Enigmatic HCl detections as a potential signature of igneous activity on Mars

Mars is reputed to lack ongoing volcanic activity. However, marsquakes were revealed recently, and clues to volcanism younger than tens of million years have accumulated. While sulfur dioxide, so far undetected, could testify to ongoing eruptive activity, evidence of active magma chambers may be better identified by seepage of halogen gases, such as hydrogen chloride (HCl). Observations by Atmospheric Chemistry Suite led to the widespread observations of HCl at part-per-billion levels in the atmosphere of Mars during the warm perihelion period, attributed to aerosol-atmospheric sources. Here, we report HCl detections during the cold aphelion season that are isolated in the Alba Mons region. We also identify fresh and narrow (<1 meter) fissures in this area, which we interpret to be potential HCl pathways rooted into magmatic bodies at depth. Our results are best explained by ongoing igneous processes in the Martian interior.