Recent Impacts on the Moon

Prior to Lunar Reconnaissance Orbiter (LRO) observations, an understanding of impact rates of meteoroids <1 m in size was based on extrapolation techniques from near-Earth object (NEO) knowledge [1,2,3], meteors in Earth's atmosphere [4], recent impacts recorded on Mars [5,6,7], and lunar 'flashes' (likely impacts) observed by teams such as those at Marshall Space Flight Center [8]. Since July of 2009, the Lunar Reconnaissance Orbiter Camera (LROC) collects meter scale Narrow Angle Camera (NAC) images, with repeat coverage in areas of high interest. Planned and serendipitous re-imaging with similar illumination conditions provides the means to detect temporal surface changes with the ultimate goal of measuring the current flux of impacts on the Moon. To easily detect a change at the surface, NAC-pairs separated in time (temporal pair), with similar illumination geometries are compared. Overlapping regions in a temporal pair are map projected and co-registered and a ratio is computed (second observation / first observation) and examined for temporal anomalies. Some changes are clearly distinguished as newly formed craters with rims and ejecta, while others are simply small (a few pixels) reflectance changes (crater not resolved). Detections are categorized as relatively high reflectance changes (HRC) or low reflectance changes (LRC) relative to the surrounding substrate. To date the LRCs outnumber the HRCs by a factor of ten. Clusters (>3) of changes were discovered in 48 temporal pairs. So far, we have identified 599 individual changes, with 547 LRCs and 48 HRCs. Of the 599 detections, sixteen represent resolved craters, and of these diameters range up to 20 m, suggesting bolide sizes up to ~1 m diameter. The total surface area examined to date is ~25,000 square km and the maximum time window between repeat images is 2.5 years, yielding an estimated minimum 364,000 new lunar craters per year (or one crater per year for every 104 square km) detectable at the scale of NAC images (secondary or primary events), assuming all temporal changes are due to impacts. The LRCs were unexpected, as fresh craters have relatively high reflectance ejecta due to exposure of immature material from the subsurface. Currently we are investigating the hypothesis that the LRCs result from shallow excavation into a fully mature substrate and thus no immature material is excavated. The origin of the clusters is enigmatic, and four mechanisms are under consideration: 1) meteoroids travel in tight clusters, 2) meteoroids break up as a result of close approaches to Earth before impacting the Moon, 3) meteoroids disaggregate upon approach to the lunar surface, 4) clusters are actually secondaries from yet undiscovered (larger) primaries. [1] Rabinowitz, et al., 2000. Nature, 403, 165-166; [2] Stuart and Binzel, 2004. Icarus, 170, 295-311; [3] Nemtchinov, et al., 1997. Icarus, 130, 259-274; [4] Brown, et al., 2002. Nature, 420, 294-296; [5] Malin, et al., 2006. Science, 314, 1573-1577; [6] Daubar et al. 2011. LPSC, #2232 [7] Daubar et al. 2013. Icarus, 225, 506-516; [8] Suggs, et al., 2013. NLSI Lunar Science Forum.