Forty-five years after astronauts landed on the moon, scientists say they have finally discovered its true shape: slightly flattened, with a bulge on one side.
“Like a lemon with an equatorial bulge,” said Ian Garrick-Bethell, a planetary scientist at University of California, Santa Cruz, and an author of the study, being published in the journal Nature. “If you can imagine a water balloon flattening out as you spin it.”
Efforts to pinpoint the moon’s exact shape have long been stymied by the presence of large craters on its surface that formed after the crust solidified. There have also been inconsistencies between its measurements and what we know about its past.
For example, the moon barely spins, yet it appears to have the sort of equatorial bulge caused by rotation. And why would a giant ball of cooled liquid be anything but spherical?
“There’s no plate tectonics like on the earth,” Dr. Garrick-Bethell said. “Why is it so deformed?”
To overcome the crater problem, he and his colleagues used highly accurate maps of the moon’s topography, made with a laser altimeter, then ran calculations to see what the surface could have looked like before the craters formed.
The measurements that emerged help explain how the moon acquired its shape, the researchers say. Its squashed appearance is probably a result of the gravitational process called tidal heating or acceleration, which stretched the moon’s crust as it was being formed. The equatorial bulge probably dates to a later period, when the moon was still spinning but was slowing down and moving away from earth, freezing a tidal surge in place.
The clues, Dr. Garrick-Bethell said, are all in the math.