Step right up and see the dark side of the moon! ... again?

The western hemisphere of the Moon, as seen by the Galileo spacecraft (Image: Johnson Space Center Collection / NASA)

BILLIONS of years ago, the man in the moon may have performed the ultimate about-face, when an asteroid flipped the moon around.

The far side of the moon never faces us, because the moon rotates once for every orbit it makes of the Earth. Yet an analysis of impact craters shows the far side may once have pointed our way.

Mark Wieczorek and Matthieu Le Feuvre at the Paris Institute of Earth Physics in France studied the relative age and distribution of 46 known craters, gouged out by impacts from debris originating in the solar system's asteroid belt.

According to earlier computer simulations, the moon's western hemisphere as viewed from Earth should have about 30 per cent more craters than the eastern hemisphere. That's because the west always faces in the direction in which the moon orbits, which makes it more likely to be hit by debris, for the same reason that more raindrops strike a moving car's front windshield than its rear.

However, when Wieczorek and Le Feuvre compared the relative ages of the craters, using data about the sequence in which ejected material was deposited on the surface, they found the opposite to be true. Although the youngest impact basins were concentrated in the western hemisphere, as expected, the older craters were mostly congregated in the east. This suggests that the eastern face had once been bombarded more than the western face (Icarus, DOI: 10.1016/j.icarus.2008.12.017).

This could be explained if a large asteroid impact had set the moon turning. Such an impact would have put the satellite's rotation rate out of whack, so that for tens of thousands of years it would have appeared to slowly turn as viewed from Earth. Eventually, it would have settled into the current position.

The handful of lunar-rock debris collected from craters formed by a big enough smash suggest that the moon turned to face the other way more than 3.9 billion years ago, says Wieczorek. Asian probes currently circling the moon could reveal additional craters that would support the about-face idea.

via Did the Moon's far side once face Earth? - 21 January 2009 - New Scientist.