Homogeneous Nucleation of Water Vapor Evidenced by Elongated Clouds on Mars

Clouds are widely thought to form by heterogeneous nucleation, in which water vapor condenses onto pre-existing dry nuclei. Homogeneous nucleation, where vapor condenses directly without any pre-existing nucleus, is theoretically possible but requires high supersaturation, so it has generally been dismissed as a viable cloud-formation process under real atmospheric conditions.

The study introduces homogeneous nucleation into a meteorological model of Mars and tests whether it can reproduce the distinctive behavior of the Arsia Mons Elongated Cloud, a recurring water ice cloud that conventional cloud microphysics had been unable to model.

With homogeneous nucleation included, the model successfully reproduces the elongated cloud's unique characteristics. This demonstrates that homogeneous nucleation does occur on Mars, marking the first evidence of homogeneous nucleation of water vapor in a planetary atmosphere.

The finding challenges current assumptions about cloud formation and has implications for understanding certain clouds on Earth and potentially on other planets.