A handful of hydrogen chloride detections above fissured ground near Mars’ Alba Mons may be evidence of ongoing activity beneath the planet’s surface. The signal is intriguing, but it does not prove that Mars is volcanically active today: several atmospheric and surface processes could also produce the gas.
ScienceAlert’s Ivan Farkas reports that astronomers using the European Space Agency’s ExoMars Trace Gas Orbiter detected hydrogen chloride, or HCl, near Alba Mons, a volcano in the northern Tharsis region. The study, published in Science Advances, describes multiple detections above cracked ground. HCl is a chlorine-containing gas that can be released from magma even after an eruption has ended.
The timing and location help explain why the detections drew attention. The report says a handful of HCl signals appeared during early northern summer, when Mars is near aphelion, its farthest point from the Sun. Northern summer is colder than southern summer, but the atmosphere is also clearer of dust and water vapor. Those conditions make an atmospheric-aerosol explanation less likely for these particular signals, and the detections appear at lower altitudes.
One proposed explanation is that early summer warmth sublimates ice covering cracks, allowing HCl to escape. The fissures may otherwise be sealed by ice during cold spells, when temperatures can plunge to about −120 degrees Celsius. This is a possible route for gas to reach the atmosphere, not a direct observation of magma flowing or an eruption taking place.
The orbiter has recorded HCl signals in different Martian seasons, and many appear connected to atmospheric or water-related processes. The report describes the signals highlighted in this study as unusually sparse and faint, at parts-per-billion concentrations. Detecting a few molecules from a potentially intermittent source while monitoring the atmosphere from orbit makes interpretation difficult.
The researchers considered several non-atmospheric explanations: a deep volcanic source; mud volcanoes releasing HCl after interacting with magma; or the breakdown of chlorine-containing compounds at the surface. They favor a local igneous source as the best fit, citing the association with small fissures. Some fissures form long swarms and have dark halos, which the report says may resemble features around Earth’s fumaroles. The idea that the cracks are vents connected to a deeper hydrothermal system remains a possibility, not a confirmed structure.
That distinction matters because Mars has a long volcanic history, and signs of comparatively recent activity have been reported in several regions. Yet evidence for activity still happening now has been difficult to establish. HCl may offer another way to investigate the question, particularly because it can persist as a volcanic signal after eruptions have ceased. The detections add a clue that fits ongoing magmatic processes, while leaving the source unresolved.
The possibility also bears on the search for past or present life. If hydrothermal systems are active, they could supply heat, nutrients and shelter from harsh surface conditions—features that could matter for microorganisms. Co-lead author Daniel Mège, a planetary scientist at the Space Research Centre of the Polish Academy of Sciences, suggested the Alba Mons fissures would be sensible places to investigate. But the reported gas detections are not evidence that life exists on Mars; they only make the region scientifically interesting to explore.