Age of Giordano Bruno crater as deduced from the morphology of its secondaries at the Luna 24 landing site

We undertook a photogeologic study of Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) images of the ejecta of the very young 22-km diameter crater Giordano Bruno (GB) and for the Luna 24 landing site region, where secondary craters from Giordano Bruno are observed. Using the technique of Basilevsky (1976) for estimating the absolute ages of small lunar craters based on their morphologic prominence and size, craters near the Luna 24 site considered to be secondaries of Giordano Bruno crater and craters superposed on ejecta of Giordano Bruno crater are found to have approximately the same age: somewhere between 5 and 10 m.y. This suggests that the craters at the Luna 24 site are indeed secondaries of GB crater and that this age range is an estimate for the time of GB crater formation. Our results generally agree with estimates derived from counts of small craters superposed on GB ejecta: 1–10 m.y. and with the exposure age of one of the samples derived from Zone I of the Luna 24 core. Plescia et al. (2010b) interpreted a large fraction of the craters superposed on GB ejecta and floor not as primary craters, but rather as secondary craters formed from the GB event itself. On this basis, it was postulated that GB may be formed by an historic impact. Our new data and analysis do not support an age as young as historical. The general agreement of our results for age determination for the GB secondaries of the Luna 24 area, and for craters superposed on the GB ejecta, with the age estimate based on crater count and with the mentioned exposition age of one of the Luna 24 samples, also validates the technique of Basilevsky (1976). A special study of the Luna 24 samples should be undertaken designed to find records of the formation age of the crater on whose rim Luna 24 landed, thus better constraining the age of GB crater.