Monday, August 03, 2009
Lunar Crater Stats Indicate Hidden Population of Asteroids
The asymmetric distribution of craters on the Moon may have been caused by an undiscovered population of near Earth asteroids
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 1980s and 1990s 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.
And let's not waste too much time about it. By some reckonings, asteroid impacts represent the greatest threat to humankind that we are able to calculate.