Despite decades of exploration and study, Mars still has its fair share of mysteries. In particular, scientists are still trying to ascertain what happened to the water that once flowed on Mars’ surface. Unfortunately, billions of years ago, the Martian atmosphere began to be stripped away by solar wind, which also resulted in the loss of its surface water over time – although it was not entirely clear where it went and what mechanisms were involved.

To address this, a team of scientists recently consulted data obtained by three orbiter missions studying the Martian atmosphere. In the process, they found evidence that the smaller regional dust storms that happen almost annually on Mars are making the planet drier over time. These findings suggest that storms are a major driving force behind the evolution of Mars’ atmosphere and its transition to the freezing and desiccated place we know today.

Dust storms are a regular occurrence on Mars and occur whenever the lower atmosphere heats up, which causes air currents to pick up dust and circulate it around the planet. This can occur when Mars is at the closest point in its orbit to the Sun (perihelion) and can also be exacerbated due to variations in temperature between the hemispheres – when one of them is experiencing summer, atmospheric circulation can dramatically increase.

These dust storms have the effect of heating the upper areas of Mars’ sparse atmosphere, preventing water molecules from freezing as they normally would and forcing them to rise even higher. In these highest reaches of the Martian atmosphere, water molecules are vulnerable to ultraviolet radiation, which causes them to undergo chemical disassociation and break into their constituent elements – hydrogen and oxygen.

Whereas the oxygen (the heavier element) will either escape to space or settle back to the surface, the hydrogen is easily lost to space. Michael S. Chaffin, a researcher at the Laboratory for Atmospheric and Space Physics at the University of Colorado at Boulder, was also the lead author on the study. As he said in a recent NASA press release:

“All you have to do to lose water permanently is to lose one hydrogen atom because then the hydrogen and oxygen can’t recombine into water. So when you’ve lost a hydrogen atom, you’ve definitely lost a water molecule.”

Scientists have long suspected that Mars lost most of its water due to dust storms but did not realize the significance of regional storms. These happen nearly every summer in the planet’s southern hemisphere, whereas the larger storms (which can encompass the entire planet) typically happen once every three to four Martian years – the equivalent of about five and a half to seven and a half Earth years.

Image taken by the MRO on Nov. 30, 2010, showing a “tower tower” (bottom center), a concentrated cloud of dust that can be lofted dozens of miles above the surface. The blue-white plumes are water vapor clouds. Credits: NASA/JPL-Caltech/MSSS

Previousily, these massive storms and the hot summer months in the southern hemisphere (when Mars is closer to the Sun) were thought to be the main drivers. However, after consulting data obtained by the three Mars orbiters, Chaffin and his colleagues found that Mars loses about twice the amount of water during a regional storm as it does during summer in the southern hemisphere without regional storms.

This data comes from NASA’s Mars Reconnaissance Orbiter (MRO), the ESA’s Trace Gas Orbiter (TGO), and the Mars Atmospheric and Volatile EvolutioN (MAVEN) orbiter. Geronimo Villanueva, a Martian water expert at NASA’s Goddard Space Flight Center (and a co-author on the paper), was also a member of the Trace Gas Orbiter’s science team. As he explained:

“This paper helps us virtually go back in time and say, ‘OK, now we have another way to lose water that will help us relate this little water we have on Mars today with the humongous amount of water we had in the past… The instruments should all tell the same story, and they do.”

Since water is one of the key ingredients for life as we know it, scientists are very interested in determining where it went and how long it existed on the surface of Mars. Essentially, they
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