Hydrogen chloride over Mars’s Alba Mons hints at magma, but volcanism is unconfirmed

Hydrogen chloride turned up in the cold season above fresh fissures on the volcano Alba Mons, at about 2 parts per billion, and the team behind the finding proposes it seeps from magma underground, though no eruption has ever been seen on Mars.

Peer reviewed · Orbital observations · Proposed interpretation

Europe’s ExoMars Trace Gas Orbiter has caught hydrogen chloride (HCl) gas in a season and place where it should not be: over fresh, narrow fissures on the flanks of the Martian volcano Alba Mons in early northern summer. A team led by Alexander Trokhimovskiy of the Space Research Institute in Moscow reports in Science Advances that the detections are best explained by magma still active beneath the surface. That would mean Mars is not volcanically dead. The catch: it is an interpretation of trace gas readings, and nobody has seen an eruption or a new lava flow.

Evidence at a glance

Is this a big deal?

It is a strong hint, not proof. If it holds up, it would be the first chemical sign of magma active on Mars today, which planetary scientists have long suspected but could not show. For now it rests on one gas, at trace levels, over one region, and on the team’s reasoning about where that gas comes from.

What scientists found

Hydrogen chloride is not new on Mars. The same orbiter’s ACS instrument first reported it in 2021, after spotting it during the 2018 global dust storm. That HCl appeared in both hemispheres at once, in the warm dusty season near perihelion, when Mars is closest to the Sun. ESA explained it then as chemistry between salty dust and water vapour, not volcanoes. (For the rest of the air, see our guide to the atmosphere of Mars.)

The new paper reports something different. During the cold aphelion season, when Mars is farthest from the Sun, HCl showed up isolated in the Alba Mons region. ESA describes it as “always in the same season and at the same place”: early northern summer, above volcanic fissures. Mège puts the concentration “of the order of two parts per billion by volume.” That is two molecules of HCl in every billion molecules of air.

How they found it

The ACS team saw the odd HCl in the atmosphere first. Kevin Olsen of the University of Oxford, a co-author, told Mège that the team suspected a geological source for some detections, according to the ESA blog. Mège, a geologist at the Space Research Centre of the Polish Academy of Sciences, looked at where the gas sat. Judging from the spatial pattern, “it became instantly obvious to me that there was something volcanic behind it, but it had to be demonstrated,” he told ESA.

Demonstrating it took two kinds of evidence. ACS, a set of infrared spectrometers, measured gas that was localised in space and time, just after winter frost had sublimated. The orbiter’s camera, CaSSIS, then imaged the ground below and found fresh, narrow fissures, under 1 metre wide according to the abstract and in some cases hundreds of metres long, with colour changes on the surface around them. One ESA example image, taken on 3 February 2023, covers about 20 by 20 kilometres centred near 64.6°N, 274.4°E on the volcano’s northern flanks.

The proposed mechanism works a bit like a frozen bottle cap. In winter, at temperatures down to about −120 °C, ice may block the fissures. As the ice turns straight to vapour in early summer, gas from below escapes, much as from fumaroles (volcanic gas vents) on Earth. The analogy breaks down in one way: nobody has measured pressure building under the ice. The pale deposits around the cracks may be minerals left by the gas, but that too is an interpretation.

“A huge part of the work consisted of examining all the possible chemical processes going on in the martian atmosphere,” Mège said in the ESA release. The team concluded that magmatic seepage was the most likely source.

Why it matters to you

This won’t change anything for anyone on Earth this year. It matters for the wonder question of whether Mars is a dead world or a sleeping one, and for where future missions might look. Mège suggests the seeping fissures would be a sensible place to search for microbial life. His reasoning is conditional: if hydrothermal systems are active there, they could offer energy and shelter to microorganisms. That is speculation, and he notes HCl itself is not a friendly gas for living things.

What the evidence does not show

It does not show an eruption. No new lava flows or eruptions have ever been observed on Mars, ESA notes, and the paper’s own abstract says sulphur dioxide (SO₂), the gas that would testify to eruptive activity, remains undetected. The case for magma rests on HCl alone.

It does not rule out every other source. ESA’s 2021 work tied seasonal HCl to dust and water chemistry, and that explanation still stands for the widespread warm-season gas; the new abstract attributes those detections to “aerosol-atmospheric sources.” Mège also notes that the detections sit at fairly high latitude, where freezing and thawing of the upper permafrost can trap and release gases with the seasons. SciQuest could not read which atmospheric explanations the team tested and rejected, or how much gas they estimate is escaping, because the full paper was not accessible.

How this fits what we already knew

Mars has the largest volcanoes in the Solar System, and some appear to have been active within the last few million years, according to ESA. Seismometers have recorded marsquakes, and other studies have pointed to a still-warm interior, such as the gravity-based estimate of a hotter mantle beneath Mars’s southern highlands. What was missing was a gas sign of magma today. Earlier spacecraft instruments were not sensitive enough to find volcanic gases at trace levels, Mège told ESA’s blog.

The new result also revisits a 2021 conclusion. Back then, HCl appeared in distant places at the same time and without other volcanic gases, so ESA ruled out volcanoes for that gas. The Alba Mons detections are the opposite pattern: tied to one place, in an unexpected season.

Independent view

SciQuest found no comment from an independent researcher in the coverage published by 8 November 2026, and we have not yet interviewed one. Every voice quoted so far, including ESA’s Trace Gas Orbiter project scientist Colin Wilson, who said “These findings may prove to be TGO’s greatest success yet,” belongs to the study team or to ESA, which runs the mission.

How other headlines framed it

• ESA: framed it as “hints” of volcanic activity, with a Mars that may be sleeping rather than dead.
• ScienceDaily (ESA text): headlined a Mars that may still be “volcanically alive,” while its body says the findings do not establish current eruptions.
• Reuters (via KFGO): led with a pungent gas hinting at ongoing volcanism, and added the life angle.
• Space.com: said recent volcanic activity “reveals” Mars is sleeping, firmer wording than the paper’s “potential signature.”
• BBC Sky at Night Magazine: stressed signs of activity and a clue for the search for life, with hedges in the body.

What happens next

Two things would firm this up. If ACS keeps finding HCl over the same fissures each early summer, the pattern gets harder to explain away. A detection of sulphur dioxide or another magmatic gas at the same spot would strengthen the case far more. For a sense of how far those missions must travel, see how far Mars is from Earth.

In one sentence

Hydrogen chloride that keeps appearing over fresh cracks on Alba Mons in the cold season may be leaking from magma underground, which would mean Mars still has a living volcanic system, but no eruption or second volcanic gas has been seen yet.

Read next

• Mars is 200 to 400 °C hotter beneath its southern highlands: more evidence of a warm interior.
• Teghaza 001 meteorite samples Mars’s early crust: what Martian rocks say about its volcanic past.
• Was Mars habitable in the past? The bigger question behind the life angle.

Sources and disclosure

1. Trokhimovskiy A., Mège D., Korablev O., Olsen K.S., Fedorova A.A., Taysum B., Tesson P.-A., Luginin M., Montmessin F. et al. (21 authors). “Enigmatic HCl detections as a potential signature of igneous activity on Mars.” Science Advances 12(45), eaeb9422 (2026). doi:10.1126/sciadv.aeb9422. Abstract and author list read via Crossref.
2. ESA, “European orbiter detects hints of volcanic activity on Mars,” 5 November 2026: esa.int (also issued 4 November via EurekAlert).
3. ESA “To Mars and back” blog, “Geological intuition for Mars,” interview with Daniel Mège, 4 November 2026: blogs.esa.int.
4. AAAS, Science Advances press release, 4 November 2026: EurekAlert.
5. ESA, “ExoMars discovers new gas and traces water loss on Mars,” 10 February 2021 (Korablev et al., Science Advances 7, eabe4386): esa.int.
6. NASA, Mars Fact Sheet (perihelion 206.65 and aphelion 249.26 million km): nssdc.gsfc.nasa.gov.

Disclosure: This article was drafted with AI assistance and checked against the sources above. The full paper was blocked by the publisher’s bot check, so SciQuest relied on the abstract and on ESA’s release and co-author interview; figures reported elsewhere that we could not trace to those sources were left out. Calculation by SciQuest: (249.26 ÷ 206.65)² ≈ 1.45, so Mars gets about 45% more sunlight at perihelion than at aphelion. The graphic is a SciQuest original. Spotted an error? See our corrections policy.