ExoMars Orbiter Links Gas Spikes and Fresh-Looking Fissures to Possible Volcanism

A European Space Agency orbiter has repeatedly detected hydrogen chloride near narrow, fresh-looking fissures around Alba Mons, raising the possibility that gas is escaping from a dormant but still-warm volcanic system beneath Mars. The combined atmospheric and surface observations strengthen the case for recent geological activity, but scientists have not observed a new eruption or lava flow.

An international team led by Alexander Trokhimovskiy detailed the findings in a Science Advances study published November 4, 2026. The investigation used atmospheric measurements from the ExoMars Trace Gas Orbiter alongside orbital imagery of the region around Alba Mons, a vast volcano on the northern edge of Mars’ Tharsis volcanic province.

A gas appearing in the wrong season

Hydrogen chloride is not new to the mission. The Trace Gas Orbiter first detected it in the Martian atmosphere in observations reported in 2021. Most of those measurements fit a broad seasonal cycle: concentrations increase during the warmer, dustier part of the Martian year and largely disappear during the opposing season.

The Alba Mons detections break that pattern. Researchers found localized hydrogen chloride during early northern summer, near the colder point in Mars’ orbit when the gas is generally absent elsewhere. Measurements over multiple Martian years placed the recurring concentrations in a narrow region around the volcano. The gas was also concentrated in the lowest atmospheric layers accessible to the orbiter, rather than broadly distributed at higher altitude.

That combination unusual timing, geographic concentration and proximity to the surface led the team to investigate whether the source might be below ground. Standard atmospheric chemistry models did not reproduce the observed concentration and localization during this season. Other proposed sources, including mud volcanism and the decomposition of chlorine-bearing salts, are thought to occur more widely across the northern lowlands and therefore do not readily explain why the detections cluster around Alba Mons.

The surface images add a second line of evidence

Images from the orbiter’s Colour and Stereo Surface Imaging System showed narrow fissures in the region beneath the atmospheric detections. Some extend for hundreds of metres, while the detailed study describes cracks less than one metre wide. Their sharp appearance, alignment with larger volcanic and tectonic structures, and associated changes in surface colour led researchers to describe them as apparently fresh.

The fissures are not evidence of a recent eruption by themselves. No surface change was identified by comparing available images from different times, and orbital instruments have not found a conclusive heat signature. Their value lies in providing plausible pathways through which small amounts of gas could move from depth into the atmosphere without producing a visible lava flow.

The proposed mechanism also accounts for the seasonal timing. Winter temperatures can fall to around minus 120 degrees Celsius, allowing water ice in the shallow subsurface or atmosphere to trap hydrogen chloride and obstruct fissures. As northern summer begins, warming reduces the ice load and causes ice to sublimate. Gas accumulated below or within the frozen material could then escape through the cracks for a limited period.

Hydrogen chloride is removed from the Martian atmosphere quickly, potentially in less than several Martian days under these conditions. That short lifetime would help explain why the gas remains localized rather than spreading around the planet. It also makes repeated detections over the same region more significant: a persistent local source fits the measurements better than gas transported over a long distance.

Volcanic seepage is not an eruption

The team interprets the observations as a potential sign of dormant igneous processes subsurface activity associated with magma rather than an erupting volcano. Hydrogen chloride can continue escaping after eruptive activity has ceased, and weak seepage through fractures would not necessarily create the thermal signal, ash or fresh lava expected from a surface eruption.

“This suggests that Mars has retained enough internal heat to support volcanic activity until recently, and possibly still does,” said Daniel Mege, a co-lead author and professor at the Space Research Centre of the Polish Academy of Sciences.

The interpretation fits a broader but still incomplete picture of Mars. Orbital studies have identified volcanism dating to the geologically recent past, while seismic instruments have detected marsquakes. Earlier work on Alba Mons dated some mapped summit-region lava flows to between 130 million and 810 million years old young compared with much of the planet’s volcanic terrain, but far older than the gas now sampled by ExoMars.

If heat and gas are still moving beneath Alba Mons, its fissures could become targets for future geological and astrobiological investigations. Magma interacting with underground water could create comparatively warm hydrothermal environments protected from the radiation and extreme conditions at the surface. Such an environment would be relevant to the search for past or present microbial life, although the orbiter has detected neither organisms nor a hydrothermal system.

The next useful evidence will come from continued seasonal searches over Alba Mons and other young volcanic regions. Repeated gas measurements, improved imaging and any independently detected heat or seismic activity could test whether the fissures truly connect to a lingering magmatic source. For now, ExoMars has found a recurring chemical signal over plausible escape routes not a Martian eruption, but a specific place to look for one of the planet’s remaining sources of internal heat.

By David Whitaker — Associate editor for AMI’s aerospace and drone systems desk, translating flight systems, aircraft programs, spaceflight, and UAV developments into accessible technical stories.