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Why the Farside is so Wrinkled?
A rock (Rock #72417) from the moon has been dated to 4.6 Billion years old. It is believed that this is some of the original stuff of which the solar system was made. This process of accretion was the first stage of the history of the moon. In the early history of the solar system accretion was quite rapid. Accretion slowly continues today as asteroids and comets impact the planets of the solar system. The origin of the moon is still debated but it seems the evidence is swinging toward the swiping collision with the earth. The currently held consensus is one where a mars sized object impacted with a still molten earth scattering pieces into orbit. These pieces reaccreted into the moon. This would have happened about 4.5 Billion Years ago. But we have a problem with the age of rock # 72417.
Instead let us speculate that early in the history of our solar system there was a crossing of juvenile solar systems. Ours and another solar system passed close enough together to trade orbiting bodies. A lot of solar system anomalies could be explained with this mechanism, but back to the moon. This event happened when the moon was about the size it is today, in a heliocentric elliptical orbit, and crossed the orbit of the crusted over but still molten earth. The impact occurred after the solar system was formed some 4.5 billon years ago because the moon had its own history before running into the earth. The impact may have happened 3.8 billion years ago due to the onset of the mare basalts. So there is a 700 to 800 million year period when the moon orbited the sun, or some other star. This was the first phase of the moons history.
The Face of the earth has changed many times since that collision, but the moon has retained the scar of the side swipe. It has been postulated that the area of the moon called South Pole-Aitken is the evidence of that grazing impact. South Pole-Aitken (SP-A) is a crater 2250 KM wide and 12 KM lower than the average "sea level". North of SP-A a hugh "flap" of crustal material has increased the crustal thickness to 120 KM for thousands of Kilometers. The average elevation of this flap is, on average, 8 Kilometers higher than the lunar "sea level". The crater was deep enough to have possibly displaced the core and possibly fragmented it. A fragmented core could possibly explain the mass concentrations under some of the maria and the wide variations in gravity, Clementine observed. The SP-A hemisphere wide impact and ejecta area exhibits a characteristic, low angle, ray pattern of much smaller impact craters. The impact is also very old, because the ejecta field is pock marked with newer crater scars. The hemisphere is the farside of the moon. This evidence was found by direct measurement by Clementines Lidar or Lasar altimeter. 72000 altitude measurements revealed the overall shape of the moon. Other measurements by other instruments, determined the microgravity variations and the crustal thickness. The SP-A impact was a very low angle grazing impact, but it did a number of things that would forever alter the future of both the moon and the earth. The basic orbit of the moon was changed from solarcentric elipsodial, to earthcentric and, eventually, almost circular. The earth now had a satellite that was a tide maker and because of that, life could possibly evolve.
This lunar topographic is from the Paul Spudis team at the University of Arizona. It shows SP-A at the bottom of the right image with the lunar maria at the top of the left image.
The other side of the moon, with respect to SP-A, is the northern hemisphere at about 20 degrees of Longitude. That is well inside the maria belt. The impact at SP-A sent shock waves through the mantle concentrating generally on the antipodal position. The antipode began to weep lava. It oozed from every fault and formed volcanoes, calderas and maria. This traumatic activity lasted about 600 million years, from 3.2 to 3.8 billion years ago. In a less than scientific mood, I call this the bleeding time. During this period, as other impacts occurred, the lava would flow. While the mantle was forced to the surface less and less pressure was exerted on the core, and it began to solidfy. This occurred many times over the 600 million year volcanic period. It was also during this period that the spin of the moon was slowing down due to the gravitational drag of the earth.
The gravity well of the earth pulled at the hardening core of the moon that had been displaced by the impact. The core had been displaced in such a way as to increase the friction of gravity, thus accelerating the slowing of the rotation of the moon. Eventually, it slowed until it presented the same face to the earth. It had reached the condition of tidal lock. Now the constant pull of earths gravity displaced the core even more, exerting pressure on the molten mantle. Inexorable, constant pressure eventually caused more episodes of vulcanism, flowing through the cracks and weak surface areas of the marias and highlands. The moon was cooling now and the displacement continued to displace lava from the mantle to the surface. The pressure supporting the surface began to diminish all over the moon except those areas over the core. Weakened areas began to subside. On the near side fractures were raised and rims began to appear. On the limbs and farside, strange collapse craters, rilles, and crevasses began to appear. Strange convoluted landscapes began to rise like those in Procellarum and those on the farside like Barbier crater and "Farside City". The tectonics of catastrophism was shaping the moon.
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