Lunar Meteorites and the Lunar Cataclysm Barbara Cohen (2001)
(Seacrest School Moon Crew, 2011, Summary discussion notes. Read these along with article and its Figures)
Notes:
We think of the moon as an unchanging place, yet 3.9 billion years ago it was pummeled with asteroids, comets, and/or meteors. 1700 craters were formed 100km or larger.
The nearside of the Moon has 2 kinds of surfaces: maria (darker and smoother) and highlands (brighter and rougher). There are about 50 gigantic basins (200 x 200 miles and larger) caused by huge impacts. These areas filled in with lava from the lunar mantle obliterating anything that used to be there. Their smooth dark materials makes them look like seas, therefore “maria.” The marerocks were formed 3.3- 3.6 billion years ago, real young guys! The highland surfaces are much older and made of different, lighter rockers than the maria. Highland rocks can be “original” lunar surface of 4.5 billion years ago. But most all the rubble on the moon’s surface is 3.9 billion years old. See Figs. On Page 3.
It appears there was a “short” period of huge bombardment, 3.9 billion years ago that shot up the entire surface of the Moon. Was there a Moon-wide shower of asteroids, comets, and meteors 3.9 billion years ago? It would help to have samples from all over the moon rather than just the Apollo landing sites.
Read page 5 on potassium argon dating: good story!
Meteorites found on earth helped solve this problem. Many of the meteors falling into to Earth may have come from the Moon. 4 definite examples are closely similar to moon rocks, but come from different areas of the moon than the Apollo landings. All 4 date from at least 3.9 billion years ago supporting the idea that at that time and lasting for about 100 million years there was an increase in large lunar impacts. If stuff was hitting the moon, it was also hitting Earth.
Note the ending section on Earth impacts. Earth is bigger with greater gravity: 10 times the impacts would have hit Earth as hit the Moon. The Moon’s cataclysm lasted 200 million years. So that is one huge (100 miles wide) crater per 100,000 years. Nothing we would notice. But during that time the Earth got smacked every 10,000 years with something huge enough to ruin the planet. It is not surprising that life didn’t take hold on Earth until after these devastating impacts were done, about 3.85 billion years ago.