A gas spotted above an ancient Martian volcano could reveal hidden activity beneath the surface of the Red Planet.
Mars's landscapes still show evidence of an intense volcanic past. Yet no one has observed a current eruption on the planet. An international team has just identified a puzzling clue near Alba Mons, a massive volcano: a gas regularly appears in its vicinity during a period when it should normally become rare.
False-color topographic map of Alba Mons and the Tharsis volcanic region on Mars. The highest elevations appear in red and white.
Credit: NASA / JPL-Caltech / Arizona State University, public domain.
The gas is called hydrogen chloride. On Earth, it can escape through openings connected to molten, extremely hot rock: magma.
What particularly intrigues researchers is the location and timing of the detections. The European ExoMars Trace Gas Orbiter probe, which orbits Mars, recorded unusual concentrations at the beginning of summer in the northern hemisphere. Normally, hydrogen chloride is more abundant during winter and then becomes barely detectable in summer.
When examining images of the region, scientists found another clue. Narrow fissures appear on the flanks of Alba Mons, sometimes stretching several hundred meters. Some seem recent on a geological timescale. Color differences around these openings could correspond to deposits left by gases rising from underground.
The researchers are considering the presence of a still-hot magma reservoir deep below the surface. Gases could escape from it and rise through fractures in the ground. During the Martian winter, frost could block some openings. When the ice changes directly into vapor as milder temperatures return, the passage could reopen.
This scenario would explain why the gas appears in the same place and during a particular season. However, the instruments detected neither magma directly nor a recent lava flow. The study, published in Science Advances, combines atmospheric measurements with observations of the terrain.
The discovery nevertheless changes which places deserve examination on Mars. If molten rock still lies beneath Alba Mons, it could warm water present underground. On Earth, some hot underground environments harbor microbes. This provides a concrete reason to study these fissures.
New observations during different seasons could determine whether the concentrations return above the same fractures. Researchers will also need to distinguish this possible underground leak from ordinary chemical reactions in the atmosphere.