The Small Crater Population on Giordano Bruno

Crater counts conducted on the proximal ejecta of lunar craters have been found to yield inconsistencies in model ages, including craters used to anchor the crater chronology of the inner solar system (e.g. [1]). Studies using Lunar Reconnaissance Orbiter Camera (LROC) images have found evidence suggesting that self-secondary cratering (e.g [2]) or variations in target properties (e.g. [3]) are contributing factors. Recent work studying crater Giordano Bruno (GB), a 22 km diameter Copernican-age crater, highlighted substantial variations in the crater size-frequency distributions (CSFDs) of the superposed crater population across GB's ejecta and concluded that a significant fraction of the craters represented self-secondary craters [4]. This is supported by the observation of partial burial of some craters by impact melt indicating the craters formed prior to the final emplacement of the melt [4][5].

Temperatures measured by LRO's Diviner reveal substantial heterogeneity in thermophysical properties of GB's ejecta [6]. Anisothermality in Diviner's IR spectral passbands, indicative of surface rock fraction, have been found to correlate with the density of craters showing suppression of craters where rock fractions were higher. This could result from inhibition of crater formation as block sizes become comparable to the impactor sizes which would influence cratering efficiency. We have found that the night temperatures of the clastic ejecta of GB and the superposed melt are substantially different from nominal lunar regolith with nighttime cooling revealing vertical layering in the impact melt more consistent with coherent rock covered with fine-grain dust.

Because of GB's young age and relatively pristine morphology, it provides a good opportunity to explore target effects and self-secondary cratering as the influence of these factors on CSFDs diminish over time. We will expand on these previous results with systematic mapping of the crater population superposed on GB and correlate these with variations in thermophysical properties revealed by Diviner observations.

[1] Hiesinger H., et al. (2012) JGR, 117

[2] Zanetti M., et al. (2017) Icarus, 298

[3] van der Bogert et al. (2017) Icarus, 298

[4] Plescia & Robinson (2019) Icarus, 321

[5] Williams et al. (2018) MAPS, 53

[6] Williams et al. (2016) Icarus, 273