A "mysterious column of smoke" that forms near an inactive Martian volcano has spread across the surface of the Red Planet for more than a month.
Space.com reports that for several weeks, a European orbiter surrounding Mars has been observing the long cloud.
The smoke has remained in place on a volcanic mountain called Arsia Mons near the Martian equator since September 13, according to a statement issued by the European Space Agency.
They said that apparently there are no volcanic processes that produce the cloud, since the volcano has not been active in about 50 million years.
Scientists say that the same spacecraft, which is called Mars Express, and its predecessors have seen similar clouds on at least three previous occasions, and those structures were formed almost at the same time in the Martian year.
And that's not a coincidence, wrote the ESA.
They noted that the cloud is filled with water ice and has been created by the flow of air along the side of the volcano.
This means that the feature is what scientists call an orographic cloud or read.
That also means that the cloud changes throughout the day as atmospheric patterns change on Mars.
Space.com added that scientists who observe the cloud have noticed that it grows throughout the morning, extending along the equator.
It can also be affected by dust still in the atmosphere of the massive dust storm that engulfed Mars earlier this year.