Martian Weather: Sorbonne Model Solves Cloud Mystery
A new study in Nature Geoscience proposes that a 1,000-mile cloud forms via a rare ice process requiring humidity levels 100,000 times higher than on Earth.
By Alina Carter ·
Jorge Hernández-Bernal and his team at Sorbonne Université in Paris have published a modeling study in Nature Geoscience that offers a potential explanation for a long-standing mystery on Mars. The researchers propose that a massive cloud forming near the Arsia Mons volcano relies on a physical process that has never been directly observed in any planetary atmosphere.
The feature, designated the Arsia Mons Elongated Cloud, was first recorded by a probe in 2018. It appears during Martian spring and summer, stretching across more than 1,000 miles. The cloud grows longer throughout the morning until it reaches a span comparable to the straight-line distance from New York to Kansas City, before dissipating. This cycle has repeated over several months, baffling scientists who struggled to find a plausible mechanism for its formation.
For years, standard cloud physics failed to account for the Martian phenomenon. On Earth, clouds typically form when water vapor condenses around tiny particles like dust or salt. The new study suggests the Martian cloud forms through homogeneous nucleation, where vapor turns directly into ice crystals without any debris to seed the process. Hernández-Bernal described the required physics as "exotic" and noted that while it appears in textbooks, it is usually treated as theoretical and unlikely to occur in nature. He compared the process to condensation droplets appearing in the middle of a room rather than on a window, calling it "wholly unexpected."
The simulations only reproduced the cloud's formation after the team incorporated this rare ice physics. The process demands extreme conditions, specifically a temperature drop of 30 degrees Celsius within 10 minutes and very high relative humidity. The Arsia Mons volcano, which rises 65,000 feet, is central to creating these conditions. When wind strikes the peak, it generates a powerful wave that lifts moist air high into the atmosphere within minutes.
The required humidity levels are more than 100,000 times greater than any value measured on Earth. Scientists had not previously known such humidity existed on Mars, though they had long assumed similar nucleation could occur in the upper atmospheres of planets like Venus. Hernández-Bernal stated that far less is known about the Martian atmosphere than Earth's, making the successful reproduction of the cloud a notable achievement. "The result is remarkable," he said.
The study marks the first time a model has successfully replicated the formation of the Arsia Mons Elongated Cloud. The findings suggest that the Martian atmosphere contains conditions previously thought too extreme for standard cloud formation, forcing a reevaluation of how weather systems operate on the red planet.
