Mars may not be as geologically dead as many scientists assumed — a study published Nov. 4 in Science Advances reports bursts of hydrogen chloride gas above fresh fissures on the flanks of Alba Mons, a giant volcano, and proposes that the gas is leaking from a dormant magma source below.

The observations come from the European Space Agency's ExoMars Trace Gas Orbiter. "Our findings suggest that Mars has retained enough internal heat to support volcanic activity until recently, and possibly still does," said Daniel Mège of the Space Research Centre of the Polish Academy of Sciences, a co-lead author, in an ESA release.

A Gas Showing Up in the Wrong Season

Hydrogen chloride, or HCl, is a common component of volcanic gas on Earth. When the orbiter first detected it on Mars in 2021, scientists hoped it was a volcanic fingerprint. But the gas turned out to be widespread and seasonal, peaking during the dusty southern summer, which pointed to chemistry between salty dust and water vapor rather than eruptions, ScienceAlert reported at the time.

The orbiter detects gases by solar occultation. As the spacecraft watches the Sun rise or set through the thin edge of the Martian atmosphere, its spectrometers measure which wavelengths of sunlight are absorbed, revealing which molecules are present and at what altitude.

That seasonal pattern is what makes the new signal stand out. HCl normally disappears during the northern summer. Yet the orbiter repeatedly found it in early northern summer, in the same place each time, above volcanic fissures on the northern side of the Tharsis plateau.

According to a conference abstract presented by the team in 2025, the orbiter's Atmospheric Chemistry Suite made two unusual detections near Alba Mons in Mars Year 35. A dedicated search in later Mars years found HCl there again, with support from a second orbiter instrument and a ground telescope in Hawaii.

Fresh Cracks and Discolored Ground

The chemistry led the team to look more closely at the ground. Images from the orbiter's CaSSIS camera showed fresh, narrow fissures, some hundreds of meters long, with color changes in the surrounding surface.

The researchers compare those color changes to mineral deposits around fumaroles, the gas vents found on active and sleeping volcanoes on Earth, such as those in Yellowstone or Iceland. The team's abstract notes that fissures in the region cut across periglacial ground patterns thought to be younger than about 1 million years, which is very recent in geological terms.

Alba Mons sits in the Tharsis region, also home to Olympus Mons. It is the largest volcano in the solar system by area, and NASA describes how its surrounding linear depressions may have formed after a massive eruption and collapse into an emptied magma chamber.

Winter Ice May Bottle Up the Gas

The team's explanation hinges on the seasons. During the Martian winter, gas rising from the interior may get trapped beneath ice and frost covering the fissures. When that ice sublimates, turning directly from solid to gas in early summer, the trapped HCl escapes in a burst that the orbiter can detect.

Mège and colleagues suggest the gas is fed by "volcanic plumbing" connected to a dormant magma chamber. "We needed to be rigorous, critical and creative to explain the presence of hydrogen chloride," Mège said, according to Space.com.

On Earth, volcanic plumes usually carry more sulfur dioxide than HCl. The researchers suggest that HCl may dominate on Mars because the planet's interior has a different chemical makeup, an idea that remains to be tested.

Strong Clues, Not Proof

This is an observational study that combines spectroscopy, imaging and modeling. It found a repeated, localized gas signal matched to young-looking fissures. It did not observe an eruption, lava flow, or measurable heat at the surface, and the magma chamber itself is inferred, not imaged.

Mège has described the signal as a handful of molecules from a perhaps intermittent source. Earlier skepticism also matters. In 2021, lead instrument scientist Oleg Korablev argued that the lack of sulfur dioxide and Mars' low seismic activity did not support a volcanic origin for HCl. The new work addresses only the unusual Alba Mons detections, not the planet-wide seasonal cycle.

Even so, the study adds to a growing body of evidence, including young lava surfaces and marsquakes recorded by NASA's InSight lander, that Mars still has internal heat.

Hidden Heat and the Search for Life

If magma still sits beneath Alba Mons, it could heat underground water or ice, creating hydrothermal systems. On Earth, hot springs and deep-sea vents host microbes that thrive without sunlight, which makes similar settings on Mars attractive targets in the search for life.

"Looking for Martian life in the seeping fissures of the Alba Mons region would make a lot of sense," Mège said in the ESA release. The researchers also note that magmatic gases were likely more abundant in the distant past, which could have thickened Mars' atmosphere and warmed its climate.

The next steps are continued seasonal monitoring by the Trace Gas Orbiter, searches for other volcanic gases at Alba Mons, and high-resolution imaging to track any change in the fissures. Each new Martian summer is a chance to test whether the gas returns on schedule.

For now, the evidence points to a planet that is quiet — not dead. The case for active volcanism is stronger than before but still unproven.

What Readers Want to Know

Is Mars volcanically active? The new study suggests it may be, but it has not proven it. No eruptions or lava flows have been observed.

What did the orbiter detect? Hydrogen chloride gas above fresh fissures at Alba Mons in early northern summer, a season when the gas normally disappears.

Why does the gas appear in summer? Researchers think gas from the interior gets trapped under winter ice and escapes when the ice sublimates.

Where is Alba Mons? In Mars' Tharsis region, near Olympus Mons. It is the largest volcano in the solar system by area.

What does this mean for life on Mars? Underground heat could create hydrothermal systems, which on Earth support microbes. The fissures may be good places to search.

What happens next? The orbiter will keep monitoring the region each summer to see if the gas returns and whether other volcanic gases appear.