Lunar Lump Explained… Kind of

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.