How scientists use the decay of potassium to date rocks on other planets

Nature · Published 3 October 2026 · Updated 25 October 2026 Swipe more like this →

In short: Potassium-argon dating is a cornerstone of geochronology, allowing researchers to determine the age of rocks by measuring the accumulation of argon gas trapped within mineral crystals. From calibrating the Earth's magnetic history to dating Martian samples, this method provides a precise clock for understanding our planet and beyond.

The K–Ar dating method relies on the radioactive decay of potassium-40 (40K) into argon-40 (40Ar). While 40K also decays into calcium-40, this pathway is rarely useful because non-radiogenic calcium is already so abundant in the Earth's crust that it masks any signal from radioactive decay. Argon, conversely, is a noble gas that typically escapes molten rock. It only begins to accumulate within a mineral's crystal lattice once the rock solidifies and cools, effectively resetting the internal clock.

To calculate the age of a sample, researchers measure the ratio of accumulated 40Ar to remaining 40K. Because 40K has a long half-life of 1.248 billion years, the technique is most effective for materials older than 100,000 years. The process requires several critical assumptions: the sample must have remained a closed system since cooling, and the initial 40K/39K ratio must be constant. If these conditions are met, the measured argon provides a reliable timestamp for when the rock last crystallized.

The method is not without challenges. Contamination from atmospheric argon or the inclusion of older, non-representative material—known as xenoliths—can skew results. To mitigate this, scientists use mass spectrometry to isolate radiogenic argon from atmospheric sources. Despite these complexities, the technique remains vital. It was used to calibrate the geomagnetic polarity time scale and helped bracket the age of archaeological deposits at sites like Olduvai Gorge. In 2013, the Curiosity rover even applied this method to date a rock on Mars, marking a milestone in planetary science.

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