Mars

Mars is a cold, dry, dust-coloured world about half Earth's diameter, but with the same total land area thanks to its absent oceans. Its red tint comes from iron-oxide dust covering most of the surface. The thin CO₂ atmosphere — under 1% of Earth's surface pressure — cannot retain heat or shield the surface from radiation. Yet ancient riverbeds, deltas, and clay minerals show liquid water flowed here for hundreds of millions of years, making Mars the most accessible candidate for past life beyond Earth.

A thin envelope averaging 6 millibars at the surface — about 0.6% of Earth's pressure. CO₂ dominates, but enough water vapour exists to form thin morning clouds and seasonal frosts. Global dust storms occasionally engulf the entire planet, as in the 2018 storm that ended the Opportunity rover. Surface winds reach 100 km/h but, in the thin air, push with little force.

Mars holds the solar system's most extreme topography. Olympus Mons rises 21.9 km — about 2.5 times the height of Everest above sea level — and spans roughly 600 km across. Valles Marineris is a canyon system over 4,000 km long and up to 7 km deep. The northern hemisphere is dominated by smooth lowlands; the southern by ancient cratered highlands. The polar caps are layered water and CO₂ ice that grow and retreat with the seasons.

Mars is the most-explored planet beyond Earth. Mariner 4 made the first flyby in 1965; Vikings 1 and 2 landed in 1976 and ran the first life-detection experiments. NASA's rovers — Sojourner (1997), Spirit and Opportunity (2004), Curiosity (2012), and Perseverance (2021) — have driven a combined ~80 km across the surface. Perseverance is caching samples for a future Mars Sample Return; the Ingenuity helicopter flew 72 times before its January 2024 retirement. China's Zhurong rover landed in 2021. Crewed missions remain a NASA, SpaceX, and CNSA goal for the 2030s-2040s.