What happened to Mars’s water?

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Carla Jiménez (4ºESO B) y Laura Mayoral (4ºESO B)

Billions of years ago, the Red Planet was far more blue. According to evidence still found on the surface, abundant water flowed across Mars and formed pools, lakes, and deep oceans. The question, then, is where did all that water go?

The answer is nowhere. Stated in new research from Caltech and JPL, a significant portion of Mars’s water (between 30 and 99 percent) is trapped within minerals in the planet’s crust (outermost layer). The research challenges the current theory that the Red Planet’s water escaped into space.

The Caltech/JPL team found that around four billion years ago, Mars was home to enough water to have covered the whole planet in an ocean about 100 to 1,500 meters deep, a volume roughly equivalent to half of Earth’s Atlantic Ocean. But, a billion years later, the planet was as dry as it is today. Previously, scientists seeking to explain what happened to the flowing water on Mars had suggested that it escaped into space as a result of Mars’s low gravity. Though some water did indeed leave Mars this way, it now appears that such an escape cannot account for most of the water loss.

«Atmospheric escape doesn’t fully explain the data that we have for how much water actually once existed on Mars,» says Caltech PhD candidate Eva Scheller, who is the lead author of a paper on the research that was published by the journal Science on March 16 and presented the same day at the Lunar and Planetary Science Conference (LPSC).

The team studied the quantity of water on Mars over time in all its forms (vapor, liquid, and ice) and the chemical composition of the planet’s current atmosphere and crust through the analysis of meteorites as well as using data provided by Mars rovers and orbiters.

The lighter-weight hydrogen has an easier time escaping the planet’s gravity into space than its heavier counterpart. Because of this, the escape of a planet’s water via the upper atmosphere would leave a telltale signature on the ratio of deuterium to hydrogen in the planet’s atmosphere: there would be an outsized portion of deuterium (chemical substance) left behind.

However, the loss of water solely through the atmosphere cannot explain both the observed deuterium to hydrogen signal in the Martian atmosphere and large amounts of water in the past. Instead, the study proposes that a combination of two mechanisms — the trapping of water in minerals in the planet’s crust and the loss of water to the atmosphere — can explain the observed deuterium-to-hydrogen signal within the Martian atmosphere.

«Atmospheric escape clearly had a role in water loss, but findings from the last decade of Mars missions have pointed to the fact that there was this huge reservoir of ancient hydrated minerals whose formation certainly decreased water availability over time,» says Ehlmann.

«All of this water was isolated fairly early on, and then never cycled back out,» Scheller says.

Ehlmann, Hu, and Yung previously collaborated on research that seeks to understand the habitability of Mars by tracing the history of carbon, since carbon dioxide is the principal component of the atmosphere. Next, the team plans to continue to use isotopic and mineral composition data to determine the fate of nitrogen and sulfur-bearing minerals. In addition, Scheller plans to continue examining the processes by which Mars’s surface water was lost to the crust using laboratory experiments that simulate Martian weathering processes, as well as through observations of ancient crust by the Perseverance rover.

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