Mars is the fourth planet from the Sun, a cold, thin-atmosphered desert world about half Earth’s diameter. Its dusty surface is stained red by iron(III) oxide — hematite — from eons of oxidation.

Despite its hostility, Mars is the most-explored world beyond Earth. It has been visited by dozens of successful orbiters, landers, and rovers since Mariner 4’s flyby in 1965. Olympus Mons, a shield volcano 21.9 km tall and 600 km wide, is the largest known volcano in the Solar System. The Valles Marineris canyon stretches over 4,000 km — a scar that would cross the entire continental United States.

Abundant geologic evidence — valley networks, deltas, hydrated minerals, subsurface water ice — confirms that liquid water once flowed on Mars’s surface, and recent discoveries by orbiting radar (MARSIS, SHARAD) and the Mars Reconnaissance Orbiter suggest vast reservoirs of subsurface water ice. Mars is the primary focus of NASA’s and China’s human spaceflight ambitions through the 2030s. The Perseverance rover is caching samples for an eventual Mars Sample Return campaign.