planetary geology . A science that applies principles and techniques of geology, geochemistry, and geophysics to the study of the nature, origin, and history of the condensed matter and gases in the solar system (usually excluding the Earth). It includes: remote-sensing observations and in situ manned exploration of other planetary bodies, such as the Moon and Mars, Venus, Mercury, and "earthlike" or "rocky" satellite moons of the giant planets (Shirley and Fairbridge, 1997); the study of the chemistry, mineralogy, and history of objects that occur on the Earth but are of known or possible extraterrestrial origin (such as meteorites and tektites) or that are returned to the Earth (such as lunar samples); and the study of the effects of extraterrestrial processes (such as meteorite impact, solar energy changes, and tides) on the Earth in the present and past. The term was first used by Lesevich (1877) for a branch of astronomy based primarily on the study of meteorites and secondarily on telescopic spectroscopy (see Milton, 1969). Syn: astrogeology, planetology.