Sorbonne Model Forces 100,000x Humidity to Explain Mars Cloud

A narrow cloud that stretches over 1,000 miles across the Martian surface each morning may rely on atmospheric physics that have never been directly observed on any planet. A new modeling study suggests the formation of the Arsia Mons Elongated Cloud, or AMEC, depends on a specific process that challenges long-standing assumptions about how clouds form in planetary atmospheres.

The AMEC appears during Martian spring and summer. It grows longer through the morning until it reaches a size comparable to the straight-line distance between New York and Kansas City, then dissipates. This cycle repeats over several months. A probe first recorded the cloud in 2018, but scientists struggled to find a plausible explanation for its existence and rapid change.

The new work, published in Nature Geoscience, is the first to successfully reproduce the cloud's formation in a computer model. Lead author Jorge Hernández-Bernal, a planetary scientist at Sorbonne Université in Paris, stated that the simulations only functioned after the team included what he termed exotic physics. He noted in a press release that while this physics appears in textbooks, it is typically treated as theoretical and considered unlikely to occur in nature.

"Once we included this physics in our simulations, the AMEC emerged just as we hoped," Hernández-Bernal said. The study proposes that the cloud forms through homogeneous nucleation. In this process, water vapor turns directly into ice crystals without needing dust or salt particles to seed the droplets. Hernández-Bernal compared the phenomenon to condensation appearing in the middle of a room rather than on a window, calling the mechanism wholly unexpected.

The process demands extreme conditions. The model requires a temperature drop of 30 degrees Celsius within 10 minutes and very high relative humidity. The 65,000-foot Arsia Mons volcano is central to creating these conditions. When wind hits the peak, it generates a powerful wave that lifts moist air high into the atmosphere within minutes.

Scientists have long assumed similar nucleation could occur in the upper atmospheres of Venus and Earth, but it has never been observed. The model suggests the humidity required on Mars is more than 100,000 times greater than any value measured on Earth. Researchers did not previously know such humidity levels existed on the Red Planet. Hernández-Bernal noted that far less is known about the Martian atmosphere than Earth's, making the successful reproduction of the cloud a notable achievement. "The result is remarkable," he said.