Fresh cracks on Mars’s widest volcano leak a gas tied to buried magma

Every early summer in the northern hemisphere of Mars, a trace of hydrogen chloride appears over the flanks of Alba Mons, the widest volcano on the planet, at a time of year when the gas has vanished from the rest of the Martian sky. Directly beneath those detections, orbital cameras have found swarms of fresh, narrow cracks that nobody had mapped before.

On Earth, hydrogen chloride is one of the gases that magma releases through volcanic vents. An international team working with the European Space Agency’s ExoMars Trace Gas Orbiter now argues that the same thing is happening on Mars, with gas from a magma body deep underground leaking through the cracks once the winter ice that seals them turns to vapour. If they are right, Mars is a sleeping world rather than a geologically dead one, and those cracks are the first place to look for a warm, wet pocket under its frozen surface.

“Our findings suggest that Mars has retained enough internal heat to support volcanic activity until recently, and possibly still does,” said Daniel Mège, a professor at the Space Research Centre of the Polish Academy of Sciences and co-lead author of the study. The work was led by Alexander Trokhimovskiy of the Space Research Institute of the Russian Academy of Sciences in Moscow and involves researchers in Poland, Russia, the United Kingdom, Germany, France, Switzerland, Italy, Ukraine and Australia.

Mars has the largest volcanoes in the Solar System, yet nobody has ever seen one erupt. Lava flows that look only a few million years old, and marsquakes recorded by NASA’s InSight lander, have hinted that the interior is not completely cold. What was missing was a gas, a chemical trace coming out of the ground today.

How the orbiter caught the gas

The Trace Gas Orbiter carries the Atmospheric Chemistry Suite, a set of infrared spectrometers that work by solar occultation. As the spacecraft watches the Sun rise or set through the thin Martian atmosphere, the instruments record which wavelengths of sunlight are absorbed on the way, and each gas leaves its own fingerprint. The method can pick out gases at the level of parts per billion and tell at what altitude they sit.

The same instrument team first reported hydrogen chloride on Mars several years ago. Since then the gas has turned up widely, but almost always in the warm, dusty season when Mars passes closest to the Sun, rising and falling with dust and water vapour. Researchers attribute that seasonal surge to chemistry between the air and airborne dust and ice, not to volcanoes.

The exception was a pair of observations in northern summer, when Mars is farthest from the Sun and the whole atmosphere is cold and dry, both north of Alba Mons. The team then ran a dedicated search in a later Martian year and found hydrogen chloride over the same region again, while orbits over other latitudes with similar viewing conditions showed none. “The Trace Gas Orbiter was able to find the smoking gun. It was like looking for a needle in a haystack,” Mège said.

Cracks less than a metre wide

To find a possible source, the researchers examined the ground beneath the detection points with three cameras: CaSSIS on the Trace Gas Orbiter, and HiRISE and CTX on NASA’s Mars Reconnaissance Orbiter. They found linear swarms of fissures, some only tens of centimetres wide and up to hundreds of metres long, grouped into belts tens of kilometres long on topographic ridges. Some are flanked by bright and dark deposits, a colour change the team compares to the crusts that build up around fumaroles, the gas vents of volcanoes on Earth.

The cracks cut through the ice-rich blanket of dust and frost that covers these northern latitudes, and through the polygon patterns that freezing ground creates, so they are geologically young, probably less than a million years old. They also line up with the large fault systems around the volcano, Tantalus Fossae and Phlegethon Catena.

The proposed mechanism is seasonal. In winter, when temperatures fall to about minus 120 degrees Celsius, ice plugs the fissures. When that ice turns straight to vapour in early summer, the trapped gas escapes, which would explain why the signal returns to the same place in the same season.

How big Alba Mons is, and how small the leak

Alba Mons is an unusual volcano. Its lava flow fields reach at least 1,350 kilometres from the summit and it spans an area comparable to the United States, yet it rises only about 6.8 kilometres, roughly a third of the height of Olympus Mons. The average slope on its northern flank is about half a degree, flatter than most motorway ramps.

The leak itself is modest. Assuming hydrogen chloride survives about one Martian day in the atmosphere, the authors estimate a seepage rate below three kilograms per second. Volcanoes on Earth release the gas at rates from a few kilograms to more than 100 kilograms per second, so the Martian source sits at the very low end of that range.

The much larger seasonal pulse seen across Mars near its closest approach to the Sun, estimated at around 180 kilograms per second, is not thought to be volcanic in itself. The team suggests, however, that some of that chlorine may have come from the interior long ago and keeps cycling between the air, surface salts and ice.

What it doesn’t prove

Nobody has seen magma. No new lava flow or eruption has ever been observed on Mars, and sulfur dioxide, the gas that would point most directly to ongoing eruptions, has not been detected. The authors argue that halogen gases such as hydrogen chloride can seep from magma bodies at depth without an eruption, and without the strong heat signature that infrared searches for active volcanism have looked for and never found.

The link between the gas and the cracks is an inference from place and timing. The detections are few, the gas is not well mixed and it disappears quickly, so its source can be located only roughly. Mège described the volcanic reading as intuition that needed proof: “Looking at the spatial distribution of the seeping, it became instantly obvious to me that there was something volcanic behind it, but it had to be demonstrated.” According to the team, most of the work went into testing and ruling out chemical processes in the atmosphere that could produce the gas instead.

Trace gases on Mars also have a contested record. Methane spikes measured at ground level by NASA’s Curiosity rover have never been confirmed from orbit, where the Trace Gas Orbiter has set very low upper limits. A repeat detection over Alba Mons, ideally by a second instrument, would make the volcanic case much harder to dispute.

Why the cracks matter for life

Magmatic gases such as hydrogen chloride may have been far more abundant in the planet’s past, helping to thicken the atmosphere and warm the climate when liquid water covered much of the surface. If heat from below still meets ground ice today, it could sustain pockets of liquid water underground.

“Looking for martian life in the seeping fissures of the Alba Mons region would make a lot of sense. If hydrothermal systems are active, they could contain the ingredients and energy needed for microorganisms to develop,” Mège said. That is a proposal for future missions, not a finding. Colin Wilson, ESA’s project scientist for the orbiter, said the results “may prove to be TGO’s greatest success yet.”

Common questions about volcanoes on Mars

Are there active volcanoes on Mars?

No eruption has ever been observed on Mars. Some lava flows appear to be only a few million years old, and the hydrogen chloride detected over Alba Mons suggests that magma underground may still be releasing gas, which would make the planet dormant rather than extinct.

What is Alba Mons?

Alba Mons is a shield volcano in the northern Tharsis region and the largest volcano on Mars by surface area. Its flows reach at least 1,350 kilometres from the summit, but it stands only about 6.8 kilometres high.

Why does hydrogen chloride point to volcanic activity?

On Earth, hydrogen chloride escapes from magma through vents and fumaroles. On Mars it normally appears in the warm season through atmospheric chemistry, so finding it in the cold season, above fresh fissures, points to a source in the ground.

Could there be life in the Alba Mons fissures?

There is no evidence of life there. The researchers suggest that if magma heat meets underground ice, hydrothermal systems could form, the kind of environment where microbes thrive on Earth, which makes the fissures a candidate target for future exploration.

The study was published in Science Advances on 4 November 2026. The first two cold-season detections came in the Martian year that ran from 2019 to 2021, and the confirming search in the one that ran from late 2022 to late 2024; one of the CaSSIS images of the fissures was taken on 3 February 2023. The next early northern summer on Mars, when the seasonal model predicts the gas should return over Alba Mons, begins around April 2027, and the Trace Gas Orbiter remains in orbit to look for it.

Reference: Trokhimovskiy et al., “Enigmatic HCl detections as a potential signature of igneous activity on Mars,” Science Advances, 2026. DOI: 10.1126/sciadv.aeb9422

Tags: astronomy, Mars, esa, Alba Mons, ExoMars Trace Gas Orbiter, hydrogen chloride