A cloud stretching 1,800 kilometres, about 1,118 miles, regularly forms over the Martian volcano Arsia Mons, and scientists say its physics should not work.

The Arsia Mons Elongated Cloud appears each Martian year, trailing westward from the giant volcano. A study published October 7 in Nature Geoscience explains the paradox.

The cloud forms through homogeneous ice nucleation, meaning water ice crystals appear without any dust particles to seed them. On Earth, clouds need such seeds. On Mars, the process happens anyway, which researchers call exotic physics.

Hernandez-Bernal of the Sorbonne and CNRS led the analysis, using observations from the European Space Agency’s Mars Express orbiter.

The findings matter for understanding the Martian water cycle and climate. If ice can form without dust, models of the planet’s atmosphere need revision.

The cloud is also a striking sight. Images show a long white plume arcing across the rust-coloured sky, one of the most dramatic weather features in the solar system.