The rains on Mars disappeared. Maybe this is where he went.

Mars was once wet, with ocean 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 rain go?

Some of it has disappeared. Water molecules, pumped by particles of solar wind, broke into hydrogen and oxygen atoms, and those, especially the lighter hydrogen atoms, were lost out of the atmosphere, lost to space. -outside.

But most of the water, a new study concludes, descends, sinking it into the rocks of the red planet. And there it is still, locked in minerals and salts. In fact, as much as 99% of the water that once flowed on Mars could still be present, the researchers estimated in a paper 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, showed a wide spread of what geologists call hydrated minerals.

“It has become very clear that it is common and uncommon to find evidence of water change,” 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 press conference Tuesday at the Lunar and Planetary Science conference, said that as rocks are transformed by melting water, water molecules will be absorbed into minerals like clay. “Water is effectively trapped in the bark,” she said.

To understand the amount of water in it, planetary scientists talk about “equal global coverage” – that is, if Mars were moved out into a featureless uniform ball, what how deep would the water be?

The scientists thought the depth would be between 100 and 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 showed that the water was almost gone 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 a long time,” said Eva Scheller, a Caltech graduate student who was the lead author of the paper Science.

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

Planet scientists have long been amazed at ancient evidence of water flowing carved into Martian surfaces – gigantic canyons, tendrils of meandering river channels and deltas where the rivers deposited sediment for lakes. NASA’s latest robotic explorer, Perseverance, which landed last month in the Jezero crater, will head to a river delta at its edge in hopes of finding traces of a past life.

Without a time machine, there is no way to see exactly how much water there was on younger Mars more than 3 billion years ago. But the hydrogen atoms floating today in the atmosphere of Mars retain a ghostly view of the ancient ocean.

On Earth, there are about 1 in 5,000 atoms of hydrogen called a deuterium that is twice as heavy as its nucleus of both neutrons and protons. (The nucleus of common hydrogen atoms, or neutrons, has only one proton.)

But on Mars, the density of deuterium is much higher, around 1 in 700. Scientists at NASA’s Goddard Space Flight Center who described this discovery in 2015 said that this could be used to measure of water that Mars once had. Mars may have started with the ratio of deuterium to hydrogen to Earth, but the fraction of deuterium increased over time as water emptied and hydrogen was lost to space, since the heaviest deuterium is less. likely to escape the atmosphere.

The problem with that story is that Renyu Hu, a scientist at NASA ‘s Jet Propulsion Laboratory and another author of the conventional Science paper, said that Mars has not been losing hydrogen fast enough. Measurements by NASA’s Mars Atmosphere and Evolution Volatile orbiter, or MAVEN, have shown that the current rate, calculated over 4 billion years, can “only account for a small fraction of the water loss,” he said. Hu. “This is not enough to explain Mars’s massive drought.”

As a result the new research concluded that most of the water entered the rocks.

“This is a very exciting new study in which many processes are combined to provide alternative conditions for how water falls on Mars,” wrote Geronimo Villanueva, a NASA scientist who earlier deuterium measurements, in e-mail. “This opens up the opportunity for an even wetter time, and the rocks on Mars now hold more water than we first thought.”

But the rain may not be of much use to settlers from Earth. “The water level in a rock is very small,” Scheller said.

To release water trapped in minerals they must be heated to a high temperature. “We had to cook some kind of rock to make anything that would be helpful,” Scheller said.

Elon Musk, founder of SpaceX and who dreams of sending settlers to Mars one day, has thought about exploding nuclear bombs on Mars to melt the ice caps and warm the planet , making it more welcoming. These explosions would also release some of the water in the hydrated minerals, although Scheller declined to estimate the level.

Michael Meyer, lead scientist for NASA ‘s Mars study program, said, “I’ll say that building a planet is not usually a good way to make it more inhabited. ”

On Earth, water is also trapped in rocks, but it never lives there. The movement of the Earth’s crust pushes rocks down into the mantle, where they melt, and then the molten rock – and water – comes back up through volcanoes. On Mars, volcanism, like melting water, seems to be long gone.