Mars' 1800-Kilometer Elongated Cloud Reveals Rare Atmospheric Physics

Image: Illustrative image · European Space Agency · CC BY-SA 3.0 igo · Source

A massive elongated water-ice cloud above Arsia Mons on Mars forms through unusual physics — homogeneous nucleation — previously considered only theoretical and unobserved in nature.

Researchers have uncovered the cause behind a remarkable atmospheric phenomenon occurring regularly over Arsia Mons, a dormant volcano on Mars. Every year, a giant elongated cloud of water ice, stretching up to 1800 kilometers, forms over the volcano during early morning hours, persists for several hours, then dissipates by midday. Since its discovery in 2018, this Arsia Mons Elongated Cloud (AMEC) has puzzled scientists because its behavior did not match established cloud formation models.

Ordinarily, cloud droplets or ice crystals require condensation nuclei—particles like dust—to form. On Mars, with its dusty surface, scientists expected ice crystals to accumulate on airborne dust grains. However, existing models failed to reproduce the observed AMEC phenomenon until physicists incorporated a rarely observed process called homogeneous nucleation. This process involves water vapor directly crystallizing into ice particles without any intermediary nuclei or dust, akin to droplets forming spontaneously in open air rather than on a surface.

Homogeneous nucleation demands extreme supersaturation of water vapor, conditions previously thought feasible only in rare situations such as on Venus or the upper layers of Earth's atmosphere. Mars, surprisingly, can reach these conditions due to the unique interaction between Arsia Mons’ topography and the planet’s thin atmosphere. The volcano’s 19-kilometer elevation acts as a giant aerodynamic barrier: winds force moist air upwards, rapidly cooling it by 30 °C within ten minutes and elevating relative humidity to levels where ice self-crystallizes.

“The exotic physics of homogeneous nucleation was essential to reproduce AMEC in our models,” explained Jorge Hernandez-Bernal, the lead author of the study. While the model does not perfectly predict every aspect of the cloud’s behavior, it convincingly demonstrates that without this process, the cloud would not form. The findings indicate that Mars can reach previously unrecognized humidity extremes.

European Space Agency’s Mars Express mission has played a crucial role in observing AMEC over several years. Mars Express’s high temporal-resolution data has provided an unprecedented detailed view of the cloud’s formation and dissipation, making it pivotal in solving the mystery.

This discovery has broader implications beyond Mars. It suggests atmospheric phenomena on other planets may operate under physical principles not typically observed or expected based on Earth’s atmosphere alone. As Colin Wilson, the Mars Express project scientist, summarized, “Understanding this exotic Martian cloud required exotic physics, and similar principles may apply to atmospheres of other planets.”