GIORDANO BRUNO: SECONDARY IMPACT CRATERING PROCESSES ON THE MOON

Abstract

Giordano Bruno is a young, Copernican-age crater on the lunar far side (35.9 ºN, 102.8 ºE). With an estimated age of 1–10 Ma, the surface is not old enough to have reached crater saturation. Since impact cratering is a stochastic process, it is expected that a random distribution of craters will be expressed on its continuous ejecta blanket due to cratering by primary impactors. However, high-resolution images acquired by the Lunar Reconnaissance Orbiter Camera (LROC) and previous crater-counting investigations revealed heterogeneities in the morphologies and spatial distribution of impact craters observed within the ejecta blanket and interior of Giordano Bruno. We counted 47,130 craters within a 2437 km2 area centered on Giordano Bruno. We present a detailed examination of the point densities, crater size–frequency distributions (CSFDs), and absolute model age (AMA) calculations in different regions of the parent crater and its ejecta. Different regions at Giordano Bruno have AMAs ranging from 100 ka to 6.8 Ma (i.e., N(1) = 8.78 × 10–8 to 5.68 × 10–6). We interpret that self-secondary cratering likely causes AMA discrepancies between melt and ejecta units in the continuous ejecta blanket. To assess the likelihood that the presence of self-secondary craters (SSCs) affects the CSFDs, the spatial distribution of the craters was characterized using 2nd-closest neighbor distances under the assumption that primary craters are random and secondary craters are clustered. Using this information, the CSFDs were filtered to remove craters that exhibit clustering > 3σ, and we calculated that the AMA for Giordano Bruno is 3.5 ± 0.2 Ma. This work demonstrates the importance of accounting for self-secondary cratering in the estimate of AMAs for young impact craters on the Moon and elsewhere in the Solar System.