Mars was once wet, with the value of an ocean of water on its surface.
Today, most of Mars is as dry as a desert except for ice deposits in its polar regions. Where did the rest of the water go?
Part of it disappeared into space. The water molecules, hit by solar wind particles, separated into hydrogen and oxygen atoms, and those, especially the lighter hydrogen atoms, quickly left the atmosphere and were lost in outer space.
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But most of the water, a new study concludes, sank and was absorbed by the rocks of the red planet. And there it remains, trapped between minerals and salts. In fact, up to 99% of the water that once flowed on Mars could still be there, the researchers estimated in a paper published this week in the journal Science.
Data from the last two decades of robotic missions to Mars, including NASA’s Curiosity rover and the Mars Reconnaissance Orbiter, showed a wide distribution of what geologists call hydrated minerals.
“It was very, very clear that it was common and not rare to find evidence of disturbance of the water,” said Bethany Ehlmann, a professor of planetary science at the California Institute of Technology and one of the authors of the paper.
Ehlmann, speaking at a news conference Tuesday at the Lunar and Planetary Science conference, said that as rocks are altered by liquid water, water molecules are incorporated into minerals such as clays. “The water is effectively trapped in the crust,” he said.
To get an idea of the amount of water, planetary scientists speak of a “global equivalent layer,” that is, if Mars were smoothed out into a uniform, featureless ball, how deep would the water have been?
Scientists estimated the depth to have been 100 to 1,500 meters, or 330 to 5,000 feet.
The most likely depth was about 2,000 feet, they said, or about a quarter of the amount of water in the Atlantic Ocean.
The data and simulations also indicated that almost all water disappeared 3 billion years ago, around the time on Earth when life consisted of single-celled microbes in the oceans.
“This means that Mars has been dry for quite some time,” said Eva Scheller, a Caltech graduate student who was the lead author of the Science paper.
Today, there is still water equivalent to a global ocean 65 to 130 feet deep, but it is mostly frozen in the polar ice caps.
Planetary scientists have long marveled at ancient evidence of flowing water carved into the Martian surface: giant canyons, tendrils of winding river channels, and deltas where rivers spew sediment into lakes. NASA’s latest robotic Mars rover, Perseverance, which landed in Jezero crater last month, will head to a river delta on its rim in hopes of finding signs of past life.
Without a time machine, there is no way to directly observe how much water was on a Mars younger than 3 billion years ago. But the hydrogen atoms floating today in Mars’ atmosphere retain a ghostly touch of the ancient ocean.
On Earth, about 1 in 5,000 hydrogen atoms is a version known as deuterium that is twice as heavy because its nucleus contains both a neutron and a proton. (The nucleus of a common variety hydrogen atom has only one proton, no neutrons.)
But on Mars, the deuterium concentration is notably higher, around 1 in 700. Scientists at NASA’s Goddard Space Flight Center who reported this finding in 2015 said this could be used to calculate the amount of water ever had Mars. Mars probably started out with a similar ratio of deuterium to hydrogen as Earth, but the deuterium fraction increased over time as the water evaporated and the hydrogen was lost to space, because heavier deuterium is less likely to escape. of the atmosphere.
The problem with that story, said Renyu Hu, a scientist at NASA’s Jet Propulsion Laboratory and another author of the current scientific paper, is that Mars hasn’t been losing hydrogen fast enough. Orbiter measurements of the Mars atmosphere and NASA’s Volatile Evolution, or MAVEN, have shown that the current rate, extrapolated to over 4 billion years, “can only explain a small fraction of the loss of water, “Hu said. “This is not enough to explain the great drying up of Mars.”
That led the new research to conclude that a vast majority of water entered the rocks.
“This is a very interesting new study in which many processes are combined to provide alternative scenarios for the fate of water on Mars,” Geronimo Villanueva, one of the NASA scientists who made the first deuterium measurements, wrote in an email. . “This opens up the possibility of an even wetter past, and that rocks on Mars now contain more water than we initially thought.”
However, the water would probably not be of much use to the settlers on Earth. “The amount of water in a rock is very small,” Scheller said.
To release the water trapped in the minerals it is necessary to heat them to high temperatures. “We would have to cook a lot of stone to have something that would be useful,” Scheller said.
Elon Musk, the founder of SpaceX who dreams of sending colonists to Mars one day, has reflected on the detonation of nuclear bombs on Mars to melt the polar caps and warm the planet, making it more hospitable. Those explosions would also release some of the water from the hydrated minerals, although Scheller declined to speculate how much.
Michael Meyer, the lead scientist for NASA’s Mars exploration program, said: “I’ll just mention that bombarding a planet is generally not a good way to make it more habitable.”
On Earth, water is also absorbed into rocks, but it does not stay there indefinitely. The movement of the earth’s crust pushes the rocks into the mantle, where they melt, and then the molten rock, and the water, returns through the volcanoes. On Mars, volcanism, like liquid water, seems to have long since disappeared.
This article originally appeared in The New York Times.
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