Geological observations suggest rivers and seas dotted the martian surface 3.5 billion years ago. The amount of water has been equated to a planet-wide ocean half-a-kilometer deep or more. For the planet to have stayed warm enough for liquid water, scientists assume that Mars had a greenhouse "blanket" of carbon dioxide atmosphere at least 1000 times thicker than what Earth has now.
That carbon dioxide is mostly gone. So is the water. "Either they went up or they went down," says Dave Brain from UC Berkeley.
By "down," Brain is referring to the subsurface of the red planet. Water ice is known to be lurking underground, while vestiges of carbon dioxide can be found in the polar ice cap and in certain mineral deposits. But many scientists expect that a large fraction of the water-soaked atmosphere was sucked "up" into space.
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