Researchers have reported analysis of data collected over the last few years by the ExoMars orbiter, a joint mission by the European Space Agency and their Russian counterpart, Roscosmos. The observations suggest the possible presence of more water reservoirs than expected on Mars, as well as the first observation of hydrogen chloride in the atmosphere of the red planet.
Both studies are published in Science Advances. In Transient HCl in the atmosphere of Mars, Oleg Korablev and colleagues report the observation of hydrogen chloride. This halogen gas was identified in observations collected in April 2019, during the late southern hemisphere summer. That particular season is notorious for the presence of impressive dust storms, some of which could become global. The one that killed Opportunity in 2018 was one of these.
The observations of hydrogen chloride were collected during such a particularly intense storm. For this reason, the team theorizes that the dust storm might be playing a crucial role in the presence of the halogen gas in the atmosphere. A possible mechanism sees water released by ice melting interacting with sodium chloride (table salt) lifted by the wind from the dust. As the two molecules interact, hydrogen chloride can form. This might stay in the atmosphere for a while, or fall back to the ground attached to dust. Further dust storms can kick it back up again.
Another possible alternative is that the hydrogen chloride comes from magmatic activity. Mars is home to the tallest volcanos in the solar system, but as far as we know, they are extinct. So magma activity is not impossible, but more evidence should be collected before this can be considered as a viable mechanism.
“It is incredibly rewarding to see our sensitive instruments detecting a never-before-seen gas in the atmosphere of Mars,” Korablev, principal investigator of the Atmospheric Chemistry Suite instrument that made the discovery, said in a statement. “Our analysis links the generation and decline of the hydrogen chloride gas to the surface of Mars.”
The second study looked at the water in the atmosphere, and at a measurement called the isotope ratio. Chemical elements come in different isotopes – these have all the same chemical properties, but a different number of neutrons in the nucleus, which changes their physical properties a bit. Hydrogen has a heavier isotope called deuterium. If you’re heard the term heavy water, related to nuclear power plants, what is meant is a water molecule with deuterium atoms instead of regular old hydrogen.
The ratio between deuterium and hydrogen in water is a very useful tool. This was used on Earth to establish that most of our planet's water did not come from comets. This method is also being employed by ExoMars to understand the water cycle on Mars.
“The deuterium to hydrogen ratio, D/H, is our chronometer – a powerful metric that tells us about the history of water on Mars, and how water loss evolved over time. Thanks to the ExoMars Trace Gas Orbiter, we can now better understand and calibrate this chronometer and test for potential new reservoirs of water on Mars,” explained Geronimo Villanueva of NASA’s Goddard Space Flight Center and lead author of the new paper.
The data was collected between April 2018 and April 2019 and tracked significant events such as the storms we mentioned above, but also the thawing of the southern polar ice cap during summer. The team observed significant changes in the isotope ratio. Most of these are probably due to the behavior of heavy water vapor, but the team doesn’t discount the possibility that multiple reservoirs of water with specific isotope ratios exist on Mars today.