A study published in Science Advances on November 4, 2026, reports six confident detections of hydrogen chloride (HCl) in the Martian atmosphere near Alba Mons. The authors say the signal’s location and season fit a possible local igneous source, but the measurements do not directly observe magma or an eruption. The study used ExoMars Trace Gas Orbiter’s Atmospheric Chemistry Suite (ACS) to measure the gas in the atmosphere.
The unusual clue is where and when the HCl appeared: near Alba Mons during Mars’s northern aphelion season.
What ExoMars TGO measured
ACS used infrared solar occultation, a technique that measures atmospheric gases as sunlight passes through the atmosphere, to retrieve HCl profiles. The observations were not samples taken from the surface.
The researchers examined 78 northern-hemisphere occultations from Martian Years (MY) 35 and 37, covering solar longitude (Ls) 100°–120°. A Martian year is one full orbit around the Sun; solar longitude marks Mars’s position in that orbit. Six observations in MY37 were confident detections. Two earlier MY35 observations near Alba Mons were inconclusive.
The MY37 detections fell at Ls 107°–119° and 43°–66° north latitude, chiefly northeast of Alba Mons. The highest reported atmospheric profiles reached 1.5 parts per billion by volume (ppbv), a measure of how much gas is present in a volume of air.
How the Alba Mons signal differs from Mars’s other HCl cycle
The Alba Mons detections appeared during early northern summer, around aphelion, when Mars is farthest from the Sun. Their localized distribution differs from a separate HCl pattern that spreads widely during southern-hemisphere spring and summer.
| HCl pattern | Season and distribution | Reported concentration | Study interpretation |
| Alba Mons detections | Early northern summer during aphelion; localized near Alba Mons | Profiles up to 1.5 ppbv | The authors favor a local igneous source to explain the location |
| Widespread seasonal cycle | Southern-hemisphere spring and summer; widespread | Peaks of about 4–5 ppbv | Mainly aerosol-related atmospheric chemistry |
The higher peak in the widespread cycle belongs to a different season and distribution. Its presence does not explain why the Alba Mons signal appeared in a localized patch during northern aphelion.
Why the authors favor a local igneous source
The study’s authors say a local igneous source best explains the Alba Mons detections’ unusual location. They also consider mud volcanism and the decomposition of chloride compounds, but say those alternatives do not account for the localization as well. The igneous explanation remains an interpretation of atmospheric measurements and nearby geology.
The study identifies fresh fissures less than 1 meter wide as possible pathways for gas seepage. The fissures’ geological setting supports the proposed explanation; the study does not report seeing HCl emerge from each fissure.
ESA says no new lava flows or eruptions have been observed on Mars in connection with the finding. Its announcement describes the result as a hint of possible volcanic activity, while the study’s evidence is the localized atmospheric HCl signal.
