Rare Freezing Process Explains Formation of Mars' Massive Morning Cloud
Researchers at Sorbonne Université successfully modeled the Arsia Mons Elongated Cloud by incorporating homogeneous nucleation, a phenomenon requiring humidity levels 100,000 times higher than those found on Earth.
By Priya Marino ·
The recurring appearance of a vast cloud near the Arsia Mons volcano on Mars may finally have a scientific explanation. A new modeling study suggests that the formation of the Arsia Mons Elongated Cloud, or AMEC, relies on a specific physical process that has never been directly observed in any planetary atmosphere to date.
The AMEC forms during Martian spring and summer, stretching across more than 1,000 miles before dissipating. This length is comparable to the straight-line distance between New York and Kansas City. The feature was first recorded by a probe in 2018, but scientists have struggled to identify a plausible mechanism for its creation until now.
The new work, published in Nature Geoscience, is the first to successfully reproduce the cloud's formation in a simulation. Jorge Hernández-Bernal, a planetary scientist at Sorbonne Université in Paris and the lead author of the study, stated that the simulations only worked after including what he termed "exotic physics." He noted in a press release that this physics appears in textbooks but is typically treated as theoretical and assumed not to occur in nature.
Unlike Earth clouds, which form when water vapor condenses around particles like dust or salt, the Martian cloud appears to form through homogeneous nucleation. In this process, vapor turns directly into ice crystals without any debris to seed the reaction. Hernández-Bernal compared the effect to condensation appearing in the middle of a room rather than on a window, describing the result as "wholly unexpected."
The mechanism requires extreme conditions, specifically a temperature drop of 30 degrees Celsius within 10 minutes and high relative humidity. The 65,000-foot Arsia Mons plays a central role in generating these conditions. When wind encounters the volcano, it creates a powerful wave that lifts moist air high into the atmosphere within minutes.
The humidity required for this process is more than 100,000 times greater than any value measured on Earth. This extreme level may explain why homogeneous nucleation has not been observed in Earth's atmosphere, despite long-standing assumptions that it could occur in the upper atmospheres of Venus and Earth. Researchers had not previously known that such humidity levels existed on Mars.
Hernández-Bernal emphasized that far less is known about the Martian atmosphere than Earth's, making the successful reproduction of the cloud a notable achievement. He described the result as "remarkable," noting that the study resolves a mystery that has persisted since the cloud's initial detection.
