January 2, 1960 (a Saturday)

John H. Reynolds

On this date, John Hamilton Reynolds, University of California/Berkeley physicist, announced that a meteorite which fell near Richardton, North Dakota in 1919 had yielded evidence that the Solar System is 4.95 billion years old. The age was determined by measuring the amount of the xenon isotope of mass 129 (or 129Xe), a rare radioactive gas, in the meteorite. The sample contained more of this rare gas than any other natural substance previously analyzed, Reynolds said.

Reynolds inferred that the 129Xe must have been produced from the radioactive decay of iodine-129 (or 129I), which was trapped in the meteor when the meteor formed.  Iodine-129 has a half-life of about 17 million years, meaning that half of any given quantity of it will turn into 129Xe in that time.  Then half of the remainder will turn into 129Xe in another 17 million years, and so on until the 129I is essentially all gone.  For most practical purposes, a radioactive material is no longer present after 10 or 20 of its half-lives. This is because 210 is about a thousand, and 220 is about a million.  So, after 20 half-lives, only one millionth of the original amount remains, too small to measure; twenty half-lives for 129I would be approximately 350 million years.

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Most elements were formed at the birth of the Universe, some 15 to 25 billion years ago, but 129I is produced in quantity in nature only by supernova explosions.  As the half-life of 129I is comparatively short in astronomical terms, the discovery of meteoritic 129Xe demonstrated that only a short time had passed between the supernova and the time the meteors had solidified and trapped the 129I. These two events (supernova and solidification of gas cloud) were inferred to have happened during the early history of the Solar System, as the 129I isotope was likely generated before the Solar System was formed, but not long before, and seeded the solar gas cloud isotopes with isotopes from a second source. A shock wave from such a supernova source may also have caused collapse of the solar gas cloud.

Since it was known that Earth was 4.6 billion years old, as measured with the uranium-lead technique by Clair Patterson, Harrison Brown, George Tilton, and Mark Inghram in 1953, it was only necessary to add 350 million years to estimate the age for formation of the solid bodies of the Solar System.

Letters in support of Reynolds’ discovery were glowing: “His work on meteorites…has revolutionized much of cosmological theory. His latest result is the most important single event in the whole field” (Willard Libby). “Reynolds has made an exceedingly important discovery, namely that there is a variation in the abundance of the isotopes of xenon in meteorites. The nature of this variation is two-fold: first, there is a special anomaly due to the decay of iodine-129 which shows that the meteorites were formed within a couple of hundred million years after the last important synthesis of the elements; and second, there is a general anomaly which indicates that nuclear processes of some kind were different for the meteorites than they were for the material of the Earth…I regard this as a very important discovery” (Harold Urey). “One can point to one particular accomplishment in his investigation of the xenon content of meteorites. The isotopic composition of xenon has led to most striking conclusions concerning the conditions under which our planetary system must have formed” (Edward Teller).

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