Hydrogen Chloride Gas Seeping From Alba Mons Fissures
“On Mars, as on Earth, the chlorine cycle initiated by volcanic degassing from the deep interior upon planetary accretion was fed by the subsequent volcanic eruptions, and, because of its high mobility and solubility, chlorine has been continuously recycled in water, sediments, permafrost, and ice clouds,” Universität Bern researcher Nicolas Thomas and his colleagues stated in the journal Science Advances. Hydrogen chloride gas has been detected in the Martian atmosphere during the cold aphelion season, pointing to ongoing igneous processes inside a planet long considered volcanically quiet.
The Atmospheric Chemistry Suite on the European Space Agency’s Trace Gas Orbiter recorded the gas at parts-per-billion levels near the Alba Mons region. Space.com noted that Alba Mons sits in the Tharsis region, home to Olympus Mons, the largest volcano in the solar system. Researchers examined high-resolution images from the HiRISE, CaSSIS, and CTX cameras, uncovering narrow fissures tens of centimeters wide and stretching for tens of kilometers along topographic ridges. These newly identified features align with regional tectonic and volcanic patterns, offering a pathway for gas escaping from deep magmatic bodies.
Evaluating Subsurface Magma Versus Surface Salts
Live Science reported that the findings represent the strongest evidence yet of gases emerging from dormant Martian volcanoes, following seismic observations by NASA’s InSight lander. Alexander Trokhimovskiy at Russia’s Space Research Institute described the atmospheric data as a complementary line of evidence pointing to subsurface magmatic systems. However, Kevin Olson, a visiting researcher at the University of Oxford, pointed out to Live Science that when TGO first spotted hydrogen chloride in 2021, scientists hesitated to link it directly to volcanism because the distribution appeared seasonal and lacked typical companion volcanic gases like sulfur.
Researchers cannot definitively prove the gas escapes directly from the newly spotted fissures, leaving open the possibility of isolated underground gas pockets rather than active magma. Hot water reacting with metal chlorides could also produce hydrogen chloride without active volcanism. Gregg suggested that the massive Tharsis magma chambers would take extended periods to fully cool and solidify, meaning serpentine gassy magma might still meander below the surface.
Atmospheric Seepage Rates and Hydrothermal Habitats
Assuming an atmospheric lifetime of one sol for hydrogen chloride, the study authors calculated a seepage rate of less than 3 kilograms per second. That metric matches the low end of fluxes recorded at terrestrial volcanoes. By contrast, the much larger seasonal release of hydrogen chloride during perihelion reaches around 180 kilograms per second, which researchers attribute to cycling between the atmosphere and surface reservoirs like perchlorates and ice rather than direct volcanic outgassing.
Daniel Mège of the Polish Academy of Sciences told Space.com that active hydrothermal systems could harbor the necessary energy and ingredients for microorganisms while shielding them from harsh surface conditions. Live Science noted that maintaining stable subterranean conditions over millions of years remains difficult to verify. Even if hidden magma drives the gas release, no immediate eruptions are expected on the planet.
Questions About Martian Volcanism
How did researchers detect hydrogen chloride on Mars?
Instruments on the ExoMars Trace Gas Orbiter measured parts-per-billion concentrations of the gas in the atmosphere, which scientists cross-referenced with high-resolution orbital camera imagery revealing narrow volcanic fissures near Alba Mons.
Could the detected gas come from sources other than active magma?
Researchers note that hydrogen chloride can also form through chemical reactions between hot water and metal chlorides stored in the Martian crust, or from trapped underground gas pockets.
Does this discovery mean Mars will erupt soon?
Scientists emphasize that the estimated seepage rate of less than 3 kilograms per second indicates dormant or sleeping volcanic systems rather than an imminent eruption.
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