An article published in the journal “Science” reports a study that explains where most of the water that the planet Mars possessed when it was young, when it was in the liquid state on its surface, has gone. A team of researchers led by Shane Stone of the University of Arizona, USA, used data collected by NASA’s MAVEN space probe to track the movements of water in the atmosphere, up to high altitudes, where there are reactions that break it down and produce atomic hydrogen that is dispersed in space. This study highlighted the role of dust storms in water loss.

Reconstructions of the history of Mars clearly indicate that liquid water used to flow in abundance on its surface, but an environmental collapse slowly transformed it into the present-day red planet. Water still exists underground, but the amount discovered is very small to justify the geological traces that still exist after a few billion years. The MAVEN space probe, which entered Mars orbit on September 22, 2014, made it possible to collect a wealth of information on the phenomena taking place in the Martian atmosphere, which today is very thin but at the time when the planet was similar to Earth was far denser. That information already made it possible to understand how the atmosphere slowly disperses into space and now this new research offers an explanation for the loss of water as well.

The models had already suggested the theory of the dispersion of the water in the atmosphere and its separation into its atoms with the subsequent loss of hydrogen into space. However, data collected by the MAVEN space probe, and in particular its NGIMS (Neutral Gas and Ion Mass Spectrometer) instrument, show traces of water at higher altitudes than predicted by the existing models. In essence, the data indicate that water reaches the high atmosphere and reacts there with ions that generate atomic hydrogen that disperses into space.

An important discovery is that the upward water transport process is accentuated when there are dust storms, which on Mars can be even global and cover the entire planet. Regional storms also have their importance because they’re a seasonal and therefore recurring phenomenon. Storms contribute to the atmosphere’s warming that leads to an upward movement of water, increasing its reactions with the ions present in the higher layers and the subsequent dispersion of hydrogen.

The image (University of Arizona/Shane Stone/NASA Goddard/Dan Gallagher) shows a depiction of water in Mars’ atmosphere with peaks during periods of both regional and global dust storms.

This discovery greatly changes the existing models, which predicted that there was a very slow and above all constant loss of water on Mars. Shane Stone explained that he and his colleagues didn’t expect to find traces of water in the high atmosphere at all. There the reactions with the ions break it into its atoms in a time that the researchers calculated as up to four hours.

Extrapolating the data to reconstruct the last billion years of water loss, the researchers calculated that this process may explain the loss of a global ocean about 43 centimeters deep. Another 17 centimeters have been lost due to the contribution of global dust storms, which increase the amount of water carried into the high atmosphere by twenty times.

That process probably didn’t work the same way over a billion years ago because at the time the characteristics of the atmosphere were different. Understanding what conditions allowed it to begin is important to reconstruct the water loss process on Mars throughout its history.