An invisible cloud that defies terrestrial meteorology

In the deserts of Mars, where atmospheric pressure amounts to a tiny fraction of what we feel on Earth, a meteorological phenomenon happens almost every day and doesn't fit in any atmospheric physics textbook. The Arsia Mons Elongated Cloud (AMEC) stretches up to 1,800 kilometers, about one-twelfth of the planet's circumference, and vanishes after a few hours, only to reappear the next day.

For decades, scientists tried to reproduce this pattern in simulations. Martian weather models can predict the behavior of dust, frozen CO2 clouds, and even small amounts of water ice. But the AMEC resisted. It was as if the planet followed rules of cloud formation that nobody could describe mathematically.

The answer came from a study published on October 7 in Nature Geoscience, conducted by Jorge Hernández-Bernal of the University of the Basque Country (then at Sorbonne), together with colleagues Anouk Määttänen and Anthony Spiga, with collaboration from François Forget at the Laboratoire de Météorologie Dynamique (IP Paris). The result is a discovery that changes what we know about how clouds form not just on Mars, but potentially on any other planet.

The problem models couldn't solve

Mars clouds, in general terms, are of the same type as Earth's: water vapor rises, cools, and condenses. However, on Earth the vapor needs a grain of sand, literally, to form ice. Tiny particles of dust, sea salt, or even fungal spores act as condensation nuclei. This process is called heterogeneous nucleation and it is universal on Earth: without a solid or liquid nucleus, water vapor cannot organize into ice crystals, even at extremely low temperatures.

On Mars, the main candidate for condensation nuclei is dust. But the AME forms in regions of the atmosphere where the dust concentration is insufficient to explain the enormous amount of ice the cloud contains. When Hernández-Bernal and his team fed the MarsWRF weather model with conventional heterogeneous nucleation parameters, the cloud simply didn't appear.

The solution came when they introduced a theoretical mechanism that exists in textbooks but has never been observed in nature in any planetary atmosphere: homogeneous nucleation.

Homogeneous nucleation -- textbook physics, unprecedented observation

In homogeneous nucleation, water vapor transforms directly into ice without any solid or liquid particle to serve as a starting point. It's the same principle that allows dew drops to form on a cold surface on a dry day, but on a planetary scale and under conditions where supersaturation needs to be extremely high, something considered practically impossible in natural atmospheres.

By introducing homogeneous nucleation into our simulations, the AMEC cloud emerged exactly as we hoped, Hernández-Bernal explained. Including this exotic physics was the key, and it shows it's happening regularly on Mars.

The mechanism works as follows: during the southern hemisphere summer on Mars, strong winds push moist air upward and over the slopes of Mons Arsia, an extinct volcano 20 km high, more than twice the height of Mount Everest. When the air current hits the summit, it generates a powerful gravity wave that lifts moist air to altitudes above 20 km in minutes. The temperature drop is brutal: about 30 degrees Celsius in 10 minutes. Under normal conditions, this rapid cooling would form clouds through heterogeneous nucleation. But on Mars, where there's so little suspended dust, water vapor ends up exceeding saturation by a factor much higher than on Earth, reaching levels that allow direct vapor-to-ice transition.

The cloud forms, stretches up to 1,800 km, and then disappears within hours when sunlight heats the surface and breaks apart the ice crystals.

Why this matters -- beyond Mars

Homogeneous nucleation is a well-established concept in the laboratory: in controlled cloud chambers, when supersaturation is sufficiently high, vapor condenses without nuclei. But in natural atmospheres, on Earth, on Venus, anywhere else, the conditions never seemed extreme enough to generate this phenomenon at observable scale.

Mars provides us with a unique laboratory, said Milena Corcos, who wrote the follow-up piece in Nature Geoscience on October 7. The combination of low gravity, extremely rarefied atmosphere, and volcanism creates conditions that simply don't exist anywhere else in the Solar System.

This has implications for understanding other worlds. On Venus, where sulfuric acid clouds cover the entire planet, formation mechanisms are still debated. On exoplanets with thin atmospheres and cold surfaces, homogeneous nucleation could be a key process for the formation of nitrogen, methane, or other compound clouds.

It's also relevant for certain clouds on Earth, the study notes. At stratospheric altitudes, where particle concentration is minimal, homogeneous nucleation may play a role we don't yet fully understand. Understanding what happens on Mars gives us clues about how to model these remote atmospheric layers.

What remains is a simple but profound revelation: physics has existed in textbooks for decades. The problem is that nature only reveals it when conditions are exotic enough, and by chance, Mars is the most exotic planet in the Solar System for this kind of phenomenon.

The question that lingers is whether other worlds with rarefied atmospheres also follow similar patterns, and whether telescopes like the James Webb will be able to detect signatures of homogeneous nucleation in exoplanet atmospheres before robotic probes reach them.

Sources: Nature Geoscience, Nature Geoscience (commentary), WIRED, Live Science

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