The argon-40/argon-39 method of radioactive dating, invented by Grenville Turner and his American colleague Craig Merrihue in the mid-1960s, provided the technique to precisely date the tiny but irreplaceable rock samples brought back by the astronauts of the Apollo moon-landing programme. Turner, who has died aged 87 from a brain tumour, pioneered the method prior to becoming one of the few British scientists to be employed by Nasa as a principal investigator on 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 ratios of each allows geologists to calculate the age of the rock sample. However, the process was somewhat cumbersome.
Turner and Merrihue realised that a more accurate result could be obtained if the rock sample was irradiated with neutrons in a nuclear reactor, which converts potassium-40 to argon-39 (which becomes a proxy for the parent isotope). Analysis becomes simpler because the ratio of argon-39 (now acting 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 moon rock samples, and also the ability to date meteorites and terrestrial rock formations, was of crucial importance to our understanding of the solar system.
Grenville Turner was born in Todmorden, West Yorkshire, the only child of Arnold, a loom overlooker in a cotton factory, and Florence (nee Pratt), a weaver. He was educated at Todmorden grammar school before earning a master’s degree in physics at St John’s College, Cambridge, and, in 1962, a PhD in nuclear physics at Balliol College, Oxford.
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 brief 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 established 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 programme after Nasa sent an “announcement of opportunity” to the UK’s Science Research Council in 1966. Nasa was looking for 12 principal investigators to undertake study into lunar samples once the Apollo programme got under way later that decade and, although he was not on the initial list, in 1967 John Reynolds, a physicist with whom Turner had worked at Berkeley, suggested he apply directly because of his geochronology experience, and he would provide supporting evidence. Turner was successful and was added to Nasa’s list.
Beyond the Apollo programme his research topics were broad. Among numerous achievements, he also invented the argon-38/argon-37 technique of cosmic ray dating, which helped date impact craters on the lunar surface and, with Reynolds, discovered isotope anomalies in the xenon present in meteorites that showed that they predated our solar system. Because of his work on the Apollo samples, Turner was also one of the few British scientists involved in analysis of moon rocks retrieved by the Soviet Union’s uncrewed Luna probes.
In addition, Turner established the UK’s first ion microprobe to examine extraterrestrial material, using it to measure oxygen-isotope variations in meteorite ALH 84001, composed of rock ejected from Mars. In 1996, the meteorite had made headlines worldwide when it appeared to contain microscopic fossils of bacteria. Bill Clinton, then the US president, had given a speech about the discovery, but later it became 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 several 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 won numerous awards including the Royal Society’s Rumford medal in 1996 (conferred 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 advancing 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 (an outcrop of rock protruding through an icefield) after him, following 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 notably modest about his achievements, preferring to share his knowledge and so encourage others to follow scientific careers. Much of his spare time was spent finding funding for, and supporting the careers of, the many young scientists who had 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.