For the Science Fellowship on September 23, Holly Stein and Judy Hannah of CSU and University of Oslo, Norway, told us about their use of the elements rhenium and osmium in dating ancient geological formations, and particularly the formations deposited at about the time of the greatest extinction event in the history of the earth, some 252 million years ago. One of the isotopes of rhenium (rhenium-187) undergoes spontaneous radioactive decay to an isotope of osmium (osmium-187) with a very long half-life (some 43 billion years). Very simplistically, if you can measure the ratio of those two isotopes in a rock sample, and you have reason to believe that neither one has been added or removed since formation of that rock, then you can use the half-life to establish the date of formation of that rock. Note that these dates are quite precise by the standards of geological time, but still have an uncertainty of many millions of years. These elements, along with many other metals, tend to get concentrated in organic matter in oil-generating formations so that those formations may be dated – information of significance in the oil and gas industry.