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The water on Mars disappeared. This could be where it went.

Mars was once wet with the water of an ocean on its surface. Today, most of Mars is as dry as a desert, with the exception of ice deposits in its polar regions. Where’s the rest of the water? Some of it disappeared into space. Water molecules hit by solar wind particles disintegrated into hydrogen and oxygen atoms, and those, especially the lighter hydrogen atoms, raced out of the atmosphere and were lost to space. Sign up for The Morning Newsletter from the New York Times. However, according to a new study, most of the water went into the red planet’s rocks. And there it stays, trapped in minerals and salts. In fact, 99% of the water that once flowed on Mars could still be there, the researchers estimated in an article published this week in Science magazine. Data from the last two decades of robotic missions to Mars, including NASA’s Curiosity Rover and Mars Reconnaissance Orbiter, showed widespread use of what geologists refer to as hydrated minerals. “It became very, very clear that it was common and not uncommon to find evidence of water changes,” said Bethany Ehlmann, professor of planetary science at the California Institute of Technology and one of the authors of the paper. Ehlmann said at a press conference on Tuesday at the Lunar and Planetary Science conference that water molecules are incorporated into minerals such as clays when the rocks are altered by liquid water. “Water is effectively trapped in the crust,” she said. To get a feel for the amount of water, planetary scientists speak of a “global equivalent layer” – that is, if Mars had been smoothed into a uniform, strange sphere, how deep would the water have been? The scientists estimated that the depth would have been 100 to 1,500 meters, or 330 to 5,000 feet. The most likely depth is about 2,000 feet, or about a quarter as much water as in the Atlantic. The data and simulations also showed that water was almost completely gone 3 billion years ago, by the time on Earth when life consisted of single-celled microbes in the oceans. “This means Mars has been dry for a long time,” said Eva Scheller, a Caltech PhD 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 that is mostly frozen in the polar ice caps. Planetary researchers have long marveled at ancient evidence of flowing water carved into the surface of Mars – gigantic canyons, tendrils of winding river channels, and deltas where the rivers ejected sediments into lakes. NASA’s newest robotic Mars explorer, Perseverance, who landed in Jezero Crater last month, will head to a river delta on the river’s edge 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 younger Mars more than 3 billion years ago. But the hydrogen atoms floating in the Martian atmosphere today retain a ghostly breath 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 hydrogen atom of the common species has only one proton, no neutrons.) On Mars, however, the deuterium concentration is significantly higher, about 1 in 700. Scientists at NASA’s Goddard Space Flight Center, who reported the finding in 2015, said it could used to calculate the amount of water Mars once had. Mars probably started out with a similar ratio of deuterium to hydrogen as Earth, but the amount of deuterium increased over time as the water evaporated and hydrogen was lost to space, as the heavier deuterium is less likely to escape the atmosphere. The problem with this story, said Renyu Hu, a scientist at NASA’s Jet Propulsion Laboratory and another author on the current science paper, is that Mars was not losing hydrogen fast enough. Measurements with NASA’s Mars Atmosphere and Volatile Evolution Orbiter (MAVEN) have shown that the current rate extrapolated over 4 billion years “can only account for a small part of the water loss,” said Hu. “This is not enough to explain the great drying up of Mars.” This led to the new investigation that much of the water flowed into the rocks. “This is a very interesting new study that combines many processes to provide alternative scenarios for the fate of water on Mars,” wrote Geronimo Villanueva, one of the NASA scientists who made the earlier deuterium measurements, in an E -Mail. “This opens up the possibility of an even wetter past, and the rocks on Mars now contain more water than we originally thought.” However, the water would probably not be of much use to settlers from Earth. “The amount of water in a rock is very small,” said Scheller. In order to release water trapped in minerals, it must be heated to high temperatures. “We’d have to boil a very large amount of stone to have something that would be helpful,” said Scheller. SpaceX founder Elon Musk, who dreams of one day sending colonists to Mars, has considered detonating atomic bombs on Mars to melt the ice caps and warm the planet and make it more hospitable. These explosions would also release some of the water in the hydrated minerals, though Scheller refused to speculate on how much. Michael Meyer, the lead scientist for NASA’s Mars exploration program, said, “I just want to mention that destroying a planet is usually not a good way to make it more habitable.” On earth, water is also absorbed in rocks, but does not stay there indefinitely. The movement of the earth’s crust pushes stones into the mantle, where they melt, and then the molten stone – and water – comes back through volcanoes. On Mars, volcanism seems to have long since disappeared like liquid water. This article originally appeared in the New York Times. © 2021 The New York Times Company

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