For decades, Mars has worn the reputation of a geologically dead world — its towering volcanoes long silent, its interior slowly surrendering the last of its ancient heat.
Then ESA’s ExoMars Trace Gas Orbiter spotted something that didn’t fit. Repeatedly, at the wrong time of year, elevated concentrations of hydrogen chloride gas were rising from a tight cluster of fractures on the slopes of Alba Mons. A planet long presumed cold and still appeared, in one quiet corner, to be exhaling.
A gas that shouldn’t be there
Hydrogen chloride on Mars isn’t a mystery in itself. Scientists have tracked it since TGO first confirmed its presence in 2021, and it follows a well-understood rhythm: concentrations climb during northern winter, then drop sharply as summer arrives. Dust and water cycles drive that pattern, and it holds reliably — almost everywhere.
The exception showed up above Alba Mons. TGO’s Atmospheric Chemistry Suite, using infrared spectrometers, kept detecting elevated HCl concentrations there in early summer — precisely when the gas should have nearly vanished. Readings were consistent across multiple passes, ruling out a random atmospheric glitch. On Earth, that kind of signal has a familiar source: hydrogen chloride is a well-known byproduct of volcanic degassing, released through vents and fissures connected to magma below.
Fresh fractures on an ancient volcano
Atmospheric data alone wouldn’t close the case. The team turned to TGO’s CaSSIS camera and pointed it at the same region. What came back was striking: narrow, relatively fresh-looking fractures stretching hundreds of meters across the surface of Alba Mons.
More telling was what surrounded the cracks. Unusual discoloration marked the edges — patterns consistent with mineral deposits left behind by escaping gases, similar to fumarolic deposits found around volcanic vents on Earth. The spatial overlap between the gas detections and the fracture locations was immediately apparent to the researchers.
The team also proposed a mechanism to explain the seasonal timing. Martian winters are brutal, with temperatures plunging to -120°C. At those extremes, ice could seal fractures shut and trap subsurface gases. When early summer arrives and temperatures climb, that ice sublimates directly into vapor — reopening pathways and allowing whatever is building below to exhale.
Is there a magma chamber beneath Alba Mons?
Before pointing to volcanism, the researchers had to eliminate every other explanation. They worked through the atmospheric chemistry systematically — all the non-volcanic reactions that could conceivably produce hydrogen chloride at those concentrations, at that time of year, in that location. None held up.
That left a dormant magma chamber beneath Alba Mons as the most consistent explanation. Such a chamber wouldn’t need to produce visible eruptions to be active — it could quietly release gases upward through the fracture network, operating entirely below the detection threshold of previous missions.
This possibility isn’t as far-fetched as it might seem. Orbital images already suggest some Martian volcanoes experienced eruptions within the past few million years, which is recent in geological terms. NASA’s InSight lander recorded marsquakes, confirming the planet still experiences seismic activity. The study, published in Science Advances, was led by Alexander Trokhimovskiy and co-led by Daniel Mège of the Polish Academy of Sciences.
What this means for the search for life
The implications reach beyond geology. If a magma chamber exists beneath Alba Mons and rising heat contacts subsurface groundwater, the result could be a hydrothermal system — warm, chemically rich, potentially long-lived. On Earth, hydrothermal environments host microbial life even in the absence of sunlight.
Underground settings like this would also offer meaningful protection from two of Mars’s harshest realities. The surface is bombarded by radiation and swings between extreme temperatures. A subsurface hydrothermal environment would buffer both, offering stability the surface has never provided.
Looking further back, volcanic gases including hydrogen chloride may have historically thickened Mars’s atmosphere and helped sustain liquid water across its surface. If that volcanism hasn’t fully ended, the present may matter too — which is why researchers now consider the fissures of Alba Mons a high-priority target for future life-detection missions.
What comes next for Mars exploration
ESA project scientist Colin Wilson described these findings as potentially TGO’s greatest scientific success to date — high praise for a mission already credited with transforming our understanding of the Martian atmosphere.
The results also demonstrate something methodologically important: trace-gas monitoring from orbit can detect signals from small, intermittent, deeply buried sources. That capability will matter wherever scientists look next.
The findings don’t confirm active eruptions, and they don’t confirm life. What they do is sharpen the scientific case for looking harder at this specific region. Future missions with surface access or deep-drilling capability could directly test whether hydrothermal activity is real beneath Alba Mons. Until then, the fractures sit quietly on the slopes of an ancient volcano — and, if the data are right, something beneath them may still be breathing.
Read the whole thing?
Get the week's signal, not the noise
Our sharpest reporting on energy, climate and nature — free, once a week.
