A wisp of water ice up to 1,800 km long forms downwind of Arsia Mons, a Martian volcano about 20 km tall, grows through the morning and evaporates before the day is out. A simulation study published in Nature Geoscience on Oct. 7, 2026 says the only way its models could reproduce that cloud was to let water vapor freeze directly into ice with no dust grain to seed it.
The lead author, Jorge Hernández-Bernal of the Laboratoire de Météorologie Dynamique in Paris, called the process “wholly unexpected.”
The Arsia Mons Elongated Cloud: 1,800 km by midday
The European Space Agency’s Oct. 7 release describes the Arsia Mons Elongated Cloud, or AMEC, as a white wisp of water ice that forms, grows and fades in a single Martian day, stretching up to 1,800 km before it evaporates. It returns every morning through the southern spring and summer dusty season, for several months at a time. ESA’s Mars Express orbiter revealed the cloud in 2018, and earlier work classed it as an orographic cloud, one that forms as wind flows over a volcano or mountain.
An earlier ESA account of the cloud’s life cycle gave the daily choreography: it starts before sunrise on the western slope of Arsia Mons, grows westward for about two and a half hours at around 45 km altitude, then detaches, drifts farther west on high-altitude winds and evaporates in late morning. At its largest it is about 1,800 km long and 150 km wide.
The cloud is a recurring feature rather than a one-off. The earlier ESA account says Arsia Mons is the only low-latitude volcano in the region with this kind of cloud at this time of year, that its full extent was hard to observe because of the atmosphere’s rapid changes and limited spacecraft orbits, and that Viking 2 images from the 1970s had partly captured it. Agustín Sánchez-Lavega, quoted in that account, said that understanding the cloud gives the chance to try to replicate its formation with models, which is the step the new paper takes.
Homogeneous nucleation: a 30-degree drop in 10 minutes
Orographic cloud models could not reproduce what Mars Express saw. The team, which also includes Aymeric Spiga and François Forget of the same Paris laboratory and Anni Määttänen of LATMOS, reported in “Homogeneous ice nucleation from water vapour suggested by elongated clouds on Mars” (Nature Geoscience 19, 1213-1217) that a homogeneous nucleation scheme, built into a Martian meteorological model, reproduces the cloud’s distinctive features where conventional cloud microphysics had struggled.
In ESA’s account, wind flowing past the volcano triggers a wave that lifts moist air several kilometers within minutes. The air cools by about 30 degrees in 10 minutes and its relative humidity spikes, to levels reported at more than 100,000 times those typical of daily life on Earth. Water vapor then freezes straight into ice, without dust or other particles to start the crystals. Hernández-Bernal said in the release that once this physics was included in the simulations, the AMEC emerged just as the team had hoped.
Researchers had proposed the process for the upper atmospheres of Earth and Venus, but it had not been spotted in either. The paper’s title carries the word “suggested” for a reason: the evidence is a simulation that matches the cloud, not a direct measurement of those humidity levels on Mars.
Simulation numbers from the Nature Geoscience paper
The paper’s runs combine the Mars Planetary Climate Model and the LMD mesoscale model with a zero-dimensional microphysics model for individual air parcels. In the reference simulation, shown at 8:05 local time near 8.7 degrees south, a cold core carries temperature anomalies of 15 to 30 kelvin below the mean at each altitude. Ice formed this way reaches a number density of about 200 particles per cubic centimeter with an effective radius near 0.25 micrometers, and the simulated cloud has an optical depth of about 0.36 near the volcano.
The match is not perfect. ESA’s release says the model reproduces the cloud’s emergence to a large degree but that some aspects do not exactly match the observations. The release does not list which ones.
Mars Express and the morning orbit problem
Morning coverage is the scarce resource. ESA notes that Mars Express and the ExoMars Trace Gas Orbiter are among the few orbiters able to observe Mars in the morning hours when the cloud is present, and that Mars Express can follow changes over hours. The study drew on Mars Express’s Visual Monitoring Camera, High Resolution Stereo Camera and OMEGA instrument. The release is the first from ESA to feature High Resolution Stereo Camera images of the cloud.
Colin Wilson, ESA’s Mars Express project scientist, said: “Mars Express discovered the AMEC, has followed up and monitored it for years, and is now helping reveal the secrets of its formation.” Sci.News and Astrobiology.com relayed the same findings on Oct. 7 and Oct. 8. The paper’s abstract adds that the result may challenge assumptions about how clouds form on Earth and other planets.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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