Each Martian spring, a water-ice cloud stretches about 1,800 kilometres above Arsia Mons. It forms through an extreme process that requires no dust or other impurities. Mars Express observations and meteorological modelling suggest homogeneous nucleation is at work, a process not previously observed in a real planetary atmosphere.
In September 2018, Jorge Hernández Bernal spotted a shadow in images from the European Space Agency's Mars Express probe. Researchers later identified it as a cloud that returns each day during the Martian spring.
The cloud reaches roughly 150 kilometres in width beside Arsia Mons. A study in Nature Geoscience, titled Homogeneous Ice Nucleation from Water Vapour Suggested by Elongated Clouds on Mars, says it forms through homogeneous nucleation. The process requires no suspended dust or other impurities. Jorge Hernández-Bernal, Annika Määttänen, Aymeric Spiga and François Forget wrote the paper. Its DOI is 10.1038/s41561-026-02089-9.
The researchers compared Mars Express observations with meteorological modelling. Their work does not measure homogeneous nucleation directly inside the cloud; the conclusion comes from the model results. A scheme based only on ordinary heterogeneous nucleation could not reproduce the cloud's observed structure.
That finding helps explain why researchers considered the mechanism highly unlikely on Mars. The planet has low atmospheric humidity and airborne dust, conditions expected to make direct ice formation difficult. An independent analysis of the study says the air had to become more than 100,000 times as supersaturated as air usually found on Earth.
Mars Express's Visual Monitoring Camera showed that the feature was not a passing shadow or a brief visual artefact. Its regular appearance identified it as a seasonal event and gave researchers a way to study the atmosphere around Arsia Mons.
The physical explanation begins with the volcano's height and the air moving around it. Arsia Mons is an ancient volcano about 20 kilometres high, but volcanic eruptions do not produce the cloud. Winds passing over the volcano create a gravity wave, lifting moist air several kilometres in a few minutes.
As the air rises, it cools quickly. The study records a temperature drop of about 30 degrees Celsius in 10 minutes, followed by a sharp rise in relative humidity. Water vapour can then freeze directly into ice particles without first becoming liquid. The cloud consists of water ice formed as atmospheric flow is forced over the terrain.
This is homogeneous nucleation. In the more familiar process, water droplets or ice form around dust and other particles. Here, no foreign surface triggers the process. The Martian atmosphere and the abrupt lift above Arsia Mons create the conditions for a process not previously observed in a real planetary atmosphere.
Ricardo Hueso, director of the Planetary Sciences Group at UPV/EHU, says the result overturns established assumptions about Martian clouds. In his assessment, clouds on Mars can behave very differently from those on Earth.
Meteorologist José Miguel Viñas links the process to the forced rise and rapid cooling of air above the volcano. The study's authors call the result the first evidence of homogeneous water-vapour nucleation in a planetary atmosphere.
Daniel Toledo of INTA points to the finding's wider scientific value. Events at the molecular scale shape the cloud's enormous size and reach. A microscopic change in how water freezes helps create a formation that spans 1,800 kilometres of the Martian sky.
The study does not establish how often homogeneous nucleation occurs elsewhere on Mars or how much it affects the planet's water cycle. It does show that low humidity and dust do not rule out the process when the terrain creates a sharp atmospheric change.
Modelling suggests the cloud's long tail could not be reproduced without homogeneous nucleation. The conventional heterogeneous-nucleation model fell short, while the updated scheme matched the features observed by Mars Express. That result could prompt revisions to cloud microphysics in Mars climate models.
Arsia Mons offers a natural setting for studying how volcanic relief shapes atmospheric motion and cloud formation. The findings could also inform research on clouds on Earth and other planets, whose models may need to account for homogeneous nucleation.
The cloud follows a strict daily cycle. It appears around local morning, grows for roughly three hours and disappears several hours later. The pattern repeats for about 80 days or longer during the relevant Martian season, giving researchers a regular test case for cloud microphysics.
