How the moon got its shape

Gregory H. Rivera, Wikipedia/Creative Commons

How the moon got its shape

Sid is a freelance science journalist.

From here on Earth, the moon looks like a perfect orb. But new data gathered by spacecraft zipping around our celestial companion reveal that it’s actually squished and slightly elongated, with the thickest portions of its crust on the areas nearest and farthest from Earth. The reason for this less-than-perfect shape? For 200 million years after the moon formed, its crust was weak and the underlying rocks were molten, which made it easy for tides caused by the gravitational pull of Earth to distort it. As the moon cooled, its warped shape froze into place, forming a “fossil bulge” that largely defines the moon’s current topography, scientists report online today in Nature. Later, episodes of large-scale volcanism—which previous studies suggest started about 4 billion years ago and lasted until about 2 billion years—gave the moon an off-center distribution of mass. Earth’s gravity tugging on this evolving lopsidedness gradually triggered a shift in the moon’s polar axis, the researchers say. The moon’s north pole is now stable, but prior to the density-shifting volcanism, the moon’s north pole was located about 36° away from where it is now. The paleopole now sits at a latitude of 54°N, on the edge of a feature called Oceanus Procellarum (the “Ocean of Storms”), a broad dark area maybe better recognized as the Man in the Moon’s right eye.

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