University of Arizona researchers have discovered a surprisingly large amount of water in the upper atmosphere, being rapidly destroyed. Another graduate student, Shane Stone, has analyzed data taken by NASA’s MAVEN mission. He explains: We know that billions of years ago, there was liquid water on the surface of Mars. There must have been a thicker atmosphere, so we know that Mars somehow lost the majority of its atmosphere to space. MAVEN is trying to characterize the processes responsible for this loss, and one portion of that is understanding exactly how Mars lost its water.

As MAVEN orbits Mars, it dips into the planet’s atmosphere every few hours. The onboard Neutral Gas and Ion Mass Spectrometer (NGIMS) instrument measures the abundance of water ions in the upper atmosphere, 100 miles up from the surface. This measurement allows scientists to infer how much water is present.

Mars’ water loss, it turns out, is mostly affected by the seasons on Mars itself. During the Martian summer in the southern hemisphere, Mars is closest to the Sun and warms up accordingly. At this time, more water changes from ice to vapor and rises into the atmosphere. Regional dust storms that occur about every ten years cause even more heating and more water loss.

Stone goes on to further explain: This is important because we didn’t expect to see any water in the upper atmosphere of Mars at all. If we compare Mars to Earth, water on Earth is confined close to the surface because of something called the hygropause. It’s just a layer in the atmosphere that’s cold enough to condense (and therefore stop) any water vapor traveling upward.

The hygropause on Mars, it seems, is too warm to prevent the water vapor from escaping to the upper atmosphere, and once there, the process is simple for breaking the vapor into ions. The research team then extrapolated these findings back one billion years and found that they can account for the loss of a global ocean about 17 inches deep. And those dust storms could have caused the loss of an additional 6.7 inches of ocean depth, which could explain why Mars is so cold and dry compared to Earth.

Prior to one billion years ago, the hygropause on Mars may have been stronger, so there is still more to be learned about the red planet and its relationship with water.

More Information

The University of Arizona press release 

“Hydrogen Escape from Mars Is Driven by Seasonal and Dust Storm Transport of
Water,” Shane W. Stone et al., 2020 Nov. 13, Science