A 1,800 km cloud on Mars points to ice born without dust, a first in nature

Every morning for a few months of the Martian year, a white streak of ice up to 1,800 km long, nearly twice the length of the United Kingdom, peels off the flank of the Arsia Mons volcano. A new study finds that the only way to rebuild it in a computer is to let water vapor freeze straight into ice crystals, with no speck of dust to start the process. Physicists call that homogeneous nucleation, and until now it existed in textbooks, not in any sky anyone had observed.

Nearly every cloud on Earth needs a seed. Water vapor condenses onto a grain of salt, soot, pollen or dust, and the droplet or ice crystal grows from there. On Mars the seeds are thought to be the fine dust that hangs in its thin atmosphere. The Arsia Mons Elongated Cloud, the most striking cloud on the planet, appears to skip that step entirely.

That matters beyond one strange cloud on one planet. Atmospheric scientists build weather and climate models, on Earth and on other worlds, on the assumption that clouds form on particles. A natural case where they do not means that assumption has an exception, and the team says the exception may apply to the highest, coldest clouds on Earth and Venus as well.

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“To create the AMEC in our modelling, we found that we needed to include some exotic physics… physics that, while included in textbooks, is treated as theoretical and usually thought not to happen in nature,” said Jorge Hernández-Bernal of the Laboratoire de Météorologie Dynamique in Paris, who led the work. “It certainly hasn’t been seen in action before.”

A cloud that forms at sunrise and runs west for hours

Arsia Mons is a shield volcano about 20 km tall, the southernmost of the three giants lined up on the Tharsis plateau. The cloud does not come from an eruption. It is water ice, and it forms about 45 km above the surface, on the volcano’s western slope.

Earlier observations by the same group pieced together its daily routine. The cloud starts to form before dawn, then stretches westward for about two and a half hours at roughly 170 meters per second. When it stops growing, the long tail detaches from the volcano and drifts on until it evaporates, before the afternoon. The cycle repeats every morning for about 80 days each Martian year, around the southern summer solstice, in a dusty season when clouds are otherwise scarce at those latitudes.

That timing is why the cloud went unexplained for so long. Most Mars orbiters cross the planet in the afternoon, when the cloud is already gone. Mars Express, the European Space Agency orbiter, and the ExoMars Trace Gas Orbiter are among the few spacecraft that can watch the morning side.

How they did it

The team, from two atmospheric laboratories in the Paris region, used a high-resolution weather model of Mars that couples the Weather Research and Forecasting solver, a code built for Earth, with the physics of the Mars Planetary Climate Model. They then compared its output with images from three instruments on Mars Express: the Visual Monitoring Camera, the High Resolution Stereo Camera and the OMEGA spectrometer.

Run with standard cloud physics, in which ice can only form on dust, the model failed. It produced the right winds but not the long, sharp tail. The missing piece was a strong mountain wave. As wind flows over Arsia Mons, the volcano’s bulk lifts parcels of moist air several kilometers in a few minutes. According to ESA, the air cools by about 30 degrees in ten minutes, and relative humidity climbs to levels more than 100,000 times those of everyday life on Earth.

At that point the air holds so much excess vapor that it freezes on its own. Once the researchers added homogeneous ice nucleation to the model, the cloud appeared where it does on Mars, grew a tail, narrowed before letting go and detached later in the morning, much like the real one. In the pre-review version of the paper, the team calculated that hundreds of ice particles per cubic centimeter form this way in the coldest pocket of air. That is enough to soak up the excess water vapor, so the tail stays stable as the wind carries it west.

Why Earth’s clouds don’t do this

The physics is not new. What is new is seeing it at work. Water vapor needs an extreme degree of supersaturation to freeze without help, and on Earth, air reaching that point almost always finds a particle first. Even in the mesosphere, where Earth’s highest clouds glow at dusk, homogeneous nucleation has been discussed only as a theoretical possibility. The same goes for the upper atmosphere of Venus.

Mars offers the right mix at one spot. The atmosphere is thin and carries relatively few dust particles at that altitude, while Arsia Mons is tall enough to force a violent, fast lift. “Water vapour turns directly into icy cloud particles without any middle step,” Hernández-Bernal said. “It’s akin to droplets of condensation appearing in the middle of a room, rather than on a window.”

Colin Wilson, ESA’s Mars Express project scientist, said the probe “discovered the AMEC, has followed up and monitored it for years, and is now helping reveal the secrets of its formation.”

What it doesn’t settle

The evidence is a model that matches the pictures, not a measurement of ice crystals forming. No instrument has sampled the cloud, and none could at 45 km above Arsia Mons. The authors’ own wording reflects that: the paper’s title says homogeneous nucleation is “suggested” by the clouds, and the abstract says the result “strongly suggests” it happens on Mars.

The match is also imperfect. Hernández-Bernal acknowledges that some aspects of the modeled cloud don’t exactly fit the observations. In the version of the paper posted before peer review, the simulated cloud was 30% to 50% narrower than the real one, started about an hour and a half late and reached only around a quarter of its observed length before detaching. The final, peer-reviewed text is not freely available yet, and the authors say it includes further analysis.

The strongest alternative is a population of very fine dust, particles smaller than 100 nanometers. Nobody has measured how much of it floats over Arsia Mons in that season. The team tested it and found that this finer dust produces broad, diffuse hazes rather than the sharp, bright tail seen by Mars Express’s high-resolution camera. Without direct data, though, that alternative is weakened rather than ruled out.

Common questions about the Arsia Mons cloud

What is the Arsia Mons Elongated Cloud?

It is a water-ice cloud that forms every morning on the western side of Arsia Mons, a 20 km tall volcano on Mars. It can stretch up to 1,800 km and repeats daily for about 80 days each Martian year, around the southern summer solstice.

Is the long cloud on Mars a volcanic eruption?

No. When images of it circulated widely, many people took it for a plume, but Arsia Mons is not erupting. The cloud is made of water ice, shaped by winds flowing over the volcano.

What is homogeneous nucleation?

It is the formation of ice or droplets directly from vapor, without a dust grain or other particle to start on. It needs extreme supersaturation, so it is rarely possible in real air. This study is the first strong case for it happening in a planetary atmosphere.

Do clouds on Earth form this way?

Almost all of them form on particles such as dust, salt or soot. Scientists have proposed that some of the highest clouds in Earth’s mesosphere might form without them, but nobody has observed it. The Mars result gives researchers a reason to look again.

What comes next

The study appeared in Nature Geoscience on October 7, 2026. The cloud returns every Martian year during the southern spring and summer, and Mars Express and the ExoMars Trace Gas Orbiter remain among the few spacecraft able to watch it at dawn. The team says its next work will test whether homogeneous nucleation is unique to Arsia Mons or happens in other Martian clouds, and whether it plays any role in the planet’s global water cycle.

Reference: Hernández-Bernal et al., “Homogeneous ice nucleation from water vapour suggested by elongated clouds on Mars,” Nature Geoscience, 2026. DOI: 10.1038/s41561-026-02089-9

Tags: astronomy, Mars, esa, Arsia Mons, clouds, Mars Express

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