Physics Question #3317

David Dexter, a 59 year old male from Mill Valley, CA, USA asks on March 8, 2006,

This came up during a homeschool astronomy lesson: Why does the back of the Moon not have large impact craters as the front does? It would seem that there should be an equal number, or more, of the larger size. It makes sense that there would be more smaller ones on the back, because of the "shielding" of the front by the Earth, but not fewer large ones. Thank you.

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The answer

Donald J. Barry answered on March 8, 2006

It's known from study of the gravity field of the Moon (by tracing the orbit of artificial satellites like the Lunar Orbiters and even the Apollo spacecraft) that the Maria (the "oceans") are the densest regions of the lunar surface, and also that the farside surface is the least dense -- and that it bulges above fits to the gravity center of the Moon. We infer that the crust of the farside is considerably thicker than the crust of the nearside -- the crust is less dense than the interior mantle/core.

Presumably this large-scale one-sided inhomogeneity is left over from the lunar formation, but it has created a variety of near/far dichotomies in the Moon's evolution. In particular, the young warm moon would have had higher temperatures at a given distance underneath the surface in areas of thin crust, so large crater impacts of a given size would have access to magma much more easily in thin-crust areas. As a result, whereas impact basins seem roughly evenly distributed between the two sides (the largest impact basin, the Aitken, is on the *far* side), except for the bottom cores of the very largest imapct craters and a small scattering of one part of farside which has thinner crust, the farside escaped the large-scale formation of maria that occurred on the thinner crust nearside.

This is verified by studies from orbit of gamma ray emissions of heavy elements like thorium which would preferentially associate with basaltic magmas typical of the lunar mantle -- these are found in most of the nearside maria but are absent from the farside -- the impacts, dredging to equal depths, did not gain access to mantle magma.

At the time of the impacts, there was probably no "nearside/farside" -- but rather simply a thin-crust and a thicker-crust side. The maria-forming impacts led to consolidation of these regions as dense "mascons" (mass concentrations) which then coupled to tidal interactions with the Earth, eventually steering the moon's "heavier" side into orbital resonance with the Earth. So now the near side *is* the thinner crust "heavier" side with mass concentrations.

At the distance of the Moon from the Earth (and even at the closer distance which the Moon occupied when the Earth/Moon system was much younger) there is no real effective "shielding" of one side or the other of the Moon.

In Shakespeare's play "Coriolanus" Aufidius complains about his weak spear thus: "My grained ash a hundred times hath broke and scarr'd the moon with splinters". Such was the early solar system, throwing junk as target practice at all the planetary bodies and moons. On our own Moon, though, only its more delicate side earned deep blemishes -- now turned towards us forever in touching sincerety.

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