Mars Cloud Formation Requires Humidity 100,000 Times Higher Than Earth
New model reproduces the 1,000-mile Arsia Mons cloud using a freezing process never before observed in planetary atmospheres.
By Sakura Novak ·
More than 100,000 times the highest relative humidity ever measured on Earth may be necessary to create a specific morning cloud on Mars.
Researchers at Sorbonne Université have published a model in Nature Geoscience that successfully replicates the formation of the Arsia Mons Elongated Cloud, or AMEC. The feature stretches over 1,000 miles, a distance comparable to the straight-line route between New York and Kansas City. It appears each morning during Martian spring and summer, grows gradually, and then dissipates by late morning. The cycle persists for several months before stopping.
A probe first detected the cloud in 2018, but scientists struggled to explain its existence. Standard cloud physics relies on water vapor condensing onto tiny particles like dust or salt. The new model suggests the AMEC forms through homogeneous nucleation, a process where vapor turns directly into ice crystals without any seed particles. Lead author Jorge Hernández-Bernal, a planetary scientist at Sorbonne Université, described this as including 'exotic physics' in the simulations. He noted that while this mechanism appears in textbooks, it is typically treated as theoretical and assumed not to occur in nature.
Hernández-Bernal compared the process to condensation appearing in the middle of a room rather than on a window, calling the result 'wholly unexpected.' The simulation only worked when this specific physics was included. Once added, the AMEC emerged in the model as predicted.
The formation depends on extreme local conditions generated by the Arsia Mons volcano. The peak stands 65,000 feet high. When wind hits the mountain, it creates a powerful wave that lifts moist air into the upper atmosphere within minutes. This rapid ascent causes the temperature to drop by 30 degrees Celsius in just 10 minutes.
The required humidity levels are so high that they explain why this type of nucleation has not been observed on Earth or Venus. Scientists previously assumed similar processes might exist in the upper atmospheres of other planets, but the extreme moisture needed on Mars was unknown until this study. Hernández-Bernal stated that far less is known about the Martian atmosphere than Earth's, making the accurate reproduction of the cloud a 'remarkable' achievement.
