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Mysterious Mars cloud reappears to haunt a volcano on the red planet

Mars isn't erupting, but this odd cloud still puts on quite a show.

These two views from July 2020 show the elongated cloud extending from the Arsia Mons volcano on Mars.

This story is part of Welcome to Mars, our series exploring the red planet.

The towering Arsia Mons volcano on Mars reaches over 12 miles (20 kilometers) high. It's impressive enough on its own, but it looks extra wild when a strange cloud forms above it. 

The European Space Agency Mars Express spacecraft has been keeping an eye on a "a mysteriously long, thin cloud" that periodically appears over Arsia Mons. On Wednesday, ESA released a new look at this cloud from observations made in July.

"This elongated cloud forms every martian year during this season around the southern solstice, and repeats for 80 days or even more, following a rapid daily cycle," said Jorge Hernandez-Bernal, a doctoral candidate at the University of the Basque Country in Spain. "However, we don't know yet if the clouds are always quite this impressive."

The cloud can stretch for over 1,100 miles (1,800 kilometers). The recent observations came around Mars' southern solstice. "In the early mornings during this period, this fleeting cloud grows for approximately three hours, quickly disappearing again just a few hours later," ESA said. Mars Express was in a prime spot to snap images of the cloud. 

In 2018, when Earthlings eyed the cloud, there was some internet speculation it indicated new volcanic activity on Mars, but that is not the case. According to NASA, Arsia Mons' last volcanic hurrah was around 50 million years ago.

The enigmatic cloud is made up of water ice. The Mars Express science team decided it needed its own name as they continue to investigate its appearances and disappearances. It's now known as the "Arsia Mons Elongated Cloud," or AMEC for short. That's catchy.