European Space Agency’s ExoMars Trace Gas Orbiter detects hydrogen chloride spikes above Alba Mons

The European Space Agency’s ExoMars Trace Gas Orbiter detected hydrogen chloride spikes above volcanic fissures on the flanks of Alba Mons during early summer in Mars’s northern hemisphere.

The gas appeared only above those fissures in early summer, when hydrogen chloride should have vanished, forming a consistent exception to the normal seasonal cycle. Scientists interpret this as gas rising from below the surface at potential active spots.

TGO’s CaSSIS camera revealed fresh, narrow fissures at Alba Mons. Some reach hundreds of meters in length, and surface color changes accompany them. The orbiter’s Atmospheric Chemistry Suite infrared spectrometers detected the gas in a strong localization shortly after winter frosts sublimated. CaSSIS imaged the Alba Mons fissures afterward.

On Mars, hydrogen chloride follows a seasonal cycle. Levels peak in northern hemisphere winter and fade in summer. Ice seals Martian fissures in winter when temperatures drop as low as minus 120 degrees Celsius, then releases chloride as the ice sublimates (turns directly from ice to gas) during early summer.

On Earth, hydrogen chloride often escapes through magmatic vents. That led researchers to ask whether the same venting happens on Mars. The orbiter first detected hydrogen chloride in the Martian atmosphere in 2021.

Scientists have long considered Mars geologically dead, but observations from spacecraft suggest its volcanoes might only be dormant. Olympus Mons, an extinct Martian volcano, stands around 16 miles (26 kilometers) above the nearby Martian plains.

Daniel Mège of the Space Research Centre of the Polish Academy of Sciences said the planet has retained enough internal heat to permit volcanic activity until recently, and possibly still does. “Our findings suggest that Mars has retained enough internal heat to support volcanic activity until recently, and possibly still does,” Mège said.

The spatial distribution of the gas seeping instantly suggested volcanism to him, but he had to demonstrate the link through rigorous examination of Martian atmospheric chemistry, Mège said.

Scientists have never observed new lava flows or eruptions on Mars. Images from orbit show some volcanoes were active within the last few million years, and seismometers have detected marsquakes.

Magmatic gases may be rising through fissures at Alba Mons, scientists suggest, with the seepage possibly connected to a dormant magma chamber under the Martian surface. The team proposes the chamber as the source of the gases and describes it as a volcanic plumbing system beneath the surface.

Earlier emissions of magmatic gases such as hydrogen chloride may have been more abundant. Those releases may have helped create a thicker atmosphere and warmer climate, allowing liquid water to cover much of Mars.

If molten rock moving through the crust meets groundwater, the result could be warm habitable zones. Active hydrothermal systems could supply ingredients and energy for microorganisms while shielding them from the harsh surface environment. That makes Alba Mons seepage a logical place to search for Martian life.

Colin Wilson, the European Space Agency project scientist for the ExoMars Trace Gas Orbiter, said the findings could turn out to be TGO’s greatest success yet. The discovery appeared in Science Advances and resulted from combined atmospheric and geological analysis.

Separate research has also examined how Mars retains internal heat. Lead researcher Alexander Berne of the University of Arizona published a Nature journal study showing heat beneath Mars’s southern mantle could keep rock at least partially molten. His research questioned the idea that planetary interiors are uniform and spherically symmetric, showing Mars’s interior dichotomy matches its surface split.

Researchers used tidal tomography—a method that infers interior temperature from tiny changes in spacecraft orbits—on archival tracking records. The records came from NASA’s Mars Global Surveyor, Mars Odyssey, and Mars Reconnaissance Orbiter. The method measured orbital velocity shifts tied to the planet’s gravitational field. The method built an inferred interior temperature model by tracking small orbital velocity shifts caused by the Sun’s gravity bending Mars through its tilted, eccentric orbit.

Mars’s southern hemisphere consists of ancient, heavily cratered highlands topped by thick crust. The northern hemisphere features smooth, low-lying plains with much thinner crust. The crustal thickness contrast between the southern highlands and northern plains, known as the Martian dichotomy, has long puzzled astronomers.

Seismic readings from NASA’s retired InSight lander showed waves fading faster through southern Mars. That pattern lines up with trapped heat energy in the south. Magnetic traces in southern iron-bearing minerals point to a warm mantle that once powered strong magnetic and volcanic activity across Mars.

Researchers disagree about what divided Mars’s heat distribution. One model says an ancient northern impact more than four billion years ago took away the upper crust and allowed the northern mantle to cool quickly. A different idea holds that thick crust in the southern highlands acted as a planetary thermal blanket, keeping heat inside for billions of years.