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Mile-Long Martian Cloud Forms Out of Nothing, Scientists Report; Textbook Physics Invoked
Researchers say a yearly ice cloud over Arsia Mons can be modelled only if a process long deemed theoretical is allowed to occur.
ARSIA MONS, MARS — A cloud that appears each year above a Martian volcano has led scientists to propose a process never before identified in a planetary atmosphere, according to a paper published in Nature Geoscience on October 7, 2026.
The Arsia Mons Elongated Cloud, or AMEC, forms in Mars's southern hemisphere during the spring and summer dust season. It consists of water ice, and the report states it can stretch up to 1,800 kilometers, forming in the early morning and dissipating later in the day. The cycle repeats over several months. The cloud has been studied since its discovery in 2018.
The authors say they have now built the first model that closely matches the cloud's observed formation and behavior. Lead author Jorge Hernández-Bernal, of LMD/CNRS/Sorbonne Université in Paris, said the model required "exotic physics" that textbooks treat as theoretical and usually not seen in nature.
The process is called homogeneous nucleation. Clouds ordinarily form around a nucleus such as dust, salt or pollen, and Martian clouds were long assumed to form around dust. In the AMEC, according to the researchers, water vapor turns directly into ice particles without any such seed. Mr. Hernández-Bernal likened it to condensation appearing in the middle of a room rather than on a window.
The team attributes the effect to the height of the roughly 12-mile-high volcano and the thin Martian atmosphere. Winds pushing past the mountain lift moist air, which cools by about 30°C in approximately 10 minutes, while relative humidity climbs sharply. Such extreme supersaturation had been thought exceptionally rare, and the authors say their finding suggests Martian humidity can indeed reach such levels.
A note of caution accompanies the announcement: the model reproduces the cloud's behavior closely, but further work is needed for an exact match with observations. ESA's Colin Wilson, whose agency's Mars Express spacecraft discovered and has monitored the cloud, concluded that understanding it required exotic physics, and that this may hold elsewhere in the cosmos.