Mars Cloud Formation Relies on Extreme Temperature Drop

A long, thin cloud that appears over a Martian volcano each morning may form through a physical process never before confirmed in a planetary atmosphere. The new explanation resolves a mystery that has persisted since the feature was first detected in 2018. The cloud, known as the Arsia Mons Elongated Cloud, stretches across more than 1,000 miles during the Martian spring and summer before disappearing.

The cloud grows in length throughout the morning, reaching a span comparable to the straight-line distance between New York and Kansas City. It then dissipates, completing a cycle that repeats over several months. Scientists previously struggled to identify a plausible mechanism for its formation because standard cloud physics could not account for its behavior.

Jorge Hernández-Bernal, a planetary scientist at Sorbonne Université in Paris, led the study published in Nature Geoscience. He stated that the simulations only succeeded after the team included what he described as exotic physics. This specific process appears in textbooks but is generally considered theoretical rather than natural. "Once we included this physics in our simulations, the AMEC emerged just as we hoped," Hernández-Bernal said in a press release.

The model relies on homogeneous nucleation. Unlike Earth clouds, which form when water vapor condenses around dust or salt particles, this Martian cloud forms when vapor turns directly into ice crystals without any debris to seed the process. Hernández-Bernal compared the effect to condensation forming in the middle of a room instead of on a window. He described the finding as wholly unexpected.

The conditions required for this process are extreme. The air must cool by 30 degrees Celsius within 10 minutes. The relative humidity must also reach levels more than 100,000 times greater than any value measured on Earth. The 65,000-foot Arsia Mons volcano plays a central role in creating these conditions. When wind strikes the mountain, it generates a powerful wave that lifts moist air high into the atmosphere within minutes.

Researchers have long suspected that similar nucleation might occur in the upper atmospheres of Venus and Earth, but they had never observed it directly. The extreme humidity required for the process likely explains its rarity. Hernández-Bernal noted that scientists did not previously know such humidity existed on Mars. He added that the Martian atmosphere is far less understood than Earth's, making the successful reproduction of the cloud a notable achievement in the field.

The study provides the first model to successfully reproduce the cloud's formation. It confirms that the feature depends on a rare combination of rapid cooling and high moisture. The work clarifies how specific atmospheric waves over Mars' largest volcano drive the creation of one of the planet's most distinct weather patterns.