A 1,100-mile-long Martian cloud forms daily above Arsia Mons, defying conventional cloud-forming physics and requiring “exotic” processes to explain its rapid growth and disappearance, according to a study published in Nature Geoscience.
The Arsia Mons Elongated Cloud (AMEC), first observed by the European Space Agency’s Mars Express orbiter in 2018, stretches up to 1,800 kilometers (1,100 miles) across Mars’ sky each morning during southern hemisphere spring and summer. Unlike Earth’s orographic clouds, which form when air rises over terrain and condenses around dust particles, the AMEC materializes through homogeneous nucleation
—a process where water vapor freezes directly into ice particles without needing atmospheric “seeds” like dust or salt.
The Daily Martian Cloud
Scientists tracked the AMEC for years, puzzled by its ability to form and vanish within hours. The cloud emerges as winds push moisture up Arsia Mons, a 20-kilometer-tall volcano, triggering a powerful atmospheric wave. This wave lifts air rapidly, causing temperatures to drop by 30 degrees in 10 minutes. Under these extreme conditions, water vapor bypasses the usual condensation steps and freezes into ice particles, creating the elongated cloud.
For the AMEC, it seems that cloud formation takes place without needing any of this 'stuff' [in the air],
said Jorge Hernández-Bernal, lead author of the study. Water vapour turns directly into icy cloud particles without any middle step. It’s akin to droplets of condensation appearing in the middle of a room, rather than on a window.
Exotic Physics Explained
The discovery hinges on “exotic physics” long theorized but never observed in a planetary atmosphere. Traditional models failed to replicate the AMEC until researchers incorporated homogeneous nucleation—a process where water vapor reaches supersaturation levels 100,000 times higher than Earth’s norms. This requires Mars’ atmosphere to hold far more moisture than previously thought, a scenario enabled by the volcano’s unique topography and wind patterns.
To create the AMEC in our modelling, we found that we needed to include some exotic physics… physics that, while included in textbooks, is treated as theoretical and usually thought not to happen in nature,
Hernández-Bernal said. Once we included this physics in our simulations, the AMEC emerged just as we hoped.
Mars Express and the Breakthrough
Mars Express provided critical data, capturing the cloud’s daily evolution since 2018. These observations revealed that the AMEC’s formation relies on a precise interplay between Arsia Mons’ height, atmospheric dynamics, and solar heating.
The new simulations mark a step forward in understanding the AMEC. The study’s findings suggest that homogeneous nucleation happens on Mars, and the work offers an explanation for one of the biggest mysteries of Mars’ atmosphere.
Implications for Planetary Science
The AMEC’s existence suggests that homogeneous nucleation may occur in the upper atmospheres of Earth and Venus, where it has been theorized but never confirmed. On Mars, the discovery reshapes understanding of its water cycle and atmospheric behavior.
Understanding this Martian cloud required exotic physics, offering insight into how clouds form throughout the Solar System.