Modeling suggests Mars’s Arsia Mons cloud may form ice without dust, pointing to a process never before observed in a planetary atmosphere.
A white cloud grows downwind of Mars’s Arsia Mons volcano each morning during spring and summer in the planet’s southern hemisphere. Rising beside a volcano 20 kilometers (about 12 miles) tall, this wisp of water ice can stretch up to 1,800 kilometers (about 1,120 miles), nearly twice the length of the UK, before quickly evaporating. The cycle repeats every morning for several months during the Martian dusty season.
Attempts to reproduce how the Arsia Mons Elongated Cloud (AMEC) forms had failed to match spacecraft images. Now, a study combining observations from the European Space Agency’s Mars Express with an advanced meteorological model suggests that the cloud may depend on an ice-forming process never before seen in a planetary atmosphere.
Cloud ice may form without dust
The AMEC is an orographic cloud, a type also found on Earth that forms as winds flow past the uneven terrain of a mountain or volcano. In the simulations, Arsia Mons triggers a powerful wave that lifts moist parcels of air several kilometers in just a few minutes. As the air rises, its temperature drops by 30 degrees in just 10 minutes, sending relative humidity soaring. Mars’s thin atmosphere and the volcano’s towering height combine to create the exceptional conditions the model needs.
Clouds usually form when moist air cools and water vapor condenses into liquid droplets or ice crystals. Typically, the vapor gathers on tiny particles already floating in the atmosphere, such as salt, pollen, soot, or dust. This process is called heterogeneous nucleation, and dust is thought to provide those starting points for clouds on Mars.
“For the AMEC, it seems that cloud formation takes place without needing any of this ‘stuff’,” says Jorge Hernández-Bernal of LMD/CNRS/Sorbonne Université in Paris, France, lead author of the new study. “Water vapor turns directly into icy cloud particles without any middle step. It’s akin to droplets of condensation appearing in the middle of a room, rather than on a window. We call this homogeneous nucleation, and we’ve never seen it before in a planetary atmosphere. It’s wholly unexpected.”
Homogeneous nucleation requires extreme relative humidity levels, more than 100,000 times those usually experienced in everyday life on Earth. Scientists had proposed that it might occur in the upper atmospheres of Earth and Venus, but it has not been spotted there. “We’ve not seen these conditions on Mars before, but our finding now strongly suggests that the planet’s humidity can indeed reach these extreme levels,” says Hernández-Bernal.
Mars Express catches the morning cloud
The observations used to build the model come from a spacecraft that has followed the cloud for years. Mars Express first revealed the AMEC in 2018, and repeated observations have allowed researchers to investigate its evolution and changing behavior. For the new study, the team used data from three cameras aboard the orbiter: the Visual Monitoring Camera, High-Resolution Stereo Camera, and OMEGA.
Mars Express can capture large stretches of the Martian surface at high resolution. Alongside ESA’s ExoMars Trace Gas Orbiter, it is also one of the few spacecraft orbiting Mars that can observe during the morning hours when the cloud is present.
“Mars Express can also track how the cloud is changing on timescales of mere hours, which gives us an unrivaled view of short-lived phenomena on the planet,” says ESA Mars Express Project Scientist Colin Wilson. “Overall, this finding is a true accomplishment for the mission and its scientists: Mars Express discovered the AMEC, has followed up and monitored it for years, and is now helping reveal the secrets of its formation.”
Exotic physics helps recreate Mars’s cloud
Reproducing those observations required the researchers to expand the physics included in their simulations. “To create the AMEC in our modeling, 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. It certainly hasn’t been seen in action before,” says Hernández-Bernal. “Once we included this physics in our simulations, the AMEC emerged just as we hoped.”
Some features of the modeled cloud still do not exactly match the observations, although Hernández-Bernal says “the result is remarkable.” He adds: “We don’t have nearly as much information about Mars’s atmosphere as we do about Earth’s, so reproducing the AMEC to this degree is a big success for the model.”
The findings suggest that processes considered unlikely may deserve attention when scientists investigate atmospheres elsewhere, including those of exoplanets. “While clouds on Earth and Mars seem to be governed by the same ‘rules’, understanding this exotic Martian cloud required exotic physics—and this may be true elsewhere in the cosmos,” says Wilson.
Reference: “Homogeneous ice nucleation from water vapour suggested by elongated clouds on Mars” by J. Hernández-Bernal, A. Määttänen, A. Spiga and F. Forget, 7 October 2026, Nature Geoscience.
DOI: 10.1038/s41561-026-02089-9
This work was supported by CNES, focused on the Mars Express and Exomars missions, and has received funding from the European Research Council under the European Union Horizon 2020 research and innovation program (grant agreement no. 835275), project ‘Mars Through Time’.
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