The mysterious bulge on the dark side of the Moon may have been explained.

Credit: NASA

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SYDNEY, 4 August 2006 - The far side of the Moon boasts an unusual bulge at the equator, whose origin has baffled scientists for centuries, but according to a new study in today's issue of the U.S. journal Science, a possible explanation could be that early in the Moon's history the orbit may have differed during the crucial stage in which the lunar magma ocean was solidifying.

This age-old mystery of the 'fossil bulge' was first brought to attention by mathematician Pierre-Simon Laplace in 1799, and since then various explanations have been proposed but have failed to fit the exact dimensions of the Moon.

Today's study showed that the Moon's shape can be justified if the Moon was in an oval, or 'eccentric' orbit, 100 million years after its formation. Ian Garrick-Bethell, co-author of the paper and a PhD student at the Massachusetts Institute of Technology in Boston described the resulting shape of the Moon as like half of an oval-shaped American football.

For the model to work, Garrick-Bethell had to take into consideration the process of how the fossil 'freeze-in', or solidification, actually works. He framed his research around the question: "how can you freeze-in a single-axis football component in a plastic Moon, when the Moon is continually spinning with respect to the Earth thereby changing the axis that gets the football deformation?"

Along with fellow colleagues, Garrick-Bethell modelled specific orbits that were possible solutions, including one similar to the present state of the inner planet, Mercury. The researchers said that if the Moon was spinning 1.5 times on its own axis for each time it orbited the Earth, instead of only once as it does now, it would have been spinning fast enough to stretch the cooling magma.

"At any point in its orbit, the Moon's rotation stretches it like a flattened basketball, while tides from the Earth stretch it like a football," said Garrick-Bethell. "We thought of different scenarios that could increase the flattening component, and one of the most straightforward ones is to simply assume that the Moon was once spinning faster."

"It is well known that [an eccentric] orbit is stable for Mercury (which has a 3:2 resonance), so we explored what values of eccentricity would give the current lunar moments of inertia in a 3:2 resonance, if any," explained Garrick-Bethell.

According to Garrick-Bethell, these findings can be used for future work along similar lines as there is still much left to be studied about the evolution of the Earth and Moon system.

"There are always new ways of looking at old problems. Very little is definitively known about the early evolution of the Earth-Moon system," he said.