Longitudinal Asymmetries in the Lunar Impact Flux
Youngmin JeongAhn1*, Renu Malhotra2, Jui-Chi Lee1, Wing-Huen Ip1
1Institute of Astronomy, National Central University, Taoyuan, Taiwan
2Lunar and Planetary Laboratory, The University of Arizona, Tucson, AZ, USA
* presenting author:Youngmin JeongAhn, email:jeongahn@astro.ncu.edu.tw
The orbital distribution of small Earth Crossing Asteroids (ECAs) is poorly known by direct observations; it is generally assumed that they have a similar distribution as the better known larger, bright objects. The non-uniform spatial distribution of fresh craters on the Moon offers an alternative means to constrain the orbital distribution of small impactors on the Moon, i.e. small ECAs. The synchronous rotation of the Moon causes an enhancement of the impact flux on its apex side but the apex/antapex asymmetry calculated by numerical simulations consistently has been smaller than the observed ratio of 1.5 (Morota & Furumoto, 2002). Here we test the hypothesis of a missing population of low-inclination ECAs as a cause of the enhanced apex/antapex asymmetry. We calculate the spatial distribution of lunar impacts using a Monte-Carlo simulation consisting of two steps. First, we calculate the lunar impact probability of each pseudo-ECA using a modified Öpik's method (JeongAhn & Malhotra, 2015). Then we generate clones moving in parallel with each pseudo-ECA to calculate the spatial distribution of impacts on the lunar surface, including consideration of lunar gravity on their orbital paths. This semi-analytical method is applied to test the hypothesis and to compare results with the recent NEOWISE observations of the ECA population.

Keywords: Cratering, Asteroids, Lunar surface, Orbital dynamics