Where did the water on Mars go? A recent theory brings new clues

Today, Mars is a frozen desert. But dry deltas and riverbanks reveal that water once flowed over the planet's surface. Where did he go? Scientists have been trying to answer this question for decades, hoping to understand how Mars became a barren wasteland while its neighbor Earth retained its water and became a biological paradise.

Now, by connecting observations of the Red Planet with new models, a team of geologists and atmospheric scientists has created a new picture of Mars' past: Much of the planet's water may have been trapped inside minerals in the bark, where it remains to this day.

Previous research suggested that most of the water on Mars escaped into space when its atmosphere was stripped away by radiation from the sun. But the recent study, published in the journal Science and presented virtually this year at the Lunar and Planetary Science Conference, concludes that water on Mars experienced both an atmospheric exodus and geological entrapment.

Depending on how much water you start with, the new model estimates that between 30 and 99 percent of it was incorporated into the minerals in the planet's crust, while the remaining fraction escaped into space. It's a wide variety, and both processes probably played a role, so "somewhere, there's reality," says Briony Horgan, a planetary scientist at Purdue University who was not involved in the new study.

If the new model is accurate, then the history of the planet's adolescence would need to be rewritten. All the water thought to be trapped in the Martian crust today would mean that the planet had much more surface water in its youth than previous models had estimated, and that early time may have been even friendlier for microbial life. than previously thought.

"This paper allows for the possibility of a Mars that was once blue, even for a short period of time," says Paul Byrne, a planetary scientist at North Carolina State University who was not involved in the new study.

From Soggy to Desiccated

A multitude of dry riverbeds, deltas, lake basins, and inland seas make it clear that Mars once had a lot of water on its surface. It may even have had one or more different oceans in its northern hemisphere, although that is a matter of intense debate. Today, apart from a possible series of subterranean brackish lakes and aquifers, most of the water on Mars is locked up in the polar ice 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, such as ice, liquid water or water vapor, would have been present at various points throughout the history of Mars. They think that during its early days, if all that water was in liquid form, it could cover the entire planet in a shallow ocean from 150 to 850 feet deep.

Mars had a more substantial atmosphere in the past, and its pressure allowed liquid water to exist on the surface. But work with NASA's MAVEN orbiter found that much of the planet's atmosphere was blown away by the solar wind—charged particles streaming in from the sun—perhaps just 500 million years after Mars formed. The reasons are unclear, although the early loss of the planet's protective magnetic field likely played a critical role.

Either way, this atmospheric decimation vaporized about 90 percent of the water on the surface of Mars, leaving the water vapor to be dissolved by ultraviolet radiation and turning Mars into a dehydrated wasteland.

Hidden Clues in Martian Jewels

At least, that's the story. But it has some inconsistencies in the plot.

The fate of the planet's ancient water was previously estimated based on the types of hydrogen found in Mars' atmosphere today. As water vapor in the air is bombarded by ultraviolet radiation from the Sun, hydrogen is stripped of oxygen in the water molecules. Being a light gas, that free hydrogen easily escapes into space. However, some of the water vapor contains a heavier version of hydrogen called deuterium, which is more likely to linger in the atmosphere.

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

Hydrogen is still leaking from Mars today, and scientists can measure the rate to calculate how much water is being permanently lost. If this rate held steady for the past 4.5 billion years, it wouldn't be close enough to explain the disappearance of so much surface water, says lead author of the new study Eva Linghan Scheller, a Caltech doctoral student.

Another clue came courtesy of all the orbiters and rovers who have examined the rocks of Mars. During the last two decades, mineral carriers including clay have been discovered. At first, only patches here and there were found. But today, "we have evidence of a large volume of hydrated minerals at the surface," Horgan says.

All of these extremely old hydrated minerals suggest that, long ago, a lot of water flowed through the ancient Martian soil, much more than the atmospheric deuterium signal indicated.

"It's taken us a while to find all of the hydrated mineral exposures that we've found and then fully recognize their importance on a global scale," says Kirsten Siebach, a planetary scientist at Rice University who was not involved in the work.

Two ways to kill a planet

One problem was that previous models didn't properly account for the crust's ability to hold water within minerals, Scheller says. She and her colleagues decided to make a new model to estimate where the water on Mars went during its entire 4.5 billion-year lifetime.

The model makes a few assumptions, such as how much water Mars had at the beginning, how much was later delivered by watery asteroids and icy comets, how much was lost to space over time, and how much volcanic activity deposited more water on the planet's surface . Depending on the values ​​of those variables, the team found that Mars may once have had enough surface water to make a global ocean from 100 to 1,494 meters deep.

Between 4.1 and 3.7 billion years ago, the amount of surface water decreased significantly as it was absorbed by minerals in the crust and as it escaped into space. None of the hydrated minerals found so far are less than three billion years old, Scheller says, implying that Mars has been a barren wasteland for most of its life.

The new model has its limits, with some key details left blurry. But it's an important step that "will surely help a lot of future research into the history of water on Mars," he says. Geronimo Villanueva, a planetary scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who was not involved in the new study.

For one thing, it helps address a discrepancy between the amount of water estimated by deuterium measurements and the myriad of 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 might have been present on Mars.

However, the research does not change the amount of water scientists believe is available on Mars today, which is not much. Astronauts may one day bake hydrated minerals on Mars to release their water, Horgan says, but that would be an energy-intensive process.

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

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

Water also bonds with minerals on Earth. But on our planet, plate tectonics recycles these minerals, constantly releasing their water through volcanic eruptions, Siebach says. Otherwise, the stagnant crust of Mars may have doomed the planet to become an extremely cold desert. Did the same world-changing process occur 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 was not involved in the work, says 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 are a host of new questions to ask and think about."