M Meteorite crater on Earth.This is the Barringer Crater, just east of Flagstaff,Arizona.The bowl has a 1 km diameter. Notice the elevated rim. It is the first terrestrial crater recognized as coming from a meteor impact. [USGS]

types. In this picture, the C type asteroids are like the carbonaceous chondrite meteorites. The S type asteroids are like the ordinary chondrites or the stones.

M Meteoroids and comets.The meteoroids are debris left by the comet along its orbit.We see a meteor shower as the Earth passes through the debris.

Radioactive dating tells us that meteorites are 4.5 billion years old. This confirms our idea that they are part of the debris left over from the formation of the Solar System. Studies are now being done of the relative abundances of various elements to understand the composition of the material out of which the Solar System formed. So far, some unusual abundances have been found. For example, xenon-129 is formed by the beta decay of iodine-129, with a half-life of 17 million years. A lot of xenon-129 has been found, suggesting a large production of iodine-129 just prior to the formation of the Solar System. However, this is still quite speculative, and some claim that they can explain the abundances without invoking any unusual events.

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