Scientists Discover Why the Earth's Moon is Shaped Like a Lemon and Not a Sphere

The moon isn't a simple sphere, and scientists have long struggled to explain why the shape of the moon deviates from a spherical shape; now, they may have an answer, and it all has to do with the way the moon was formed in the first place.

As the moon cooled and solidified about 4 billion years ago, the sculpting effects of tidal and rotational forces became frozen in place. The idea that this freezing created a frozen tidal-rotation bulge, known as the "fossil bulge" hypothesis, was first put forth in 1898. Yet this fossil bulge process can't fully account for the current shape of the moon.

That's why scientists decided to take a closer look. In the past, efforts to analyze the moon's overall shape have met with some hang-ups due to the large basins and craters created by powerful impacts that deformed the lunar crust and ejected large amounts of material.

"When we try to analyze the global shape of the moon using spherical harmonics, the creators are like gaps in the data," said Ian Garrick-Bethell, one of the researchers, in a news release. "We did a lot of work to estimate the uncertainties in the analysis that result from those gaps."

In this case, the researchers found that the variations in the thickness of the moon's crust caused by tidal heating during its formation can account for most of the large-scale topography. The rest is consistent with a frozen tidal-rotational bulge that formed later.

"In 2010, we found one area that fits the tidal heating effect, but that study left open the rest of the moon and didn't include the tidal-rotational deformation," said Garrick-Bethell. "In this paper we tried to bring all those considerations together."

Tidal heating and tidal-rotational deformation gave the moon a slight lemon shape with a bulge on the side facing the Earth and another bulge on the opposite side. The two processes left distinct signatures in the moon's gravity, as well. More interesting is that the scientists found that the moon's overall gravity field is no longer aligned with the topography, as it would have been when the tidal bugles were frozen into the moon's shape.

The findings reveal a little bit more about the moon and how it first acquired its shape. More specifically, it reveals a bit more about the shape of the moon today.

The findings are published in the journal Nature.