Modern Mars is cold, desolate, and dry. Its atmosphere contains a mere 0.03% of water vapor. Water can’t survive as a liquid. Only at the planet’s poles do the temperature and pressure let water exist, as ice, on the surface.
But Mars was once wetter. Giant canyons, networks of gullies, layers of sedimentary rocks, even features that resemble shore lines are visible on the surface. All these structures formed by the action of water. Carving all the water-worn features required a lot of water. According to Mike Carr of the US Geological Survey, the amount needed is equivalent to an ocean 500 meters deep covering the entire planet—far more than the atmosphere or ice caps currently store. Where the water went is one of the biggest Martian mysteries.
Now, a suite of nuclear physics instruments on board NASA’s Mars Odyssey spacecraft has identified what is almost certainly a new reservoir...