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The Earth is so close to the Moon and yet the number of craters on the Moon are far more abundant. Have meteoroid and asteroid bombardments been selective? That is, have they been pounding the lunar surface but somehow sparing Earth’s? Not really.
Earth is definitely receiving its fair share of meteoroid and asteroid assaults. In fact, meteoroids rain nonstop on both the Earth and the moon every minute. However, our planet is protected by its atmosphere. The moon, unfortunately, doesn’t have one.
When small meteoroids pass through the atmosphere, friction causes them to ignite and disintegrate; the larger ones simply explode. The shooting stars and meteor showers that we see are the more visible ones. Surely you didn’t think meteoroids only struck at night, did you?
On the moon, even the tiniest meteoroids strike the surface unhindered, forming the smaller-sized craters on the moon. These tiny craters even leave marks on moon rocks and are easily spotted on the rock samples brought home by the Apollo missions.
The tiny craters on the Apollo moon rocks are among the most substantial evidence that the Apollo landings actually took place. You can’t hope to find rocks with the same features here on Earth.
So far, we’ve explained the presence of small craters on the Moon. How about the larger ones? The giant craters on the Moon were obviously formed by much larger astronomical objects, like asteroids or comets.
Bombardments from comets or asteroids are not common – both on Earth and on the Moon. However, why do we see more of them on the Moon’s surface?
The reason for this is because of the absence of water (in liquid form) and climates on the lunar surface. These factors cause erosions, which subsequently conceal the evidence of such major bombardments. It would be safe to say that the large lunar craters fashioned by asteroid and comet impacts were formed millions of years ago.
This is consistent with the theory that the dinosaurs were wiped out by this kind of impact some 65 million years ago. While the evidence of such an impact, as well as even older impacts, are well preserved on the Moon, the same thing cannot be said for the Earth.
The Earth’s high plate tectonic activity is also one major factor that has concealed large craters. Although we are not yet sure whether the Moon has any plate tectonic activity, it is believed that, if the Moon in fact has any such activity, the Earth’s plate tectonic movement is still far more active.
Come October 9, 2009, LCROSS will perform a lunar impact. Find out which crater NASA has chosen for the impact. If you want to know more about the largest crater on the Moon, NASA’s got the right stuff.
There are some interesting episodes from Astronomy Cast that we’d like to recommend: