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NASA's MAVEN orbiter is beginning to help planetary scientists unravel the mystery of Mars' missing water – once abundant on the red planet early in its history, but now long gone, mainly lost to space over billions of years.

Evidence is mounting that during that early, wet period, Mars hosted conditions that were hospitable to microbial life, although so far no clear evidence has emerged that life ever gained a foothold there. That's a far cry from the dry, desolate planet astronomers see today.

Two general observations that predate MAVEN lend credence to the idea that the planet lost its water to space, researchers say.

One is the apparent dearth of large deposits of minerals that formed in water (think Britain's White Cliffs of Dover). So Mars itself didn't soak up much over time.

The second comes courtesy of NASA's Mars rover Curiosity. It has measured the relative abundance of two forms of argon in the atmosphere from the floor of Gale Crater, where the rover currently is operating. One form of argon is heavier than the other. The heavier form, or isotope of argon, dominates, suggesting that the lighter ones escaped to the upper atmosphere to be swept away via the sun's solar wind.

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