Some elements like potassium, uranium, and thorium are naturally radioactive and give off gamma rays as they decay. Electromagnetic radiation from these isotopes can be detected by a Gamma-Ray Spectrometer (GRS) dropped toward the planetary surface or observed from orbit. An orbiting instrument can map the surface distribution of many elements for an entire planet.
Uncrewed spacecraft programs such as Venera and the Vega program have flown to Venus and sent back data allowing estimates of the K/U ratio of the surface rocks.
The Lunar Prospector mission used a GRS to map the Earth's Moon.
To determine the elemental makeup of the Martian surface, the Mars Odyssey used a GRS and two neutron detectors.
These GRS readings can be compared to direct elemental measurements of chondrites meteorites, Earth, and Moon samples brought back from Apollo program missions, as well as to meteorites that are believed to have come from Mars.