A 1,000-mile-long cloud that stretches across the Martian sky every morning before completely vanishing may be driven by a rare atmospheric phenomenon never before observed in any planetary atmosphere. First spotted by an orbiter in 2018, the recurring morning formation near the Arsia Mons volcano has baffled planetary scientists until new meteorological modeling pointed toward an exotic physical process.
Arsia Mons Morning Cycles and the 1,000-Mile Stretch
Every morning during the Martian spring and summer, the wispy formation emerges near the 12-mile-tall extinct volcano Arsia Mons. As increased solar radiation triggers strong surface winds, the structure gradually elongates until it reaches over 1,000 miles in length—roughly the distance from New York City to Kansas City as the crow flies—before completely evaporating within a few hours. The European Space Agency’s Mars Express orbiter first recorded the anomaly in 2018, noting that it repeats its cycle across several months while scientists spent years unable to find a plausible explanation for the recurring atmospheric event.
Live Science noted that the phenomenon, formally known as the Arsia Mons Elongated Cloud or AMEC, can stretch up to 1,100 miles long, covering about one-twelfth of Mars’ circumference. The Red Planet is mostly devoid of atmospheric water vapor, making the appearance of such vast, puffy cloud formations exceptionally rare.
Simulating Exotic Physics Over Mars
To finally decode the mechanics behind the persistent cloud, researchers utilized data gathered by the Mars Express orbiter to build new atmospheric simulations. The initial runs failed to recreate the formation using standard meteorological principles.
The team eventually turned to theoretical equations to crack the simulation. Jorge Hernández-Bernal, a planetary scientist at Sorbonne Université in Paris at the time of the study and later at the University of the Basque Country in Spain, explained the breakthrough in a press release.
Hernández-Bernal said that the team found they needed to include exotic physics in their simulations—physics that, while included in textbooks, is treated as theoretical and usually thought not to happen in nature and has not been seen in action before. He added that once they included this physics in their simulations, the AMEC emerged just as they hoped.
While the AMEC is classified as an orographic cloud—similar to the Levanter cloud that forms over the Rock of Gibraltar on Earth when winds push moisture over a raised structure—the formation mechanism differs entirely.
Homogeneous Nucleation Without Atmospheric Dust
On Earth, orographic clouds typically form through heterogeneous nucleation. This process requires moist air to cool and condense into ice crystals around tiny atmospheric particles such as dust, salt, pollen, or soot.
Mars possesses an abundance of atmospheric dust, yet that particulate matter plays no role in generating the AMEC. Hernández-Bernal noted that cloud formation occurs without needing any of those suspended particles in the air. Water vapor converts directly into icy cloud particles without any intermediate step, a process equivalent to condensation droplets appearing spontaneously in the middle of a room rather than collecting on a window pane.
Researchers identify this mechanism as homogeneous nucleation. According to the study published in Nature Geoscience, this specific type of phase transition has never been directly observed in action within any planetary atmosphere.
