How did Mars lose the surface water that was plentiful on its surface 3 to 4 billion years ago?  New research says it did not leave the planet but rather was incorporated on a molecular level into Martian minerals.  (NASA)

Once it became clear in the past decade that the surface of ancient Mars, the inevitable question arose regarding what happened to it all since the planet is today so very dry.  And the widely-accepted answer has been that the water escaped into space, especially after the once thicker atmosphere of Mars was stripped away.

But NASA-funded research just made public has a new and bold and very different answer:  Much of the water that formed rivers, lakes and deep oceans on Mars, the research concludes, sank below the planet’s surface and is trapped inside minerals in the planet’s rocky crust.

Since early Mars is now thought to have had as much surface water as half of the the Earth’s Atlantic Ocean — enough to cover most of Mars in at least 100 meters of water — that means huge volumes of water became incorporated into the molecular structure of clays, sulfates, carbonates, opals and other hydrated minerals.

While some of the early water surely disappeared from Mars via atmospheric escape, the new findings, published in the latest issue of Science, conclude that atmospheric loss can not account for much or most of its water loss — especially now that estimates of how much water once existed on the surface of the planet have increased substantially.

“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 and a doctoral candidate at the California Institute of Technology.  The rate of water loss was found to be too slow to explain what happened.

Scheller and others at Caltech set out to find other explanations. Based on modeling and data collected by Mars orbiters, rovers and from meteorites, they concluded that between 30 and 99 percent of that very early Martian surface water can now be found trapped in the minerals of the planet’s crust.

Mars mudstone, as imaged by the Curiosity rover.  (NASA/JPL-Caltech)

As described in a release for NASA’s Jet Propulsion Laboratory, the team studied the quantity of water on Mars over time in all its forms (vapor, liquid, and ice) and the chemical composition of the planet’s current atmosphere and crust through the analysis of meteorites as well as using data provided by Mars rovers and orbiters.  They looked in particular at the ratio of deuterium to hydrogen (D/H) in the atmosphere.

Eva Scheller is a doctoral student in planetary science from Denmark working in the Caltech group of John Grotzinger, the lead NASA scientist during the first two years of the Curiosity rover’s time on Mars. Scheller is a student collaborator on the Mastcam-Z investigation for the Mars 2020 mission’s Perseverance rover. (Caltech)

Water is made up of hydrogen and oxygen: H2O, but all hydrogen atoms are not created equal. There are two stable isotopes of hydrogen. The vast majority of hydrogen atoms have just one proton within the atomic nucleus, while a tiny fraction (about 0.02 percent) exist as deuterium, or so-called “heavy” hydrogen, which has a proton and a neutron in the nucleus.

The lighter-weight hydrogen (also known as protium) has an easier time escaping the planet’s gravity into space than its heavier counterpart. Because of this, the escape of a planet’s water via the upper atmosphere would leave a telltale signature on the ratio of deuterium to hydrogen in the planet’s atmosphere: there would be an outsized portion of deuterium left behind.

The loss of water solely through the atmosphere cannot explain both the observed deuterium to hydrogen signal in the Martian atmosphere and the apparently large amounts of water in the past, the team concluded.   But when the large deposits of minerals formed only in the presence of water are taken into account, all the water loss can be explained.

What happens in this process is that water interacts with rock and a chemical “weathering” takes place that changes the initial mineral and traps the hydrogen and oxygen of the water inside the new structure.  Chemical weathering is a gradual and ongoing process.  The mineralogy of the rock adjusts to the near surface environment and over time creates a deposit of hydrated minerals — rocks that scientists know can only be formed in the presence of water.

 

While it was previously suspected that most of Mars’s water was lost to space, a significant portion–between 30 and 90 percent–has been lost to hydration of the crust, according to a new study. Some water was released from the interior via volcanism, but not enough to replenish the planet’s once significant supply. Evidence for the water’s fate was found in the ratio of deuterium to hydrogen in the planet’s atmosphere and rocks. (Caltech)

This process occurs on Earth as well as on Mars. But because Earth is tectonically active, old crust continually melts into the mantle and forms new crust at plate boundaries, recycling water and other molecules back into the atmosphere through volcanism. Mars, however, is tectonically inactive for the most part, and so the “drying” of the surface, once it occurs, is permanent.

The result is that the Martian surface is quite similar today to what it was like 3-4 billion years ago, when the planet was gradually lost its surface water.  This is why Mars rovers — and especially the Perseverance rover which landed in February — can study geological and geochemical  environments on Mars that are older and far less modified  than anything on Earth.

So while minerals are hydrated in similar ways on both  Mars and Earth, their later histories are far different.

“The hydrated materials on our own planet are being continually recycled through plate tectonics,” said Michael Meyer, lead scientist for NASA’s Mars Exploration Program at the agency’s headquarters in Washington. “Because we have measurements from multiple spacecraft, we can see that Mars doesn’t recycle, and so water is now locked up in the crust or been lost to space.”

Mars orbiters identified deposits of these telltale minerals decades ago, but it’s difficult to assess the size of the deposit from orbit.

“It’s only by having measurements in particular places on the surface with your rovers or landers, or your occasional view of a fresh crater, that you get an idea of how thick the particular spot is on the planet for the hydrated minerals that you’re looking at,” Meyer said. “So the answers are there, but they slowly build through time as you gain more data.”

More data is also needed to get a handle on more precisely how much Martian water went off into space and how much was sucked up by the planet’s crust and incorporated into the structure of minerals.  Lead author Scheller said the estimate of how much water sank into the crust ranges so widely — that figure of between between 30 and 99 percent of the water loss — because of uncertainty over the rate at which Mars lost water to space in the distant past.
She said that the Perseverance rover, now exploring a fossil delta in Jezero Crater, can help refine these estimates, “as it is going to one of the most ancient parts of the Martian crust, and so can help us nail down the past process of water loss to the crust much better.”
The paper was presented at a virtual Lunar and Planetary Institute conference.