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If the moon didnít get chicken pox, then
Scars that look like miniature moon craters on your skin are not normally a mystery: a childhood case of the chicken pox is often responsible.
What caused the craters on the moon, Mars, Earth, Venus and Mercury is not so obvious. In fact, this "whodunnit" is one of the mysteries of astronomy.
In a new study, scientists describe a likely answer to this mystery: two groups of rocky space objects called asteroids created the craters when they crashed into the moon and Earth, Mars, Venus and Mercury.
The first of the two groups of asteroids to crash into these planets and the moon is from the "main asteroid belt" which can be found between Mars and Jupiter, according to Robert Strom from the University of Arizona in Tucson, AZ and his colleagues. These asteroids seem to be responsible for a short, intense burst of crater making that occurred about 3.9 billion years ago.
The asteroids that have crashed into the moon and these planets after the period of heavy hitting let up about 3.9 billion years ago are from a group called the "near Earth asteroids." These asteroids are smaller than the asteroids from the main belt.
When you look up at the moon and notice its darker and lighter areas, you are looking at some of the raw material the scientists used to make their discovery.
The darker areas on the moon are made of lava. This lava is younger than the lighter parts of the moon which are as old as the moon itself Ė about 4.5 billion years old.
The scientists compared the craters on the older, lighter parts of the moon to the craters on the younger, darker parts of the moon.
They found that the sizes of the craters on the older parts of the moon are about the same as the sizes of the asteroids from the main asteroid belt. This means that asteroids that caused the super intense round of crater formation that occurred about 3.9 billion years ago probably came from the main belt.
The craters on the younger, darker parts of the Moon's surface are smaller and less common. In terms of size, these craters match up with the asteroids in the near Earth asteroid belt. This suggests that the near Earth asteroid belt has been supplying the asteroids that have been hitting the moon as well as the Earth, Mars, Venus and Mercury since the short period of heavy asteroid hitting ended about 3.9 billion years ago.
This new research should help astronomers better understand parts of the history of our early solar system. For example, discovering that asteroids from the main belt were responsible for the first round of crater formation helps astronomers understand when Jupiter and Saturn changed their orbits around the sun.
This astronomy research appears in the 16 September, 2005 issue of the journal Science.