Unfamiliar sight: The far side of the Moon as captured by one of NASA's Apollo missions in the 1970s. The Moon is locked in a synchronous rotation with the Earth, meaning that we always see the same side of it.
Credit: NASA
LONDON: The far side of the Moon may have once faced Earth, says a new study which argues that a massive asteroid impact billions of years ago could have flipped it through 180º.
From Earth we only ever see one side of the Moon, as its rotation is locked with ours and it spins once for every orbit around the Earth.
Moon flip
However, crater patterns on the lunar surface suggest this orientation may not have always been the case, say scientists Mark Wieczorek and Mathieu Le Feuvre from the Paris Institute of Earth and Physics in France.
The idea of a Moon flip is not totally new. In 1975 researchers in the U.S. proposed that if a large enough asteroid slammed into the satellite, it would wobble back and forth like a pendulum before settling down in a locked rotation with only one face visible from Earth.
Until now, though, there has been no evidence to back up the theory.
For the new study, detailed this month in the journal Icarus, Wieczorek and Le Feuvre scoured images of the Moon's cratering patterns.
Recent work has suggested that as the Moon orbits Earth, its leading side – the western hemisphere – is 30 per cent more likely to get hit by asteroids that the other. "It's really like a car moving through the rain, there'd be more rain falling on the front windscreen than the back," explained Wieczorek.
Dating craters
The researchers compared the ages of craters across the surface. As expected, that the younger asteroid impacts were on the western side. But to their surprise, the oldest craters were on the eastern side – the back windscreen – suggesting that the Moon's orientation may once have been reversed.
The age of the craters was estimated by measured by looking at layers of debris thrown out from multiple impacts, and also by the number of craters superimposed on top of one another.
But where's the evidence for a massive Moon-flipping asteroid impact? Wieczorek claims there are six giant basins that could mark the site of the blow.
The current favourite is the Smythii basin that lies on the Moon's equator "so it would have more ability to spin up the Moon than, say, if it was on the north pole." The experts estimates the direct hit at over 3.9 billion years ago.
This about-turn could also mean a major rethink about how Moon craters are aged. Previous estimates have assumed asteroid impacts were totally random, but if the forwards impact effect holds true, and then the Moon flipped, many old measurements will now be inaccurate, the researchers said.
"It seems plausible that the Moon was originally orientated in the opposite direction to what we see today," commented Katherine Joy, a lunar geologist from University College London in England.
Major rethink
This discovery also "has important consequences for understanding the impact cratering record in the whole of the inner Solar System… cratering age calculations may have a new degree of ambiguity," she said.
"This is the first real evidence that [a Moon flip] could have occurred," added planetary geophysicist Jay Melosh from the University of Arizona in Tucson; the expert who first came up with the theory in 1975.
The find also raises the prospect that other satellites in our Solar System might have flipped too, said Melosh. "The moons of Saturn and Jupiter currently face their primary direction, but they may have undergone reversal at some point."
The next stage will be finding more craters to back up Wieczorek's findings, and the answers may not be far away – Indian and Japanese Moon probes currently in orbit and the U.S.'s GRAIL (Gravity Recovery and Interior Laboratory) mission, slated for launch in 2011, could uncover vital new evidence for the about-turn.
