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What Meteorites Tell Us


I've never found a meteorite nor even held one in my hand, but I would like to. While these cosmic rocks are pretty cool to watch as a streak of light in a pithc-black sky, their real value lies in what they can tell us about early Earth.


Much of what we assume about the chemical composition of the primitive Earth comes from our study of meteorites, which may be found on our planet in places like Antarctica or the Sahara or the Mohave Desert in California where conditions are favorable for the preservation of meteorites. Have you ever been meteorite hunting in the desert? (Get off your dirt bike and look around every once and a while instead of tearing up the desert pavement!)

One class of meteorite, the chondrites, is among the oldest rock in our solar system. They contain inclusions (minerals within their stony matrix called chondrules) that have been dated at about 4.5647 + 0.6 billion years old, based on lead isotope radionuclide dating by Lawrence Livermore National Laboratories (see Science, 2002). The simplest explanation for the origin of our planet assumes that these chondrites (and/or similar materials) are the same stuff from which our planet formed. By examining the isotopic composition of these chondrites (and other types of meteorites) and comparing them to the oldest rocks found on earth, scientists can infer a great deal about the geological and chemical evolution of our planet from the time of its formation.

You see, as Earth cooled, some elements and their isotopes (same element with a different number of neutrons) were selectively rejected from the cooling mass and some elements were preferentially included in the molten mass. As a result of these selective chemical processes, called chemical fractionation, modern-day scientists (the ones who were not present at the time of the Earth’s formation), are able to piece together an idea of how and when the Earth’s crust formed. The fractionation of rocks imparts on them a particular isotopic composition and when those are compared to meteorites or rocks formed under very specific conditions (often determined in the laboratory), then scientists can say something reasonably intelligent about the origin of the rocks and their chemical evolution as they were subjected to heat and pressure in the interior of the Earth.

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