Mars Express modelling suggests the 1,800 km Arsia Mons cloud freezes straight from vapour

The study is a comparison run inside a model. Its paper in Nature Geoscience is careful with its verb: the title says the elongated clouds "suggest" homogeneous ice nucleation ^[1]. ESA says earlier simulations did not reproduce the cloud seen in the Mars Express images ^[2]. When the team added one process, the cloud appeared ^[3]. The runs without that process work as the control.

"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," said Jorge Hernandez-Bernal of LMD/CNRS/Sorbonne Universite in Paris, the study's lead author ^[3][11]. "Once we included this physics in our simulations, the AMEC emerged just as we hoped." ^[3]

The process they added skips the dust grain entirely. "Water vapour turns directly into icy cloud particles without any middle step," Hernandez-Bernal said. "We call this homogeneous nucleation, and we've never seen it before in a planetary atmosphere." ^[4]

In the proposed account, the volcano supplies the conditions. As wind flows past Arsia Mons, its bulk sets up a wave that lifts moist air several kilometres in a few minutes ^[13]. Temperatures fall by 30 degrees in 10 minutes, about 3 degrees a minute, and relative humidity spikes until the vapour freezes outright ^[13][17].

Homogeneous freezing needs relative humidity more than 100,000 times what people experience in daily life on Earth ^[6]. On Mars, the study's evidence for humidity that high is the model's success. "We've not seen these conditions on Mars before, but our finding now strongly suggests that the planet's humidity can indeed reach these extreme levels," Hernandez-Bernal said ^[12].

The match is partial. According to ESA, some aspects of the modelled cloud do not exactly match the observations ^[9]. Hernandez-Bernal called the result "remarkable" and set it against how little is known: "We don't have nearly as much information about Mars's atmosphere as we do about Earth's, so reproducing the AMEC to this degree is a big success for the model." ^[9] ESA's release does not say which aspects diverge, or whether dust-seeded runs with other assumptions about dust and vapour were tried and failed.

The thing this doesn't tell you is how far the result travels beyond one volcano. In my view, the evidence earns a strong claim about this cloud and very little yet about the rest. The researchers frame it narrowly too, placing the cloud in a unique position where Mars's thin atmosphere and the height of Arsia Mons together produce the conditions the process needs ^[16].