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How is the moon tidally locked? (self.askscience)

submitted 2 years ago by Hypersapien to c/askscience

19 comments fedilink hide all child comments

The moon rotates once per revolution around the Earth, but that's not a coincidence. Somehow the rotation and revolution are connected to each other. Some force is keeping them the same. How exactly does that work?

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[–] jasparagus 9 points 2 years ago* (last edited 2 years ago) (1 children)

Here's a good explainer:

What is tidal locking? https://phys.org/news/2015-11-tidal.html

Basically, the moon acted like a spinning (unbalanced) wheel, and eventually stopped with the "heavy" side pointing "down" towards Earth. I.e. think of the moon as orbiting Earth with the heavy side staying pointed at Earth.

[–] Zadkine 2 points 2 years ago (1 children)

Is that also why more ‘interesting’ side, with more features such as lunar maria, is pointed at us? The far side of the moon looks pretty boring compared to the near side. Or is that just a coincidence?

[–] jasparagus 3 points 2 years ago

Looks like it's a common question, and is still under debate!

https://en.m.wikipedia.org/wiki/Lunar_mare

The reason that the mare basalts are predominantly located on the near-side hemisphere of the Moon is still being debated by the scientific community. Based on data obtained from the Lunar Prospector mission, it appears that a large proportion of the Moon's inventory of heat producing elements (in the form of KREEP) is located within the regions of Oceanus Procellarum and the Imbrium basin, a unique geochemical province now referred to as the Procellarum KREEP Terrane.[10][11][12] While the enhancement in heat production within the Procellarum KREEP Terrane is most certainly related to the longevity and intensity of volcanism found there, the mechanism by which KREEP became concentrated within this region is not agreed upon.[13]

It is commonly suggested that there is some form of link between the synchronous rotation of the Moon about the Earth, and the mare basalts. However, gravitational torques that result in tidal despinning only arise from the moments of inertia of the body (these are directly relatable to the spherical harmonic degree-2 terms of the gravity field), and the mare basalts hardly contribute to this (see also tidal locking). (Hemispheric structures correspond to spherical harmonic degree 1, and do not contribute to the moments of inertia.) Furthermore, tidal despinning is predicted to have occurred quickly (in the order of thousands of years), whereas the majority of mare basalts erupted about one billion years later.