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Mars geological documentation suggests the planet once experienced a humid climate that may have helped support extraterrestrial life. But much of its water currently exists as ice, at the poles or below the surface. Compared to the past, today’s thinner, cooler, drier Martian atmosphere cannot withstand liquid water.
The isotopic evidence shows a high ratio of deuterium to hydrogen in the planet’s atmosphere, which has led scientists to conclude that water vapor undergoes photodissociation reactions in the lower atmosphere below the minimum of water vapor, called a hygropause. Most of the molecular hydrogen product, containing either hydrogen or deuterium, then diffuses through that barrier into the upper atmosphere and eventually escapes into space; hydrogen leaves more easily than heavier deuterium.
Now Shane Stone and Roger Yelle of the University of Arizona at Tucson and their colleagues have compiled observations and conducted simulations that suggest that water may have left the atmosphere of Mars more quickly than previously thought. Recent data from NASA EXPERT OF Spacecraft show seasonal variations in the abundance of water in the upper atmosphere that are punctuated by waves of water during dust storms.
The figure below shows an increase in the abundance of water in the upper atmosphere during a global dust storm in 2018; the amount of molecular hydrogen, however, remains constant (not shown). The high abundance of water vapor in the upper atmosphere indicates a weakening of the hygro pause.
To better understand how water vapor moves through the atmosphere and eventually escapes, Yelle and her graduate student Daniel Lo used the EXPERT OF data as input into a one-dimensional photochemical model they built. Previous simulations have reported that neutral photolysis reactions dissociate water in the lower atmosphere to produce molecular hydrogen. The H2 it spreads into the upper atmosphere, where ions break it down into atomic hydrogen which then escapes into space. But with the help of EXPERT OF data, the new model shows that water is transported directly to the upper atmosphere, where it quickly reacts with ionized chemical species to form atomic hydrogen.
Calculations of the various reaction rates indicate that before the water is destroyed, it lasts for only four hours or so in the upper atmosphere, which is about an order of magnitude less than the photolysis of water in the middle atmosphere. The discovery means that water vapor can escape from the atmosphere in the form of atomic hydrogen faster than previously thought. According to estimates by Stone and his colleagues, a single sandstorm could have caused more hydrogen loss than would have happened in an entire average Martian year. (SW Stone et al., Science 370, 824, 2020.)
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