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What do meteorites tell us about life on other planets?

I wanted to know why meteorites are important to the study of life. Also, I wanted to know what meteorites consist of and where I can find more information on this topic.

When rocks originating elsewhere in the solar system land on Earth, they carry with them information about the planets from which they come. These meteorites could also potentially bring evidence for the past existence of other life forms. One very controversial meteorite is called ALH84001 and was discovered in Antarctica in 1984. A group of scientists at NASA led by David McKay published articles in the mid 90's claiming to have found evidence of bacteria having lived on the meteorite when it existed as a rock on Mars. Most now believe that ALH84001 was contaminated with this "evidence" after having reached Earth (and thus the bacteria were not from Mars), but no conclusive proof of this suggestion has been found.

Scientists can help determine the most probable origin of a meteorite by looking at how the rock is crystallized. The crystallization reveals the size of the meteorite's original home (i.e. it answers the question: did it come from something the size of a planet or an asteroid?). Comparing the composition of gas bubbles trapped within the meteorite with compositions of atmospheres on other planets can offer clues as well. For more information on meteorites, particularly what they're made of, check out:

http://seds.lpl.arizona.edu/nineplanets/nineplanets/meteorites.html

http://www.ess.ucla.edu/research/cosmochem/meteorite.asp

January 2006, Sabrina Stierwalt (more by Sabrina Stierwalt) (Like this Answer)

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