For the first time, scientists have seen hydrogen chloride on Mars. The source of this colorless gas remains unclear, but current theories include volcanic activity or a previously undiscovered 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 the 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 – was discovered in 2004. Hydrogen chloride is not associated with life (quite the contrary, actually), but like methane, its presence on Mars is now a question to be answered.
Kevin Olsen, a co-author of the study and a research scientist from the Department of Physics at Oxford University, says there are two possibilities: Either the gas is produced by magmatic activity below the surface. or through complex chemical interactions with surface dust and atmospheric gases. Whichever one is right, it will be an exciting result.
“If there is evidence of the proposed chemical cycle linking minerals in the surface dust with gases in the atmosphere, it will be the first known direct link between the surface and the atmosphere, other than ice formation,” Olsen explains in an email. “On the other hand, if some type of venting is found to be the source of HCl – such as volcanoes or other magmatic outgassing – then this is one of the first evidence of active geological processes to be found.”
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Indeed, NASA’s InSight probe, through its discovery of Marsquakes, has suggested the presence of unknown geological processes on Mars. The aforementioned discovery of methane also points to unknown geological – or possibly biological – processes. However, should a chemical cycle of surface materials and atmospheric gases be involved in HCl, that would still be a big win for science and the ExoMars Trace Gas Orbiter (TCO), because this is just the kind of thing 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 it has been in orbit around Mars since 2016. The primary 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 possible interplay of these and other compounds could provide evidence of previously undiscovered chemical processes taking place Mars. The reported discovery of HCltherefore represents a slam dunk for the Trace Gas Orbiter.
Data collected by TCO’s Atmospheric Chemistry Suite spectrometer revealed a spectral sequence consistent with HCl. The team discovered “multiple spectral features, a pattern of characteristic strengths and positions” that enabled them to “unmistakably identify HCl,” said Oleg Korablev, a planetary scientist at the Space Research Institute in Moscow and the study’s lead author. 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 the Earth’s atmosphere – is invisible at room temperature, but it produces white vapors of hydrochloric acid when it comes into contact with atmospheric water vapor.
“Near the surface, it is formed from evaporated seawater and is linked to acid formation, and in the upper atmosphere it plays a role in the destruction of ozone,” said Olsen. “It is also emitted by volcanoes, which is why we looked for it on Mars – a sign that there is active volcanic activity. But we don’t think volcanoes are the cause of what we’ve seen. W.We think there are other atmospheric chemistries involved. “
Olsen and his colleagues suspect this because the behavior of HCl and water vapor appears to be related. This water vapor comes from the south polar ice cap, which leaks evaporated water into the atmosphere during Mars summer in the southern hemisphere. Indeed, the HCl was detected in April 2019, which is late summer in the southern hemisphere of Mars.
“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,” said Olsen.
Importantly, the HCl signatures were also detected during a epic dust storm that enveloped the planet in 2018 – the same one-time storm that is permanent exhausted NASA’s Opportunity Rover. The global dust storm resulted in a temporary greenhouse effect, drawing water from near the surface to higher altitudes. These were the conditions, “a warm, dusty, and humid atmosphere,” that may have led to the formation of HCl on Mars, Olsen said. But as the scientists noted over the following year, HCl formation “can take place under normal, seasonally dusty conditions,” he noted..
At the same time, evidence of a volcanic origin for HCl remains weak. Other and “expected to be more abundant volcanic gases”, such as sulfur dioxide, “Are not detected on Mars,” Korablev said. “The distribution of our detections around 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, “disagree with the volcanic origin of HCl.”
Strangely, however, the HCl disappears quickly. It was seen during and after the global dust storm and also during the dusty season, but then it’s gone, and the researchers don’t know why.
“Our understanding of how HCl behaves does not explain this,” said Olsen. “It won’t condense and freeze like carbon dioxide or water, it shouldn’t break down as quickly, and there’s too much of it to go anywhere our instruments don’t measure. We expect there will be interactions with solid and ice particles, but how HCl can be removed from the atmosphere as quickly as we see is a mystery, ”he said.
That HCl exists on Mars is no big surprise, since perchlorates (another chlorine compound), found it back in 2008, the presence of this gas hinted. If the researchers are right about a chemical source for HCl, and if chlorine switches between mineral and gas phases, “it will affect perchlorate formation, but we have yet to see how much,” said Olsen. To which he added, “HCl is also very reactive and plays an important role in the Earth’s atmosphere, and we see it at much higher levels than predicted, so it will affect how we change the chemistry of the Martian atmosphere. viewing and modeling. “
The team is now looking forward to searching the TCO data collected during the next Mars year, when a global dust storm did not appear. The team will study how the appearance and disappearance of HCl is related to dust and atmospheric vapors and the possible ingredients involved in the proposed gas-mineral reaction. At the same time, the team also anticipates “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 second-best after Earth, but studies like this one remind us how inhospitable and strange this place really is. There is a really funky chemistry going on, with no obvious analogues of processes observed on Earth. Mars, with no running water on the surface, a painfully thin atmosphere filled with carbon dioxide and hugely variable temperatures, is home to exotic processes that are hard to understand. Suffice to say, we will not be living there again soon.