It appears at first glance to show a volcanic eruption on Mars, a long white trail of smoke billowing from a red planet peak more than twice the height of Mt Everest.
But the latest fascinating photo sent back by the Mars Express probe is an illusion, astronomers say.
In a release, the European Space Agency described the fog a "curious cloud formation".
The long white cloud stretches 1500km across the Martian sky, and its position - appearing to come from the 22km-high Arsia Mons - is no coincidence.
Eldar Noe Dobrea of the Planetary Science Institute in Arizona told the New York Times Arsia Mons' massive size is to blame. Water-laden wind is pushed up into the sky by the mountain, where it cools and forms clouds - just like on Earth.
"The air on Mars is much thinner than on Earth, but the rules of weather physics also apply there," wrote Times science reporter Kenneth Chang.
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Similar clouds have been spotted in 2009, 2012 and 2015. The current one is so big, it can be seen by anyone with a decent telescope on Earth, the ESA says.
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