A peer-reviewed study published in Science Advances on November 4, 2026, reports six confident detections of hydrogen chloride (HCl) in Mars’s atmosphere near Alba Mons during Martian Year 37 (MY37). The authors argue that a local igneous source best explains the signal’s unusual location and season, but present that source as an interpretation. Read the study.
The unusual clue is a localized HCl signal near Alba Mons during early northern summer.
What TGO detected near Alba Mons
The ExoMars Trace Gas Orbiter’s Atmospheric Chemistry Suite (ACS) found HCl in six MY37 observations taken during the aphelion season. Across the MY35 and MY37 search, the team examined 78 northern-hemisphere solar occultations between solar longitude (Ls) 100° and 120°. Two earlier MY35 observations near Alba Mons were inconclusive because the data were limited.
The six confident MY37 detections fell between Ls 107° and 119°. Four were at 55°–66°N and two at 43°–48°N. The atmospheric profiles reached concentrations of up to 1.5 parts per billion by volume (ppbv), with HCl concentrated in the lowest layers measured.
How ACS measured the gas
ACS used infrared solar occultations: as the Sun rose or set behind Mars, the instrument measured sunlight absorbed as it passed through the atmosphere. Researchers used those absorption measurements to calculate profiles of HCl at different altitudes.
Aerosols can weaken the solar signal at lower altitudes, so the lowest level ACS could reach varied with local conditions, from a few kilometers to a few tens of kilometers. The measurements describe HCl in the lower atmosphere; they are not direct samples of the Martian surface.
Two seasonal HCl patterns on Mars
The Alba Mons detections differ from Mars’s widespread HCl cycle during the warmer perihelion season. The study places that cycle’s peak at about 4–5 ppbv during southern-hemisphere spring and summer, around Ls 230°–330°, and attributes it mainly to chlorine-bearing aerosols and atmospheric chemistry.
| Pattern | Season | Distribution | Reported concentration | Authors’ explanation |
| Widespread perihelion HCl | Southern-hemisphere spring and summer, around Ls 230°–330° | Widespread, especially at southern high latitudes | Peaks around 4–5 ppbv | Mainly aerosol and atmospheric chemistry |
| Alba Mons aphelion HCl | Early northern summer; MY37 detections at Ls 107°–119° | Localized near Alba Mons | Profiles reached up to 1.5 ppbv | A local igneous source best fits the authors’ analysis |
Here, ppbv means parts per billion by volume, a way to express the proportion of a gas in the atmosphere. The two patterns occur in different seasons and distributions, and the study treats them as separate phenomena.
Why the authors propose a local igneous source
The study’s authors say the atmospheric conditions and chemistry models they assessed do not explain why the aphelion detections cluster near Alba Mons. They conclude that a local igneous source best fits their analysis. They also discuss mud volcanism and the breakdown of chloride compounds as possible alternatives.
Orbital imagery shows fissures and other structures associated with the region’s tectonic and volcanic geology. The study considers these possible pathways, while the HCl measurements themselves are atmospheric profiles. The European Space Agency said no new lava flows or eruptions had been observed in its release about the study.
Further detections noted in MY38
The paper also reports HCl detections over the same area during MY38 aphelion, corresponding to terrestrial summer 2025. It defers detailed analysis of those observations to a separate study.
