By combining observations from three international spacecraft at Mars, scientists were able to show that regional dust storms play a huge role in drying out the Red Planet.

Dust storms heat up higher altitudes of the cold Martian atmosphere, preventing water vapor from freezing as usual and allowing it to reach farther up. In the higher reaches of Mars, where the atmosphere is sparse, water molecules are left vulnerable to ultraviolet radiation, which breaks them up into their lighter components of hydrogen and oxygen. Hydrogen, which is the lightest element, is easily lost to space, with oxygen either escaping or settling back to the surface.

“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,” said Michael S. Chaffin, a researcher at the Laboratory for Atmospheric and Space Physics at the University of Colorado at Boulder. “So when you’ve lost a hydrogen atom, you’ve definitely lost a water molecule.”

Scientists have long suspected that Mars, once warm and wet like Earth, has lost most of its water largely through this process, but they didn’t realize the significant impact of regional dust storms, which happen nearly every summer in the planet’s southern hemisphere. Globe-enveloping dust storms that strike typically every three to four Martian years were thought to be the main culprits, along with the hot summer months in the southern hemisphere when Mars is closer to the Sun.

But the Martian atmosphere also gets heated during smaller, regional dust storms, according to a new paper published August 16 in the journal Nature Astronomy. The researchers, an international team led by Chaffin, found that Mars loses double the amount of water during a regional storm as it does during a southern summer season without regional storms.

“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,” said Geronimo Villanueva, a Martian water expert at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and co-author on Chaffin’s paper.

A dust tower on Mars
The yellow-white cloud in the bottom-center of this image is a Mars "dust tower" — a concentrated cloud of dust that can be lofted dozens of miles above the surface. The blue-white plumes are water vapor clouds. Olympus Mons, the tallest volcano in the solar system, is visible in the upper left corner, while Valles Marineris cab be seen in the lower right. Taken on Nov. 30, 2010, the image was produced by NASA's Mars Reconnaissance Orbiter's Mars Color Imager.