The European Space Agency’s Mars Express, a camera-equipped Mars orbiter, is keeping close tabs on a mysterious “elongated cloud” appearing over the 20 kilometer tall Arsia Mons volcano near the Red Planet’s equator.

The spacecraft, and other ones like it, have been following the strange cloud’s evolution since 2009. Now, new images revealed this week by the ESA show that the cloud has persisted.

Martian Mystery

Scans suggest that the mystery cloud is not linked to volcanic activity in any way — the last time Arsia Mons was active was about 50 million years ago, according to NASA. Instead, scientists suspect it’s a cloud made up of water ice flowing down the sloping sides of the volcano.

The cloud, as spotted in the latest images taken on July 17 and 19 by Mars Express’s Visual Monitoring Camera (VMC), appears to be up to 1,800 kilometers in length.

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