Hydrogen chloride on Mars


The ExoMars Trace Gas Orbiter examines the composition of the atmosphere on Mars. (Image: ESA)

Even if Mars is one of the best-researched planets in the solar system – knowledge about its atmosphere is still very sketchy. The ESA ExoMars Trace Gas Orbiter space probe has now discovered a completely new class of gases in the Martian gas envelope: It has detected the gas hydrogen chloride (HCl) for the first time. Apparently, this halogen compound is formed when salty dust is blown into the atmosphere during storms, as the time course of the HCl measurement data suggests. However, it is still unknown why the hydrogen chloride is then broken down again surprisingly quickly.

The atmosphere of a planet can reveal a lot about the chemical processes taking place on its surface and below. The existence of life and biological activity can also make itself felt through the release of certain gases – this is one of the reasons why the possible detection of phosphine in the Venusian atmosphere recently caused a sensation. But of all places on our neighboring planet Mars, measurement data and observations from Mars probes keep raising new questions. The oxygen content of the Martian gas envelope seems to fluctuate in a hitherto inexplicable way and the gas methane also shows abrupt local and global changes for which there are no clearly apparent causes.

First detection of a halogen gas

In this respect, it is hardly surprising that planetary researchers have now discovered a class of gas in its atmosphere that is completely new to Mars. “We discovered hydrogen chloride on Mars for the first time. This is also the first finding of a halogen gas in the Martian atmosphere, ”reports co-author Kevin Olsen from the University of Oxford. Hydrogen chloride (HCl) is a highly corrosive gas that can turn into hydrochloric acid when it comes into contact with water. On earth, this gas is produced in small quantities during volcanic eruptions, but above all when salty sea spray is hurled into the atmosphere, where it is photochemically converted into hydrogen chloride. If the gas rises into the stratosphere, it can release highly reactive chlorine radicals that contribute to ozone depletion. Hydrogen chloride has also been detected in the gas envelope of Venus, but not on Mars.

But now measurement data from the ExoMars Trace Gas Orbiter (TGO) space probe from the European space agency ESA and the Russian Roskomos space agency have detected this gas in the Martian gas envelope for the first time. “The discovery of the first new trace gas in the Martian atmosphere is an important milestone for the TGO mission,” says Håkan Svedhem from ESA. “This is the first new gas class that has been determined since the discovery of methane by the ESA Mars Express probe in 2014.” The spectral signal of the hydrogen chloride was shown in data that the two spectrometers of the probe from summer 2018 measured into spring 2019. During this time, they detected an average of one to two parts per billion (ppb) at altitudes between 15 and 25 kilometers in the northern hemisphere of the planet. In the southern hemisphere it was two to three ppb at an altitude of 20 to 30 kilometers. “This means that the concentrations of hydrogen chloride on Mars are comparable to those in the upper stratosphere and mesosphere of the earth,” explains the team led by Olsen and first author Oleg Korablev from the Russian Space Research Institute in Moscow.

Where does the hydrogen chloride come from?

What is surprising about these values, however, is that the hydrogen chloride does not seem to come from some local sources, as is expected for volcanic outgassing, for example. Instead, the gas was found at the same time in very distant places and was distributed almost globally. In addition, other compounds typical for volcanic emissions were missing, as Korablev and his colleagues explain. This raises the question of where the hydrogen chloride could have come from. The researchers found a possible answer in the course of the HCl detection: The first signatures appeared on Mars in the summer of 2018 with the start of the global dust storm. At the end of the storm, the level of hydrogen chloride also fell until it was barely detectable. “This suggests that physical or chemical processes in the Martian storms cause the gas to be released from the blown dust,” the scientists write.

Specifically, the team suspects that salty dust and perchlorates are thrown into the Martian atmosphere during the dust storms. Similar to the sea spray on earth, reactions then take place in the atmosphere through which the salts become hydrogen chloride. “You need steam to release the chlorine and you need the by-products of water – hydrogen – so that hydrogen chloride can form. Water is essential in this chemical reaction, ”says Olsen. It could be fitting that the gas appeared when it was summer in the southern hemisphere of Mars and therefore more water ice was evaporating. So far, however, it has not been possible to explain why the hydrogen chloride disappeared so unexpectedly quickly from the Martian atmosphere after the dust storms ended. “This loss indicates an unexpected chemical sink for this gas,” the researchers state. “The temporary nature of hydrogen chloride tells us that we are not yet seeing the full picture – and that we are missing out on an important breakdown process for the chloride.” Its chemical cycle now needs to be understood. Further measurements, laboratory tests and global atmospheric simulations should now help.

Source: Oleg Korablev (Space Research Institute (IKI), Moscow) et al., Science Advances, doi: 10.1126 / sciadv.abe4386

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