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Scientists have suspected for decades that ancient Mars was a much warmer, wetter environment,

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but estimates of just how much water Mars has lost over its four-and-a-half-billion-year history vary widely.

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Now, researchers at NASA's Goddard Space Flight Center have made the best estimate to date

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by measuring the atmospheric ratio of normal to heavy water molecules near the Martian polar caps.

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The new measurements suggest that at least twenty percent of the Martian surface was once covered by water,

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contained in an ocean with a maximum depth of about one mile.

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Over time, nearly ninety percent of this ocean was lost to space, with the remainder locked up today in Mars' north and south polar caps.

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This new picture of early Mars is considerably wetter than many previous isotopic estimates,

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and suggests that water persisted on the Martian surface for billions of years,

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raising the odds for the ancient habitability of the Red Planet.

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