For the first time ever, scientists have observed hydrogen chloride on Mars. The source of this colorless gas remains unclear, but ongoing theories include volcanic activity or a previously unmarked chemical cycle attached to the epic dust storms of the Red Planet.
New research published today in Science Advances, is the first to document hydrogen chloride (HCl) and its associated chlorochemistry in the Martian atmosphere. This is the first detection of a new class of molecules on Mars since methane – a potential biosignature – was discover in 2004. Hydrogen chloride is not associated with life (on the contrary, 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 researcher in 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 by complex chemical interactions that include surface matter and atmospheric gases. Whatever is correct, it will be an exciting result.
“If evidence grows for the proposed chemical cycle that connects minerals in the surface substance with gases in the atmosphere, it will be the first known direct link between the surface and atmosphere, other than ice formation,” Olsen explained in an email. “On the other hand, if it is determined that a type of discharge is the source of HC1 – such as volcanoes or other magmatic exhaust gases – it is one of the first evidences of active geological processes found.”
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Indeed, NASA’s InSight probe through its discovery of marsquakes, suggested the presence of unknown geological processes on Mars. The above discovery of methane also indicates unknown geological – or possibly biological – processes. If a chemical cycle with surface materials and atmospheric gases is involved in HCl, it will still be a great victory for science. and the ExoMars Trace Gas Orbiter (TCO), because that’s exactly the kind of thing it’s designed to detect.
TCO, used to detect HCl on Mars, is a joint mission of the European Space Agency and that of Russia Roscosmos, and it’s 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 potential interaction of these and other compounds may provide evidence of previously undetected chemical processes. March. The reported discovery of HClthus representing a slam dunk for the Trace Gas Orbiter.
Data collected by the TCO’s Atmospheric Chemistry Suite spectrometer revealed a spectral sequence corresponding to HCl. The team detected ‘multiple spectral features, a pattern of distinctive strengths and positions’ that enabled them to ‘unambiguously identify HCl’, said Oleg Korablev, a planetary scientist at the Space Research Institute in Moscow, and the first author of the study said. 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 in contact with atmospheric water vapor.
“Near the surface, it is formed from evaporated seawater and it is associated with acid formation, and in the upper atmosphere it plays a role in ozone depletion,” Olsen said. ‘It is also released from volcanoes, which is why we searched for it on Mars – a sign that there is active volcanic activity. But we do not think volcanoes are the cause of what we have seen. WI think there is other atmospheric chemistry going on. ”
Olsen and his colleagues suspect this because the behavior of HCl and water vapor appear to be related. This water vapor comes from the southern ice cap, which evaporated into the atmosphere during the Mars summer on the southern hemisphere. And indeed, the HCl was detected in April 2019, which is in Mars’ southern hemisphere in late summer.
“Our observations are the effects that the seasonal freeze-thaw cycle of the icebergs has on the atmosphere and climate of Mars,” Olsen said.
It is important that the HCl signatures also during a epic dust storm that enveloped the planet in 2018 – the same storm that was permanent once in a decade knocked out NASA’s Opportunity Rover. The global dust storm led to a temporary greenhouse effect that drew water from near the surface to higher altitudes. It was the conditions, “a hot, dusty and humid atmosphere”, that possibly led to the formation of HCl on Mars, Olsen said. But as scientists noted during the ensuing year, HCl formation can “occur in regular, seasonal dusty conditions,” he noted..
At the same time, the evidence of a volcanic origin for HCl remains weak. Other and “expected more volcanic gases”, such as sulfur dioxide, “Not detected on Mars,” Korablev said. “The distribution of our observations across the planet does not support any local source why HCl is concentrated,” while NASA’s InSight lander “found that seismic activity on Mars was low.” All of these facts, he said, “do not agree with the volcanic origin of HC1.”
Oddly enough, however, the HCl disappears quickly. It was spotted during and after the global dust storm and also during the dusty season, but then it’s gone, and the researchers do not know why.
“Our understanding of how HCl behaves does not explain that,” Olsen said. ‘It will not condense and freeze like carbon dioxide or water, it should not break down as fast, and there is too much to move somewhere where our instruments do not measure. “We expect there to be interactions with solids 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, because perchlorate (another chlorine compound), found back in 2008, alluding to the presence of this guest. “If the researchers are correct about a chemical source for HCl and if chlorine moves between minerals and gas phases, it will have an impact on the formation of perchlorate, but we have yet to see how much it is,” Olsen said. To which 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 predicted, so it will have an impact on how we view the chemistry of Mars. atmosphere and model. ‘
The team is now looking forward to collecting TCO data collected during the next March year, when no global dust storm appeared. The team will study how the occurrence and disappearance of HCl is related to dust and atmospheric fumes and the possible ingredients involved in the proposed gas-mineral reaction. At the same time, the team also expects ‘new developments in the modeling of atmospheric chemistry and laboratory studies related to chlorochemistry on Mars’, Korablev said.
We like to think of Mars as the next best thing on earth, but studies like this are a reminder of how inhospitable and strange this place really is. There is a lot of funky chemistry going on, without clear analogues to processes seen on earth. Mars, without flowing water on the surface, a thin atmosphere filled with carbon dioxide, and very fluctuating temperatures, are home to exotic processes that we struggle to understand. Suffice it to say: we will not live there soon.