This curious cloud requires no solid particles to seed its formation — a theoretical phenomenon that’s never been observed until now.

The Arsia Mons Elongated Cloud, as seen by the European Space Agency's Mars Express orbiter.

ESA / DLR / FU Berlin

In theory, clouds can form out of thin air. Mind you, we’ve never seen it happen: Clouds observed in Earth’s atmosphere, as well as on Venus and Mars, form via nucleation , in which droplets of water or ice condense onto tiny, solid particles, such as floating dust on Mars.

Now, for the first time, scientists have reason to think they’ve seen a cloud formation form spontaneously, condensing out of vapor without the help of such seeds. The results appear in the October issue of Nature Geoscience .

The cloud in question is a long, thin cloud extending some 1,800 kilometers (1,100 miles) from the bulge of the Arsia Mons volcano, one of three distinctive volcanoes near the equator.…