Scientists analyzing data from the European Space Agency’s Mars Express spacecraft have discovered that Mars’s massive Arsia Mons Elongated Cloud forms through an extreme physical process never before observed in any planetary atmosphere, relying on unprecedented humidity levels and spontaneous vapor-to-ice transformation without atmospheric dust.
Cloud Reemerges Daily Near the 20-Kilometer Volcano
Every spring and summer in the southern hemisphere during the dusty season, a distinct white wisp of water ice emerges downwind of the 20-kilometer-tall Arsia Mons volcano. First spotted in 2018 by the Mars Express spacecraft, this cloud stretches up to 1,800 kilometers long—nearly twice the length of the United Kingdom—before quickly evaporating under morning sunlight. The cycle repeats daily for several months.
Researchers Find Water Vapor Freezes Without Dust
Jorge Hernández-Bernal of LMD/CNRS/Sorbonne Université in Paris, lead author of the study published in Nature Geoscience, described the rarity of the mechanics at play. On Earth and Mars alike, clouds typically form when moist air cools and water vapor condenses around suspended particles like dust, pollen, or salt through a process called heterogeneous nucleation. For the Arsia Mons Elongated Cloud, however, standard atmospheric models consistently failed to match spacecraft imagery.
Researchers determined that water vapor skips intermediate stages entirely, turning directly into icy cloud particles in open air.
This mechanism requires extreme relative humidity levels exceeding 100,000 times those typically experienced on Earth. While theoretical models suggested the process might occur in the upper atmospheres of Earth and Venus, it had never been directly observed in a real planetary atmosphere until now.
Topography Triggers Rapid Temperature Drops
The unique geography of the Arsia Mons region creates the precise conditions required for this rare atmospheric state.
This sequence forces water vapor to freeze spontaneously into cloud particles.
Scientists Rethink Exoplanetary Atmospheres
ESA Mars Express Project Scientist Colin Wilson emphasized that understanding the martian cloud demands a view of atmospheric rules across the cosmos. The discovery demonstrates that planetary meteorology can involve mechanisms previously dismissed as too unlikely to occur naturally.
When modeling the atmospheres of distant planets outside our solar system, scientists may now need to account for homogeneous nucleation processes previously considered too rare to matter, changing how researchers interpret meteorological data across planetary science.