Sorbonne model explains Mars cloud with unseen ice physics

A new modeling study offers the first successful simulation of a recurring Martian cloud, identifying a physical process that has never been directly observed in any planetary atmosphere. The research, published in Nature Geoscience, explains the formation of the Arsia Mons Elongated Cloud, a feature that has puzzled scientists since its first detection in 2018.

The cloud appears during Martian spring and summer, stretching more than 1,000 miles before dissipating. This length is comparable to the straight-line distance between New York and Kansas City. The phenomenon repeats over several months, growing gradually throughout the morning before vanishing.

Jorge Hernández-Bernal, a planetary scientist at Sorbonne Université in Paris, led the study. He stated that the simulations only produced the cloud after incorporating what he termed exotic physics. While this specific process appears in textbooks, it is generally treated as theoretical and unlikely to occur in nature. Hernández-Bernal noted that the result is remarkable given the limited understanding of the Martian atmosphere compared to Earth.

On Earth, clouds form when water vapor condenses around particles like dust or salt. The study proposes that the Martian cloud forms through homogeneous nucleation, where vapor turns directly into ice crystals without any debris to seed the process. Hernández-Bernal described this mechanism as wholly unexpected, comparing it to condensation appearing in the middle of a room rather than on a window.

This process requires extreme conditions, specifically a temperature drop of 30 degrees Celsius within 10 minutes and high relative humidity. The 65,000-foot Arsia Mons volcano plays a central role in creating these conditions. When wind meets the volcano, it generates a powerful wave that lifts moist air high into the atmosphere within minutes.

The humidity required for this nucleation is more than 100,000 times greater than any value measured on Earth. Scientists had previously assumed similar nucleation could occur in the upper atmospheres of Venus and Earth, but the extreme humidity needed explains why it had never been observed. Researchers also did not previously know that such humidity levels existed on Mars.

The study marks a shift in understanding Martian meteorology by reproducing a complex atmospheric feature through a rare physical mechanism. The findings suggest that the conditions for cloud formation on Mars are far more extreme than previously modeled.