Age estimation and boulder population analysis of the West crater at Apollo 11 landing site using Orbiter High Resolution Camera on board Chandrayaan-2 mission

Abstract

Orbiter High Resolution Camera (OHRC) on-board Chandrayaan-2 had acquired a high-resolution image (∼0.26 m) covering Apollo 11 landing site, showing the Lander Module (LM) along with other small-scale features, such as boulders, etc. In the east of Apollo 11 LM lies the West crater, which has a rich population of boulders spread around it. We studied this boulder distribution around the West crater to estimate the age of the West crater and to understand the aspects related to the impact cratering and space weathering through the analysis of boulder distribution. For this, we identified and mapped >8500 boulders around the West crater using the OHRC image, out of which 6466 boulders (>∼1 m in size) lying within 1–5 crater radius were used for further analysis. For estimating the age of the West crater, we applied various methods based on crater morphology, boulder distributions and Diviner Rock Abundance. The age was estimated to be in the range of 80 Ma to 120 Ma with the most probable age close to 100 Ma, the same as that expected from the cosmic ray exposure dating. Boulder distribution around the West crater was found to be highly anisotropic with majority of boulders lying in the eastern to north-eastern direction, suggesting an impact at very low angle. Relationship between crater size and largest boulder size also pointed towards primary origin of the West crater. The Height (H) to Diameter (D) ratio is estimated to be ∼0.25 considering all the boulders with diameter ≤ 4m, going up to 0.38 with comparatively poor correlation for boulders with diameter > 4m. The H/D ratio from Apollo mission photos for boulders around Apollo 11 LM is found to be approximately double that of the corresponding measurements from the OHRC image. We also fit a Power law and the Weibull distribution to the cumulative Boulder Size Frequency Distribution (BSFD) curve with Maximum Likelihood (ML) method and Kolmogrov-Smirnoff (KS) statistics. Both fit well with R² of 0.999, but varying minimum diameter (d_KS) of 1.85 m and 1.64 m, respectively. The good fit by Power-law with high slope value ($b$ = 4.61) describes the fragmentation due to the West crater forming impact as complex. However, the higher diameter tail was better explained by the Weibull fit implying sequential fragmentation of boulders with time.