A chemical element (K) used as a tracer for volatile loss in planetary formation.Potassium is a chemical element with symbol K (from Latin kalium) and atomic number 19. It is a soft, silvery-white alkali metal that is highly reactive, especially with water. In planetary science, potassium is used as a tracer for volatile loss because it is moderately volatile and its abundance relative to more refractory elements, like uranium, can reveal how much heating and evaporation a planetary body has undergone during its formation.
Potassium was discovered in 1807 by Humphry Davy, who isolated it by electrolysis of potash. Its use as a tracer in planetary science began with the Apollo missions in the 1960s and 1970s, when samples from the Moon were analyzed for their elemental composition.
Potassium's volatility means it tends to evaporate at high temperatures, so planets that formed in hotter environments or experienced large impacts will have less potassium relative to elements that condense at higher temperatures. The ratio of potassium to thorium or uranium, measured by gamma-ray spectroscopy from orbiters, provides a proxy for the thermal history of a planet. For example, the Moon has a lower K/Th ratio than Earth, suggesting it lost more volatiles, consistent with a giant impact origin.
Potassium is used in fertilizers, glass production, and as a heat-transfer medium in some nuclear reactors. In space exploration, its abundance is measured by instruments like the Gamma Ray Spectrometer on NASA's Dawn mission to map the composition of Vesta and Ceres.
Potassium-40, a radioactive isotope with a half-life of 1.25 billion years, is used for dating rocks and is a major source of heat in the Earth's interior, contributing to plate tectonics.
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