Is Mars a planet whose geological activity has ended, or is magma still active deep underground? New clues to this long-standing question have emerged from atmospheric observations. A research team analyzing data from the ExoMars Trace Gas Orbiter (TGO), a Mars probe operated by the European Space Agency (ESA) and Roscosmos, has found localized hydrogen chloride (HCl) in the atmosphere above the area near Alba Mons, a giant volcano in Mars's northern hemisphere. The findings were published in the journal Science Advances on November 4, 2026 (DOI: 10.1126/sciadv.aeb9422).
However, what was confirmed is hydrogen chloride in the atmosphere. No lava flows or high-temperature fumaroles have been found. Alexander Trokhimovskiy, the paper's lead author, and colleagues do not present the discovery as proof of current volcanic activity. Instead, they report it as a promising clue: when the distribution of the gas in the atmosphere is considered together with the surrounding geological structures, it may be of igneous origin.
Trace amounts of hydrogen chloride detected above Mars
The discovery used the mid-infrared channel (MIR) of ACS (Atmospheric Chemistry Suite), the atmospheric chemistry instrument aboard TGO. ACS MIR observes infrared light at wavelengths of 2.2 to 4.4 μm and determines the types and concentrations of trace gases from the absorption spectra produced when sunlight passes through the Martian atmosphere.
The observations relied on solar occultation, which uses the moment when the Sun sets below or rises above the Martian horizon as seen from the spacecraft. Because sunlight travels a long path through the atmosphere, even slight absorption by trace gases is easier to detect. The team closely examined the wavenumber range of 2894 to 3239 cm⁻¹, where absorption lines characteristic of hydrogen chloride appear, and derived concentrations and their distribution by altitude.
The observations covered Ls (solar longitude) 100° to 120° in Martian Years (MY) 35 and 37. A Martian year is equivalent to about 687 Earth days, and this period falls in northern-hemisphere summer, when Mars is moving away from the Sun, near aphelion. Of the 78 occultation observations in the northern hemisphere analyzed by the team, hydrogen chloride was clearly detected in 8: two in MY35 and six in MY37. The MY35 data were initially inconclusive on their own.
データを表で見る
| 観測数 (件) | |
|---|---|
| HClを有意に検出 | 8 |
| 検出上限0.2 ppbv以下 | 70 |
Notably, the detections were concentrated in a narrow range between 96°W and 117°W. Elsewhere, even near Alba Mons, no hydrogen chloride was detected, with the upper limit from observations at 0.2 ppbv (0.2 parts per billion by volume) or less. At the detection sites, by contrast, concentrations reached up to 1.5 ppbv. The hydrogen chloride was also concentrated near the lowest observable altitudes, and its concentration dropped sharply over a rise of 10 km in altitude.
Solar occultation does have observational limitations. Because dust and ice particles (aerosols) in the atmosphere block sunlight, observations normally cannot reach down to the surface, and the lowest observable altitude is limited to a few to several tens of kilometers. This time, aerosols were locally sparse, so measurements at unusually low altitudes were possible, but the resulting altitude-dependent concentration profiles still carry analytical uncertainty. The team therefore could not directly pinpoint where on the surface the hydrogen chloride was released from.
Features that differ from dust-storm-driven hydrogen chloride production
This is not the first time hydrogen chloride has been found in the Martian atmosphere. Its presence has been reported from TGO observations since 2021. But the phenomenon observed around Alba Mons differs significantly from the previously known formation process.
Hydrogen chloride has mainly been observed during perihelion season, when Mars is closest to the Sun. This is the southern-hemisphere summer, when large dust storms occur and dust and water vapor are carried to high altitudes. Chloride-bearing dust and water vapor are thought to react through electrostatic and photochemical processes, producing hydrogen chloride over wide areas.
The aphelion season, when the current observations were made, is on average about 30 K colder than perihelion season (a temperature difference of about 30 °C). Water vapor stays at low altitudes and there is less suspended dust. These conditions are very different from those in which the wide-ranging production associated with dust storms becomes active.
| 比較項目 | 従来の近日点期の塩化水素循環 | アルバ山周辺の遠日点期の異常 | 解釈上の注意点 |
|---|---|---|---|
| 観測時期 | 南半球の夏。砂嵐が頻発し、比較的温暖 | 北半球の夏。近日点期より平均約30 K寒冷 | 季節的な氷の変化が検出条件に影響した可能性 |
| 地理的分布 | 惑星規模の広い範囲 | 西経96°〜117°に局在 | 観測範囲の偏りによる見落としの可能性 |
| 水蒸気と砂塵 | 水蒸気や砂塵が高高度まで輸送される | 水蒸気は低高度に集中し、砂塵は少ない | 既存モデルでは急激な濃度勾配を再現できない |
| 想定される生成・供給源 | 砂塵中の塩化物と水蒸気の反応 | 地下マグマの脱ガスと断裂系を通じた浸出 | 火山起源は仮説であり、共存ガスは未検出 |
| 推定供給量 | 火星全体で約180 kg/s | 局所的な浸出量は3 kg/s未満と試算 | いずれもモデルに依存し、直接測定値ではない |
The existing atmospheric chemistry models examined by the team could not reproduce a phenomenon in which hydrogen chloride is concentrated in a specific region and forms a steep concentration gradient at low altitude, as seen here. In the models, aphelion-season hydrogen chloride is predicted to be spread thinly over a wide area of the north polar region, or to appear along water vapor transport paths away from Alba Mons. Explaining the observed localized rise in concentration requires considering a different source.
Key to understanding this localization is the lifetime of hydrogen chloride in the atmosphere. Water-ice aerosols in the Martian atmosphere adsorb and remove hydrogen chloride, and the effective lifetime of hydrogen chloride at levels of a few ppbv is thought to be less than a few sols (Martian days) in environments where ice particles are present. One sol is about 24 hours 39 minutes, so if hydrogen chloride is removed quickly, gas released from a local source could be lost before it spreads across the planet.
Still, differences in concentration may not arise from differences in source alone. It is also possible that seasonal declines in ice particles made hydrogen chloride less likely to be removed and therefore easier to detect. However, when the team examined the state of the surrounding atmosphere, it found no evidence of unusual warming or complete disappearance of ice only around Alba Mons. At the very least, it is difficult to explain the localized distribution this time with known atmospheric chemical processes alone.
The amount of hydrogen chloride supplied to the entire Martian atmosphere during the conventional perihelion season is estimated at about 180 kg/s. This differs by nearly two orders of magnitude from the upper limit of the local seepage rate assumed here, suggesting the two may arise from different mechanisms. This difference alone, however, does not prove that igneous activity plays no role at all in broad-scale hydrogen chloride production.
Fractured terrain around Alba Mons suggests possible subsurface activity
If atmospheric chemical reactions alone cannot explain the observations, attention turns next to the geological structures at the surface. The team investigated what kind of terrain lies directly beneath the area where hydrogen chloride was detected.
Analysis of images from the high-resolution imagers HiRISE (High Resolution Imaging Science Experiment) and CaSSIS (Colour and Stereo Surface Imaging System) showed numerous thin cracks near the detection sites. The cracks are only tens of centimeters wide, and form fracture groups extending for tens of kilometers along ridges up to 30 m high.
These fractures could serve as pathways for gas to travel from underground to the surface. Particularly notable is that the cracks cut through what is known as the "latitude-dependent mantle," an ice-rich surface deposit. The deposit is estimated to have formed about 630,000 years ago (0.630 Ma). However, this age refers to the deposit, and does not directly indicate when the cracks formed or when current gas release began.
The team examined publicly available high-resolution images covering a radius of about 1 degree, or about 60 km on the Martian surface, centered on the sites where hydrogen chloride was detected at low altitude. Comparing images taken at different times, however, it found no clear changes in terrain or new collapses. And because the cracks are extremely thin, estimating their formation age by crater counting is also difficult to apply. On Mars, small cracks can be buried by wind-blown dust, so the preservation of sharp shapes suggests relatively recent geological activity, but does not establish when they formed.
Based on these observations, the team proposed a hypothesis that hydrogen chloride released from magma deep underground is seeping to the surface through a fracture system. The envisioned mechanism is that hydrogen chloride produced by magma degassing reaches the atmosphere through a two-layer subsurface fracture system while interacting with shallow ice-bearing layers. In this case, only gas from underground could leak out, without large-scale ice melting or conspicuous thermal anomalies at the surface. This is also consistent with the fact that no high-temperature fumaroles were found in these observations.
Still, the source of the hydrogen chloride is not necessarily magma. The team also considered non-igneous possibilities, such as mud volcanism and the thermal or radiolytic decomposition of perchlorates in Martian soil. But these materials and phenomena are widely distributed across the northern lowlands of Mars, and cannot adequately explain why hydrogen chloride was detected only in a limited area around Alba Mons. These alternative hypotheses have not, however, been completely ruled out by experiments or on-site observations.
What is needed to confirm volcanic activity on Mars
The discovery is an important clue for considering whether geological activity continues inside Mars today. But concluding that magma is active underground from the presence of hydrogen chloride still involves uncertainties in both observational data and models.
Assuming hydrogen chloride is present at about 1 ppbv in a local region equivalent to less than 1% of the Martian atmosphere, with an effective lifetime of 1 sol, the team estimated the supply rate from the surface. The upper limit of the seepage rate came out at less than 3 kg/s. This is not a figure measured at a surface vent, but one back-calculated from assumptions about atmospheric concentration and gas lifetime. It can be compared with hydrogen chloride emissions from volcanoes on Earth, but a simple comparison is not possible because gravity, atmospheric pressure, magma water content, and chemical composition all differ.
How long the activity has continued is also unknown. That hydrogen chloride was detected at widely separated times, MY35 and MY37, is consistent with an interpretation in which a source has existed over multiple years. But it cannot be determined whether the activity has persisted for tens to hundreds of thousands of years, or whether this is a temporary release of gas.
The team further reports that it obtained a new hydrogen chloride detection signal around Alba Mons during the MY38 aphelion season, which corresponds to summer 2025 in Earth time. However, detailed analysis of this observation has not been completed, and it must be distinguished from quantitatively confirmed results. If further analysis reconfirms detection in the same area, it would be an important piece of evidence for considering the existence of a continuing source.
Another major unresolved problem is that volcanic gases such as sulfur dioxide (SO₂) have not been detected. At volcanoes on Earth, sulfur-bearing gases are often released along with hydrogen chloride. In these observations, however, no co-existing gas that would independently support a magmatic interpretation has been confirmed. Whether subsurface aquifers or permafrost layers are absorbing sulfur components, whether the magma has a different chemical composition, or whether the source is not volcanic at all remain issues for future verification.
The discovery also does not show that life exists on Mars, or that hydrothermal environments capable of supporting life exist underground. What was observed is an extremely small amount of hydrogen chloride, at most 1.5 ppbv, and from that alone it is impossible to judge subsurface temperatures, the state of water, or the potential for life.
To confirm whether igneous activity continues on Mars today, it will be necessary to monitor whether hydrogen chloride detections around Alba Mons recur, track changes at the surface with high-resolution images, search for temperature anomalies with thermal infrared observations, and detect co-existing gases such as sulfur dioxide. Obtaining two independent clues, trace gas in the atmosphere and geological structures at the surface, is significant, but it has not yet been proven that the two are linked by current magmatic activity. Follow-up observations are expected to reveal whether this localized hydrogen chloride reflects activity inside Mars.