STRATIGRAPHY OF THE SOUTH POLE-AITKEN BASIN BASED ON CRATER SIZE-FREQUENCY DISTRIBUTION MEASUREMENTS

We present new crater size-frequency distribution (CSFD) measurements of the largest and possibly the oldest preserved lunar impact structure, the South Pole-Aitken (SPA) basin. Data from the Lunar Reconnaissance Orbiter allowed us to perform detailed and systematic CSFD measurements over the entire SPA basin in order to derive absolute model ages (AMA) of the basin itself, as well as several superposed impact basins, light plains deposits, and mare basalts. The derived AMAs are discussed in the context of lunar sample ages [1,2] and with respect to the proposed lunar cataclysm [e.g., 3]. Our CSFD measurements indicate that the SPA basin is ~4.26 (±0.03) Ga old (N(1) = 3.70x10^-1). Craters Planck and Oppenheimer formed nearly at the same time, i.e., ~4.09 (+0.02/-0.03; N(1) = 1.11x10^-1) and ~4.04 Ga (±0.01; N(1) = 8.43x10^-2) ago. According to our AMA of ~3.92 Ga (±0.02; N(1) = 3.74x10^-2), Schrödinger is younger. Both Planck and Schrödinger show underlying older ages of 4.26 (+0.07/-0.18; N(1) = 3.70x10^-1) Ga and 4.19 Ga (+0.08/-0.24; N(1) = 2.26x10^-1), which are close in age to SPA. The CSFD of Apollo is likely disturbed by Orientale ejecta and/or secondary craters. Consequently, we could only derive a poorly constrained age of 3.91 Ga (+0.04/-0.06; N(1) = 3.46x10^-2). Fourteen light plains units were dated in the Apollo/Oppenheimer region and exhibit AMAs between 3.43 (+0.07/-0.15) and 3.81 (+0.06/-0.11) Ga. We conclude that SPA is older than 4 Ga. Such an age is consistent with [2], who suggested that a significant impact event(s) occurred at ~4.2 Ga that affected not only the K-Ar system in plagioclases and pyroxenes, but also the U-Pb of zircons. If SPA is indeed older than 4 Ga, it is likely too old to be consistent with some models for lunar cataclysm. Some of the superposed craters such as Schrödinger and Planck only incompletely resurfaced the SPA basin and exhibit underlying older ages that are similar to the age of SPA. To unambiguously determine the age of SPA and excavation depth of material exposed at the surface a dedicated sample return mission is required.

[1] Garrick-Bethell et al., (2008) Early Solar System Impact Bombardment. [2] Fernandez et al., (2008) Early Solar System Impact Bombardment. [3] Tera and Wasserburg, (1973) LPS 4.