Mars's Strangest Cloud May Freeze Straight From Vapour With No Dust Seed, a New Study Suggests
A 1,800-kilometre ribbon of ice that forms each morning over a Martian volcano defied standard cloud physics until researchers let water vapour crystallise on its own.
Thursday, October 29, 2026/2 min read
For a few months around the southern summer solstice on Mars, a long, thin cloud stretches downwind from the volcano Arsia Mons most mornings and fades by evening. It is about 1,800 kilometres long, and since Mars Express first imaged it in 2018 nobody has been able to explain it. A paper in Nature Geoscience, published earlier this month and covered by Science News on Thursday, now offers an answer that involves physics never confirmed in a planetary atmosphere.
The team, led by Jorge Hernández-Bernal of Sorbonne University and including François Forget of the Laboratoire de Météorologie Dynamique in Paris, argues that the Arsia Mons Elongated Cloud forms without any seed particles at all. Their model only reproduces the cloud when that process is switched on, according to Sky & Telescope.
Why can't ordinary cloud physics explain it?
On Earth, water vapour condenses onto specks of dust or salt long before the air becomes extremely oversaturated. Earlier Martian simulations used the same logic and failed to reproduce either the cloud's tail or its daily cycle. Dust is plentiful on Mars, but at the roughly 45 kilometres where this cloud sits, the atmosphere is so thin that few particles stay aloft to do the job, Science News reports.
What is homogeneous nucleation?
It is the freezing of water vapour into ice crystals with nothing to start the process. It has been proposed for Earth and Venus, but Sky & Telescope says it had not been observed before, and it needs an extreme setting. In the model, wind over the volcano lifts a parcel of moist air, and it expands and cools, by about 30 degrees Celsius in a few minutes. That leaves it holding roughly 100,000 times more moisture than it can stay stable with, and the vapour freezes into ice. Winds then draw the crystals into a long streamer.
Simulations of other Martian peaks, including Olympus Mons, produced no similar clouds, which suggests Arsia Mons has unusually suitable conditions. Hernández-Bernal, who has studied the cloud for eight years, told Sky & Telescope, "It's such a beautiful cloud."
How certain are the researchers?
Not fully. The model reproduces the cloud's daily recurrence but does not perfectly match satellite observations, Science News says, and the explanation rests on simulations matching what spacecraft saw, not on watching nucleation happen. Samuel Atwood of the University of Maryland, Baltimore County and NASA Goddard, who was not involved, called it a compelling story but only one perspective, and said future measurements would add certainty. Hernández-Bernal plans to test alternative explanations.
If the idea holds, it may send researchers back to Earth's upper atmosphere, where meteor smoke is thought to seed some clouds but some studies have suggested homogeneous nucleation could also play a part. The next step on Mars is more observations of the cloud through its next season.
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