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The potassium/thorium ratio (K/Th ratio) compares the ratio of two elements that differ in volatility, i.e., boiling points: potassium is more volatile and thorium more of a refractory material. Within the solar system, a lack of potassium (as compared to thorium) suggests at some time, the temperature was such that potassium boiled away. Their long-half-life radioactive isotopes allow them to be detected by gamma-ray spectrometers from some distance, e.g., by planetary probes with or without landing.