Newly documented traces of hydrogen chloride (HCl) – a stinky gas often produced by magmatism – in the Martian atmosphere corroborates emerging evidence that suggests the planet is still volcanically active. As recorded by the ExoMars Trace Gas Orbiter, the gas appears locally above a region with fresh surface fissures. For a long time, planetary scientists have been unsure whether and to what degree Mars is still volcanically active. Yet, scientists recently discovered it experiences marsquakes, implying that there is ongoing volcanic activity. One way to corroborate this evidence is by looking for volcanic outgassing, or when magmatic activity creates emissions of gases such as sulfur dioxide and HCl. Here, Alexander Trokhimovskiy and colleagues have analyzed data from the Atmospheric Chemistry Suite on the ExoMars Trace Gas Orbiter, observing HCl at parts-per-billion levels in the Martian atmosphere. They documented these higher-than-expected concentrations during the cold aphelion season near the Alba Mons region. The HCl levels appeared above newly discovered fissures in the Martian surface, which Trokhimovskiy et al. hypothesize reach down to active magma. "Detections of HCl at low altitudes, unexplained by observed atmospheric state and models, together with the geological context of the region, hint at an igneous origin of this gas," Trokhimovskiy et al. write.
For reporters following updates on the red planet's roiling interior, a 2025 Science study found that Mars has a highly mixed and jumbled mantle, born from early collisions and erratic convection: www.science.org/doi/10.1126/science.adk4292
