Introduction: Prior to the Lunar Reconnaissance Orbiter (LRO) observations, an understanding of impact rates of meteoroids <1m in size was based on extrapolation techniques from near-Earth object (NEO) knowledge [1-3], meteors in Earth’s atmosphere , recent impacts recorded on Mars , and lunar “flashes” (likely impacts) observed by teams such as those at Marshall Space Flight Center. Since July of 2009 the Lunar Reconnaissance Orbiter Camera (LROC) has collected Narrow Angle Camera (NAC) images at resolutions ranging from 0.2 to 4.2 meters/pixel. Intentional and accidental reimaging with similar illumination conditions provides a reliable dataset to detect changes on the surface over time with the ultimate goal of measuring the current flux of impacts on the Moon.
Methodology: To easily detect a change in the surface at a site (the overlap region between two NAC-pairs separated in time, identified as a NAC temporal set) – the incidence angle should be nearly the same. Since October 2011 targeting of existing NAC coverage under like-illumination conditions became routine. Overlapping regions between temporal sets were map projected with the more recent NAC-pair being co-registered and warped to overlay the older NAC-pair, then a ratio was computed. The ratio images are examined for temporal anomalies to record the location, size, and relative reflectance difference. Some anomalies are clearly distinguished as craters with rims and ejecta, while others are too small to resolve the crater. Temporal anomalies are seen as relatively high reflectance (HRA) or low reflectance anomalies (LRA) relative to the local substrate. The LRAs were a surprise as fresh craters usually have relatively high reflectance ejecta due to exposure of immature material from the subsurface. The LRAs do not come from known cryptomare, so their origin is enigmatic at this time. Additionally, we observe multiple LRAs occurring in a NAC temporal set.
Results: So far 69 temporal anomalies are recorded with 53 LRAs and 16 HRAs. LRAs tend to have a larger and more diffuse “ejecta,” whereas the HRAs have a constant reflectance to the edge of the ejecta. Crater diameters range up to 7.3-m suggesting bolide sizes up to ~1.0-m diameter. Thirteen of the LRAs and two of the HRAs have discernable crater rims. The total surface area examined to date is 5830 km2 and the total time window observed is 2.5 years yielding an estimated 180,000 lunar impacts per year detectable at the scale of NAC images. Separating the LRAs and HRAs yields 138,000 and 42,000 new impacts per year, respectively.
Conclusions: Currently we have only analyzed 31 NAC temporal sets and discovered 69 temporal anomalies in 21 of the temporal sets (68%). As more temporal sets are analyzed, reliable estimates of current impact rates of bolides in the Earth-Moon system at 10 cm diameter and above will be possible.
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