Sorbonne University model identifies hidden physics behind Mars' daily cloud

A new model explains the formation of a massive cloud that appears daily on Mars.

The Arsia Mons Elongated Cloud stretches across more than 1,000 miles each morning before disappearing. It first appeared in records from 2018 and persists during the Martian spring and summer. The feature grows until it spans a distance comparable to the straight line between New York and Kansas City.

Researchers at Sorbonne University in Paris published the findings in Nature Geoscience. The study marks the first successful reproduction of the cloud's lifecycle in a simulation. Lead author Jorge Hernández-Bernal stated that the model only worked after adding specific physical processes.

Hernández-Bernal described these processes as exotic physics found in textbooks but rarely assumed to occur in nature. He noted that the physics is typically treated as theoretical. "Once we included this physics in our simulations, the AMEC emerged just as we hoped," he said in a press release.

The model proposes that the cloud forms through homogeneous nucleation. Unlike Earth clouds, which rely on dust or salt particles to seed droplets, this Martian cloud forms when water vapor turns directly into ice crystals. Hernández-Bernal compared the process to condensation forming in the middle of a room rather than on a window.

The 65,000-foot volcano Arsia Mons drives the conditions needed for this reaction. Wind striking the peak creates a wave that lifts moist air rapidly into the atmosphere. This action triggers a temperature drop of 30 degrees Celsius within 10 minutes.

The process requires extreme relative humidity. The levels needed in the model are more than 100,000 times greater than any value measured on Earth. Scientists had previously assumed this type of nucleation might exist on Venus or Earth, but they had never observed it. Hernández-Bernal called the result remarkable because far less is known about the Martian atmosphere than Earth's.