Up to 99 per cent of the water once in the ancient lakes and oceans on Mars is trapped in the planet’s crust and was not lost to space, according to a study in Science this week.
Scientists at the California Institute of Technology (Caltech) and NASA’s Jet Propulsion Laboratory reckon the water has been sucked into minerals in the Red Planet’s crust. This suggestion is at odds with the current accepted theory that pretty much all the moisture evaporated and floated away after Mars lost its atmosphere billions of years ago.
“Atmospheric escape doesn't fully explain the data that we have for how much water actually once existed on Mars," said Eva Scheller, lead author of the published study and a PhD candidate at Caltech.
The team estimated how much water Mars had and how much it is left, by inspecting the chemical composition of its atmosphere, polar ice caps, and meteorite samples. That left much water unaccounted for.
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Crucially, the US team had to find the ratio of deuterium to hydrogen. Water is made up of two hydrogen atoms bonded to one oxygen atom. Hydrogen has more than one isotope; the most common form, known as protium, has one proton for a nucleus, and deuterium has one proton and one neutron. A tiny proportion of hydrogen atoms – an estimated 0.02 per cent on Earth – exists in the heavier deuterium form.
Water made up of protium atoms is lighter and can escape Mars's weak gravity more easily. By measuring the amount of water consisting of deuterium atoms left on Mars, the team estimated how much water made out of protium atoms must have escaped.
“Atmospheric escape clearly had a role in water loss, but findings from the last decade of Mars missions have pointed to the fact that there was this huge reservoir of ancient hydrated minerals whose formation certainly decreased water availability over time," said Bethany Ehlmann, professor of planetary science at Caltech and a research scientist at JPL.
Instead, they believe that somewhere between 30 and 99 per cent of Martian water has been absorbed by minerals in the planet's crust to form clay. On Earth, such water is recycled by tectonic shifts and eventually makes its way back into the atmosphere, though not so on the Red Planet.
Mars isn’t geologically active. Once the water disappears into its crust, it isn’t replaced, leaving the alien world with the dry and arid climate it has today. “All of this water was sequestered fairly early on, and then never cycled back out," said Scheller.
The boffins may be proved right by regolith samples scraped from the surface by NASA’s Perseverance rover on the unforgiving dust world. The team reckons Mars may have been completely covered by an ocean 100 to 1,500 metres deep. ®