Thursday, November 13, 2008

Pocked Face Moon

Look at the moon through even the most modest of telescopes—as Galileo did—and you are impressed first and foremost by the craters that pock its surface. Most craters are the result of asteroid and meteoroid impacts. Only about a hundred craters have been identified on Earth, but the moon has thousands, great and small. Was the moon just unlucky? No. Many meteoroids that approach Earth burn up in our atmosphere before they strike ground. And the traces of those that do strike the ground are gradually covered by the effects of water and wind erosion as well as by plate tectonics. Without an atmosphere, the moon has been vulnerable to whatever comes its way, and preserves a nearly perfect record of every impact it has ever suffered.
Meteoroid collisions release terrific amounts of energy. Upon impact, heat is generated,
melting and deforming the surface rock, while pushing rock up and out and creating an ejecta blanket of debris, including large boulders and dust. It is this ejected material that covers the lunar surface.
It is believed that the rate of meteoroid impact with the moon (and with other objects in the solar system) was once much higher than it is now. The rate dropped sharply about 3.9 billion years ago— at the end of the period in which it is believed that the planets of the solar system were formed—and, some time later, lunar volcanic activity filled the largest craters with lava, giving many of them a smooth-floored appearance.

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