Something Weird is Seeping Out of Mars

The inner solar system is brimming with volcanic fire. Earth has around 40 volcanoes erupting at any one moment. And scientists generally agree that Venus, despite being shrouded in thick clouds, has its own lava-spewing mountains. But Mars, once covered in active volcanoes, is now silent. Nobody has ever seen any definitive signs of volcanic activity there.

But that doesn’t mean that Mars is volcanically dead—and some tantalizing new evidence suggests we shouldn’t count it out just yet.

Over the past few years, the European Space Agency’s ExoMars Trace Gas Orbiter spacecraft has detected bursts of hydrogen chloride, a particularly stinky gas reminiscent of vinegar mixed with a dangerously strong toilet cleaner, in the Martian atmosphere. While the gas can originate from other sources, it has been linked to volcanic outbursts on Earth. On Mars, it also seems to be coming from a series of cracks on the slopes of a colossal volcano, and it’s possible that these vapors are eking out of sheets of molten rock bubbling about beneath the surface.

This evidence, reported in a new paper in the journal Science Advances, is not definitive. But “altogether,” says Daniel Mège, a planetary scientist at the Polish Academy of Sciences in Wrocław and one of the authors of the new study, signs “point to a volcanic origin.”

The find plays into a longstanding debate over how small planets like Mars generate heat to fuel volcanoes—and potentially support microbial life.

Today, the surface of Mars is frigid and arid, but ancient volcanoes, such as the behemoth Olympus Mons, covered the planet in lava and ashy, pumice-rich debris. Heat left over from its formation and from the decay of radioactive elements drove these eruptions. Unfortunately for Mars, its small mass, just one-tenth that of Earth, couldn’t sufficiently insulate that internal heat, which quickly escaped into space. Its limited reserves of radioactive elements also couldn’t make up for this heat loss. Its geologic heart rapidly cooled down, and the planet’s era of profuse volcanism eventually ran out of steam billions of years ago.

But lava didn’t stop appearing altogether, with some flows appearing—at least based on orbital imagery—to be just one or two million years of age, perhaps younger. And there have been hints that Martian volcanoes weren’t dead but merely sleeping.

(Today, it’s a massive volcano on Mars—but it may have once been an island.)

A pivotal moment came in 2018, when NASA’s InSight lander touched down in Mars’s Elysium Planitia region. There, its seismometer detected a series of odd marsquakes, which indicated that fluid might be moving around below the surface. “Some folks think it’s magma; others think it’s water,” says Tracy Gregg, a planetary volcanologist at the University at Buffalo who wasn’t involved with the new work. “I think it might be both.”

To date, the evidence for ongoing volcanism on Mars remains equivocal. But Europe’s Trace Gas Orbiter has now added another alluring clue to the mystery.

In 2021, scientists revealed that the orbiter detected tiny amounts of hydrogen chloride, or HCl, in the Martian atmosphere. This colorless, corrosive and atrociously pungent gas can be emitted by volcanoes on Earth when the magma below rises and decompresses. Most of this HCl probably isn’t linked to volcanism, though. Instead, it’s connected to Mars’s truly alien weather.

During the warmer spring and summer months on Mars, HCl pervades the entire southern hemispheric sky. Some of it may come from the vaporization of the Martian ice, which releases trapped HCl into the atmosphere. When the southern hemisphere's colder autumn arrives, ice particles start forming. The lofted HCl sticks to this falling ice and gets dragged down to the surface.

And yet, the new study “identified several very unusual detections” of HCl in the northern hemisphere, says Alexander Trokhimovskiy, a researcher at the Russian Space Research Institute in Moscow and one of the authors of the new study.

The same seasonal spike shouldn’t be happening in the northern hemisphere, where it’s still pretty cold in the summer. Mars’s orbit is not as circular as Earth’s, which means it’s sometimes closer to the Sun, and sometimes further away. The southern summer happens to coincide with proximity to the sun, while the northern summer coincides with a distant part of the orbit. That means the northern hemisphere doesn’t undergo an intense cycle of vaporizing, then freezing, of Martian ice, as happens in the southern hemisphere.

Weirdly, the northern hemisphere’s HCl appears as distinct, sharp pulses. “The really intriguing thing is that these observations occur in the same place: the northern flank of the Alba Mons volcano,” says Kevin Olsen, an atmospheric physicist at the University of Oxford and one of new study’s authors.

Alba Mons is roughly the same area as the contiguous United States, making it by far the most expansive volcano on Mars. It’s also covered in youthful lava flows, indicating it was volcanically active in the recent geologic past.

Using additional orbital imagery, the team conducted a careful examination of its gassy northern flank. They spotted a series of strange fissures, each around 330 feet in length. These fissures “are thought to be associated with vertical magma sheets,” says Mège. The team speculate that there could be magma down there, and that this molten rock may be releasing HCl into the sky. The signal is only detected during the summer months, because during the winter the ground is covered in ice, which prevents the gas from escaping.

(This 280-mile-long volcano may have been hiding in plain sight.)

The team acknowledges that this isn’t a smoking gun for ongoing volcanism. They cannot be certain that the HCl is seeping out of these exact fissures. And “the nature of the source is unknown,” says Olsen. “Is it just pockets of gas underground, or is there actually active magma?”

HCl can also originate from chemical reactions; those between hot water and metal chlorides can easily make it. Chlorides have previously been detected on Mars, and the warmer crust is likely to harbor a few hydrothermal labyrinths here and there.

The InSight lander also never detected any convincing magmatic seismic signals coming from Alba Mons, or Tharsis in general, the ginormous volcanic province on which the volcano sits. That doesn’t mean the region is, volcanically speaking, definitively dead. “The Tharsis region was home to the planet’s largest and likely longest-lived magma chambers, and they would take a long time to fully cool down, degas, and freeze solid,” says Gregg.

Serpents of gassy magma may still be meandering about down there, somewhere—and this latest study may support that notion. If so, it might mean that Mars is still capable of producing eruptions, albeit very sporadic ones. It may also lend credence to the idea that, beneath the surface, magmatic activity – the sort driving the flow of hot water and certain chemicals – might create a subterranean environment conducive to microbial life.

Even if the HCl is coming from hidden magma, nobody’s forecasting an imminent Martian eruption. As for whether Mars might repaint its surface further into in the future, ultimately, we still don’t know. “Planets are big, complicated bodies and unpredictable things can always happen,” says Gregg. “In volcanology, we’ve learned to never say never.”