Mars's Arsia Mons Cloud Points to Rare Homogeneous Ice Nucleation
Modeling of Mars Express images suggests the Arsia Mons Elongated Cloud forms by water vapor freezing directly, with no dust needed, a first for a planet.
Mars’s most striking cloud may form through a process never before seen in a planetary atmosphere. A team led by Jorge Hernández-Bernal of LMD/CNRS/Sorbonne Université in Paris combined images from ESA’s Mars Express orbiter with an advanced Mars weather model. The study is published in Nature Geoscience, and ESA announced it on October 7, 2026.
The cloud is the Arsia Mons Elongated Cloud, or AMEC. Each spring and summer in Mars’s southern hemisphere, during the dusty season, a wisp of water ice forms downwind of Arsia Mons, a volcano about 20 km tall. It grows to as much as 1,800 km long, nearly twice the length of the UK, then evaporates, and the cycle repeats every morning for several months. Mars Express first revealed it in 2018 and has watched it repeatedly since. It is an orographic cloud, the kind that forms when wind is forced over mountains or volcanoes.
The puzzle was that earlier simulations could not reproduce what the images show. The team found that the model only produced the AMEC once it included homogeneous nucleation. On Earth, clouds usually form by heterogeneous nucleation: vapor condenses onto specks such as dust, salt or soot. In homogeneous nucleation, water vapor turns directly into ice particles with no seed needed. Hernández-Bernal calls the result “wholly unexpected.”
This requires extreme conditions, with relative humidity more than 100,000 times what people usually experience on Earth. In the model, wind hitting Arsia Mons drives a powerful wave that lifts moist air several kilometers in a few minutes. The air cools by about 30 degrees in 10 minutes, humidity spikes, and the vapor freezes spontaneously into the cloud. The team says the finding strongly suggests Martian humidity can reach such levels. Some details of the modeled cloud do not match the observations exactly, though the team calls the overall agreement remarkable.
The model drew on data from three Mars Express instruments: the Visual Monitoring Camera, the High Resolution Stereo Camera and OMEGA. Mars Express can observe in the morning, when the AMEC is present, and can track changes over a few hours.
ESA’s Colin Wilson notes the lesson may reach beyond Mars, including to exoplanets: unlikely processes should not be dismissed. For more on Mars and its neighbors, see our solar system guide and the solar system news archive.
Primary source
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