[P-PS08] Lunar Science and Exploration

[PPS08-17] Relationship between morphology of boulders and compositions on lunar craters: assessing Wadell roundness and circle ratio sphericity

*Ayame Ikeda¹, Satoru Yamamoto¹ (1.Geological Survey of Japan, AIST)

Keywords:the Moon, Boulder morphology

Understanding how boulders are fragmented into smaller fragments and regolith provides important information for crater degradation and regolith formation processes. Lunar boulders have been thought to form through impact cratering (Plescia et al., 2023) and thermal fatigue (Molaro et al., 2015). The morphology of boulders depends on their parent rock composition, but few studies considered the relationship between the boulder shape/morphology and their composition. Ikeda et al. (2025, LPSC abstract) investigated the distribution and morphology of boulders, using panchromatic images taken by Narrow Angle Camera (NAC) installed on Lunar Reconnaissance Orbiter (LRO). They estimated the axis ratio of boulders, where they approximated ellipsoid, and they indicated that morphology of boulders varies with their composition. However, boulders actually found on lunar slopes have complex morphologies, and the elliptical approximation alone cannot be used to obtain boulder shapes. So, we estimated two parameters that characterize boulder shape: Wadell roundness (Wadell, 1932) and circle ratio sphericity (Mitchell et al., 2005). Our study area is the central peaks and crater wall of lunar craters and basin (central peaks of Copernicus crater, Tsiolkovskiy crater, Bullialdus crater, and crater wall of Schrödinger basin), where the dominant rock minerals are already estimated. We used LROC NAC images to detect the outlines of boulders with polygon and calculated the roundness and sphericity using an algorithm of Zheng et al. (2015). In this presentation, we will report how the morphology depends on the composition of boulders.