Dust storms starve Mars of water

The geologic record of Mars suggests that the planet once experienced wet weather that may have helped sustain extraterrestrial life. But much of its water presently exists as ice, either at the poles or beneath the surface. Relative to the past, today’s thinner, cooler, drier Martian atmosphere cannot support liquid water.

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 water vapor minimum, called the hygropause. Most of the molecular hydrogen product—containing either hydrogen or deuterium—then diffuses across that barrier to the upper atmosphere and eventually escapes to space; the hydrogen leaves more readily than the heavier deuterium.

Now Shane Stone and Roger Yelle of the University of Arizona in Tucson and their colleagues have compiled observations and conducted simulations that suggest water may have left Mars’s atmosphere more quickly than previously thought. Recent data from NASA’s MAVEN spacecraft show seasonal variations in the abundance of water in the upper atmosphere that are punctuated by surges of water during dust storms.

The figure below shows an increase in the abundance of water in the upper atmosphere during one 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 hygropause.

To better understand how the water vapor moves through the atmosphere and eventually escapes, Yelle and his graduate student Daniel Lo used the MAVEN data as inputs in a one-dimensional photochemical model they constructed. Previous simulations reported that neutral photolysis reactions dissociate water in the lower atmosphere to yield molecular hydrogen. The H₂ diffuses to the upper atmosphere, where ions break it up into atomic hydrogen that then escapes to space. But with the help of the MAVEN data, the new model shows that water is transported directly to the upper atmosphere, where it rapidly reacts with ionized chemical species to form atomic hydrogen.

The calculations of the various reaction rates indicate that before water is destroyed, it has a lifetime of only four hours or so in the upper atmosphere, which is about an order of magnitude shorter than the photolysis of water in the middle atmosphere. The finding means that water vapor may escape the atmosphere in the form of atomic hydrogen faster than previously thought. By Stone and his colleagues’ estimates, a single dust storm could have instigated more hydrogen loss than would happen in an entire average Martian year. (S. W. Stone et al., Science 370, 824, 2020 .)