Mars cloud model points to ice forming without dust

The Arsia Mons Elongated Cloud appears in the morning during the Martian spring and summer, lengthens and then dissipates. Ars Technica describes a study published in Nature Geoscience in which researchers reproduced its formation using homogeneous nucleation: water vapor forming ice crystals without particles such as dust serving as a starting point.

Lead author Jorge Hernández-Bernal said the cloud emerged in simulations after the team included this process. In the proposed mechanism, wind striking Arsia Mons generates a powerful wave that lifts moist air. The required conditions include a temperature drop of 30°C within 10 minutes and high relative humidity.

The article says the cloud was first recorded by a probe in 2018, but scientists had lacked a plausible explanation for its formation. The extreme humidity implied by the new model was not previously known to exist on Mars; that does not establish when those conditions began.

The reported modeling result offers a mechanism for the cloud and points to exceptionally humid conditions in the relevant Martian air. It is a simulation-based explanation, not a direct measurement establishing that this ice-forming process occurs there.

The article distinguishes this mechanism from the particle-assisted formation typical of clouds on Earth. It says homogeneous ice nucleation has not been directly observed in any planetary atmosphere, making the Martian cloud a potential test of a process whose operation in nature remains unconfirmed.