The Arsia Mons Elongated Cloud — Mars’ longest cloud at up to 1,800 km, nearly twice the length of the UK — may form through physics never observed in a planetary atmosphere.

A study in Nature Geoscience found that models could only reproduce the cloud using homogeneous ice nucleation: water vapour freezing directly into ice particles without dust or other nuclei. On Earth and Mars, clouds normally need particles to condense onto.

The cloud forms each morning in Mars’ southern spring and summer downwind of the 20-km-tall Arsia Mons volcano, then evaporates. Lead author Jorge Hernandez-Bernal called the required physics “exotic” but textbook — just never seen in action.

The finding challenges assumptions about cloud formation on Mars and could apply to exotic clouds on Earth and exoplanets too.

The team wants to test whether the same ice-freezing process explains other mysterious Martian clouds. If confirmed, textbooks on planetary atmospheres will need a new chapter on cloud formation without condensation nuclei.