. J. Evans was affiliated with the Lamont-Doherty Earth Observatory at the Columbia University and the Southwest Research Institute.