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A question about the structure of the Moon.


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A couple of days ago, I did a large mosaic of the terminator of the Moon. While staring at the laptop screen, making small adjustments to the scope to keep the chosen field exectly centred for the three minute AVI's, I began to ponder about the structure of the cratered areas and the plains.

I think the generally accepted theory of the Moon's creation was a collision between the Earth and another Solar System body, which blasted off a big chunk of the Earth. Apollo missions brought back evidence that the Lunar rock is a very similar composition to that of the Earth.

Now, the question is almost here... After the collision, both bodies would be pretty hot, and began to cool, solidifying and then being subject to further bombardments, substantially cratering the Moon and the Earth. Due to the geology and weather on Earth, most of those craters were erased. On the Moon, they stayed. There are however, some lava plains ( I'm guessing lava ) which resurfaced parts of the surface of the Moon, after the major bombardment had ended, but were subject to light bombardment after they formed.

Here's it comes....Does anyone know why these certain areas of the Moon stayed molten after the rest solidified, or why those areas re-melted? Wiki says "This is thought to be due to a concentration of heat-producing elements under the crust on the near side, seen on geochemical maps obtained by Lunar Prospector's gamma-ray spectrometer, which would have caused the underlying mantle to heat up, partially melt, rise to the surface and erupt.".

So why a different structure in this one side of the Moon compared to the evenly cratered far side. Would it be to do with the geological make up of the surviving part of the body that hit us, or that the rest of the Moon came from the other body, and the heat retaining elements were those blasted from the Earth? Is there a reason ( density? ) that these areas ended up facing us?

OK, there's a few questions in there, but can anyone enlighten me a little further?

The is a small version of the picture that started my train of thought.
x3mosaic600.jpg

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Does it have something to do with tidal locking resulting in the far side of the moon always being in the line of fire, so to speak. Therefore erasing any lava lakes that solidified on that side? albeit over a verly long time period?

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It could have been as MarkMayf says above together with the fact that the Moon was a lot closer to the earth when it originally formed. This with its slightly eccentric orbit could have caused tidal flexing which kept the Moon hotter for longer. As the Moons rotation slowed, together with the Earths, any residual heat may have been kept towards the near side once it became tidally locked.

Then again it may just be that is where the Clangers had their furnaces!  :icon_clown:

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To be honest, the Soup Dragon was my first thought, but I'm not sure how much scientific evidence there is for that, which is why I was looking for other explanations, but your explanation certainly seems logical.

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I would have thought that the moon formed from the ejection material from the Earth/Body collision, and that this ejected material would have been cool when it finally coalesced into the Moon. In effect the stuff that formed the moon would not have been hot but cool. So I would question the statment that both bodies would have been hot. I think "the Moon" would initially have been lots of rocks (big rocks which may have been Everest sized) orbiting the resultant Earth then became the Moon as we know it.

Lava fields later - big meteor hits moon, lots of energy, melts moon rock, makes lava, lava cools into a lava field..

After that AAF's idea, ask the soup dragon. Would mushroom soup set like a lava field?

Damn I miss the Clangers.

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So why a different structure in this one side of the Moon compared to the evenly cratered far side.

x3mosaic600.jpg

It's a great question! I honestly had never thought about this, so I had a little peak on the net and found this possible reply to investigate.

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I believe one theory is that a further collision occurred..  with an object that was semi-molten and broken up, which sort of went 'splat' custard pie style over one face of the moon.  Gravity then smoothed things out.   Possibly the moon was still molten beneath the solid surface, with an impact from a broken (shoemaker levy 9 style) impactor punching holes letting lava out so creating the 'seas' we see today.

Sorry no link.

Derek

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It's a great question! I honestly had never thought about this, so I had a little peak on the net and found this possible reply to investigate.

I like the article but it still doesn't quite explain as it mentions no solution as to why there are differing densities on either side of the Moons crust. But in my head at least a much higher velocity orbit due to close proximity of the Earth during formation as aforementioned would seem to make a feasible solution. Interestingly Mars also has considerable variances in the depth of its crust and was thought for some time (maybe still is) to have been on the receiving end of a impact / collision.

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  • 1 month later...

I have read a reputable theory explaining the difference between the topography of the near and far side. the near side is relatively flat and the far side  relatively mountainous....I cannot confirm that from personal observation ! the theory states that the earth was hit by a mars sized object and this actually caused a coalescence into two moons. over the eons they came closer and eventually collided. the far face copped the brunt of the collision and became hilly....

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The core of the Moon is not in its centre but offset towards the Earth making the crust thinner on the Earth side so lava was able to reach the surface more easily through cracks.
 
The Moon's crust may be thicker on the far side. So there was less volcanic activity. So there is less lava to cover up craters. Lava flows form the "maria". 

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