A gas seeping from cracks on a giant Mars volcano hints the planet may not be volcanically dead

On the northern slopes of Alba Mons, one of the largest volcanoes on Mars, there are thin, fresh cracks in the ground. Each early northern summer, a European spacecraft keeps smelling the same gas above them.

On 4 November, scientists reported in the journal Science Advances that ESA’s ExoMars Trace Gas Orbiter (TGO) found hydrogen chloride over those cracks. They think it may rise from magma deep below. If they are right, Mars volcanic activity is not only ancient history. The planet may still hold some heat inside.

That matters beyond one volcano. Heat underground can mean warm, wet places, and those shape where future missions will look for signs of life.

A faint gas, always in the same place and season

The orbiter’s Atmospheric Chemistry Suite, an infrared spectrometer, measured about 2 parts per billion of hydrogen chloride in the air, ESA says. That is tiny, but more than expected.

The gas appeared in early northern summer, the cold “aphelion” season when Mars is farthest from the Sun. In that season this gas is normally absent from Mars’s air, according to the University of Oxford. Yet it kept returning to the same spot, at the same time of year.

The orbiter’s CaSSIS camera photographed the area on 3 February 2023. The pictures show narrow fissures, less than a metre wide and some hundreds of metres long, with colour changes around them.

How a frozen lid could let a volcano breathe

The team’s idea is simple to picture. A dormant body of magma sits deep under Alba Mons and slowly gives off gas. In winter, ice seals the cracks like a lid. In early summer, the ice turns to vapour and the trapped gas escapes. The coloured ground may be mineral deposits, like those around fumaroles, the gas vents on volcanoes on Earth.

“The Trace Gas Orbiter was able to find the smoking gun,” said co-lead author Daniel Mège of the Space Research Centre of the Polish Academy of Sciences and the University of Bern, in the Oxford release. Colin Wilson, ESA’s TGO project scientist, said in the same release that the findings “may prove to be TGO’s greatest success yet.”

The lead author is Alexander Trokhimovskiy of the Space Research Institute (IKI) of the Russian Academy of Sciences. Co-authors include Oleg Korablev and Kevin Olsen, who has moved from Oxford to the Canadian Space Agency.

Is Mars still volcanically active? Not proven yet

Here the scientists are careful. No lava flows or eruptions have been seen. The orbiter found no sulphur dioxide, a gas usually linked to eruptions. The paper says the results “hint at” a volcanic origin. It also discusses another idea: frozen ground, or permafrost, could trap the gas and release it later.

The hint fits a wider picture of Mars volcanic activity in the recent past. NASA’s InSight lander recorded marsquakes, and some lava plains look young. Neither proves an active volcano today. Together, they suggest Mars is quiet, not completely dead.

TGO first found this gas on Mars in 2021, and ESA linked it then to salty dust lifted from the surface. The new paper argues that, at Alba Mons, there may be a deeper source.

What a sleeping volcano could mean for the search for life

For students and space fans, this is a clear lesson in how science works: a strange signal, years of checking, then a careful “hint”, not a headline claim. It also comes from a wide team: an ESA spacecraft, with scientists from Russia, Poland, Switzerland, the UK and Canada.

For everyone, the bigger question is water. Where hot rock meets underground water, warm and possibly habitable pockets could form, Oxford notes. Mège is clear, though, that the conditions for life may never have been met there.

Why a careful “maybe” is worth more than a big claim

On balance, this is a meaningful step, and the team deserves credit for its restraint. A gas that returns to the same cracks every early summer is hard to explain away. The fresh fissures make the volcanic reading more likely.

Still, one gas at a few parts per billion is thin evidence. Without sulphur dioxide or a visible eruption, the permafrost idea stays alive. In practice, results like this usually need several more seasons of watching before they settle. It also fits what we keep learning about the planet’s past: one of the oldest Mars meteorites shows water once moved through its crust. Further out, Webb is studying a lava planet far beyond our Solar System.

The likelier outcome is patience, not a quick answer.

Next northern summer, the orbiter will look again. If the gas returns to Alba Mons on schedule, Mars may be telling us it is only sleeping.