Paris Model Replicates Martian Cloud Using Extreme Humidity and Rare Ice Physics

A 1,000-mile fog that forms daily near a Martian volcano may finally have a scientific explanation. Researchers at Sorbonne Université in Paris published a modeling study in Nature Geoscience that successfully reproduces the formation of the Arsia Mons Elongated Cloud, or AMEC.

The cloud appears during Martian spring and summer. It stretches longer through the morning until it spans a distance comparable to New York to Kansas City before vanishing. A probe first detected the feature in 2018, but scientists have struggled to explain its existence for years.

Lead author Jorge Hernández-Bernal, a planetary scientist at Sorbonne Université, stated that the simulations only worked after adding what he called exotic physics. This specific process is found in textbooks but is generally considered theoretical rather than natural. He noted that once the team included this physics, the AMEC emerged as expected.

The model proposes that the cloud forms through homogeneous nucleation. In this process, water vapor turns directly into ice crystals without dust or salt particles to seed the formation. Hernández-Bernal compared the phenomenon to condensation appearing in the middle of a room instead of on a window. He described the result as wholly unexpected.

The formation requires extreme conditions. The model shows a temperature drop of 30 degrees Celsius within 10 minutes combined with high relative humidity. The 65,000-foot Arsia Mons volcano creates these conditions when wind hits its peak, generating a wave that lifts moist air high into the atmosphere in minutes.

The humidity required is more than 100,000 times greater than any value measured on Earth. Scientists had long assumed similar nucleation could occur in the upper atmospheres of planets like Venus, but it has never been directly observed. The extreme levels found in the model explain why the process had remained hidden until now.

Hernández-Bernal emphasized that far less is known about the Martian atmosphere than Earth's. This lack of data makes the successful reproduction of the cloud a major achievement for the team. He called the result remarkable, noting that the discovery clarifies a long-standing mystery in planetary science.