A giant cloud on Mars that stretches over 1,000 miles long forms through physics never before seen in a planetary atmosphere, scientists have found.

The Arsia Mons Elongated Cloud is a wisp of water ice that appears downwind of the 20-km-tall Arsia Mons volcano. It grows up to 1,800 km long, then evaporates, repeating the cycle every morning for several months. ESA’s Mars Express orbiter has tracked it since 2018.

Computer models long assumed the cloud formed the way Earth clouds do, with water vapour condensing onto dust particles. But those simulations could never reproduce what the orbiter actually saw.

The new study found the missing ingredient: homogeneous nucleation, where water vapour turns directly into ice particles without any dust to cling to. Textbooks treat this as theoretical, and it has never been observed in action before.

Lead author Dr Jorge Hernandez-Bernal said the finding challenges assumptions about how clouds form, with implications for certain Earth clouds and other planets too. It is a reminder that Mars, for all its familiarity, still holds genuine surprises.