What Is Really Happening?

Every morning, during spring and summer in the Martian southern hemisphere, a white plume stretches across the sky. It grows to nearly 1,800 kilometres long—roughly twice the length of the UK—and then vanishes within hours. It has been doing this for decades, possibly centuries. And until now, scientists couldn’t fully explain how it formed.

The Arsia Mons Elongated Cloud (AMEC) is the most visually striking cloud on Mars. It emerges downwind of Arsia Mons, a 20-kilometre-tall extinct volcano that dwarfs anything on Earth. It stretches up to 1,800 kilometres—about one-twelfth of Mars’ circumference, or the distance between New York City and Miami. And it does this every single morning for several months, before evaporating completely within about three hours.

Scientists first spotted the AMEC in 2018, using the European Space Agency’s Mars Express orbiter. But the cloud had actually appeared in archival images as far back as NASA’s Viking 2 mission in the 1970s—it simply escaped notice because of its ephemeral nature.

For years, researchers struggled to explain it. They knew it was an orographic cloud—a type of cloud that forms when wind flows past a mountain or volcano. Earth has these too. But when they tried to simulate the AMEC in computer models, the simulations never matched what Mars Express was seeing.

Something was missing. And that something turned out to be a form of physics that had never been observed in a planetary atmosphere before.

The Cloud That Shouldn’t Exist

To understand why the AMEC was so puzzling, you have to understand how clouds normally form.

On Earth—and, scientists assumed, everywhere else—clouds form through a process called heterogeneous nucleation. Water vapour cools and condenses around tiny particles suspended in the atmosphere: salt, pollen, soot, or dust. Without those particles, water vapour would have to reach absurdly high supersaturation levels before it could freeze into ice crystals. And in most planetary atmospheres, that simply doesn’t happen.

On Mars, the assumption was that dust kicked up from the surface would serve as those nuclei. Mars is a dusty planet. There’s plenty of material floating around.

But when researchers modelled the AMEC with dust as the nucleation seed, they couldn’t reproduce its long tail. The cloud kept collapsing. Something was wrong.

The breakthrough came when they tried something different. They removed the dust entirely.

“To create the AMEC in our modelling, we found that we needed to include some exotic physics,” said Dr Jorge Hernández-Bernal of Sorbonne University, the study’s lead author. “Physics that, while included in textbooks, is treated as theoretical and usually thought not to happen in nature”.

That physics is called homogeneous nucleation—the direct freezing of water vapour into ice particles without any pre-existing particles to cling to.

“Water vapour turns directly into icy cloud particles without any middle step,” Hernández-Bernal explained. “It’s akin to droplets of condensation appearing in the middle of a room, rather than on a window”.

The study, published in Nature Geoscience, represents the first time homogeneous nucleation has been observed in a planetary atmosphere. Scientists had suggested it might occur in the upper reaches of Earth’s or Venus’s atmosphere, but it had never been confirmed.

Why It Happens on Mars

Homogeneous nucleation requires extreme conditions—specifically, relative humidity levels over 100,000 times higher than anything typically experienced on Earth. So how does Mars, a planet with almost no water vapour in its atmosphere, produce them?

The answer lies in the unique combination of Arsia Mons and Mars’ thin atmosphere.

When winds flow past Arsia Mons, the volcano’s enormous size generates a powerful atmospheric wave that pushes moist air upwards very quickly—several kilometres in just a few minutes. That rapid ascent causes the air to cool dramatically, with temperatures dropping by 30 degrees in just 10 minutes.

The water vapour, suddenly chilled and lifted to high altitude, reaches the extreme supersaturation required for homogeneous nucleation. It freezes directly into ice crystals, forming the cloud. The tail then stretches downwind, driven by winds of up to 373 miles per hour.

“It’s not the heat,” as Mashable put it. “It’s the humidity”.

The Daily Life of a Martian Cloud

The AMEC’s daily cycle is remarkably consistent.

It begins at dawn, around 5:7 local time, when the first sunlight hits the volcano’s slopes. The cloud expands rapidly from its origin point next to Arsia Mons, growing for about 2.5 hours at velocities of around 170 metres per second. By about 8:30 local time, it reaches its maximum extent. Then, as the sun rises higher and the air warms, the ice crystals sublimate back into vapour. Within a few hours, the cloud is gone.

The entire cycle repeats every morning for several months, between solar longitudes of 220° and 320°—Mars’ spring and summer in the southern hemisphere.

It is, in a sense, a daily heartbeat of the Martian atmosphere. Predictable, enormous, and until now, inexplicable.

What This Means for Planetary Science

The discovery has implications that extend far beyond one strange cloud on Mars.

First, it changes how we model cloud formation on other planets. Heterogeneous nucleation has been the default assumption for decades. The AMEC shows that homogeneous nucleation is not only possible but can occur on a planetary scale under the right conditions. Climate models for Mars, Venus, and even exoplanets may need to be revisited.

Second, it tells us something new about Mars’ water cycle. The AMEC requires extreme humidity levels to form. The fact that it does form means Mars’ atmosphere can reach those levels, at least locally and temporarily. That has implications for understanding how water moves between the surface, atmosphere, and space on Mars.

Third, it’s a reminder that other planets can surprise us. “The actual answer seems obvious now, but at first we were quite skeptical of ourselves,” Hernández-Bernal told Mashable. The physics that explained the AMEC had been sitting in textbooks for decades. It just hadn’t been observed in nature. Mars, it turns out, is a natural laboratory for testing theoretical physics that Earth’s atmosphere never quite reaches.

The Arsia Mons

 Elongated Cloud is not just a curiosity. It is a phenomenon that, until 2026, could not be fully explained by the principles of atmospheric physics that scientists had been applying to every planet in the solar system.

It required “exotic” physics—a process that was theorised but never seen. And it happens every single morning, on a planet that is, by cosmic standards, right next door.

The AMEC is a reminder that even in our own solar system, there are still wonders that defy easy explanation. And sometimes, the answer is something we thought we understood all along.

The study, “Homogeneous ice nucleation from water vapour suggested by elongated clouds on Mars,” was published October 7, 2026, in Nature Geoscience.