Distributions of Superposed Impact Craters on Lunar Basins

Determining the distribution of impact craters on the Moon is vital for understanding inner solar system impactor populations and their evolution. The Moon is such an important keystone for this area of research because the crater distribution, especially from early solar system history, is readily observed and has not been heavily modified like it has been on the Earth. Furthermore, radiometric dating of Apollo samples provide the only extraterrestrial set of absolute ages for specifically identified planetary surfaces. Therefore the Moon is the only place in the solar system that provides a calibrated absolute crater chronology for a large fraction of solar system history [e.g., 1-4]. These chronologies, however, assume that the impactor distribution has not changed significantly over time (possibilities include the Late Heavy Bombardment [5] and asteroid break-ups [6]). In addition, crater counts on many areas of the Moon have not been performed for at least 30 years. In order to further constrain lunar impactor populations and their temporal evolution, we have begun measuring craters superposed on lunar basins from different eras and regions of the Moon (e.g., Birkhoff, Imbrium and Orientale). Currently, we are utilizing newly digitized Lunar Orbiter (LO) images [7]. The LO-IV and -V missions collected many valuable images of the near and far side at resolutions useful for compiling crater databases with a large dynamic range: 30-80 m/pixel, which converts to a minimum measurable diameter (D) of ~ 200 m up to several tens of kilometers. These resolutions are also comparable to the resolutions of Lunar Reconnaissance Orbiter Wide Angel Camera (LRO-WAC). When rectified LRO-WAC images become available, we will use them to compare with results from LO images and examine new regions. LRO-WAC images have better dynamic range and coverage, and lack the image blemishes of LO images. Preliminary results from Birkhoff basin (center = 59°N, 147°S, D = 325 km, [8]) indicate that secondary cratering may be prolific for craters with diameters < ~ 20 km. The distribution displays a relative deficiency of craters with D ~ 20 km (on a binned R-plot). This relative decrease in craters in a narrow size range could be indicative of two combined populations, one with a deficiency of larger craters (i.e., secondaries) and another with a deficiency of small craters (main asteroid belt, NEO’s?). If corroborated in other regions of the Moon, this result could reduce the utility of small craters for determining absolute ages of terrains on the Moon. REFERENCES: [1] Neukum, G., et al. (2001) Space Sci. Rev. 96, 55-86. [2] Hartmann, W.K., et al. (1981) in Basaltic Volcanism on the Terrestrial Planets. Pergamon Press, New York, 1049-1127. [3] Ivanov, B.A., et al. (2002) in Asteroids III. Univ. of Arizona Press, Tuscon, AZ, 89-101. [4] Stöffler, D., et al. (2006) Rev. Mineral. Geochem. 60, 519-596. [5] Kring, D.A. and B.A. Cohen (2002) J. Geophys. Res. 107, 10.1029/2001JE001529. [6] Bottke, W.F., et al. (2008). Early Solar System Impact Bombardment. Abst #3005. [7] Weller, L., et al. (2007). Lunar Planet. Sci. XXXVIII. abst #2092. [8] Wilhelms, D.E. (1987) The Geologic History of the Moon. USGS, Paper 1348. Washington, DC.