Perchlorate, a negative ion consisting of one chlorine atom bonded to four oxygen atoms, occurs naturally in some soils on Earth, and is especially abundant in Martian soil. As a powerful oxidizer, perchlorate is also manufactured and used in solid rocket fuel, fireworks, munitions, airbag initiators for vehicles, matches and signal flares. It is a byproduct in some disinfectants and herbicides.

Because of its ubiquity in both soil and industrial goods, perchlorate is a common water contaminant that causes certain thyroid disorders. Perchlorate bioaccumulates in plant tissues and a large amount of perchlorate found in Martian soil could make food grown there unsafe to eat, limiting the potential for human settlements on Mars. Perchlorate in Martian dust could also be hazardous to explorers. Current methods of removing perchlorate from water require either harsh conditions or a multistep enzymatic process to lower the oxidation state of the chlorine element into the harmless chloride ion.