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Our Moon is the only solar system body for which we have both crater size-frequency distributions (SFDs) through most of bombardment history and ages of samples that are reasonably associated with known terrains. These are keystones for understanding the crater production function through time. Previous work on this topic is decades old [e.g., 1, 2]. New imaging from Lunar Reconnaissance Orbiter Camera (LROC) and results from dynamical calculations of plausible impactor populations [e.g., 3, 4] encourage a reevaluation, especially of smaller diameter craters (D < 20 km). For this purpose, we have compiled SFDs for smaller craters superposed on the floors of 40 craters with diameters primarily between 80-100 km and locations across the lunar surface.
Within each larger crater, superposed craters are measured on the LROC Wide Angle Camera mosaic (pixel scale=100 m/pixel) using the 3-point crater tool in JMARS for the Moon (http://jmars.asu.edu/), which outputs center latitude and longitude, and crater diameter. Degradation class is assigned to each crater, ranging from 1 (fresh) to 4 (most degraded). A crater may also be identified as an “obvious secondary” by being part of an obvious cluster or chain. Ages of the crater floors are computed using the Model Production Function (MPF) chronology developed by Marchi et al. [5].
An intriguing result is that many of our computed ages for the crater floors (which may be the original crater floor or reflect later modification) are older than indicated by previous work [e.g., 6]. This could be the result of several factors. First are issues with estimation of ages in previous work, which include use of poor Lunar Orbiter images (especially away from the near side) and application of the unreliable “DL” method, which involves simplified assumptions about how craters degrade. Further issues include assumptions in the absolute age model, such as not incorporating short, intense bombardments from asteroid breakups in the impact rate or including unrecognized secondaries in the superposed SFD. Further study is needed into these differences in ages and what they may mean for our understanding the impact rate over time.
Many of our superposed crater SFDs have shapes matching the MPF, which relies solely on the observed and modeled population of impacting bodies that create primary craters and does not include secondary craters. For the SFDs that match, we find two trends. First, for younger terrains (< ~ 3.5 Ga) the superposed crater SFDs can be represented by a single, steep differential slope of b ~ 5, [N(Da,Db) = aD-b] for D ~ 0.7-1.5 km. Then, as terrains get older (> ~ 3.5 Ga), the steep slope remains for small craters, but a shallow SFD for large craters (D > 1.5 km), with b ~ 2, is added. This shallow slope might be consistent with a primary crater population already observed and named “Population 1” by Strom et al. [7] for larger craters (D > 20 km). Strom et al. [7] were also able to correlate “Population 1” SFD with the MBA SFD, implying this may represent the production population of large primary craters on the Moon. However, we CANNOT necessarily extrapolate “Population 1”, and its suggested representation of the MBA population, to the small craters we are observing. First, many of the shallow portions of the SFDs are represented by 1 or 2 large craters with the associated large uncertainties; therefore, the statistics are too poor to understand if the shallowing of the SFD is a real feature. Second, we find that most of these SFDs are composed of degraded craters (classes 3 & 4), implying the superposed small craters are in degradation equilibrium. Degradation at different rates for different diameters alters the initial production SFD to a new equilibrium population that is not representative of crater production.
References: [1] Hartmann, W.K., et al. (1981) in Basaltic Volcanism on the Terrestrial Planets. Pergamon Press, New York, 1049-1127. [2] Neukum, G., et al. (2001) Space Sci. Rev. 96, 55-86. [3] Gomes, R., et al. (2005) Nature 435, 466-469. [4] Bottke, W.F., Jr., et al. (2005) Icarus 175, 111-140. [5] Marchi, S., et al. (2009) Astron. J. 137, 4936-4948. [6] Wilhelms, D.E. (1987) The Geologic History of the Moon. USGS, Paper 1348. Washington, DC. [7] Strom, R.G., et al. (2005) Science 309, 1847-1850.
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