That exposed the planet to the solar wind, which clawed away at the atmosphere; and that in turn allowed the planet's water to sputter off into space. To look at Mars today is to see a desert world, stamped with the dry riverbeds, delicate deltas and deep ocean basins hinting at the water that is no more.

At least, that's the long-accepted view. But according to a study published Mar. 16 in Science by a team of researchers at the California Institute of Technology, that scenario might be all wrong. Mars is dry, alright—or at least it appears to be. But the researchers say much of its water—from 30% to a staggering 99% of it—is still there. It simply retreated into the martian rocks and clay rather than escaping into space.

Just how much water once flowed across the surface of Mars is expressed by a unit of measure known as "global equivalent layer" (GEL)—the depth that the water would be if it were not sequestered in basins and rivers, but instead were spread evenly across the entire planet. The best estimates for Mars's original GEL is anywhere from 100 to 1,500 meters (330 to 4,900 ft). That's an awfully wide range, but today it's considerably narrower: the modern-day water on the planet's surface—almost entirely trapped in its polar ice caps—has a GEL of just 20 to 40 m.

When Mars lost its atmosphere, all that original water had to go somewhere. The evaporation-to-space route was always the easiest explanation—but it's a flawed one, too. The problem, as the Caltech researchers knew, involves hydrogen. As Martian water molecules rise into and then escape from the atmosphere, they disassociate into free hydrogen and oxygen atoms. An oxygen atom in water is just an oxygen atom, but hydrogen comes in two forms: ordinary hydrogen (with a single proton in its nucleus) and deuterium (with a proton and a neutron). Water molecules made of heavier deuterium instead of ordinary hydrogen are known, straightforwardly enough, as heavy water.