Hydrogen chloride seeping through fissures near Mars’ Alba Mons points to a source of heat beneath the planet’s surface. The ExoMars Trace Gas Orbiter’s detection suggests Martian volcanism may have continued until recently—and could persist today, despite the absence of observed eruptions or lava flows.
The finding centers on the Tharsis region, which contains both Alba Mons and Olympus Mons. Olympus Mons rises about 16 miles (26 kilometers) above the surrounding plains and stretches roughly 374 miles (600 kilometers) across. Alba Mons is the largest Martian volcano by surface area.
A gas signal in an unexpected season
The European Space Agency’s Trace Gas Orbiter (TGO) detected hydrogen chloride above fissures in the Alba Mons region. On Earth, the gas can escape from magmatic vents, where it originates in hot material below the surface. Its presence on Mars offers a possible chemical trace of dormant volcanic activity.
The timing helped narrow down the source. Hydrogen chloride generally peaks in Mars’ northern hemisphere during winter and fades in summer, following the planet’s water and dust cycle. Yet the TGO recorded spikes during northern summer, including above volcanic fissures on Alba Mons’ flanks.
Follow-up observations revealed fresh, narrow fissures in the area, some hundreds of feet long. The team argues that the combination of the unusual gas signal and these features supports a volcanic explanation.
“Our findings suggest that Mars has retained enough internal heat to support volcanic activity until recently, and possibly still does,” Daniel Mège, a co-leader of the research team and a scientist at the Space Research Centre of the Polish Academy of Sciences, said in a statement sent to Space.com by email. He described the TGO detection as “the smoking gun” and “like looking for a needle in a haystack.”
Mège said the team considered other chemical processes that might account for the gas. “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,” he said. “We needed to be rigorous, critical and creative to explain the presence of hydrogen chloride. The data were complex, but also excellent brain food.”
The researchers propose that gases rise from Mars’ interior during winter and become trapped by ice. When the ice sublimates in summer—changing directly from solid to gas—it releases the stored hydrogen chloride into the atmosphere. The TGO’s ability to detect a signal that may come from an intermittent source while continuously monitoring the atmosphere helped reveal the pattern, Mège said.
What dormant magma could mean for life
The team links the gas to a dormant magma chamber beneath the surface, part of what Mège described as Mars’ “volcanic plumbing.” If hydrothermal systems remain active near the fissures, they could provide warmth, chemical ingredients and energy—conditions that could support microbial life while sheltering it from the harsh surface environment.
Volcanic gases may also have mattered more in Mars’ past. The researchers suggest that greater quantities of gases such as hydrogen chloride could have thickened the atmosphere and warmed the planet, allowing liquid water to persist across more of the surface. Where lava met groundwater, the resulting warm environments may have created habitable niches.
“Looking for Martian life in the seeping fissures of the Alba Mons region would make a lot of sense,” Mège said. The fissures now offer a specific place to investigate as researchers assess whether subsurface heat and water may still provide conditions favorable to microorganisms.
Earlier orbital images had indicated that some Martian volcanoes may have been active within the past few million years, while seismometers have recorded marsquakes. The new gas detection adds a chemical clue to that picture. The team’s research was published in Science Advances on Wednesday (Nov. 4).