Radioactivity, the discovery of time and the earliest history of the Earth
The discovery of radioactivity about 100 years ago permitted the measurement of absolute time in the distant past and transformed our understanding of the evolution of our planet from the origin of the Solar System to the development of Homo Sapiens. Geologists were already convinced that the Earth had to be very old but Lord Kelvin had placed stringent limits on its antiquity by measuring how fast it appeared to be losing heat. Radioactivity not only provides the heat necessary to keep the Earth geologically active for 100 times longer than Kelvin had realized, it also equips the scientist with a powerful tool for deciphering the absolute timing of events and rates of natural processes in the ancient past. We can now date individual microscopic mineral grains weathered from past continents, determine the rates at which magma is accumulating beneath volcanoes, study the events surrounding the collapse of the solar nebula and the formation of planets and test theories for climate change by measuring the history of sea level, dust circulation and ocean temperature. However, the largest challenge remains finding ways to decipher the first 500 million years of Earth history, a period from which not a single indigenous rock appears to have survived. Most of the new insights are being gleaned from the record of the extinct short-lived nuclides that were alive in the early Solar System.