Where did Mars's liquid water go? A new theory holds fresh clues.

Oceans’ worth of ancient water may have been locked up in minerals in Mars's crust, increasing estimates for the total amount of that once flowed on the red planet.

Today, Mars is a frigid desert. But dried up deltas and riverbanks reveal that water once flowed over the plant’s surface. Where did it all go? Scientists have been trying to answer this question for decades, hoping to understand how Mars became an arid wasteland while its neighbor, Earth, kept hold of its water and became a biological paradise.

Now, by plugging observations of the red planet into new models, a team of geologists and atmospheric scientists has come up with a new picture of Mars’s past: Much of the planet’s ancient water could have been trapped within minerals in the crust, where it remains to this day.

Prior research suggested that most of Mars’s water escaped into space as its atmosphere was stripped away by the sun’s radiation. But this new study, published today in the journal Science and virtually presented at this year’s Lunar and Planetary Science Conference, concludes that Mars’s water experienced both an atmospheric exodus and a geologic entrapment.

Depending on how much water you start with, the new model estimates that anywhere between 30 and 99 percent of it was incorporated into minerals in the planet’s crust, while the remaining fraction escaped into space. It’s a big range, and both processes likely played a role, so “somewhere in there, the reality lies,” says Briony Horgan, a planetary scientist at Purdue University who wasn’t involved with the new study.

If the new model is accurate, then the story of the planet’s adolescence needs a rewrite. All of the water thought to be trapped in the Martian crust today means that the planet had far more surface water in its youth than previous models had estimated—and that early epoch may have been even more amiable to microbial life than previously thought.

“This paper allows for the possibility of a once-blue Mars, even for a short period of time,” says Paul Byrne, a planetary scientist at North Carolina State University who wasn’t involved with the new study.           

From drenched to desiccated

A multitude of dried up riverbeds, deltas, lake basins, and inland seas make it clear that Mars once had a lot of water on its surface. It may have even had one or several different oceans in its northern hemisphere, although that’s a matter of intense debate. Today, aside from a possible series of briny underground lakes and aquifers, most of Mars’s water is locked up in the polar caps or in ice buried below the surface.

By looking at the chemistry of Martian meteorites of various ages, and by using NASA’s Curiosity rover to study ancient rocks and measure the current Martian atmosphere, scientists have been able to estimate how much surface water—as ice, liquid water, or water vapor—would have been present at various points throughout Mars’s history. They think that during its earliest epochs, if all that water were in liquid form, it could cover the whole planet in a shallow ocean 150 to 800 feet deep.

Mars had a more substantial atmosphere in the past, and its pressure allowed liquid water to exist on the surface. But work using NASA’s MAVEN orbiter found that much the planet’s atmosphere was stripped away by the solar wind—charged particles streaming from the sun—perhaps just 500 million years after Mars formed. The reasons why are not clear, although the early loss of the planet’s protective magnetic field probably played a critical role.

Either way, this atmospheric annihilation vaporized around 90 percent of Mars’s surface water, leaving the water vapor to be broken up by ultraviolet radiation and making Mars a dehydrated wasteland.

Clues hidden in Martian jewels

At least, that’s how the story goes. But it has some plot holes.

The fate of the planet’s ancient water was previously estimated based on the types of hydrogen found in Mars’s present atmosphere. As water vapor in the air is bombarded by ultraviolet radiation from the sun, hydrogen gets stripped away from the oxygen in water molecules. Being a light gas, that free hydrogen easily escapes into space. Some of the water vapor, however, contains a heavier version of hydrogen called deuterium, which is more likely to remain in the atmosphere.

Scientists know what the natural ratio of hydrogen and deuterium should be on Mars, so the amount of deuterium left behind can be used to determine how much of the lighter version was once present on the planet. Deuterium therefore acts as a ghostly fingerprint that reveals the amount of past water that ultimately escaped into space.

Hydrogen is still escaping from Mars today, and scientists can measure the rate to work out how much water is being permanently lost. If this rate held steady over the past 4.5 billion years, it would be nowhere near enough to explain the disappearance of so much surface water, says lead author of the new study Eva Linghan Scheller, a doctoral student at Caltech.

Another clue came courtesy of all the orbiters and rovers examining Mars’s rocks. Over the past two decades, a lot of water-bearing minerals have been discovered, including plenty of clays. At first, only patches were found here and there. But today, “we see evidence for a huge volume of hydrated minerals on the surface,” Horgan says.

All those extremely old hydrated minerals suggest that, long ago, plenty of water was flowing across the ancient Martian soil—much more than the atmospheric deuterium signal indicated.

“It’s taken a while to find all the hydrated mineral exposures that we have found, and then to fully acknowledge their importance on a global scale,” says Kirsten Siebach, a planetary scientist at Rice University who wasn’t involved with the work.

Two ways to kill a planet

One problem was that previous models didn’t adequately take into account the crust’s ability to lock up water inside minerals, Scheller says. She and her colleagues decided to make a new model to estimate where Mars’s water went over its entire 4.5-billion-year lifetime.

The model makes some assumptions, such as how much water Mars had to begin with, how much was delivered later by watery asteroids and icy comets, how much was lost to space over time, and how much volcanic activity deposited more water onto the planet’s surface. Depending on the values of those variables, the team found that Mars could once have had enough surface water to make a global ocean 330 to 4,900 feet deep.

Between 4.1 and 3.7 billion years ago, the amount of surface water decreased significantly as it was soaked up by minerals in the crust and as it escaped into space. None of the hydrated minerals found so far have been younger than three billion years, Scheller says, which implies that Mars has been an arid wasteland for most of its lifetime.

The new model has its limits, with some key details remaining fuzzy. But it is an important step that “will surely assist many future investigations about the history of water on Mars,” says Geronimo Villanueva, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who wasn’t involved with the new study.

For one thing, it helps address a discrepancy between the amount of water estimated by the deuterium measurements and the myriad water features that have been left on the surface. It wasn’t clear how so many rivers and lakes could emerge from so little water, Siebach says, but this new model offers a solution to that mystery by identifying additional water that could have been present on Mars.

However, the research doesn’t change how much water scientists think is available on Mars today—which isn’t much at all. Astronauts may one day bake hydrated minerals on Mars to unleash their water, Horgan says, but that would be an energy-intensive process.

“What this study does is that it says you have more water to play with early in Mars’s history, and that’s when Mars was most habitable,” Siebach says. Microbes, if they ever existed, may have spread through all that available water, but they would have struggled to survive by the time most of it vanished three billion years ago.

The idea that a significant volume of water can vanish into the crust also has implications for other rocky worlds, says Byrne of North Carolina State University.

Water binds to Earth’s minerals, too. But on our planet, plate tectonics recycles these minerals, constantly unleashing their water through volcanic eruptions, Siebach says. By contrast, Mars’s stagnant crust may have doomed the planet to become a bitterly cold desert. Did the same world-changing process happen on Venus? Does water end up locked in the crust of exoplanets far from our solar system?

Scott King, a planetary scientist at Virginia Tech who wasn’t involved with the work, says that the model paves the way for an even richer understanding of how Mars and other rocky worlds evolve through the ages.

“Now,” he says, “there’s a whole wealth of new questions that can be asked and thought about.”

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