A previously undetected chemical reaction has been detected on Mars


The Martian South Pole.

The Martian South Pole.
Illustration: ESA / DLR / FU Berlin / Bill Dunford

For the first time, scientists have detected hydrogen chloride on Mars. The source of this colorless gas remains unclear, but current theories include volcanic activity or a previously undetected chemical cycle linked to the Red Planet’s epic dust storms.

New investigation published today in Science Advances is the first to document hydrogen chloride (HCl) and its associated chemistry with chlorine in the Martian atmosphere. This is the first detection of a new class of molecule on Mars since methane, a possible biological signature, was discovered in 2004. Hydrogen chloride is not associated with life (quite the opposite, actually), but, like methane, its presence on Mars is now a question that needs an answer.

Kevin Olsen, a co-author of the study and a research scientist at the Department of Physics at the University of Oxford, says there are two possibilities: either the gas is produced by magmatic activity below the surface. or through complex chemical interactions involving surface dust and atmospheric gases. Whichever is correct, it will be an exciting result.

“If evidence grows for the proposed chemical cycle linking minerals in surface dust with gases in the atmosphere, this will be the first known direct link between the surface and the atmosphere, in addition to ice formation,” Olsen explained. in an email. “On the other hand, if some type of ventilation is determined to be the source of HCl, such as volcanoes or other magmatic degassing, this is one of the first evidence of active geological processes that has been found.”

Graph of the proposed chlorine cycle on Mars.

Graph of the proposed chlorine cycle on Mars.
Picture: THAT

In fact, NASA’s InSight probe, through its discovery of marsquakes, has suggested the presence of unknown geological processes on Mars. The aforementioned methane discovery also points to unknown geological, or possibly biological, processes. However, if a chemical cycle involving surface materials and atmospheric gases were involved with HCl, it would still represent a great victory for science. Y ExoMars Trace Gas Orbiter (TCO), as this is precisely the kind of thing it was designed for.

TCO, which was used to detect HCl on Mars, is a joint mission of the European Space Agency and Russia Roscosmos, and it has been in orbit around Mars since 2016. The main goal of the ExoMars project is to catalog rare gases in the lower atmosphere of Mars, things like water vapor, nitrogen dioxide, acetylene and methane. The discovery and potential interaction of these and other compounds could provide evidence of previously undetected chemical processes occurring in Mars. The reported discovery of HClTherefore, it represents a dunk for the Trace Gas Orbiter.

Data collected by TCO’s atmospheric chemistry suite The spectrometer revealed a spectral sequence consistent with HCl. The team detected “multiple spectral features, a pattern of strengths and characteristic positions” that allowed them “to identify HCl unmistakably,” said Oleg Korablev, a planetary scientist at the Moscow Space Research Institute and first author of the study. in an email. “We even recognized two isotopes with different atomic weights of Cl, 35Cl and 37Cl,” he added.

HCl, a very important gas in Earth’s atmosphere, is invisible at room temperature, but produces white fumes of hydrochloric acid when it comes in contact with atmospheric water vapor.

“Near the surface, it forms from evaporated seawater and is linked to the formation of acids, and in the upper atmosphere it plays a role in ozone depletion,” Olsen said. “It is also emitted by volcanoes, which is why we have been looking for it on Mars, a sign that there is active volcanic activity. But we do not believe that volcanoes are the cause of what we have seen. WWe think there is another atmospheric chemistry at play. “

Olsen and his colleagues suspect this because the behavior of HCl and water vapor appear to be related. This water vapor comes from the ice sheet of the south pole, which, during the Martian summer in the southern hemisphere, leaks evaporated water into the atmosphere. And indeed, HCl was detected in April 2019, which is late summer in the Martian southern hemisphere.

“Our observations are of the effects that the seasonal freeze-thaw cycle of the polar ice caps has on the atmosphere and climate of Mars,” Olsen said.

Importantly, HCl signatures were also detected during a epic dust storm that engulfed the planet in 2018, the same storm that occurs once in a decade that permanently knocked out NASA’s Opportunity rover. The global dust storm resulted in a temporary greenhouse effect, pulling water from near the surface to higher altitudes. These were the conditions, “a hot, dusty and humid atmosphere,” that may have led to the formation of HCl on Mars, Olsen said. But as the scientists observed over the following year, HCl formation “can occur under regular seasonal dusty conditions,” he noted..

At the same time, evidence for a volcanic origin of HCl remains weak.. Others and “more abundant expected volcanic gases” such as sulfur dioxide“They are not detected on Mars,” Korablev said. “The distribution of our detections on the planet is not compatible with any local source around which HCl is concentrated”, while NASA’s InSight lander “found seismic activity on Mars to be low.” All these facts, he said, “are at odds with the volcanic origin of HCl.”

However, strangely, the HCl disappears quickly. It was detected during and after the global dust storm. and also during the dusty season, but then it disappears and the investigators don’t know why.

“Our understanding of how HCl behaves doesn’t explain this,” said Olsen. “It won’t condense or freeze like carbon dioxide or water, it shouldn’t break down that fast, and there’s too much to move somewhere our instruments won’t measure. We expect there to be interactions with solid dust and ice particles, but how HCl can be removed from the atmosphere as quickly as we see it is a mystery, ”he said.

That HCl exists on Mars is not a big surprise, since perchlorates (a different chlorine compound), he found in 2008, it hinted at the presence of this gas. If the researchers are correct about a chemical source of HCl, and if chlorine circulates between the mineral and gas phases, “then this will have an impact on the formation of perchlorate, but we have yet to see how much,” Olsen said. To which he added: “HCl is also very reactive and plays an important role in Earth’s atmosphere, and we are seeing it at much higher levels than anticipated, so it will affect the way we view and model chemistry. of the Martian atmosphere “.

The team now hopes to analyze TCO data collected during the following Martian year, when no global dust storms appeared. The team will study how the appearance and disappearance of HCl are related to atmospheric dust and vapor. and the potential ingredients involved in the proposed gas-mineral reaction. At the same time, the team also expects “new developments in atmospheric chemistry models and laboratory studies related to the chemistry of chlorine on Mars,” Korablev said.

We like to think of Mars as the best alternative to Earth, but studies like this are a reminder of how inhospitable and strange this place is. Some really strange chemistry is going on, with no clear analogues to the processes seen on Earth. Mars, with no running water on the surface, a painfully thin atmosphere filled with carbon dioxide, and extremely fluctuating temperatures, is home to exotic processes that we are struggling to understand. Suffice it to say, we won’t be living there anytime soon.

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