MadSci Network: Astronomy

Re: Is it true that the moon may have prevented the earth from ending?

Date: Mon Dec 13 12:45:54 1999
Posted By: Denise Kaisler, Grad student, Astronomy, UCLA, Division of Astronomy
Area of science: Astronomy
ID: 944573406.As

Even though the moon *does* move further from the Earth every year, the 
distance it moves is actually minimal. So it would not have been 
shielded the earth from impacts much better than it does today.

Also, a lot of things have hit the earth in past. In fact, if the earth 
had no oceans to hide impact craters, and if weather and geological 
processes (volcanoes, earthquakes, shifting of the continental plates) 
didn't erase the signs of impacts, the surface of our planet  would look 
just like the surface of the moon.

However, the moon does shield the Earth from impacts to some extent
and it is probably why the far side is more heavily cratered than the
near side.

Finally, one point to clear up: meteor showers (even violent ones)
are not what produced the big craters we see on the moon.
Meteor showers come about when the earth's orbit crosses the path of a 
bygone comet. Comets leave little chunks of rock behind as the sun's heat
melts or sublimates (turns ice directly into steam) the comet's surface.
When these grains hit the earth's atmostphere, they heat up, causing
streaks of light to appear in the sky. The main point is that these
chunks of rocks are *small*. They start out about the size of your
fist. Most of them burn up completely before they hit the ground. Even
though the moon has no atmosphere, these little space rocks wouldn't
make a big crater. Those are caused by asteroids, or planetesimals --
chunks of rock that are meters to kilometers in size. 

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