The currently accepted view of the moon's formation is that a mars sized planet plowed into the earth very early on, and a bunch of material was ejected from the collision into lunar orbit. Because much of the dense minerals of the earth were already headed to the core, the ejecta was lighter crustal material. So good so far.
Now the lunar orbit after collision was much closer to the earth than the current orbit (it was then about 20K kilometers away). Both bodies were blazingly hot and molten. The moon aggregated all the ejecta to become the size that it is today, and became fairly quickly tidally locked because the orbit was so close (the closer two orbiting bodies are, the more quickly the smaller one gets tidally locked).
The tidal locking explains why the crust on the side of the moon facing the earth is much thinner than the crust on the far side. Because they were so hot, the moon was venting lots of gas. Because the earth was so hot, the near side of the moon was hotter than the far side, so the gases solidified on the far side. The deposition of hot gases thus made the far side of the moon's crust thicker than the near side of the moon's crust. Again, no mystery.
However. The near side of the moon has most of the mascons (high gravity areas). The moon (and the earth) were bombarded by lots of asteroids. The biggest asteroids that hit the moon penetrated the thinner lunar crust side permitting heavier molten material to flow up and out, thus making mascons - whereas on the thicker crust side they didn't penetrate the crust, thus did not permit heavier molten material to flow up and out. The "mares" on the moon are all on the side facing the earth (the thinner crust side), and the mares correspond quite well to the mascons. Again, perfectly reasonable.
(Interesting side tidbit - it is because of the lunar mascons that it is hard to keep anything in orbit around the moon for more than a couple years. The mascons mess with lunar orbits and thus bring lunar orbiters crashing down into the moon unless whatever orbits is regularly making adjustments - which means firing retro rockets, which means any long term lunar orbiter has to carry a bunch of fuel just to keep in orbit.)
Here's the mystery. The big bombardment happened around 4 billion years ago - when the moon orbited only around 30K kilometers away from the earth, unlike today when it orbits an average of 240K kilometers away. The earth shielded the near side of the moon from asteroid strikes (because it was so close to the earth). Since the moon was already tidally locked to the earth, how did big asteroids manage to hit the near side of the moon, since these asteroid strikes occurred when the moon was much closer to the earth?
Edited 1 time(s). Last edit at 29-Mar-18 23:54 by Race Jackson.
|A true lunar mystery||990||Race Jackson||29-Mar-18 23:53|
|Re: A true lunar mystery||265||carolb||30-Mar-18 00:33|
|Re: A true lunar mystery||239||Spiros||30-Mar-18 12:31|
|Re: A true lunar mystery||196||Race Jackson||30-Mar-18 16:14|
|Re: A true lunar mystery||220||D-Archer||30-Mar-18 13:35|
|Re: A true lunar mystery||260||carolb||30-Mar-18 16:22|
|Re: A true lunar mystery||218||carolb||02-Apr-18 17:34|
|Re: A true lunar mystery||385||Itatw70s||03-Apr-18 16:25|