...a vast body of water larger than the Arctic Ocean graced the surface of Mars some 4.5 billion years ago. The primitive ocean covered 19 percent of the Red Planet’s surface and had a volume of more than 5 million cubic miles, according to a paper published this month in Science. But today almost all of that water is gone. The only evidence that an ocean ever existed there is in the planet's polar ice caps.

So what happened to the ocean on Mars?

“That’s one of the big mysteries,” said Michael Meyer, an astrobiologist and lead scientist for NASA's Mars Exploration Program. But he and other scientists have theories. One way a planet could lose its ocean is from a meteorite or asteroid strike—one that doesn’t obliterate the planet but instead rips apart its atmosphere. “There’s a saying that ‘comets can giveth and comets can taketh away’—that refers to how comets can give water and life, or take it away,” Meyer said.

But the prevailing theory, he told me, is that solar winds wick away Mars’s water from its atmosphere. The sun constantly blasts charged particles from its hot surface toward its celestial bodies. Some planets, like Earth, are protected from the plasma onslaught because they have a magnetic shield that diverts incoming particles around the planet and to its poles. (This is the mechanism that creates dazzling auroras on Earth.) But Mars, unlike Earth, lost its magnetic field at some point in its history. Without the invisible shield, the planet is susceptible to bombarding solar winds. These same winds, the theory goes, are the ones that split exposed water molecules on the surface of Mars's ocean and knocked them into space—like a cosmic cue ball hitting billiard balls into the side pockets.