By Michael D. Lemonick, for National Geographic Astronomers using some of the world's most powerful telescopes have determined that an ocean at least a mile deep covered a significant fraction of the Martian surface four billion years ago. The research, reported Thursday in the journal Science, reinforces reams of earlier evidence that water once existed on the surface of the red planet, leaving traces such as stream pebbles, ancient shorelines, river deltas, minerals that must have formed in a watery environment, and more. (Video: NASA's Mars Phoenix once detected snow falling on Mars.) This time, the evidence comes from an analysis of the water vapor that lingers today in the Martian atmosphere. A team of scientists led by Geronimo Villanueva, of NASA's Goddard Space Flight Center, took a series of measurements of sunlight reflecting off the Martian surface and through the atmosphere, which revealed the chemical composition of the atmosphere's vapor. The hydrogen in water, whether it's on Mars or on Earth, comes in two varieties: ordinary hydrogen, whose nuclei contain just a proton, and "heavy" hydrogen, also known as deuterium, with a proton and a neutron. More via Nat Geo.