For the first time, scientists have seen hydrogen chloride on Mars. The source of this colorless gas remains unclear, but ongoing theories include volcanic activity or a previously undetected chemical cycle related to the Red Planet’s epic dust storms.
New research published today in Science Advances is the first to document hydrogen chloride (HCl) and its associated chlorine chemistry in the Martian atmosphere. This is the first detection of a new class of molecules on Mars since methane – a potential biosignature – found in 2004. Hydrogen chloride is not associated with life (quite the opposite, in fact), but, like methane, its presence on Mars is now a question that needs an answer.
Kevin Olsen, co-author of the study and a researcher in the Department of Physics at Oxford University, says there are two possibilities: Either the gas is produced by magmatic activity beneath the surface or through complex chemical interactions involving surface dust and atmospheric gases. Any of these is correct, it will be an interesting result.
“If evidence of the proposed chemical cycle linking surface dust minerals to atmospheric gases increases, this will be the first known direct link between the surface and the atmosphere, other than ice formation,” Olsen said in an email. “On the other hand, if it is established that some kind of aeration is the source of HCl – such as volcanoes or other magmatic aspirations – then this is among the first evidence of active geological processes that have been found.”
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Indeed, NASA’s InSight spacecraft, through its discovery marches, suggested the presence of unknown geological processes on Mars. The above-mentioned discovery of methane also indicates unknown geological – or possibly biological – processes. If a chemical cycle involving surface materials and atmospheric gases were involved with HCl, however, it would still be a great gain for science. and ExoMars Trace Gas Orbiter (TCO) because this is exactly the kind of thing it was designed to detect.
TCO, which was used to detect HCl on Mars, is a joint mission of the European Space Agency and Russia Roscosmos and has been orbiting Mars since 2016. The main goal of the ExoMars project is to catalog rare gases in Mars’ lower atmosphere – things like water vapor, nitrogen dioxide, acetylene and methane. The discovery and potential interaction of these compounds and other compounds could provide evidence of previously undetected chemical processes. Mars. Reported discovery of HCltherefore, it is a slam dunk for Trace Gas Orbiter.
Data collected by TCO 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 “unmistakably identify HCl,” said Oleg Korablev, a planetary scientist at the Moscow Space Research Institute and the first author of the study, in an email. “We even recognized two isotopes with atomic weights other than Cl, 35Cl and 37Cl,” he added.
HCl – a very important gas in the Earth’s atmosphere – is invisible at room temperature, but produces white vapors of hydrochloric acid when in contact with atmospheric water vapor.
“Near the surface, it is made up of evaporated seawater and is linked to acid formation, and in the upper atmosphere it plays a role in destroying ozone,” Olsen said. “It is also emitted by volcanoes, which is why we looked for Mars – a sign that there is active volcanic activity. But, we don’t think volcanoes are the cause of what we’ve seen. WI think there is another atmospheric chemistry at play. “
Olsen and colleagues suspect this because the behavior of HCl and water vapor seems to be related. This water vapor comes from the southern polar cap, which, during the Martian summer in the southern hemisphere, drains water evaporated into the atmosphere. And indeed, HCl was detected in April 2019, which is the end of summer in the southern Martian hemisphere.
“Our observations are about the effects that the seasonal freeze-thaw cycle of polar ice caps has on the atmosphere and climate of Mars,” Olsen said.
Importantly, HCl signatures were also detected during a the epic dust storm which enveloped the planet in 2018 – the same storm that occurred in a decade permanently beaten NASA’s Opportunity Rover. The global dust storm resulted in a temporary greenhouse effect, drawing water near the surface to higher altitudes. These were the conditions, “a warm, dusty and humid atmosphere,” that could have led to the formation of HCl on Mars, Olsen said. But, as scientists noted the following year, HCl formation “can occur in regular, seasonal dust conditions,” he noted..
At the same time, evidence of a volcanic origin for HCl remains weak. Other “and probably more abundant” volcanic gases, such as sulfur dioxide“They are not detected on Mars,” Korablev said. “The distribution of our detections on the planet does not support any local source around which HCl is concentrated.” while NASA’s InSight lander “found that seismic activity on Mars is low.” All of these facts, he said, “are at odds with the volcanic origin of HCl.”
Strangely, however, HCl disappears quickly. It was observed during and after the global dust storm and also during the dusty season, but then disappeared and researchers do not know why.
“Our understanding of how HCl behaves does not explain this,” Olsen said. “It will not condense and freeze like carbon dioxide or water, it should not decompose so quickly and there is too much of it to move somewhere where our instruments do not 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.
The fact that HCl exists on Mars is not a huge surprise, because perchlorates (a compound other than chlorine), found in 2008, suggested the presence of this gas. If the researchers are right about a chemical source for HCl and if chlorine circulates between the mineral and gaseous phases, “then this will have an impact on the formation of perchlorate, but we still haven’t seen how much,” Olsen said. He added: “HCl is also very reactive and plays important roles in the Earth’s atmosphere and we see it at much higher levels than expected, so it will impact the way we look at and shape the chemistry of the Martian atmosphere.”
The team is now looking forward to the TCO data collected next March, when no global dust storm occurred. The team will study how the appearance and disappearance of HCl is related to dust and atmospheric vapors and the potential ingredients involved in the proposed gas-mineral reaction. At the same time, the team also expects “new developments in atmospheric chemistry modeling and laboratory studies related to chlorine chemistry on Mars,” Korablev said.
We like to think of Mars as the next best thing for Earth, but studies like this remind us how inhospitable and alien this place really is. There is a very strange chemistry there, with no clear analogies to the processes seen on Earth. Mars, with no running water on the surface, a painful and thin atmosphere, filled with carbon dioxide and fluctuating wild temperatures, hosts exotic processes that we strive to understand. Suffice it to say, we won’t be living there any time soon.