Scientists Identify Exotic Physics Behind Daily Giant Cloud Over Mars' Arsia Mons

A towering cloud that appears each spring and autumn downwind of the 20-km-high Arsia Mons volcano on Mars has finally been explained. The formation, known as the Arsia Mons Elongated Cloud, can extend about 1,800 km before dissipating within hours, baffling researchers since its 2018 discovery. In a study published in Nature Geoscience, Dr Jorge Hernández-Bernal of Sorbonne University reports that the cloud arises from homogeneous nucleation, where water vapor freezes into ice particles without needing dust nuclei.

The volcano’s massive height generates a strong atmospheric wave that lifts moist air, causing a rapid temperature drop of roughly 30 °C and a spike in relative humidity, conditions required for this rare process. When the team added this exotic physics to their Mars climate model, the simulated cloud matched the real observations. The result implies that Martian humidity can reach extreme levels, making the planet’s climate more complex than previously believed.