Sorbonne Model Resolves Mars Cloud Mystery

A team of planetary scientists has identified the physical mechanism behind a massive cloud that forms daily on Mars. The feature, known as the Arsia Mons Elongated Cloud or AMEC, spans more than 1,000 miles each morning before dissipating. A new study published in Nature Geoscience offers the first model to successfully reproduce this phenomenon.

The cloud appears during Martian spring and summer. It grows longer through the morning, reaching a distance comparable to the straight-line span from New York to Kansas City, before vanishing. A probe first detected the AMEC in 2018, but researchers lacked a clear explanation for its existence for years.

Jorge Hernández-Bernal, a planetary scientist at Sorbonne Université in Paris, led the research. He stated that standard atmospheric models failed to generate the cloud. The simulations only worked after the team incorporated a specific type of physics that textbooks describe as theoretical and rarely seen in nature.

The study proposes that the cloud forms through homogeneous nucleation. This process allows water vapor to turn directly into ice crystals without dust or salt particles to seed the formation. On Earth, clouds typically require debris to trigger condensation. Hernández-Bernal described the Martian process as wholly unexpected, comparing it to condensation appearing in the middle of a room rather than on a window.

The mechanism relies on extreme local conditions. Wind hitting the 65,000-foot Arsia Mons volcano creates a powerful wave that lifts moist air high into the atmosphere. This rapid lift causes the temperature to drop by 30 degrees Celsius within 10 minutes. The study indicates that this sudden cooling is necessary for the vapor to freeze without seeds.

The model requires relative humidity levels more than 100,000 times greater than any value measured in Earth's atmosphere. 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 accurate reproduction of the cloud a notable achievement.

The research suggests that similar nucleation processes may occur in the upper atmospheres of other planets, including Venus and Earth, though they have never been directly observed. The AMEC cycle repeats over several months during the Martian year, providing a consistent natural experiment for atmospheric physics.