Imagine all the water in the Arctic Ocean. Now, imagine all that and more on Mars. If you visited the Red Planet roughly 4.3 billion years ago, that's what you likely would have found, say NASA scientists. Their new study arrived at that "solid estimate ... by determining how much water was lost to space," says lead author Geronimo Villanueva. The Guardian reports the scientists used three powerful infrared telescopes to analyze two forms of water in Mars' atmosphere: H2O and HDO, in which deuterium (aka "heavy" hydrogen) has taken the place of one hydrogen atom. As National Geographic explains, Mars' gravity is weaker than our own; that allowed hydrogen to escape from the atmosphere and into space over time, boosting the amount of deuterium in its water. Scientists compared the ratio of HDO to H2O in Martian water today with that in water from a 4.5 billion-year-old Mars meteorite to arrive at their conclusions, a press release states.
They ultimately calculated that Mars was once home to 20 million cubic kilometers of ocean. It could have covered every inch of the planet in water 450 feet deep, but the scientists think a different scenario is more likely: that the ocean covered roughly a fifth of the planet, was located in the northern hemisphere, and was as much as a mile deep. While National Geographic observes the findings back up "reams of earlier evidence that water once existed on the surface" of Mars, another study author points out the assessment of the degree of water lost indicates something new. "The planet was very likely wet for a longer period of time than was previously thought, suggesting it might have been habitable for longer," a scientist says. (Mysterious plumes on Mars are stumping scientists.)