At around the same time that Arthur Holmes published his ideas for the age of the Earth, Harrison Brown, a professor at the University of Chicago, was developing a new method for counting lead isotopes in igneous rocks. Brown thought this method of counting was incredibly tedious but very easy, so he assigned it to Patterson as his dissertation project in 1948.
Prior to beginning his research, Patterson had worked on the Manhattan Project during World War II, showing that, by the time he began his research, he had much experience in the field. Patterson’s work consisted of using samples of old rock to measure uranium-lead ratios to determine their ages. The main problem with using this method of dating was that Patterson needed ancient rocks that contained crystals bearing both uranium and lead. Additionally, these lead- and uranium-bearing crystals would have had to be as old as the Earth. Finding these kinds of rocks on Earth proved to be a nearly impossible task, as during that time nobody could account for why extremely old rocks are not found at the Earth’s surface, or where they would be if not at the surface. In order to move past this problem, Patterson looked for answers in rocks beyond the Earth; he turned to meteorites.
Canyon Diablo Meteorite
In using meteorites to calculate the age of the Earth, Patterson made two assumptions about rocks that proved to be correct. He assumed, just as Holmes did, that meteorites were leftover materials from the beginning of the solar system and that by being in space, they would maintain an unchanged interior chemistry. Patterson believed, and rightfully so, that if he were to measure the age of one of these meteorites, then he would have an age close to that of the Earth. Patterson was able to acquire these rare meteorite samples, which contained zircon crystals. These crystals were small and difficult to isolate, but they contained lead and uranium, the materials necessary for Patterson to conduct his research. However, he always found that they were contaminated with atmospheric lead when exposed to air. This contamination became a problem in solving the age of the rocks, so he created the world’s first sterile lab at the California Institute of Technology in 1952. This allowed him to make more accurate measurements with clean samples. Patterson had received his Ph.D. in chemistry from the University of Chicago in chemistry in 1951, and he still continued his work.
In 1953, Patterson travelled to the Argonne National Laboratory in Illinois and was given permission to use their state of the art mass spectrometer on his samples. This mass spectrometer was able to detect and measure minute amounts of lead and uranium inside of his zircon crystals. The mass spectrometer worked by using a magnet to separate out the elements contained in a sample so that the amounts of each could be quantified. After using this and isolating the sample from any outside contamination, Patterson could accurately measure the amount of lead and uranium in the sample. With the calculated ratio of uranium to lead, Patterson would be able to determine the age of the zircon crystals and thus, the age of the Earth. Patterson’s final meteorite specimens were from the Canyon Diablo iron meteorite. Before Patterson, the oldest determination for the age of the Earth was about 3.3 billion years old, made by Arthur Holmes. With the sample Patterson possessed, he was at last able to determine the age of the Earth and, at a meeting in Wisconsin in 1953, he announced the age of the Earth to be 4.550 billion years, with an error of about 70 million years. The currently accepted value today is 4.54 billion years old, with an error of 0.05 billion years. Patterson’s methods were accurate and have received few changes since his numbers were published.