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Sorry for the maybe too large pictures. They do make the surface structures of both sides of the moon visible though. Is it just a coincidence that on the side facing us is so much sea is visible?
It looks as if on the far side more meteorites have hit the surface.
On the other hand, some pretty big ones seem to have hit the side we see. You can see there are overlapping circles containing flat and dark grounds, the seas.

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Deschele Schilder
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    Not the same question, but contains an answer to this question too https://astronomy.stackexchange.com/questions/41264/why-would-the-side-of-the-moon-that-faces-earth-be-as-dark-as-the-far-side-of-th/41266#41266 ... The near side cooled more slowly, and so has thinner crust, resulting in lava flows creating maria. – James K Jun 08 '21 at 20:07
  • @JamesK Is this caused by huge meteorites? These kinds of huge impacts seem to be absent on or side. Well, they are huge but not too huge. – Deschele Schilder Jun 08 '21 at 20:14
  • Somewhat related: https://en.wikipedia.org/wiki/Mass_concentration_(astronomy)#Lunar_mascons & https://en.wikipedia.org/wiki/Gravitation_of_the_Moon The Moon's lumpiness affected the angle that the Moon is tidally locked at. – PM 2Ring Jun 08 '21 at 20:21
  • @PM2Ring Once upon a time: The late heavy bombardment https://en.wikipedia.org/wiki/Late_Heavy_Bombardment Is there an early one too? – Deschele Schilder Jun 08 '21 at 20:30
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    There were lots of collisions in the early phases of the Solar System's formation, as the protoplanetary disk condensed and planetesimals formed. But the Late Heavy bombardment happened well after those early chaotic times, when most stuff in the inner system had settled into nice stable (relatively) low eccentricity, low inclination orbits. – PM 2Ring Jun 08 '21 at 20:55
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    @PM2Ring If the Late Heavy Bombardment happened at all (or as severely as was previously thought). There are doubts now. – David Hammen Jun 08 '21 at 23:42
  • Perhaps all the rocks coming from one direction hit the moon, and all the rocks coming from the other direction hit the Earth. – Strawberry Jun 10 '21 at 17:04
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    @Strawberry That idea was proposed shortly after the discovery of the near side / far side asymmetry, but it was quickly tossed because numerical simulations showed that the Earth isn't very good as a meteor shield. – David Hammen Jun 11 '21 at 15:25

3 Answers3

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The current configuration has the Moon's center of mass closer to the Earth than the Moon's center of figure by a couple of kilometers. The reason for that disparity is that the Moon's far side has a much thicker crust than does the Moon's near side. The near side in places has no crust at all.

This is a stable tidally locked configuration. Other configurations, with the Earth's center of mass, the Moon's center of mass, and the Moon's center of figure not being more or less co-linear are not stable configurations.

There is an alternate configuration that is stable. If what is now the far side was facing the Earth and what is now the near side was facing away from the Earth, that would also a stable tidally locked configuration. However, the current configuration is slightly favored energetically / entropically than is the alternate configuration.

Update

This answer does not directly answer the question. There currently is no definitive answer to the question of why the near side and far side of the Moon are so very distinct geologically. There are instead many competing conjectures. Science is cool with "we don't know". Or, as Isaac Asimov put it, “The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka! ' but 'That's funny…'”

David Hammen
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    But why is the crust thicker on one side? Because of meteorites? Is the probability that a meteorite hits one side bigger (or less) than for the other side? Due to the presence of Earth? It seems that on our side bigger (but less) meteorites have smashed on the surface. – Deschele Schilder Jun 08 '21 at 23:50
  • @Barbierium That's an open mystery as far as I know. It was once thought that the Late Heavy Bombardment resulted in more impacts on the near side than the far side. That idea has fallen out of favor. Some think that the Moon alternated between the two stable configurations as the Moon migrated away from the Earth. Some think that two massive collisions (rather than one last giant impact) were needed to make the Moon form the way it is. These are all hypotheses (guesses). There are lots of hypotheses regarding the formation of the Moon and the marked disparity between the near and far sides. – David Hammen Jun 09 '21 at 00:04
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    What is known is that there is a marked disparity between the near and far sides. This was a startling result of photos of the far side by the Soviet Union (and later, the US) in the 1960s. What is known is the center Moon's center of figure and center of mass are displaced by a couple of kilometers. Why? Right now, we don't know why. Sometimes, "we don't know" is one of the best answers in science. Having mysteries for future scientists is what makes science science. – David Hammen Jun 09 '21 at 00:16
  • One idea proposes that the Earth originally had two moons from the Theia collision, both sharing the same orbit. The second, smaller moon eventually caught up to what is now the far side and there was a slow merger on the far side only which produced the thicker crust there. The "Big Splat" was proposed in 2011 by Erik Asphaug and Martin Jutzi . I am not sure where that idea currently stands in the scientific community. – JohnHunt Jun 09 '21 at 07:59
  • But how this answer the question? – Alchimista Jun 09 '21 at 09:18
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    @Alchimista Why the Moon has a marked disparity between the near side and far side cannot be answered definitively, at least not now. The list of competing hypotheses would go on and on and on. Given that the marked disparity exists, energy and entropy explain why we see the Moon with huge maria on the near side and a thick crust on the far side. – David Hammen Jun 09 '21 at 09:53
  • Thanks. Now I see what you meant. But it is a completely different point of view in the sense that there is no causality considered – Alchimista Jun 09 '21 at 10:27
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    While this is nice information, the answer here doesn't actually draw a causal relationship between what is stated and why "on the far side of the moon almost no seas are visible". In other words, this doesn't actually answer the question which was asked. The information provided might lead to an answer, but the text here doesn't actually provide it. It explains that the disparity between the center of mass and center of figure is that the far side has a thicker crust, but doesn't relate that to the visibility of the seas. – Makyen Jun 09 '21 at 13:19
  • @JohnHunt: How could there possibly be a slow merger? They would have to collide at basically lunar escape velocity, no? OTOH, suppose the collision was on the near side, and all the maria are the result of that collision... – jamesqf Jun 09 '21 at 15:38
  • @Makyen yes that is what I have noticed, too – Alchimista Jun 10 '21 at 08:32
  • @jamesqy: The idea (and it wasn't mine) is that a chunk 1/30th the mass of the moon would have formed at a Lagrange point either ahead or behind the larger moon by 60 degrees. This would have been a stable relationship until the outward migration of their orbits due to tidal forces from the Earth made their orbital relationship unstable. Complex gravitational interactions (4 body) would then have slowed one, sped up the other until a very slow motion collision taking hours took place. https://www.nature.com/articles/news.2011.456 – JohnHunt Jun 10 '21 at 17:34
  • What does "center of figure" mean? – Daddy Kropotkin Jun 11 '21 at 14:49
  • @DaddyKropotkin Another name for the same concept is the geometrical center, but center of figure is a more widely used term. – David Hammen Jun 11 '21 at 15:14
  • Ohhhh you mean the centroid? Thanks! – Daddy Kropotkin Jun 11 '21 at 15:19
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The asymmetry in impact craters from the inward facing and outward facing sides of the Moon are almost certainly not a coincidence.

A hot near side Moon may have limited/erased impact signatures from the most ancient asteroids.

One major hypothesis for the asymmetry is that the near side cooled far more recently than the far side. If this is true, even though both sides might have a similar number of asteroid strikes, the most ancient asteroids may have splashed into magma oceans and been covered by volcano emissions on the near side. Since there were more asteroids earlier in the Solar System's existence, this effect may have had a more striking effect on the lunar crater record.

But why would the inner facing side of the Moon be hotter longer?

  1. Roy et. al claim the asymmetry between the near and far faces is due to the early tidal locked Moon to cool more slowly on the lunar face pointing to the molten cooling Earth. From their abstract:

The lunar farside highlands problem refers to the curious and unexplained fact that the farside lunar crust is thicker, on average, than the nearside crust. Here we recognize the crucial influence of Earthshine, and propose that it naturally explains this hemispheric dichotomy. Since the accreting Moon rapidly achieved synchronous rotation, a surface and atmospheric thermal gradient was imposed by the proximity of the hot, post-giant impact Earth.

  1. Elardo et. al claim the asymmetry is due to a greater abundance of radioactive isotopes on the near side of the Moon, which add heat and lower material melting temperatures. They call the mix KREEP. Lower melting temperatures would have allowed longer persistence of lunar magma oceans and more recent volcanic activity. From their abstract:

The Moon’s Earth-facing hemisphere hosts a geochemically anomalous region, the Procellarum KREEP Terrane, which is widely thought to have provided radiogenic heat for mantle melting from ~3.9 to ~1 billion years ago. [...] Our results demonstrate that the anomalous enrichment in incompatible elements of this nearside reservoir dramatically lowers the melting temperature of the source rock for these magmas and may have resulted in 4 to 13 times more magma production under the nearside crust, even without any contribution from radioactivity.

Edit: As David Hammen points out, this is still an open question in Astronomy. The hypotheses tend to fall into two categories:

  1. More asteroid strikes on the far side (most recent simulations don't support this).
  2. Equal number of asteroid strikes, but less evidence on the near side due to magma "splash" craters and fill-in by lava. All the references sited in my answer and the notes are attempting to explain why the near side was hotter longer. Also, these hypotheses are not mutually exclusive. Each mechanism could contribute.
Connor Garcia
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  • So it's the Earth that is the cause? – Deschele Schilder Jun 09 '21 at 09:11
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    @Barbierium The two hypotheses described in this answer are but two of many competing hypotheses. We do not know which of the many hypotheses is correct. In science, there's nothing wrong with saying a question cannot be answered definitively. This question is one of those. – David Hammen Jun 09 '21 at 09:48
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    A couple of other competing hypotheses: (1) Evection resonance, proposed by Rufu and Canup, (2) Preferential tidal heating on the near side vs far side, proposed by Quillen et al.. There are many, many others. – David Hammen Jun 09 '21 at 10:11
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    isn't "far more recently" the same as "later"? I'm asking as the crater density suggests to me, that the near side cooled later than the far side. And you seem to argue the same in your text... so mix-up in that sentence? – planetmaker Jun 09 '21 at 14:22
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    @DavidHammen The hypothesis that the near side was hotter for longer, erasing some ancient impact evidence is fairly well agreed on. The open scientific question is why it was hotter. See: https://en.wikipedia.org/wiki/Lunar_mare#Distribution_of_mare_basalts – Connor Garcia Jun 09 '21 at 15:39
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I once read a science speculation article in Analog Science Fiction Science Fact with a title something like "The Brush That Painted the Face of the Man in the Moon". That might have been about 30 years ago.

The article had a theory that most of the lunar maria were created at the same time. The theory said that a giant but loosly held together asteroid passed closed to Earth, within the Roche limit, and was pulled apart into still large pieces which gradually spread away from each other.

Since the asteroid swung close to Earth, its direction was greatly changed by Earth's gravity, and it was now headed outwards from Earth in the direction of the Moon. When the cloud of asteroid fragments passed the Moon's orbit, some of them smashed into the near side of the Moon blastng away large amounts of the surface material and cuasing lava flows which filled up the lowlands to make the maria.

I think that one point of the theory was that asteroids attracted by Earth's gravity and falling toward Earth would be much more likely to hit the Moon - if at all - on the far side than on the near side, but there are far more maria on the near side than on the far side.

I don't know how probable that theory is, but I thought that I would mention it.

M. A. Golding
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