Rutherford, 33, realized that one way to date rocks was to measure the amount of helium (in bubbles) present within them, due to the decay of uranium into thorium-90, and then into radium, spewing out an alpha particle. Radium then generates polonium, which generates lead, which is stable. Helium does not react with other elements; it is either attracted or repelled, so all the helium in a rock is due to radioactive decay, and the more there is, the older the rock. The initial findings contradicted the young age of the Earth estimated by the respected Lord Kelvin (80), which instilled in Rutherford a certain fear in revealing the discovery. Rutherford kept a low profile until Kelvin’s death in 1907, and then quickly demonstrated the relationship between helium and uranium and the age of the Earth, dating one of the rocks he had available to him to be at least 500 million years old (already considerably older than the 2 million years estimated by Lord Kelvin). Within a few years, with the help of geologists bringing older rocks, he arrived at an estimate of 2 billion years. Today we know the Earth is 4.5 billion years old.
