Mars' 'impossible cloud' rewrites the rules of planetary atmospheres

For years, a colossal cloud stretching 1,800 kilometers across Mars has appeared every morning, only to vanish by afternoon. Nicknamed the Arsia Mons Elongated Cloud (AMEC), this ethereal streamer has defied explanation, earning its moniker as the Red Planet's 'impossible cloud.' How could such a massive, consistent phenomenon persist on a world so different from our own, yet resist conventional atmospheric modeling? The answer, as a Spanish-led team has now revealed, lies in a form of exotic physics known as homogeneous nucleation, a process previously thought impossible in a planetary atmosphere.

Led by investigator Jorge Hernández-Bernal of the Universidad del País Vasco and Sorbonne Université, an international team has published their findings in Nature Geoscience. Their work details how this 1,800-kilometer-long and 150-kilometer-wide cloud, first spotted by the European Space Agency's Mars Express in 2018, forms daily during the Martian spring and summer, downwind of the 20-kilometer-tall Arsia Mons volcano.

The Unprecedented Physics of Martian Clouds

On Earth, clouds typically form through what is known as heterogeneous nucleation: water vapor condenses around airborne particles like dust, salt, or pollen. This is why our skies are filled with countless cloud formations. But the AMEC doesn't follow these rules. Instead, it forms via homogeneous nucleation, a process where water vapor spontaneously freezes directly into icy cloud particles without any intermediary liquid phase or requiring any pre-existing particles. "Water vapor turns directly into icy cloud particles without any middle step," explains Hernández-Bernal, likening it to condensation appearing in the middle of a room, not on a window, as reported by ESA.

This mechanism was considered theoretically possible but had never been observed in a planetary atmosphere. Scientists even believed it to be "absolutely impossible" on Mars, given the planet's low humidity and abundant dust, which typically absorbs moisture and facilitates heterogeneous nucleation. The extreme conditions required for homogeneous nucleation—relative humidity levels over 100,000 times those typically found on Earth—were simply not thought to exist.

Arsia Mons: The Catalyst for the Impossible

The key to this extraordinary phenomenon lies in the unique interaction between Mars' thin atmosphere and the towering Arsia Mons volcano. The AMEC is an orographic cloud, a type also found on Earth, which forms when wind flows past elevated terrain. However, the Martian version is distinct. As winds rush past Arsia Mons, the volcano's massive bulk generates a powerful wave that rapidly lifts moist air parcels several kilometers into the atmosphere within minutes. This rapid ascent causes temperatures to plummet by 30 degrees Celsius in just 10 minutes, leading to an extraordinary spike in relative humidity. Under these specific conditions, the water vapor spontaneously freezes into cloud particles, forming the AMEC.

At the cloud's altitude, approximately 28 miles above the ground, the Martian atmosphere contains significantly less dust, allowing these extreme humidity levels to be reached without heterogeneous nucleation depleting the water vapor. This finding compels us to reconsider our assumptions about how clouds form on other worlds.

Rethinking Planetary Atmospheres

This discovery does more than just solve a Martian mystery; it profoundly impacts our understanding of atmospheric processes across the cosmos. It challenges the conventional view that cloud formation always requires nucleation particles and suggests that "exotic physics" may be more common than we previously thought on other planets, and even in certain conditions in Earth's upper atmosphere. As Hernández-Bernal told elDiario.es, this prompts questions about whether this process occurs in more places on Mars and if it influences the planet's water cycle. Furthermore, it reminds us that, as scientists, we should never discount unlikely processes when exploring the vastness of the universe.

In our view, this research highlights the dynamic and often surprising nature of planetary science. It's a testament to the fact that even on a seemingly well-studied planet like Mars, fundamental processes can still defy our expectations and rewrite our textbooks, pushing the boundaries of what we consider possible. Indeed, it underscores that understanding other planets can offer new insights into our own Earth.