Model Replicates Daily Martian Cloud Using Rare Ice Physics

The Arsia Mons Elongated Cloud, a feature that stretches over 1,000 miles across the Martian morning sky, appears to form through a mechanism never before directly observed in a planetary atmosphere. A new study published in Nature Geoscience presents a model that successfully reproduces the cloud's daily appearance and disappearance.

The cloud, known as AMEC, emerges during the Martian spring and summer. It grows throughout the morning until it spans a distance 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 phenomenon in 2018, but scientists had struggled to find a plausible explanation for its formation until now.

Jorge Hernández-Bernal, a planetary scientist at Sorbonne Université in Paris, led the research team. He stated that the simulations only succeeded when they incorporated what he termed "exotic physics." According to Hernández-Bernal, this process appears in textbooks but is generally treated as theoretical, with scientists usually assuming it does not occur in nature. "Once we included this physics in our simulations, the AMEC emerged just as we hoped," he said.

On Earth, clouds typically form when water vapor condenses onto 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 seed particles. Hernández-Bernal compared the process to condensation appearing in the middle of a room rather than on a window, describing the finding as "wholly unexpected."

The formation 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 encounters the volcano, it generates a powerful wave that lifts moist air high into the atmosphere within minutes.

The model requires humidity levels more than 100,000 times greater than any value measured on Earth. Scientists previously assumed similar nucleation could occur in the upper atmospheres of planets like Venus and Earth, but the extreme humidity requirements likely explain why the process had not been observed. Researchers did not previously know that such humidity existed on Mars.

Hernández-Bernal noted that far less is known about the Martian atmosphere than Earth's, which makes reproducing the cloud's formation a notable achievement. "The result is remarkable," he said. The study provides the first successful model-based explanation for the AMEC's existence and behavior.