Article: "Lunar Crater Stats Indicate Hidden Population of Asteroids"

From the article...

Many moons are locked in synchronous rotation with their mother planets. Examples include the Galilean moons of Jupiter, Neptune's moon Triton and our own Moon.

In the 80s and 90s astronomers noticed that the distribution of craters on these objects was asymmetric: they were more heavily cratered on their leading hemispheres which makes sense since it seems obvious that these areas should be struck more often.

It wasn't until 2003, however, that the same asymmetric crater distribution was measured on our Moon. Now Takashi Ito at the National Astronomical Observatory in Japan and Renu Malhotra at the University of Arizona have asked an interesting question. of the data. Can the asymmetric distribution of craters on the Moon be explained by the known distribution of near Earth asteroids that are thought to have caused them? Their answer is a cautious "no".

To properly explain the crater distribution, Ito and Malhotra say some other factor must have been involved. One possibility is that we simply haven't seen all the craters yet: the ongoing lunar mapping missions may help on that score.

Another idea is that the Earth's tidal forces tear Earth-crossing asteroids apart, creating a higher number of impacts than might otherwise be expected.

But the most exciting and potentially worrying possibility is that there exists a previously unseen population of near Earth asteroids that orbit the Sun at approximately the same distance as the Earth. These have gone unnoticed because they are smaller or darker than other asteroids, say Ito and Malhotra.

"More complete observational surveys of the near-Earth asteroids can test our prediction," they say.

"Lunar Crater Stats Indicate Hidden Population of Asteroids"
03 August 2009

Link: Technology Review Article

Abstract:

Asymmetric impacts of near-Earth asteroids on the Moon
Authors: Takashi Ito, Renu Malhotra
(Submitted on 17 Jul 2009)

Abstract: Recent lunar crater studies have revealed an asymmetric distribution of rayed craters on the lunar surface. The asymmetry is related to the synchronous rotation of the Moon: there is a higher density of rayed craters on the leading hemisphere compared with the trailing hemisphere. Rayed craters represent generally the youngest impacts. The purpose of this paper is to test the hypotheses that (i) the population of Near-Earth asteroids (NEAs) is the source of the impactors that have made the rayed craters, and (ii) that impacts by this projectile population account quantitatively for the observed asymmetry. We carried out numerical simulations of the orbital evolution of a large number of test particles representing NEAs in order to determine directly their impact flux on the Moon. The simulations were done in two stages. In the first stage we obtained encounter statistics of NEAs on the Earth's activity sphere. In the second stage we calculated the direct impact flux of the encountering particles on the surface of the Moon; the latter calculations were confined within the activity sphere of the Earth. To represent NEAs' initial conditions, we considered two populations: one is the currently known NEAs, and the other is a synthetic population created by debiasing the orbital distribution of the known NEAs. We find that the near-Earth asteroids do have an asymmetry in their impact flux on the Moon: apex-to-antapex ratio of 1.3-1.4. However, the observed rayed crater distribution's asymmetry is significantly more pronounced: apex-to-antapex ratio of ~1.67. Our simulations suggest the existence of an undetected population of slower (low impact velocity) projectiles, such as a population of objects coorbiting with Earth.

Link: Abstract: "Asymmetric impacts of near-Earth asteroids on the Moon"

Link: Paper: Asymmetric impacts of near-Earth asteroids on the Moon [PDF]