Last year, NASA researchers looked at water in the Red Planet's atmosphere and compared it to water trapped in ancient meteorites from Mars. The research suggested that roughly 87 percent of the water escaped to space. But there's ample evidence out there that Mars was once soaked in water. For example: All three rovers that landed on the surface have found evidence of it in various forms - minerals formed in water, or ancient streambeds.
If an ocean did exist, it would be where the rovers are - in the northern plains, since it is low-lying ground. But how did that water get there in the first place?
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An emerging hypothesis suggests that a few billion years ago, much of Mars was covered in water ice (similar to the poles of today). The ice likely came from asteroids depositing it on to the surface. Then came an event known as the Late Heavy Bombardment, where asteroid debris pummeled the inner solar system roughly four billion years ago. The asteroids could have melted the Martian ice and created an ocean for as long as 200 million years.
The research comes from Timothy Parker, a research scientist at NASA's Jet Propulsion Laboratory, who has researched the possibility of a past ocean on Mars since the 1980s. That's when the Viking missions were beaming back black-and-white images of Mars at what today would be considered extremely low resolution. Even then, Parker told Discovery News, he could see what he described as "wave-refracted shorelines" in the northern plains of Mars, shorelines that appeared similar to what is on Earth.
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As better, more high-resolution data arrived in the 1990s and beyond, Parker noticed something odd. The supposed Martian ocean interiors are lower than the shorelines. This is opposite to what happened on Earth, when isostatic rebound (the ground rising up after large icebergs melt) made the interiors higher than the shorelines. What accounted for the mystery?