Rare Freezing Physics Explains Mars' 1,000-Mile Morning Cloud

A narrow cloud stretching more than 1,000 miles across Mars each morning likely forms through a physical process that has never been directly observed in any planetary atmosphere.

The feature, known as the Arsia Mons Elongated Cloud or AMEC, appears during Martian spring and summer. It grows longer through the morning until it reaches a size comparable to the straight-line distance from New York to Kansas City, then dissipates. This cycle repeats over several months.

Scientists first recorded the cloud in 2018 but struggled to explain its formation. A new study published in Nature Geoscience is the first to successfully reproduce the cloud's behavior in a computer model. The research team at Sorbonne Université in Paris included what lead author Jorge Hernández-Bernal called exotic physics in their simulations. He noted that this physics appears in textbooks but is usually treated as theoretical and unlikely to occur in nature. Once included, the model produced the AMEC as expected.

Earth clouds typically form when water vapor condenses into droplets or ice crystals around tiny particles like dust or salt. The new model proposes that the Martian cloud forms through homogeneous nucleation, where vapor turns directly into ice crystals without any debris to seed the process. Hernández-Bernal compared the phenomenon to droplets of condensation appearing in the middle of a room rather than on a window, describing the result as wholly unexpected.

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

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