Lunar thorium distribution. Map showing the distribution of the element thorium on the Moon and the location of KREEP. Thorium is a radioactive trace element. KREEP, an acronym built from the letters K (the atomic symbol for potassium), REE (Rare Earth Elements) and P (for phosphorus), is a geochemical component of some lunar impact breccia and basaltic rocks and an important key to lunar igneous history. Imaged by NASA's Lunar Prospector mission.