The argon-40/argon-39 method of radioactive dating, invented in the mid-1960s by Grenville Turner and his American colleague Craig Merrihue, provided the technique to date the small but irreplaceable rock samples taken by the Apollo astronauts -moon was brought back, to be precisely dated -landing program. Turner, who has died of a brain tumor aged 87, pioneered the method before becoming one of the few British scientists appointed by Nasa as a principal investigator of the Apollo programme.
Argon-40/argon-39 dating is similar to an earlier technique called potassium/argon dating. In volcanic rocks and minerals, any of the isotope potassium-40 (the parent isotope) that is present decays over time to the isotope argon-40 (the daughter isotope). Measuring the amounts and proportions of each allows geologists to calculate the age of the rock sample. However, the process was somewhat cumbersome.
Turner and Merrihue realized that a more accurate result could be obtained if the rock sample was irradiated in a nuclear reactor with neutrons, which convert potassium-40 to argon-39 (which becomes a proxy for the parent isotope). Analysis becomes simpler because the ratio of argon-39 (which now acts as a proxy for potassium-40) to argon-40 (which remains the daughter isotope) can be used to calculate a more precise age. This contribution to the accurate dating of the Apollo lunar rock samples, and also the ability to date meteorites and terrestrial rock formations, was crucial to our understanding of the solar system.
Grenville Turner was born in Todmorden, West Yorkshire, the only child of Arnold, a loom in a cotton factory, and Florence (née Pratt), a weaver. He was educated at Todmorden Grammar School before gaining an MA in physics at St John’s College, Cambridge, and a PhD in nuclear physics at Balliol College, Oxford in 1962.
His first academic post was at the University of California, Berkeley, between 1962 and 1964, before becoming professor of physics at Sheffield University in 1964, which he combined with a short stint at the California Institute of Technology. He joined Manchester University’s Department of Earth Sciences as Professor of Isotope Geochemistry in 1988, where he remained for the rest of his academic career. There he founded the University of Manchester Isotope Cosmochemistry Group to investigate the use of isotopes to determine the age of the solar system and its bodies.
He was recruited to work on the Apollo program after Nasa sent an “announcement of opportunity” to the UK Science Research Council in 1966. Nasa was looking for 12 principal investigators to undertake studies of lunar samples once the Apollo program got underway later that decade and, although he was not on the initial list, in 1967 John Reynolds, a physicist with who Turner worked at Berkeley suggested that he apply directly because of his geochronology experience, and he would provide supporting evidence. Turner was successful and was added to Nasa’s list.
Outside of the Apollo program, his research topics were wide-ranging. Among numerous achievements, he also invented the argon-38/argon-37 technique of cosmic ray dating, which helped date craters on the lunar surface and, together with Reynolds, discovered isotope anomalies in the xenon present in meteorites which showed that they before our solar system. Because of his work on the Apollo samples, Turner was also one of the few British scientists involved in the analysis of lunar rocks found by the Soviet Union’s unmanned Luna probes.
In addition, Turner established the UK’s first ion microprobe to probe extraterrestrial material, using it to measure oxygen isotope variations in meteorite ALH 84001, composed of rock ejected from Mars. In 1996, the meteorite made headlines worldwide when it appeared to contain microscopic fossils of bacteria. Bill Clinton, the US president at the time, made a speech about the discovery, but later accepted that the formations in the meteorite were almost certainly inorganic. Turner’s results were a key component of the new interpretation.
Turner was elected a fellow of the Royal Society in 1980, serving on various committees and the society’s council between 1990 and 1992. He was also a fellow of the Meteoritical Society and the American Geophysical Union. He has won numerous awards, including the Royal Society’s Rumford Medal in 1996 (awarded for important discoveries in the field of thermal or optical properties of matter), the Urey Medal of the European Association of Geochemistry in 2002 (for outstanding contributions which geochemistry over a career) and the Gold Medal of the Royal Astronomical Society in 2004 (for his lifetime contribution to geophysics). In 2020, the British Antarctic Survey named a nunatak (a rock protruding through an ice field) after him, after the discovery of a meteorite there. He also became an honorary citizen of his home town of Todmorden in 2013. Apart from his academic work, he enjoyed photography, walking and visiting the theatre.
Turner was particularly modest about his achievements, preferring to share his knowledge and thus encourage others to pursue scientific careers. Much of his spare time was spent finding funding for, and supporting the careers of, the many young scientists who studied under him.
He married Kate Morris, an English teacher, in 1961. She and their daughter, Charlotte, and son, Patrick, survive him, as do his grandchildren, William, Emily, Ruby and Finlay.
