It's well known that the near side of the Moon and the far side have different topographical features and that if the Moon had a core it might have been shifted to the near side that faces Earth due to a large impact it might have sustained in the past, but do we know which side is heavier? I could guess that if the core was shifted that might make the near side heavier, but I haven't found a definite answer. Please attach any evidence that supports your answer!
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asked Aug 2, 2017 at 19:51
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5$\begingroup$ This is a question for Astronomy $\endgroup$– UweCommented Aug 2, 2017 at 20:19
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2$\begingroup$ @Uwe, it also falls under Space Exploration. $\endgroup$– Rickest RickCommented Aug 2, 2017 at 23:01
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9$\begingroup$ Despite popular, but incorrect usage, the so called 'dark' side of the Moon is properly referred to as the far side. Just because we don't see it, doesn't mean it doesn't get illuminated & that it is in a state of perpetual darkness. $\endgroup$– FredCommented Aug 3, 2017 at 2:43
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1$\begingroup$ Good observation @Fred. $\endgroup$– Rickest RickCommented Aug 3, 2017 at 3:45
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$\begingroup$ Possible duplicate of Why is the Far Side of the Moon so different from the Near Side? $\endgroup$– David HammenCommented Aug 3, 2017 at 10:04
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3 Answers
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[...] current understanding of the structure of the Moon involves an iron-rich, aluminum-poor mantle whose center of mass is offset about 10 km from the center of mass of an aluminum-rich, iron-poor crust. The direction of offset is toward the Earth, about 14 degrees to the east of the Earth-Moon line. Such a model accounts for the basaltic maria which face the Earth, and the aluminum-rich highlands on the Moon's far side, and for a 2 km offset between the observed center of mass and center of figure for the Moon.
Will, https://arxiv.org/abs/1403.7377 , p. 51
According to this description, the average density of the half of the sphere that is closer to the earth would be greater, because it would contain more than half the volume of the denser mantle, and less than half of the less dense crust.
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According to this NASA page:
The mass of the Moon is not evenly distributed; mass concentrations, called Mascons, lie beneath many of the lunar basins, and the center of mass of the Moon is displaced several kilometers towards the Earth.
This indicates that the near side is slightly heavier.
answered Aug 2, 2017 at 21:54
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The near side needs to be slightly heavier to maintain a fixed tidal locking over eons with no rotational drift but a stable equilibrium like a toy “Russian doll” even while the moon is getting further away by several centimeters per year.
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1$\begingroup$ It only needs to vary from radial symmetry. If it was stretched into an ellipsoid, it would have two stable positions, and would stay in one of them unless disturbed enough to rotate it more than 90 degrees with respect to Earth. And even with such an off-center mass, it could be stable with the heavy side outward...to the first order, it doesn't matter which end is heavier, just that the tidal forces on the object have a local maximum in a given orientation. $\endgroup$ Commented Oct 5, 2020 at 1:43
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$\begingroup$ Thankyou for expert correction. That seems quite an intriguing and counterintuitive possibility of a heavier mass joined to an inner lighter mass in a stable equilibrium! Not wishing to sound stubborn, one might still wonder if that configuration is really as likely in real world situations. Perhaps other factors may swing the balance as it were and end with the heavier part inward usually. Point taken nevertheless. Thanks. $\endgroup$– blanciCommented Oct 5, 2020 at 3:01