European Orbiter Finds Fresh Hints Mars Volcanoes May Still Be Stirring

Hydrogen chloride was turning up in the Martian atmosphere at the wrong time of year. On Mars the gas follows a seasonal rhythm, peaking in northern hemisphere winter and mostly vanishing in summer, a pattern tied largely to the planet's dust and water cycles. But researchers analyzing data from the ExoMars Trace Gas Orbiter noticed spikes in early summer, in the same place, every time: above volcanic fissures on the flanks of Alba Mons.

The orbiter, run by the European Space Agency, first reported hydrogen chloride in the Martian atmosphere in 2021. On Earth the gas commonly escapes from magmatic vents, which raised the question of whether something similar was happening on Mars. An international team led by Alexander Trokhimovskiy now argues that it is. Their paper, published in Science Advances, proposes that the gas comes from hot magma beneath the surface, a sign of dormant volcanism rather than a dead interior.

The finding came from combining atmospheric and geological work. First, infrared spectrometers in the orbiter's Atmospheric Chemistry Suite detected hydrogen chloride strongly localized in space and timed just after the winter frosts sublimate. Then the orbiter's CaSSIS camera looked closely at the Alba Mons region and found fresh, narrow fissures, some hundreds of meters long, associated with color changes on the surface.

The team's explanation runs like this. Magmatic gases may rise through the fissures much as they do at some fumaroles on Earth. In winter, with temperatures falling as low as -120°C (-184°F), ice blocks the openings. When the ice sublimates in early summer, the fissures channel the chloride out. The discoloration around them could be deposits left by escaping gases, as around volcanic vents on Earth. The researchers suggest the seepage connects to a dormant magma chamber, a kind of volcanic plumbing system under the surface.

Daniel Mège, a professor at the Space Research Centre of the Polish Academy of Sciences and co-lead author, said the findings suggest Mars kept enough internal heat to support volcanic activity until recently, and possibly still does. He described the detection as finding a needle in a haystack: the orbiter caught a handful of molecules from a perhaps intermittent source while moving continuously along its orbit to monitor the whole atmosphere. He said the spatial pattern made him feel at once that something volcanic lay behind it, but that this had to be demonstrated. Much of the work, he said, went into examining every possible chemical process in the atmosphere that could explain the gas.

The context is a long-open question. Mars has the largest volcanoes in the solar system, yet how active they are remains unknown. Orbital imagery suggests some volcanoes were active within the last few million years, and seismometers have recorded marsquakes, both hints of ongoing activity. No new lava flows or eruptions have ever been observed, however.

There is also an astrobiological angle. Magmatic gases such as hydrogen chloride may have been more abundant in the past, contributing to a thicker atmosphere and warmer climate that let liquid water cover much of the surface. If rising lava meets groundwater today, it could create warm, habitable niches. Mège said searching the seeping fissures of Alba Mons for life would make sense: active hydrothermal systems could hold the ingredients and energy microorganisms need, and shield them from the hostile surface environment. Colin Wilson, ESA's project scientist for the orbiter, said the results may prove to be its greatest success yet.

What to watch: whether further observations confirm the early-summer pattern and whether the fissures become a target in the search for Martian microbes.