ExoMars Orbiter detects hydrogen chloride near Alba Mons

Researchers using the European Space Agency’s ExoMars Trace Gas Orbiter detected hydrogen chloride around Alba Mons and reported the findings in Science Advances on November 4, 2026.

The international team was led by Alexander Trokhimovskiy, and hydrogen chloride concentrations near Alba Mons reached about 1.5 parts per billion. Across two Martian years, researchers recorded eight confident hydrogen chloride detections among 78 relevant northern-hemisphere observations, all within a narrow longitude range around Alba Mons.

Alba Mons is the largest volcano on Mars by surface area, sprawling across roughly the area of the contiguous United States, with a summit about 6.8 kilometers above the reference elevation. It sits in the northern Tharsis Plateau, northeast of Olympus Mons, which towers roughly 22 kilometers above the surrounding terrain. The volcano should have been inactive for about 3.2 billion years.

The images revealed narrow fissures, some only tens of centimeters wide, in places forming swarms tens of kilometers long cutting through ice-rich surface deposits estimated to be about 630,000 years old. The fissures are each around 330 feet in length and are thought to be associated with vertical magma sheets.

The hydrogen chloride appeared during early northern summer near aphelion, Mars’s farthest point from the Sun, a colder season when the gas is usually difficult to detect because water ice crystals absorb it.

Warmth in early summer turns the ice directly into vapor, letting hydrogen chloride leak along cracks toward the interior even when cold snaps freeze the ground to minus 120 degrees Celsius. The proposed mechanism involves chlorine-bearing material interacting with subsurface ice and remaining stored before seasonal warming releases the gas as ice changes directly from solid to gas.

The researchers say their results are best explained by ongoing igneous processes in the Martian interior, with a local magmatic source offering the best explanation for the localized hydrogen chloride detections. The team stresses that the presence of hydrogen chloride alone does not establish magma, since other chemical processes can produce it, but the location, timing, and surrounding geology point beyond the usual atmospheric cycle.

Signs of underground heat, surface changes, and sulfur gases have been elusive or mixed so far. Hydrogen chloride continues to seep from magma even after eruptions have ended. The researchers considered mud volcanism, decomposition of chlorine-containing surface compounds, and breakdown of perchlorates, but argue these processes do not explain why the unusual detections cluster near Alba Mons.

Many hydrogen chloride signals detected across Mars at various seasons are associated with hydrological and atmospheric processes. Hydrogen chloride can originate from chemical reactions between hot water and metal chlorides; chlorides have previously been detected on Mars, and the warmer crust likely harbors hydrothermal labyrinths.

Assuming an atmospheric lifetime of one sol, a Martian day, the authors estimate a hydrogen chloride seepage rate of less than 3 kilograms per second, comparable to the low end of fluxes at terrestrial volcanoes. Mars has a widespread seasonal hydrogen chloride cycle during southern spring and summer, when concentrations can reach roughly four to five parts per billion, while a larger perihelion release estimated at around 180 kilograms per second is not considered directly volcanic.

The study did not identify individual gas vents, observe fresh lava, or detect a definite local heat anomaly accompanying the gas detections, and the proposed magmatic system lies beneath the surface beyond what cameras can directly inspect. High-resolution image coverage is uneven, and the connection between the fissures and the gas remains interpretive. The team cannot be certain that hydrogen chloride is seeping out of the exact fissures, and the nature of the source is unknown.

Without something maintaining them, the fissures would have disappeared eons ago under accumulating dust or seasonal landscape deformations; they may be chimneys of hydrothermal systems connected to a volcanic plumbing system deep within Mars. The researchers interpret the fractures as possible pathways for gas moving through the crust, not as direct images of gas escaping. If magma interacts with groundwater or ice, it could create hydrothermal systems that might provide ingredients and energy for microorganisms and shield them from hostile surface conditions, though the study does not demonstrate liquid water or a habitable environment beneath Alba Mons.

NASA’s InSight lander touched down in 2018 in Mars’s Elysium Planitia region; its seismometer detected odd marsquakes indicating fluid might be moving below the surface. The InSight lander recorded more than 1,300 marsquakes over its mission, and some of those tremors could come from magma moving below the surface. But InSight never detected convincing magmatic seismic signals coming from Alba Mons or Tharsis in general. “These observations have been puzzling us for a long time,” Kevin Olsen said.

Tracy Gregg, a planetary volcanologist at the University at Buffalo who was not involved with the study, said some researchers think the fluid is magma, others think it is water, and she thinks it might be both. Tracy Gregg says the Tharsis region hosted the planet’s largest and likely longest-lived magma chambers, which would take a long time to fully cool down, degas, and freeze solid. Nobody is forecasting an imminent Martian eruption.

Hydrogen chloride is colorless, corrosive and atrociously pungent, and can be emitted by volcanoes on Earth when magma below rises and decompresses. Sulfur dioxide has not been observed, yet its presence would point to current eruptions; halogen gases including hydrogen chloride seeping from below could instead reveal active magma chambers.

The Martian atmosphere contains hydrogen chloride virtually everywhere, but during warmer seasons it behaves as an aerosol, defined as fine solid particles or liquid droplets.

The researchers propose continued searches around Tharsis and Cerberus, regions already associated with relatively recent activity, and want to compare observations with atmospheric models. Additional hydrogen chloride detections near Alba Mons occurred during another northern summer, though detailed analysis will be reported separately.