a suite of lunar lavas, relatively enriched in certain elements, that were identified in the analysis of rock samples that Apollo (Apollo program) astronauts brought back from the Moon. The elements include potassium (chemical symbol K), rare-earth elements, and phosphorus (P), from which the acronym KREEP is derived. Lunar scientists have interpreted the enrichment to be a signature of the cooling history of lunar rocks from an ancient magma ocean more than four billion years ago. The elements are called incompatible because they do not fit well into the crystal lattices of the most common lunar minerals such as pyroxene, olivine, and plagioclase, which crystallized relatively early from the magma ocean. Being preferentially excluded, the incompatable elements thus became part of the last liquids to solidify.

Because the incompatible elements also include radioactive ones such as thorium, which spontaneously emits gamma rays, it is possible to map the distribution of KREEP by using gamma-ray spectrometry from spacecraft in lunar orbit. Lunar Prospector, which orbited the Moon in 1998–99, provided the first such map of the entire lunar surface. The data revealed chemical variations that, when correlated with other remote-sensing data, enabled scientists to develop models of regional differences in the evolution of lunar landforms and in the Moon's thermal and mineral history.