Sorbonne Scientists Model Arsia Mons Cloud Using Humidity 100,000 Times Earth Levels

A cloud that stretches across more than 1,000 miles above Arsia Mons each Martian morning forms through a process scientists have never observed in any planetary atmosphere, according to a study published in Nature Geoscience. The feature, called the Arsia Mons Elongated Cloud or AMEC, lengthens through the morning until it spans a distance comparable to the trip from New York to Kansas City, then disappears. The cycle repeats for several months during the Martian spring and summer.

The model requires a temperature drop of 30 degrees Celsius within 10 minutes, combined with high relative humidity. Arsia Mons rises 65,000 feet, and when wind strikes the volcano it generates a wave that lifts moist air high into the atmosphere within minutes. A probe first recorded the cloud in 2018, and researchers had not reproduced its formation in a simulation before this work.

Jorge Hernández-Bernal, a planetary scientist at Sorbonne Université in Paris and lead author of the study, said the simulations succeeded only after researchers included what he called "exotic physics." He said in a press release that the process appears in textbooks but is treated as theoretical and usually thought not to occur in nature. "Once we included this physics in our simulations, the AMEC emerged just as we hoped," he said.

The proposed mechanism is homogeneous nucleation, in which water vapor turns directly into ice crystals without dust or salt particles to seed the process. On Earth, clouds typically form when vapor condenses around such particles. Hernández-Bernal compared the Martian case to condensation appearing in the middle of a room rather than on a window, calling it "wholly unexpected."

Humidity is the constraint. The levels required exceed any value measured on Earth by more than 100,000 times, and researchers had not previously established that such humidity exists in the Martian atmosphere. Scientists have long assumed the same nucleation process could occur in the upper atmospheres of Venus and Earth, but it has not been observed there.

Hernández-Bernal said far less is known about the Martian atmosphere than Earth's, which he said makes reproducing the cloud to this degree a significant achievement. "The result is remarkable," he said.