Water on Mars May Be Trapped in the Planet’s Crust, Not Lost to Space

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      eridani
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      https://www.scientificamerican.com/article/water-on-mars-may-be-trapped-in-the-planets-crust-not-lost-to-space/

      Mars had water—until it didn’t. Scientists thinks that about four billion years ago, the planet had substantial amounts of liquid water on its surface, enough to form rivers, lakes, seas, and even oceans—and perhaps also to support life. But something happened in the following billion years, triggering the loss of this water from the surface until all that was left was the cold, dry wasteland of a world that we see today. Why and how that happened remains somewhat of a mystery. “We don’t exactly know why the water levels decreased and Mars became arid,” says Eva Scheller of the California Institute of Technology.

      In recent years, results from NASA’s Mars-orbiting MAVEN spacecraft suggested the driver of this water depletion may have been atmospheric loss. Long ago, for reasons unknown, Mars lost its strong magnetic field, exposing the planet to atmosphere-eroding outbursts from the sun. As a result, much of Mars’s air escaped to space, presumably carrying away most of the planet’s water with it. But in a new paper published this week in the journal Science, Scheller and her colleagues argue this process alone cannot explain Mars’s modern-day aridity. Instead they say that a substantial amount of the planet’s water—between 30 and 99 percent—retreated into the crust, where it remains today, in a process known as crustal hydration.

      “That loss [to space] would have to be very large to explain the loss of all of Mars’s water,” said Bethany Ehlmann of Caltech, a co-author on the study, in a press briefing at this week’s virtually hosted Lunar and Planetary Science Conference , where the research was presented. “We realized we needed to pay attention to the evidence from the last 10 to 15 years of Mars exploration in terms of the nature of water in the Martian crust.”

      Crustal hydration—in which water is incorporated into the crystalline structure of minerals—is a natural choice for that explanation. And in fact, it was previously proposed as an important mechanism for Martian water loss. Various lines of evidence convincingly show that the process must have occurred at certain points in the planet’s history. For example, results from a neutron spectrometer instrument on NASA’s Mars Odyssey spacecraft, which arrived at the planet in 2001, showed that, “basically everywhere, the crust had at least 2 percent water,” Ehlmann says. “In the equator, that’s water in soils and rocks.” Later findings from NASA’s Mars Reconnaissance Orbiter corroborated those results, mapping hydrated minerals on the surface of Mars. “It became very clear that it was common, and not rare, to find evidence of water alteration,” she adds.

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