What is this strange cloud that has been hovering over Mars for weeks?

For more than a month, Mars' orbiters have been watching a strange Martian cloud that looks like a booger and is so big that it can be seen with telescopes on Earth.

Suspended on the western slope of Arsia Mons, a huge volcano located near the equator of the Red Planet, the elongated cloud stretches for about 930 miles. It was spotted for the first time on September 13th by the Mars Express orbiter of the European Space Agency (ESA). Like the kind of volcanic plume blown by Earth's active volcanoes, Arsia Mons has been extinct for a long time – its last eruption would have occurred about 50 million years ago.

The volcano still plays a role in the formation of the water ice cloud, with atmospheric dust levels and Martian seasons.

Image: ESA / GCP / UPV / EHU Bilbao

With a peak of more than 12 km and a diameter of nearly 400 km, Arsia Mons is 30 times larger than the largest volcanoes on the planet. Its enormous volume condenses and cools air currents as they pass through the summit, creating this "orographic cloud", a nephological formation that tends to form on leeward (leeward) slopes, on the west side of the volcano.

Similar clouds observed in 2009, 2012 and 2015 coincided with the winter season. It is therefore not surprising that this one arrived a few weeks before the Martian winter solstice of October 16th. In addition, Mars was recently hit by a dust storm that surrounded the planet in June and July, which likely increased the visibility and size of the cloud.

Read more: This "crater" could actually be a former Martian Supervolcan

Maybe one day, we will be able to see this rare event of the surface through the eyes of a rover, an undercarriage or even a human crew near the airport. Arsia Mons, who inspired science fiction stories because of his possible cave system. Meanwhile, Mars Express will continue to capture raw images of the cloud until it disappears, so you can track its progress safely from the Earth.

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