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Tidal Forces Caused The Moon To Be Shaped Like A Lemon, New Research Reports

August 1, 2014
Image Credit: Thinkstock.com

redOrbit Staff & Wire Reports – Your Universe Online

More than four decades after US astronauts first visited the lunar surface, scientists have finally determined both the actual shape of the moon and the forces responsible for giving it that form, according to a new study published this week in the journal Nature.

As it turns out, the moon is somewhat flattened with a bulge on one side, the authors told Douglas Quenqua of the New York Times. University of California, Santa Cruz planetary scientist Ian Garrick-Bethell, one of the researchers involved in the study, said it was similar to “a lemon with an equatorial bulge… imagine a water balloon flattening out as you spin it.”

Previous efforts to determine the exact shape of the moon have been hampered by the large craters present on its surface – craters which formed after the crust solidified, the Times noted. There have also been a series of inconsistencies between the measurements and what experts know about its past.

For instance, since the moon barely spins, why does it appear to have the type of equatorial bulge that is usually the result of rotation, and how can a massive ball of cooled liquid wind up as anything but a spherical object? The answer, Garrick-Bethell and his colleague report, lies in the tidal effects that were active early on in the moon’s history.

As the moon began cooling and solidifying over four billion years ago, the sculpting effects of tidal and rotational forces were locked in place, according to the study author. Using Garrick-Bethell’s previous water balloon comparison, he explained that the concept of a frozen tidal-rotational bulge (also known as the “fossil bulge” hypothesis) involved the “balloon” becoming flatter at its poles and bulging at the equator.

The “fossil bulge” hypothesis ultimately creates a lemon-like shape, with the long axis of the lemon pointing towards the Earth, but this notion alone cannot fully account for the moon’s current shape. So the authors of the new paper incorporated other tidal effects into their analysis. They took into account the large impact basins that have shaped the moon’s topography, as well as the moon’s gravity field and its topography.

“When we try to analyze the global shape of the moon using spherical harmonics, the craters are like gaps in the data. We did a lot of work to estimate the uncertainties in the analysis that result from those gaps,” Garrick-Bethell said in a statement Wednesday.

Their findings indicate that variations in the thickness of the moon’s crust resulting from tidal heating during its formation accounts for the majority of its large-scale topography, while the remainder is consistent with a frozen tidal-rotational bulge that formed later on in the course of lunar history.

According to the Times, the data they collected during the investigation “explain how the moon acquired its shape,” The “squashed appearance” is most likely the 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.”

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Source: redOrbit Staff & Wire Reports - Your Universe Online



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