Lecture21.doc - AgeofMeteors

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Age of Meteors Meteors can be age-dated by the decay of radioactive elements found within them by  means of half-life. For example, half of the atoms in a sample of Uranium-238 decay to Thorium-234 in  4.50 x 10 years.  The Thorium then decays to other elements, ultimately decaying into  Lead, where the process stops.  By measuring the amount of the various decay products,  it becomes possible to determine the age of the sample of rock, since it was last heated.   If a section of rock contains 50% Uranium and 50% Lead, we may conclude that  the rock is 4.50 x 10 years old, if it contained no Lead originally.  This can be checked  by finding in what proportions the intermediate decay products are found, and by looking  in different separate formations in the rock. IMPORTANT:   This method of dating is incontrovertible and unambiguous.  However: it only dates a rock since it was last heated. Meteors are thought to be original pieces of the Solar System; they are measured  to be 4.5 x 10 years old (the similarity to the half-life of Uranium is a coincidence).  This  is actually the age of the chrondules within the meteors. This age is one of the fundamental pieces of knowledge we have about the history  of the Solar System.  It shows that the geologic changes and the evolutionary changes we  observe on the Earth had enough time to develop.   Oldest rocks on Earth are about 3.5 x 10 years old.  Is Earth younger? NO! This  is the time since last reheating and (melting of the rocks).  Some Moon rocks are younger  than this.   Note: Age dating of Solar System from all measurements is totally consistent. Meteor Impacts
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This note was uploaded on 03/02/2011 for the course ASTRO 10 taught by Professor Norm during the Spring '06 term at University of California, Berkeley.

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Lecture21.doc - AgeofMeteors

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