Geochemistry is the study of the chemical and structural composition and evolution of Earth and its component parts, including the atmosphere, hydrosphere, crust, mantle, and core, as well as extraterrestrial matter, such as meteorites and comets, other planets, the sun, and distant stars. It concerns itself with the distribution and migration of elements within the Earth and its atmosphere. Over time, geochemistry has evolved from a descriptive science to one increasingly concerned with the mechanisms behind its observations.Geochemistry today is diversified into many subfields, including aqueous geochemistry, cosmochemistry, inorganic geochemistry, isotope geochemistry, organic geochemistry, and trace-element geochemistry. Since most geological processes involve chemical reactions, geochemistry and geochemical data are also used to explain many of these processes in other areas of the geosciences.At Caltech GPS, geochemistry has been used since 1952 to study the distribution of chemical elements in the Earth and the solar system, to develop methods to date earth and solar system processes, to discover and observe the chemical composition of terrestrial and extraterrestrial materials, and to study chemical reactions in the interior of the Earth as well as its surface and in the solar system outside of the Earth. Activity in environmental geochemistry at Caltech occurs in several academic divisions on campus as well as at the Jet Propulsion Laboratory located a few miles off campus. Below is a list of Caltech professors and key scientists from JPL who participate in geochemically oriented environmental studies.