Modeling suggests Mars’s Arsia Mons cloud may form ice without dust, pointing to a process never before observed in a planetary atmosphere.

A white cloud grows downwind of Mars’s Arsia Mons volcano each morning during spring and summer in the planet’s southern hemisphere. Rising beside a volcano 20 kilometers (about 12 miles) tall, this wisp of water ice can stretch up to 1,800 kilometers (about 1,120 miles), nearly twice the length of the UK, before quickly evaporating. The cycle repeats every morning for several months during the Martian dusty season.

Attempts to reproduce how the Arsia Mons Elongated Cloud (AMEC) forms had failed to match spacecraft images. Now, a study combining observations from the European Space Agency’s Mars Express with an advanced meteorological model suggests that the cloud may depend on an ice-forming process never before seen in a planetary atmosphere.