Mars Cloud Needs Extreme Humidity

More than 100,000 times the humidity measured on Earth is required to form a specific cloud on Mars, according to a new model. The figure describes the atmospheric conditions needed to create the Arsia Mons Elongated Cloud, or AMEC, a feature that spans over 1,000 miles.

A research team from Sorbonne Université in Paris published findings in Nature Geoscience stating that this simulation is the first to successfully reproduce the cloud's formation. The AMEC appears during Martian spring and summer, growing longer through the morning before dissipating. The cycle repeats for several months, reaching a length comparable to the distance from New York to Kansas City.

The model relies on a process called homogeneous nucleation. In this physical mechanism, water vapor turns directly into ice crystals without dust or salt particles to seed the formation. Jorge Hernández-Bernal, the study's lead author, described the phenomenon as "wholly unexpected." He noted that while the physics appears in textbooks, it is typically treated as theoretical and not expected to occur in nature.

The formation of the cloud depends on the massive Arsia Mons volcano, which stands 65,000 feet high. When wind strikes the mountain, it creates a powerful wave that lifts moist air high into the atmosphere within minutes. This rapid movement causes the temperature to drop by 30 degrees Celsius in just 10 minutes, creating the extreme environment necessary for the rare ice formation.

Scientists previously assumed that similar nucleation processes might occur in the upper atmospheres of Venus and Earth, but they had never been observed. The extreme humidity levels required on Mars likely explain the lack of prior detection. Hernández-Bernal said that far less is known about the Martian atmosphere than Earth's, making the accurate reproduction of the cloud a notable achievement for the field.

The AMEC was first recorded by a probe in 2018. Researchers had struggled to find a plausible explanation for the feature since that initial discovery. The new work suggests that the cloud's existence points to atmospheric conditions on Mars that were previously unknown to scientists.