Hydrogen chloride (HCl) gas has been detected in Mars’ thin atmosphere during the planet’s cold aphelion season, when its presence was not expected. Researchers say the observation, together with newly identified surface cracks near the Alba Mountains, may indicate that igneous activity beneath Mars is still supplying chlorine to the atmosphere, even though no clear heat signature has been found.
A view of Mars from the North Pole to the Equator, with Mount Alba, a vast shield volcano, near the center of the image. Image credit: ESA / DLR / FU Berlin / CC BY-SA 3.0 IGO.
Mars is widely considered volcanically inactive, but chemical signals in its atmosphere suggest that the planet’s interior may not be completely dormant.
“On Mars, as on Earth, the chlorine cycle started by volcanic degassing from the deep interior during planetary accretion is fed by subsequent volcanic eruptions, and due to its high mobility and solubility, chlorine is continuously recycled in water, sediments, permafrost, and ice clouds,” said University of Bern researcher Nicholas Thomas and his colleagues.
“However, in contrast to Earth, the lack of recycling by plate tectonics means that the main inputs to Mars’ surface atmospheric system may be volcanic sources,” they added. “The question addressed here is whether volcanic activity still contributes to this cycle on Mars today.”
Unexpected hydrogen chloride
The researchers analyzed data from the Atmospheric Chemistry Suite aboard ESA’s Trace Gas Explorer and detected hydrogen chloride at parts-per-billion concentrations in the Martian atmosphere.
Higher-than-expected concentrations were recorded during the cold aphelion season near the Alba Mountains. The team also examined high-resolution images from the HiRISE, CaSSIS and CTX cameras covering the area beneath the detection site.
Those images revealed narrow cracks, some only tens of centimeters wide, as well as clusters of cracks extending for tens of kilometers along topographic ridges. The researchers say the features have not previously been observed on Mars and are consistent with large-scale tectonic and volcanic activity in the region.
Possible pathways from the interior
The cracks could provide pathways for hydrogen chloride to rise from deep magma bodies, possibly after the gas interacts with ice-rich permafrost. This mechanism would not necessarily produce a strong thermal signature, which could explain why heat-based searches for active volcanism have not detected it.
Assuming that hydrogen chloride remains in the atmosphere for one sol, the authors estimate a permeation rate of less than 3 kilograms per second. That rate is comparable to the lower end of estimated surface volcanic fluxes, which range from a few kilograms per second to more than 100 kilograms per second.
The researchers estimate that the seasonal hydrogen chloride release at perihelion—approximately 180 kilograms per second—is not directly caused by the volcano. However, they suggest that some of the chlorine could ultimately have an igneous origin, cycling between the atmosphere and surface reservoirs including perchlorates and ice.
“Overall, the presence and variability of hydrogen chloride in the Martian atmosphere appears to be controlled by interactions between episodic volcanic activity, atmospheric processing, and long-term sequestration in surface salts, perchlorates, and hydrogen chloride-containing ice,” the researchers said.
The study, “Mysterious HCl detection as a potential sign of Martian magmatism,” was published in Science Advances.
Alexander Trokhimovsky et al. 2026. “Mysterious HCl detection as a potential sign of Martian magmatism.” Science Advances 12(45). doi: 10.1126/sciadv.aeb9422.
Source: www.sci.news
