Every Morning, Mars Builds a Giant Cloud Out of Thin Air

Every southern spring and summer on Mars, a streak of water-ice cloud unrolls beside a giant volcano and stretches 1,800 kilometers (1,120 miles) across the sky, almost two United Kingdoms end to end. It fades by day. The next morning it is back, for months.

ESA's Mars Express orbiter first spotted the Arsia Mons Elongated Cloud, named for the volcano it trails from, in 2018. It is an orographic cloud, the kind Earth makes when wind spills over a mountain. But when researchers tried to rebuild it in computer models, nothing matched the pictures.

On Earth, cloud droplets need a seed. Cooling vapor condenses onto a speck of dust, soot or sea salt, and on Mars, dust was assumed to do that job. This cloud seems to skip the seed entirely. Lead researcher Jorge Hernández-Bernal, based in Paris, likened it to condensation beading up midair in a room instead of on the window.

The process, homogeneous nucleation, appears in textbooks but was generally treated as theoretical. The team says no planetary atmosphere has ever been caught doing it. Their models produced the cloud only once they added it.

Here is how the team explains it. Wind sweeping past the enormous volcano sets off a strong wave that hurls damp air several kilometers (a few miles) upward within minutes. The air chills fast and the vapor freezes straight into ice, helped by the planet's wispy air and the volcano's sheer height.

The catch is humidity. The process demands moisture more than 100,000 times what Earth usually sees. Nobody has measured air like that on Mars, though the team says the cloud strongly suggests it exists.

Above one Martian volcano, mornings may get muggy enough to make a Chicago July feel bone dry.

Read the full story at BBC Sky at Night Magazine, October 9, 2026

Hot Take: Every textbook cloud diagram starts with a speck of dust. Mars just earned itself a 1,120-mile footnote.