The Thousand-Mile Cloud That Shouldn't Exist

A Streak Beside the Volcano

On 13 September 2018, the Visual Monitoring Camera aboard ESA's Mars Express sent back an image with something odd in it: a long, bright white feature stretching away from Arsia Mons, a 20-kilometre volcano near the Martian equator^1,2,7. The camera was never built for serious atmospheric science. It had been installed mainly to watch the Beagle 2 lander separate from the spacecraft years earlier, and its job since then had mostly been wide, workmanlike shots of the planet^2,8.

Nothing about the feature screamed danger. It did not behave like smoke or ash, and nobody thought Arsia Mons, long dormant, had come back to life. But the shape was wrong for an ordinary cloud: too long, too fast, too exactly tied to one slope of one mountain. It did not look like anything else anyone had catalogued on Mars.

Arsia Mons sits at the southern end of the Tharsis Montes, a line of three giant shield volcanoes near the planet's equator, and it ranks among the largest volcanoes in the solar system, roughly thirty times the volume of Hawaii's Mauna Loa⁹. Mars Express has orbited the planet since 2003, carrying several cameras, and by 2018 scientists had already seen hints of a seasonal cloud near Arsia Mons in archived images from 2009, 2012, and 2015². What made September 2018 different was that researchers were now watching closely enough to catch it building in real time, during the tail end of a planet-wide dust storm whose fading haze seems to have made the cloud unusually vivid that year⁹.

The cloud kept growing through the following weeks. By 21 September it had been photographed clearly enough to document its structure¹⁰, and by 10 October it reached about 1,500 kilometres west of the volcano, roughly six times the volcano's own 250-kilometre diameter^1,2. ESA scientists, including Agustin Sanchez-Lavega of the University of the Basque Country and Mars Express project scientist Dmitri Titov, described it publicly as a recurring, seasonal water-ice cloud tied to the volcano's leeward slope, not to any eruption².

A sequence of images taken between 20 October and 1 November 2018 pinned down its daily rhythm. The cloud began growing before sunrise on the volcano's western slope, expanded westward for about two and a half hours at over 600 kilometres per hour, then detached from the volcano and drifted further on high-altitude winds before evaporating by late morning as the sun warmed the air⁴. Researchers eventually measured its maximum extent at about 1,800 kilometres long and 150 kilometres wide, at an altitude near 45 kilometres, nearly twice the length of Great Britain, vanishing and reforming every day for months each Martian year^3,5.

Mars Express kept returning to it. A 2021 study combining years of camera data confirmed the cloud's seasonal pattern around the southern solstice¹¹, and in June 2024 the orbiter's High Resolution Stereo Camera photographed it again, catching a 980-kilometre version of the same feature in exquisite detail⁶. By then the AMEC had been observed, in one form or another, across more than a decade of Martian years.

What still wasn't settled was how a cloud could form this fast, this far from any dust, at an altitude where the Martian atmosphere is almost unbearably thin. In October 2026, a team led by Jorge Hernandez-Bernal of LMD/CNRS/Sorbonne Universite published modelling results in Nature Geoscience that gave an answer nobody had expected. On Earth and, as far as anyone knew, everywhere else, clouds form around something: a speck of dust, soot, or salt that water vapour can cling to. The AMEC's water vapour appeared to skip that step entirely, freezing directly into ice crystals in open air, a process called homogeneous nucleation that textbooks describe but that had never actually been observed happening in any planet's atmosphere³. Hernandez-Bernal compared it to condensation appearing in the middle of a room instead of on a window³. The model showed wind hitting Arsia Mons generating a wave that shoves moist air several kilometres upward within minutes, dropping its temperature by 30 degrees in ten minutes and spiking the humidity to levels over 100,000 times anything routinely experienced on Earth³.

It took Mars Express, a camera never meant for this job, eight years of patient repeat visits to turn one strange white streak into evidence of a kind of cloud formation nobody had caught in the act before^3,2.

Timeline

1. 13 September 2018Mars Express's Visual Monitoring Camera first captures the elongated cloud beside Arsia Mons.
2. 21 September 2018A clear image documents the cloud's structure during orbit 18627.
3. 10 October 2018The cloud is photographed extending about 1,500 km westward of the volcano.
4. 20 October - 1 November 2018A VMC image sequence reveals the cloud's full daily growth-and-evaporation cycle.
5. 2019ESA reports continued monitoring, confirming the cloud as a regular seasonal feature.
6. 2021A long-term study using VMC and other orbiter data measures the cloud's maximum size at about 1,800 x 150 km.
7. 24 June 2024Mars Express's High Resolution Stereo Camera photographs a 980 km version of the cloud in detail.
8. October 2026New modelling proposes homogeneous nucleation as the cloud's formation mechanism.

Pictures