Although Mars’ atmosphere is very thin and consists almost entirely of carbon dioxide, there are also weather phenomena in it: dust storms can cover the entire planet, winds and gusts of wind stir up dust, and even thin clouds of ice can form at high altitudes. However, one of these clouds is extraordinary – and huge in size: the Arsia Mons Long Cloud (AMEC) is an around 1,800 kilometer long whitish plume of water ice at a height of around 45 kilometers, which continually forms on the western slope of the Arsia Mons volcano.
“Every day, this cloud forms at sunrise, expands rapidly horizontally over the next three hours and then detaches from the volcano,” report Jorge Hernández-Bernal from Sorbonne University in Paris and his colleagues. A short time later the cloud dissipates – only to grow back again the next morning. Images from Mars probes show that the recurring cycle of the Arsia-Mons long cloud occurs in the spring and summer of the Martian southern hemisphere and lasts for several months.
This image from the Mars Express spacecraft shows the Arsia Mons long cloud in June 2024. — © ESA/DLR/FU Berlin/CC-by-sa 4.0
How does the exotic Martian cloud form?
But how does this exotic Martian cloud form? On Earth, clouds typically form when moist air cools and water vapor condenses into liquid droplets or ice crystals. However, condensation germs are necessary to trigger this – particles such as salt, pollen, soot or dust. The water molecules attach to these and this is what enables drops or ice to form. This process is called heterogeneous nucleation. It typically occurs where moist air rises quickly and therefore cools down – for example on mountain slopes.
The recurring Martian cloud also originates from a mountain – the 20 kilometer high volcano Arsia Mons. And there is actually enough dust there as cloud seeds. But when researchers tried to reconstruct the formation of this cloud in the model, they failed. “None of the proposed mechanisms could convincingly reproduce the elongated tails of this cloud,” explain Hernández-Bernal and his colleagues.
The problem of condensation
That’s why the team considered a different mechanism of cloud formation – homogeneous nucleation. During this process, water vapor condenses spontaneously without the need for aerosols or other solid surfaces to be present in the air. “Water vapor transforms directly into ice crystals, without an intermediate step. This is as if condensation droplets were formed in the middle of a room and not at a window,” explains Hernández-Bernal.
The catch: “Such a homogeneous nucleation is considered impossible under real atmospheric conditions,” explain the researchers. This spontaneous condensation required an extreme supersaturation of the air with water vapor that never occurs on Earth. “This process is therefore in the textbooks, but has never been observed in a planetary atmosphere. It was considered impossible on Earth,” says the team. And this was also considered extremely unlikely on Mars.
“Impossible” nucleation in the test
“Early analyzes showed that for homogeneous nucleation there, a relative humidity that would be 100,000 times higher than the typical values on Earth would be necessary,” explain Hernández-Bernal and his team. But Mars and its thin gas shell are actually far drier than Earth’s atmosphere. Nevertheless, the planetary scientists decided to test this exotic mechanism in the model.
To do this, they fed data on the temperatures and composition of the atmosphere around the Martian volcano Arsia Mons into a climate model of Mars and also took into account the topography and its influence on air mass movements. “To create the long cloud in our model, we had to incorporate some exotic physical processes,” explains Hernández-Bernal.
Comparison of real Mars cloud formation (above) with the simulation. — © ESA/EHU (above); Hernandez-Bernal et al., Nature Geoscience 2026 (below)
Longcloud puzzle solved
The surprising result: If the researchers assumed homogeneous nucleation, they were able to understand in their model how the recurring Arsia Mons long cloud forms. “As soon as we included this physics in our simulations, the AMEC emerged exactly as we expected,” reports Hernández-Bernal. Conditions at the Arsia Mons volcano were found to create unique conditions, including extreme supersaturation of the local atmosphere with water vapor.
Specifically, the simulations revealed that when Martian winds hit the Arsia Mons volcano, its enormous size causes a series of quasi-stationary gravity waves that quickly lift the air and cool it significantly. The temperatures drop by 30 degrees within ten minutes, while at the same time the relative humidity rises sharply. “This creates a compact, cold zone west of Arsia Mons with a water vapor saturation ratio of the order of 100,000,” reports the team.
Exotic physics for an exotic cloud
This means that exactly where the unusual Martian cloud forms, the atmosphere of the Red Planet meets the exotic conditions for homogeneous nucleation. “This is completely unexpected,” says Hernández-Bernal. “Such conditions have not been observed on Mars so far, but our results suggest that humidity on Mars can indeed reach these extreme values.” Mars could therefore be the first planet in whose atmosphere the exotic process of homogeneous nucleation has been detected.
According to the researchers, this underlines how important it is to take seemingly impossible or unlikely processes into account when exploring other planets. “Although clouds on Earth and Mars appear to follow the same basic rules, understanding this exotic Martian cloud required exotic physics – and this could also be the case elsewhere in the cosmos,” comments Colin Wilson of the European Space Agency (ESA).
Source: Jorge Hernández-Bernal (Sorbonne Université, Paris) et al., Nature Geoscience, 2026; doi: 10.1038/s41561-026-02089-9