A puzzling white streak over Arsia Mons

Every spring and summer on the Red Planet, a narrow, luminous band appears just after sunrise on the slopes of Arsia Mons, the southernmost of three massive volcanoes. The formation stretches up to 1,800 kilometres—roughly the distance between Amsterdam and Lisbon—only to evaporate within the same daylight. First captured by the European Mars Express orbiter in 2018, this fleeting phenomenon has sparked intense debate among planetary scientists.

Earthly clouds need a seed

On Earth, water vapour rarely condenses on its own. Tiny particles such as soot, sea salt, or pollen act as nuclei, allowing droplets to coalesce. Aircraft contrails illustrate this principle: exhaust provides the necessary condensation cores. Mars, with its dusty environment, was assumed to offer plenty of such scaffolding, yet the Arsian cloud seemed to ignore the rule.

Volcanic uplift as a cooling engine

Arsia Mons towers about 20 kilometres above the surrounding plain. When the thin Martian air sweeps over the summit, it is forced upward, expanding and cooling dramatically. Within ten minutes, temperatures can drop by roughly 30 °C, creating a pocket of supersaturated vapour. In this state, excess moisture can freeze spontaneously, forming ice crystals without any external particles—a process known as homogeneous nucleation.

Modelling the impossible

Research led by Jorge Hernández‑Bernal in Paris challenged the conventional requirement for dust. By allowing vapour to nucleate homogeneously once the atmosphere became sufficiently supersaturated, their computer simulations reproduced a cloud matching the observed dimensions and lifespan. The model demonstrates that, under Mars’ low pressure—less than one‑hundredth of Earth’s—the rapid cooling can push the air beyond its saturation limit, prompting immediate ice formation.

Observations versus simulations

No instrument has directly sampled the cloud itself; the evidence rests on the success of the numerical experiment. The authors acknowledge discrepancies between the simulated and actual cloud, and they admit that other, yet‑unknown mechanisms could be at play. Nevertheless, the study provides a plausible explanation for a phenomenon previously deemed inexplicable.

Implications for planetary climatology

If homogeneous nucleation can occur on Mars, it may also be relevant for other worlds with thin atmospheres, such as Venus or exoplanets with similar conditions. Climate models that assume every cloud requires a condensation nucleus might need revision, potentially altering our understanding of atmospheric dynamics beyond Earth.

While further measurements are essential to confirm the hypothesis, the research opens a new window onto how clouds can arise in extreme environments, challenging long‑held assumptions about planetary weather.

Source: https://scientias.nl/deze-raadselachtige-ochtendwolk-op-mars-krijgt-een-opvallende-verklaring/