Nearly every Mars mission since Mariner 9 has found evidence for past water. The Viking orbiters found dry riverbeds, canyons, and valleys that cut into the planet, though they are a far cry from the supposed “canals” astronomer Percival Lowell reported seeing on the Red Planet starting in the 1890s.
Mars Global Surveyor — which arrived in 1997, becoming the first successful mission to the planet since Viking — discovered gullies. On Earth, water typically carves such features. But Mars’ current atmosphere is much too thin for water to survive on the surface for long, so a contentious debate has developed as to what causes the gullies. Some scientists think dry ice or natural sedimentary movements could be at the root; others think seasonal flows of water could stick around just long enough to create the features. The orbiter also found evidence for subsurface glaciers and old lake beds.
Mars Odyssey and Mars Reconnaissance Orbiter (MRO), both still operating after more than a decade circling the planet, have discovered their share of intriguing geologic features. MRO famously found “recurring slope lineae,” brine-filled fluids that some researchers identify as seasonal flows of water, albeit in a salty slush that eventually dries into toxic salts — far from friendly to life. Odyssey has mapped several regions where water ice seems to exist just below the martian surface, out of reach of the Sun’s sublimating gaze.
But the biggest gains arguably have come from landers and rovers. Despite some notable failures — Deep Space 2, Mars 96, Mars Polar Lander, and Beagle 2 — the bulk of the landing missions in the past 20 years have been unqualified successes. It started with Mars Pathfinder, whose Sojourner probe proved the viability of rovers on Mars in 1997. The Phoenix lander, which touched down in 2008, found abundant water ice near the north pole of Mars.
In 2007, the Spirit rover found bizarre structures made of opaline silica on the floor of Gusev Crater. On Earth, such features appear as microbial colonies near geysers, though scientists are loath to make the correlation without further evidence. The two most impressive rovers, the ongoing Opportunity and Curiosity, have spent several years exploring craters and ancient lake beds, all but confirming that Mars was a lush water world billions of years ago. In fact, Opportunity has been exploring Mars since 2004 after being given an initial 90-day mission. (Its twin, Spirit, lasted “only” until 2011.)
But where did the water go?