Here is a cloud with a strict schedule. In spring and summer in Mars’ southern hemisphere, a long, thin cloud begins forming each morning on the western slope of Arsia Mons, a volcano about 20 kilometers tall. It grows downwind, stretching up to 1,800 km according to the European Space Agency, which is almost twice the length of the UK, and then it fades away. The next morning it does the same thing again, and it keeps this up for several months. ESA’s Mars Express orbiter first spotted it in 2018 and has been watching it ever since. Its name is the Arsia Mons Elongated Cloud, or AMEC, which is about as plain as names get.
The new claim is that it may be forming in a way nobody has ever seen happen in nature. A study in Nature Geoscience, led by Jorge Hernández-Bernal of the LMD laboratory at CNRS and Sorbonne Université, suggests the cloud forms through homogeneous nucleation, as Tereza Ondráčková reported for Wired, in a piece republished by Ars Technica. In homogeneous nucleation, water vapor turns directly into ice crystals all by itself, with nothing to form on.
That is not how clouds usually work. On Earth, water vapor in cooling air almost always needs a little help: a speck of dust, a grain of salt, something with a surface where droplets or ice crystals can get started. The particle does the hard first step, and the water does the rest. Take the particle away and the vapor mostly just stays vapor, even under conditions where you’d expect it to condense. Hernández-Bernal put it this way:
It’s akin to droplets of condensation appearing in the middle of a room, rather than on a window. It’s wholly unexpected.
So what does a planet have to do to get water to skip the window? Something fairly extreme. The process needs the temperature to fall by 30° Celsius within 10 minutes, which works out to 3 degrees a minute, and it needs very high relative humidity. That’s where the volcano comes in. When the wind runs into Arsia Mons (about 65,000 feet tall), it sets up a powerful wave that throws moist air high into the atmosphere within minutes. The air rises, cools fast, and ends up holding far more water than it can carry at that temperature. Ars Technica puts the humidity at more than 100,000 times greater than any value measured on Earth. ESA’s version is more than 100,000 times the levels typical of everyday life on Earth. Either way, it’s a big number.
The model and the books
The evidence is a simulation. Nobody has measured ice crystals forming over Arsia Mons. I think the easiest way to understand the result is to treat it like a reconciliation. You put the standard ingredients into a model of the Martian atmosphere: wind, the volcano’s shape, the vapor, ordinary dust-seeded ice. You check whether it produces the cloud Mars Express photographs every morning. It doesn’t quite. Then you add homogeneous nucleation, and the numbers balance. Hernández-Bernal told ESA that to reproduce the cloud “we needed to include some exotic physics.” (An earlier modeling paper on the same cloud gives a sense of how long people have been working on it.)
This inference has a slightly funny shape, and the researchers don’t hide it. Until this modeling, nobody knew Mars had humidity levels this high. So the model needs extreme humidity to make the cloud, and the cloud is the main evidence for the extreme humidity. That isn’t a flaw, exactly. It’s how you reason about places you can’t put instruments. But it explains why the paper’s title says the clouds “suggest” homogeneous nucleation and doesn’t say they prove it. ESA also notes that some features of the modeled cloud still don’t match the observations exactly.
The researchers frame the underlying problem honestly. “We don’t have nearly as much information about Mars’ atmosphere as we do about Earth’s, so reproducing the AMEC to this degree is a big success,” Hernández-Bernal said, adding: “the result is remarkable.”
Sure. And it is. Scientists have long assumed something like this could happen in the upper atmospheres of other planets, including Venus (whose yellow haze turned out to have its own surprising origin) and Earth itself. But it has never been observed. Given how humid the air would need to be, that’s not shocking. On Mars, it seems one specific volcano with the right height, wind and timing can push the air past that threshold over and over. The authors’ broader lesson, according to ESA, is that researchers studying other worlds, exoplanets included, shouldn’t rule out processes just because they seem unlikely.
There is also a practical reason this took so long. The cloud lives only in the morning, and Mars Express is one of the few orbiters, along with ESA’s ExoMars Trace Gas Orbiter, that can observe Mars at that time of day. The study used data from all three of its cameras. One of the strangest clouds in the solar system has been forming every morning for months at a time, and most of the spacecraft able to see it simply weren’t around in the mornings.