The water on Mars is gone. This could be where he went.

Mars was once humid, with the equivalent of an ocean of water on its surface.

Today, most of Mars is as dry as a desert, with the exception of ice deposits in its polar regions. Where did the rest of the water go?

Part of it has disappeared into space. The water molecules, battered by the particles of the solar wind, shattered into hydrogen and oxygen atoms, and these, especially the lighter hydrogen atoms, expelled from the atmosphere, are lost in space.

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But most of the water, a new study concludes, has descended, sucked into the rocks of the Red Planet. And it stays there, trapped in minerals and salts. Indeed, up to 99% of the water that once flowed on Mars could still be there, the researchers estimated in an article published this week in the journal Science.

Data from the past two decades of robotic missions to Mars, including NASA’s Curiosity rover and the Mars Reconnaissance Orbiter, have shown a wide distribution of what geologists call hydrated minerals.

“It became very, very clear that it was common and not uncommon to find evidence of water alteration,” said Bethany Ehlmann, professor of planetary science at the California Institute of Technology and one of the authors of the article.

Ehlmann, speaking at a press briefing Tuesday at the Lunar and Planetary Science Conference, said that when rocks are changed by liquid water, water molecules are incorporated into minerals like clays. “The water is actually trapped in the crust,” she says.

To get an idea of ​​the amount of water, planetary scientists speak of an “equivalent global layer” – that is, if Mars had been smoothed into a uniform, featureless ball, how deep would it have been? some water?

Scientists estimated the depth to have been 100 to 1,500 meters, or 330 to 5,000 feet.

The most likely depth was around 2,000 feet, they said, or about a quarter of the amount of water in the Atlantic Ocean.

Data and simulations also indicated that water was almost entirely gone 3 billion years ago, around the time on Earth when life consisted of single-celled microbes in the oceans.

The story continues

“This means that Mars has been dry for quite a long time,” said Eva Scheller, a Caltech graduate student who was the lead author of the scientific paper.

Today, there is still water equivalent to a 65-130-foot-deep global ocean, but which is mostly frozen in the polar ice caps.

Planetary scientists have long marveled at ancient evidence of flowing water carved into the Martian surface – gigantic canyons, twists of winding channels, and deltas where rivers disgorged sediment into lakes. NASA’s latest robotic explorer on Mars, Perseverance, which landed in Jezero Crater last month, will head to a river delta aboard in hopes of finding signs of past life.

Without a time machine, there is no way to directly observe the amount of water contained on a younger Mars more than 3 billion years ago. But the hydrogen atoms floating in Mars’ atmosphere today retain a ghostly hint of the ancient ocean.

On Earth, about 1 in 5,000 hydrogen atoms is a version known as deuterium which 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 concentration of deuterium is significantly higher, around 1 in 700. Scientists at NASA’s Goddard Space Flight Center who reported the discovery in 2015 said it could be used to calculate the amount of water. that Mars once had. Mars likely started with a deuterium to hydrogen ratio similar to Earth’s, but the fraction of deuterium increased over time as water evaporated and hydrogen was lost in space, because heavier deuterium is less likely to escape the atmosphere.

The problem with this story, said Renyu Hu, a scientist with NASA’s Jet Propulsion Laboratory and another author of the current science article, is that Mars did not lose hydrogen quickly enough. Measurements made by NASA’s Mars Atmosphere and Volatile Evolution orbiter, or MAVEN, have shown that the current rate, extrapolated over 4 billion years, “may only represent a small fraction of the water loss.” Hu said. “This is not enough to explain the great drying up of Mars.”

This led to new research concluding that a large majority of the water went into 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,” wrote Geronimo Villanueva, one of the NASA scientists who have carried out the previous measurements of deuterium, in an e-mail. “This opens up the possibility of an even wetter past, and rocks on Mars now hold more water than we initially thought.”

Water, however, would probably not be of much use to Earth’s settlers. “The amount of water that is in a rock is very small,” said Scheller.

To release water trapped in minerals, they must be heated to high temperatures. “We kind of had to bake a very large amount of stone to have anything that would be useful,” Scheller said.

Elon Musk, the founder of SpaceX who dreams of sending colonists to Mars one day, considered detonating nuclear bombs on Mars to melt the ice caps and warm the planet, making it more hospitable. These explosions would also release some of the water in the hydrated minerals, although Scheller declined to speculate on the amount.

Michael Meyer, senior scientist for NASA’s Mars Exploration Program, said, “I’ll just mention that bombing a planet is usually 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 rocks into the mantle, where they melt, and then molten rock – and water – rises through volcanoes. On Mars, volcanism, like liquid water, seems to be long gone.

This article originally appeared in The New York Times.

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