Physical Science Blog

Lunar Lump Explained… Kind of

A mysterious moon bulge may have been caused by Earth

December 4, 2010

This holiday season, rest assured that you aren’t the only one hiding an unsightly bulge. It turns out the Moon carries a little extra paunch on its far side, where earthly stargazers could never find it…

Okay, we did find it. Decades ago, in fact, when the Apollo 15 mission measured lunar altitudes using a laser beam in 1971. But no one could figure out why the far side’s crust is thicker and its elevation 3.7 miles higher than on the side that always faces Earth. After doing some mathematical modeling, a team of scientists at UC Santa Cruz thinks they may now have the answer. They say it’s a result of tidal forces acting on lunar crust, which once floated on a liquid core.

‘Wait a minute!’ you might be saying, ‘I thought the moon causes tides?’ So it does, here on Earth—the moon’s gravity pulls at our planet, drawing the water forward in a huge, wet pair of love handles around the Earth’s waist. But the Earth’s gravity, which is greater than the moon’s, tugs back on the lunar surface at the same time. The scientists posit that the lunar bulge formed some 4.4 billion years ago, when the moon’s interior was more molten and more stretchable than it is now.

On Earth, lunar tides effect both sides of the planet that are in line with the moon. Similarly, you’d expect the moon to have two symmetrical bulges—one on each side. But there’s only one, and the tidal model cannot explain this. The authors can only guess that after the bulges hardened, topographic changes to the near-side—like meteors or volcanic activity—erased the original symmetry.

One thing’s for sure: it wasn’t caused by mashed potatoes and pumpkin pie. If you catch your own far side growing between Thanksgiving and New Year’s, just do what these scientists did: blame it on the tides.

About the Author

Sarah Fecht

Sarah Fecht is a native of Syracuse, NY. She has loved Biology since a 7th-grade “Life Science” class and was one those rare people who went into college knowing what they wanted to study. She got a B.S. in Biology from Binghamton University, but got scared away from a research career at the prospect of narrowing her scientific interests into a thesis. Since then, she has retreated into the world of science journalism, where her interests have broadened to encompass astronomy, physics, conservation, technology and more.


1 Comment

Eat The Sheeple says:

Someone correct me if I’m wrong, but doesn’t the moon rotate. How would you explain ANY kind of singular bulge formation on an object with a continual rotational movement. If tidal forces caused bulging in a previously molten moon shouldn’t we be seeing a nice symmetrical circular bulge around the entire moon?

I’d like to see if there are any correlations between the bulge and meteor impacts.

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