The moon's far side bulges at its equator, a peculiarity that has long puzzled investigators. Scientists conjectured that the bulge formed when magma oceans that covered the young moon solidified while deformed by gravity and lunar spin, but the hypothesis failed to match theories of the moon's early orbit with its precise dimensions. Researchers at the Massachusetts Institute of Technology now calculate that they could explain the bulge if the moon's orbit 100 million to 200 million years after the moon formed was about twice as close to Earth and more oval. This orbit, resembling present-day Mercury's in completing three rotations for every two revolutions, would have proved ideal to help freeze the bulge in place. The findings, appearing in the August 4 Science, also suggest a time on Earth when the moon cycled through its phases in just 18 hours and raised tides four times a day at up to 10 times the strength.
This article was originally published with the title "Fat Side of the Moon" in Scientific American 295, 4, 32 (October 2006)