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[ music ]

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Beneath the thin atmosphere of Mars lies an enigma: a desert landscape shaped by flowing water.

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In the distant past, Mars must have had a warmer, wetter climate,

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but scientists wonder: just how wet was ancient Mars?

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[ Villanueva ] So in the ancient past, we have some indications that water was flowing on the surface, but how much water was there?

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Are we talking about oceans, are we talking about small rivers, a little rain?

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So these definitions of how much water was on the planet was very undefined.

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[ Mumma ] A major question has been: how much water did Mars actually have when it was young, and how did it lose that water?

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To answer this question, a team of researchers at NASA's Goddard Space Flight Center

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used infrared telescopes on Earth to study water molecules in the Martian atmosphere.

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[ Mumma ] We used the world's three major telescopes for infrared astronomy.

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From the ground we could actually take a snapshot of the whole hemisphere of the planet on a single night.

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The new infrared maps reveal the atmospheric ratio of normal to heavy water molecules at different locations and seasons on Mars.

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Heavy water molecules contain a heavy isotope of hydrogen called deuterium,

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which remains trapped in the Martian water cycle while normal hydrogen is lost to space.

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The researchers found that water from the polar ice caps is highly enriched in deuterium,

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indicating that Mars has lost a tremendous quantity of water.

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Now we know that Mars water is much more enriched than terrestrial ocean water in the heavy form of water, the deuterated form.

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Immediately that permits us to estimate the amount of water Mars has lost since it was young.

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The findings indicate that only 13% of an ancient ocean remains on the planet today, now stored in the polar ice caps.

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87% of this ocean has been lost to space. This means that early Mars would have looked much different than it does today,

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with a significant portion of its surface covered by water.

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[ Mumma ] So the really interesting question is, could it form a sea or an ocean? And indeed, it would.

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In the northern plains, which is a relatively flat region but depressed from the rest of the planet,

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it would form an ocean that was approximately 20% of the planet's surface area.

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And so that is a respectable ocean.

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By combining Martian topography with the new estimate for water loss,

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the researchers were able to simulate Mars' ancient ocean, and its escape to space.

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As Mars lost its atmosphere over billions of years, it lost the pressure and heat needed to keep water liquid,

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causing the ocean to shrink and recede northward.

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The remaining water eventually condensed and froze over the north and south poles, giving Mars the ice caps that we see today.

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This new scenario means that Mars would have stayed wet for longer than previously thought, expanding its ancient habitability.

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We now know that Mars was wet for a much longer time than we thought before.

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Curiosity shows it was wet for one-and-a-half-billion years, already much longer

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than the period of time needed for life to develop on Earth,

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and now we see that Mars must have been wet for a period even longer.

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[ music ]

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[ satellite beeping ]