Scientists have known for hundreds of years that the Moon’s rotational and tidal bulges are much larger than expected. The deformation is thought to be a remnant from when the Moon orbited much closer to Earth than it does today. The problem is, the bulges we see require an unusual eccentric orbit—one that scientists do not think the Moon ever had. Keane and Matsuyama solved this problem by discovering a new component to the Moon’s global figure.

The Moon is not perfectly spherical because strong forces pull it in different directions. There are two main forces: lunar rotation, which causes bulging at the equator, and tidal forces between the Earth and the Moon, which cause bulging on the nearside and farside. The observed lunar deformation is much larger than scientists would expect, given the Moon’s current orbit and rotation rate. Scientists believe that this extra deformation arises because the Moon “froze” its shape billions of years ago when it orbited closer to Earth and thus had much larger forces pulling on it.

The authors investigated whether impact basins, volcanic plains, and other lunar gravity anomalies could contribute to the lunar figure. They found that one single impact basin—the giant South Pole–Aitken basin on the farside of the Moon (and the largest impact basin in the inner solar system)—could explain most of the Moon’s anomalous figure.

Accounting for the South Pole–Aitken basin (and 30 other large gravity anomalies), they found that the Moon’s figure actually formed on a low-eccentricity, synchronous orbit, consistent with what scientists believe for the early orbit of the Moon. Furthermore, the formation of the basin reoriented the entire Moon approximately 20°.

Thus, not only did the South Pole–Aitken basin reshape the Moon, but it even slightly changed which side of the Moon we see from Earth. (Geophysical Research Letters, doi:10.1002/2014GL061195, 2014)

—Catherine Minnehan, Freelance Writer

Citation: Minnehan, C. (2015), How did the Moon get its shape?, Eos, 96, doi:10.1029/2015EO031667. Published on 23 June 2015.