Constraints on the source of lunar cataclysm impactors

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.12.013 Byline: Matija Auk (a), Brett J. Gladman (b), Sarah T. Stewart (a) Keywords: Moon; Moon, Surface; Cratering; Asteroids, Dynamics; Planets, Migration Abstract: Multiple impact basins formed on the Moon about 3.8 Gyr ago in what is known as the lunar cataclysm or Late Heavy Bombardment. Many workers currently interpret the lunar cataclysm as an impact spike primarily caused by main-belt asteroids destabilized by delayed planetary migration. We show that morphologically fresh (class 1) craters on the lunar highlands were mostly formed during the brief tail of the cataclysm, as they have absolute crater number density similar to that of the Orientale basin and ejecta blanket. The connection between class 1 craters and the cataclysm is supported by the similarity of their size-frequency distribution to that of stratigraphically-identified Imbrian craters. Majority of lunar craters younger than the Imbrium basin (including class 1 craters) thus record the size-frequency distribution of the lunar cataclysm impactors. This distribution is much steeper than that of main-belt asteroids. We argue that the projectiles bombarding the Moon at the time of the cataclysm could not have been main-belt asteroids ejected by purely gravitational means. Author Affiliation: (a) Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA (b) Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC, V6T 1Z1, Canada Article History: Received 25 June 2009; Revised 5 November 2009; Accepted 6 December 2009