The excess material around the lunar equator has been known since 1799 when French mathematician Pierre-Simon Laplace first noticed it. The reason, however, has been a mystery until now.
The moon's peculiar shape can be explained if the satellite moved in an eccentric oval-shaped orbit 100 million years after its violent formation, when the satellite hadn't yet solidified, the researchers say.
It was like a big ball of molasses and all around the equator it got deformed, study team member Ian Garrick-Bethell of the Massachusetts Institute of Technology told SPACE.com.
Around that time, conditions, such as orbit shape and position, were optimal for this "ball of molasses" to cool down and become the solid moon that we now know.
Today, the moon's orbit around the Earth is nearly circular.
To predict the moon's position and orbit millions of years ago, Garrick-Bethel and colleagues extrapolated backwards from ancient records of the timing of historical solar eclipses and of changes in the distance between the Earth and moon.
This finding will be detailed in the August 4 issue of the journal Science.