Acid Gas Over Mars' Biggest Volcano Hints Magma Still Lurks

Mars is widely seen as a volcanically dead planet, but a new study suggests its interior may not be fully quiet. Researchers report that an orbiter has picked up traces of hydrochloric acid gas high above one of the planet's largest volcanoes at a time of year when none should be there, according to Universe Today.

The team argues the gas could be seeping from magma chambers that still exist underground. The authors describe the evidence as a hint, not proof.

What the orbiter found

The ExoMars Trace Gas Orbiter has been measuring hydrochloric acid (HCl) in the Martian atmosphere at parts-per-billion levels. Earlier detections were tied to perihelion, the point in Mars' orbit when it is closest to the Sun. Scientists attributed those readings to seasonal warming and interactions between the atmosphere and airborne particles.

The new work, published in Science Advances and led by Alexander Trokhimovskiy of the Space Research Institute at the Russian Academy of Sciences, focuses on detections made during aphelion, the cold season when Mars is farthest from the Sun. Those readings were isolated in the region around Alba Mons and were the most recent ones, made in 2025. The authors say they stand in sharp contrast to the prolonged, widespread detections seen at perihelion.

The researchers also say the gas was found at low altitudes, and that neither current models nor the observed state of the atmosphere explain it.

Why Alba Mons stands out

Olympus Mons is the largest volcano and tallest mountain in the solar system, but Alba Mons is the largest volcano by surface area. Its lava flows stretch more than 1300 km from the central summit, and it is known for an enormous system of fractures. Both volcanoes sit in the Tharsis region.

The team also reports finding fresh, narrow fissures, less than 1 meter wide, in the same area. They interpret these as possible pathways that could carry HCl up from magma bodies at depth. The authors say such features had not been identified on Mars in earlier work. They note that a systematic mapping of the northern hemisphere did not consider these landforms.

To test the idea, the researchers calculated how much gas would need to seep out to account for the aphelion detections and compared it with the perihelion readings, factoring in how long HCl survives in the Martian atmosphere. According to Universe Today, their analysis rules out atmosphere-and-particle interactions as the source of the aphelion gas. It also rules them out for the perihelion gas. The authors argue that steady seepage from magma chambers would not follow the seasons, which fits that result.

Why it matters

Whether Mars is geologically alive says a lot about how the planet evolved. Universe Today notes that Mars was once warm, wet and likely habitable, and that its interior cooled and its magnetic field failed, after which most of its atmosphere was lost to space. Learning that heat and molten rock persist underground would change the picture of how quickly a planet like Mars winds down.

It could also matter for future exploration. Active magma could mean heat sources and gas pathways beneath the surface, though the study does not draw conclusions about habitability.

What remains uncertain

The authors acknowledge that Mars is believed to be volcanically dormant and that there is still no clear understanding of its modern activity. Other evidence has been accumulating. NASA's InSight lander detected more than 1300 Marsquakes, though many of those were attributed to meteorite impacts rather than volcanic processes.

No eruption has been seen, and sulfur dioxide, a gas that would more directly signal eruptions, has not been detected so far. The authors write that seepage of halogen gases such as HCl may be a better way to spot active magma chambers.

The HCl detections alone may not be enough to overturn the view that Mars is dormant, according to the report. The fissures, imaged by the Trace Gas Orbiter's CASSIS camera and by NASA's Mars Reconnaissance Orbiter, add context, but confirming a magmatic source would require further observations.

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Around Prague, based on reporting by Universe Today