[meteorite-list] Is that a bulge at your equator, or...

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Thu Aug 3 21:40:34 2006
Message-ID: <t795d25rj599uhpj9g80amo187kaq8va0d_at_4ax.com>

http://www.cosmosmagazine.com/node/515

Solved! Mysterious Moon bump explained by eccentric orbit

by Marie Theresa Bray
Cosmos Online

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 PhD student from the Massachusetts
Institute of Technology, USA described the resulting shape of the moon as like
half an 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 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.
Received on Thu 03 Aug 2006 09:41:37 PM PDT