Mysterian 1,000-Mile Cloud Over Arsia Mons Finally Modeled

Each Martian spring and summer, a thin, wispy cloud forms near the 65,000-foot volcano Arsia Mons, stretches more than 1,000 miles, and then dissipates after several months. First detected by a probe in 2018, the phenomenon—dubbed the Arsia Mons Elongated Cloud (AMEC)—has long defied explanation.

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A research team led by planetary scientist Jorge Hernández-Bernal of Sorbonne Université published a study in *Nature Geoscience* that reproduces the cloud’s formation for the first time. The model required inclusion of “exotic physics” normally treated as theoretical: homogeneous nucleation, in which water vapor directly becomes ice crystals without any particulate nuclei. Hernández-Bernal explained that this process demands an abrupt 30 °C temperature drop within ten minutes and relative humidity exceeding Earth’s highest measured values by more than 100,000 times. When strong winds encounter Arsia Mons, they generate a powerful atmospheric wave that lofts moist air rapidly to high altitudes, creating the conditions for homogeneous nucleation and the ensuing elongated cloud.

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• Cloud length: >1,000 miles (?1,600 km)

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• Volcano height: 65,000 feet (?19.8 km)

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• Required temperature change: 30 °C in 10 minutes

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• Humidity needed: >100,000 times Earth’s maximum measured value

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The successful simulation suggests that Mars can host atmospheric processes never observed elsewhere, expanding the range of possible cloud-formation mechanisms on other planets. If similar humidity spikes exist in the upper atmospheres of Venus or Earth, they could produce comparable, yet undetected, phenomena. The findings also provide a new test case for atmospheric models, potentially refining predictions for future Mars missions and informing the search for analogous processes on exoplanets.

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