A Strange Bulge On The Dark Side Of The Moon

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The dark side of the Moon is home to mysterious highlands that have baffled scientists for decades. It turns out Earth's gravity created them, back when the lunar surface floated on a magma ocean.

About 4.4 billion years ago, soon after the Moon formed, it possessed a giant ocean of liquid rock that separated the outer crust from the inner mantle. With nothing solid connecting the exterior and interior the Moon, its outer regions essentially floated on the magma, leaving them vulnerable to gravitational influences from Earth. Our planet's gravitational pull helped heat up the lunar crust and caused a phenomenon known as tidal flexing, essentially squishing the Moon out of shape.


At the lunar poles, the tidal flexing and heating was strongest, causing the crust in those areas to become thinner. Meanwhile, around the lunar equator, the crust became thicker. In particular, the parts of the equator that lined up with Earth - in other words, the region of the Moon closest to Earth, and then its corresponding point on the dark side - were most strongly affected, creating these great highlands.

This theory is the work of planetary scientist Ian Garrick-Bethell, who has discovered a remarkably simple mathematical function that can account for much of the Moon's basic shape. There is, he admits, one problem. Like I mentioned earlier, the regions closest and furthest from Earth should have experienced tidal bulges...and yet we only know about one on the dark side of the Moon. Doesn't that put a damper on his theory? This is where 4.4 billion years of intervening time comes in very handy, as he explains:

"You would expect to see a bulge on both sides, because tides have a symmetrical effect. It may be that volcanic activity or other geological processes over the past 4.4 billion years have changed the expression of the bulge on the nearside."


His mathematical function can explain far more than just the bulge - Garrick-Bethell says it can account for as much as 25% of the Moon's shape, which can help set up a larger mathematical framework for further modeling of the lunar surface.