Paris Model Reproduces Daily Martian Cloud Using Rare Ice Physics

100,000 times. That is the ratio of relative humidity required to form the Arsia Mons Elongated Cloud on Mars compared to any value ever measured on Earth, according to a new model published in Nature Geoscience.

The narrow, wispy formation appears each morning during the Martian spring and summer near the Arsia Mons volcano. It stretches more than 1,000 miles, a distance comparable to the straight-line span between New York and Kansas City, before dissipating. A probe first recorded the feature in 2018, but researchers have struggled to explain its formation until now.

Jorge Hernández-Bernal, a planetary scientist at Sorbonne Université in Paris, led the study. He stated that previous simulations failed to replicate the cloud because they omitted a specific physical process. The team incorporated what Hernández-Bernal termed "exotic physics," referring to homogeneous nucleation. In this process, water vapor transforms directly into ice crystals without condensing around dust or salt particles. He compared the phenomenon to droplets of condensation appearing in the middle of a room rather than on a window.

The model indicates that the volcano itself creates the necessary conditions. When wind encounters the 65,000-foot peak, it generates a powerful wave that lifts moist air high into the atmosphere. This lift triggers a temperature drop of 30 degrees Celsius within 10 minutes, a rapid change that facilitates the direct formation of ice.

Hernández-Bernal noted that while scientists have long assumed such nucleation could occur in the upper atmospheres of planets like Venus and Earth, it has never been directly observed. The extreme humidity levels required on Mars explain the lack of prior detection. He added that far less is known about the Martian atmosphere than Earth's, making the successful reproduction of the cloud a major achievement. "The result is remarkable," he said.

The study marks the first time a model has successfully replicated the daily cycle of the Arsia Mons Elongated Cloud. The research, originally reported by WIRED Czech Republic and Slovakia, suggests that textbook physics previously considered theoretical may be active in planetary atmospheres.