A study published on Oct. 7, 2026, suggests that homogeneous ice nucleation may help explain the Arsia Mons Elongated Cloud (AMEC), a recurring water-ice cloud on Mars. In homogeneous nucleation, ice forms from water vapor without a pre-existing particle such as dust. A Mars weather model incorporating that process reproduced distinctive characteristics of the cloud, but the proposed mechanism remains an inference from modeling and observations rather than a direct observation of ice forming.
The Arsia Mons cloud and the model-based explanation
The AMEC forms downwind of Arsia Mons, a volcano that stands 20 kilometers tall. The cloud can stretch up to 1,800 kilometers. The study’s proposed explanation combines the volcano’s effect on airflow with a separate question: how ice particles form inside the cloud.
What homogeneous ice nucleation means
In the more familiar process called heterogeneous nucleation, water vapor condenses on a pre-existing particle, such as dust. Homogeneous nucleation is different: water vapor forms ice without that particle acting as a starting point.
That particle-level distinction is the proposed twist in the Arsia Mons study. Including homogeneous ice nucleation allowed the meteorological model to reproduce distinctive features of the AMEC. The model result strongly suggests that the process may occur on Mars; it does not amount to a direct observation of the process in the atmosphere.
How Arsia Mons may create the cloud’s conditions
Winds passing Arsia Mons are proposed to generate an atmospheric wave that lifts moist air. As the air rises, it cools and its relative humidity increases. The model then suggests that water vapor could form ice particles through homogeneous nucleation.
The wave and the nucleation process describe different parts of the proposed sequence: the volcano shapes the rising air, while nucleation concerns how ice particles may form within it. The recurring cloud is the visible result associated with that sequence—not a claim that the particle-formation step has been directly witnessed.
What Mars Express contributed
The meteorological model used observations from three Mars Express instruments: the Visual Monitoring Camera (VMC), the High Resolution Stereo Camera (HRSC) and OMEGA. ESA says some features of the modeled cloud do not exactly match observations, so the model’s fit is not complete.
Mars Express first revealed the cloud in 2018
Mars Express first revealed the AMEC in 2018, according to ESA. The cloud forms, grows and fades on a daily cycle that returns each morning for several months.
