For the first time ever, scientists have spotted hydrogen chloride on Mars. The source of this colourless gas remains unclear, but going theories include volcanic activity or a previously undetected chemical cycle tied 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 molecule on Mars since methane — a potential biosignature — 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 in need of an answer.
Kevin Olsen, a co-author of the study and a research scientist from the Department of Physics at the University of Oxford, says there are two possibilities: Either the gas is being produced by magmatic activity beneath the surface or through complex chemical interactions involving surface dust and atmospheric gases. Whichever one is correct, it’ll be an exciting result.
“If evidence grows for the proposed chemical cycle linking minerals in the surface dust with gases in the atmosphere, this will be the first known direct link between the surface and atmosphere, other than ice formation,” explained Olsen in an email. “On the other hand, if some sort of venting is determined to be the source of HCl — such as volcanoes or other magmatic outgassing — then this is among the first evidence of active geological processes that has been found.”
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 likewise points to unknown geological — or possibly biological — processes. Should a chemical cycle involving surface materials and atmospheric gases be involved with HCl, however, that would still represent a big win for science and the ExoMars Trace Gas Orbiter (TCO), as this is precisely the sort 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’s Roscosmos, and it’s been in orbit around Mars since 2016. The primary goal of the ExoMars project is to catalogue rare gases in Mars’s lower atmosphere — things like water vapour, nitrogen dioxide, acetylene, and methane. The discovery and potential interplay of these and other compounds could provide evidence of previously undetected chemical processes happening on Mars. The reported discovery of HCl, therefore, represents a slam dunk for the Trace Gas Orbiter.
Data collected by the TCO’s Atmospheric Chemistry Suite spectrometer revealed a spectral sequence consistent with HCl. The team detected “multiple spectral features, a pattern of characteristic strengths and positions” which allowed them “to identify HCl unmistakably,” Oleg Korablev, a planetary scientist at the Space Research Institute in Moscow and the first author of study, said in an email. “We even recognised 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 it produces white fumes of hydrochloric acid when in contact with atmospheric water vapour.
“Near the surface, it’s formed from evaporated seawater and it’s linked to acid formation, and in the upper atmosphere it plays a role in ozone destruction,” said Olsen. “It is also emitted from volcanoes, which is why we have been looking for it on Mars — a sign that there is active volcanic activity. But, we don’t think that volcanoes are the cause of what we’ve seen. We think there is other atmospheric chemistry at play.”
Olsen and his colleagues suspect this because the behaviour of HCl and water vapour appear to be related. This water vapour is coming from the south polar ice cap, which, during the Martian summer in the southern hemisphere, leaks evaporated water into the atmosphere. And indeed, the 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 have on the atmosphere and climate of Mars,” said Olsen.
Importantly, the HCl signatures were also detected during an epic dust storm that enveloped the planet in 2018 — the same once-in-a-decade storm 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 warm, dusty, and moist atmosphere,” that may have led to the formation of HCl on Mars, said Olsen. But as the scientists observed during the subsequent year, HCl formation “can occur in regular, seasonal dusty conditions,” he noted.
At the same time, evidence of a volcanic origin for HCl remains weak. Other and “expectedly more abundant volcanic gases,” such as sulphur dioxide, “are not detected on Mars,” said Korablev. “The distribution of our detections over the planet does not support any local source around which HCl concentrates,” while NASA’s InSight lander “found that the seismic activity on Mars is low.” All these facts, he said, “are in disagreement with the volcanic origin of HCl.”
Strangely, however, the HCl quickly disappears. It was spotted 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 doesn’t explain this,” said Olsen. “It won’t condense and freeze out like carbon dioxide or water, it shouldn’t break down that fast, and there’s too much of it to move somewhere where our instruments don’t measure. We expect there to be interactions with solid dust and ice particles, but how HCl can be removed from the atmosphere as fast as we see it is a mystery,” he said.
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That HCl exists on Mars is not a huge surprise, as perchlorates (a different chlorine compound), found back in 2008, hinted at the presence of this gas. If the researchers are correct about a chemical source for HCl, and if chlorine is cycling between mineral and gas phases, “then this will have an impact on perchlorate formation, but we have yet to see by how much,” said Olsen. To which he added: “HCl is also very reactive and plays important roles in Earth’s atmosphere, and we are seeing it at far higher levels than predicted, so it will impact how we view and model the chemistry of the Martian atmosphere.”
The team is now looking forward to scouring TCO data gathered during the following Martian year, when no global dust storm appeared. The team will study how the appearance and disappearance of HCl is related to dust and atmospheric vapour and the potential ingredients involved in the proposed gas-minerals reaction. At the same time, the team also expects “new developments in atmospheric chemistry modelling and laboratory studies related to chlorine chemistry on Mars,” said Korablev.
We like to think of Mars as the next best thing to Earth, but studies like this are a reminder of how inhospitable and alien this place really is. Some really funky chemistry is going on out there, with no clear analogues to processes seen on Earth. Mars, with no flowing water on the surface, an achingly thin atmosphere filled with carbon dioxide, and wildly fluctuating temperatures, is home to exotic processes we’re struggling to understand. Suffice it to say, we won’t be living there any time soon.