A2 - Constraining the age of the South Pole–Aitken basin and the lunar landing sites – implications for the lunar chronology

Research areas: Planetology, remote sensing, age determinations

Summary

Accurately defining the lunar cratering chronology is important for deriving absolute model ages across the lunar surface and throughout the solar system. Radiometric and exposure ages from returned Apollo and Luna samples have been correlated with crater size–frequency distribution (CSFD) measurements or the cumulative number of craters of a certain reference diameter, usually ≥1 km in diameter (Ncum, where D ≥ 1 km), at the landing sites to anchor the lunar cratering chronology. However, the lunar chronology is only constrained by a few data points over the last 1 Ga. i.e., Copernicus, Tycho, North Ray, and Cone craters and there are no calibration data available between 1 and 3 Ga or beyond 3.9 Ga. Recently, radiometric and exposure ages determined from returned samples were carefully reviewed; that is, the x-axis of the lunar cratering chronology was revisited (e.g., Stöffler et al., 2006). Here, we propose to take this reinvestigation further by revisiting the y-axis of the chronology: we will perform new crater size–frequency distribution measurements of the landing sites and the South Pole–Aitken basin to test the validity of the chronology.

We have already published data on the young calibration points of the chronology (Copernicus, Tycho, North Ray, and Cone craters) and found a reasonably good agreement with Neukum et al.’s (2001) calibration points of the chronology, including the Copernicus data point, which previously did not fit the chronology. When reinvestigating the crater size–frequency distributions of the older calibration points, other researchers found significant differences from the reference measurements that were used to define the initial chronology (e.g., Marchi et al., 2009; Robbins, 2014). Thus, they proposed a new lunar chronology that differs by up to 1 Ga compared with the previous chronology (Robbins, 2014). However, the newly proposed chronology suffers from several shortcomings.

First, the count areas for the measurements are very large and most likely violate the prerequisite to date homogeneous units that were emplaced in one discrete event. Second, the accuracy of the new chronology is likely compromised by the fact that it is based on the production function of Neukum et al. (2001), instead of developing an updated model-specific production function. Third, the new chronology applies a quadratic fit to the data points, which is physically less plausible than the fit of the Neukum et al. (2001) chronology function. Therefore, it is now crucially important to revisit the crater size–frequency distribution measurements of the areas used to define the old portions of the lunar chronology.

Another issue that is still heavily debated is the existence of the so-called cataclysm; that is, a peak in basin-forming events at about 3.9-4.0 Ga. In this respect, the South Pole–Aitken (SPA) basin is of particular interest because knowing its age will shed light on the plausibility of the terminal cataclysm. Such a cataclysmic late heavy bombardment has been proposed to explain the large number of ~3.9 Ga impact ages documented in the Apollo and Luna sample collection. So, provided the age of the SPA basin is ~3.9-4 Ga, this would support some models of the lunar cataclysm hypothesis.

Unfortunately, the age of this basin is not well constrained. Ancient lunar samples from the Apollo 16 and 17 landing sites, which clearly predate the local geology at these sites, as well as lunar farside meteorites Dhofar 489 and Yamato 86032 have been interpreted to indicate the formation of the SPA basin at 4.23 Ga (Garrick-Bethell et al., 2008). However, it remains unknown whether these samples were indeed emplaced by the SPA impact or by other impact(s). We propose to perform new crater size–frequency distribution measurements to date the SPA basin and to further investigate and test the cataclysm hypothesis.