A Curious Elongated Cloud Just Reappeared On Mars

It may form every year, but it only just got a name. Meet the Arsia Mons Elongated Cloud, or AMEC. ESA/GCP/UPV/EHU Bilbao

For those of you checking off your 2020 bingo card, don’t worry, this strange, long, thin cloud that has appeared over Mars wasn’t totally unexpected. In fact, it turned up right on time, although not to be outdone by this year's events, astronomers are unsure if it is usually this impressive.

“We have been investigating this intriguing phenomenon and were expecting to see such a cloud form around now,” said Jorge Hernandez-Bernal, a PhD candidate and lead author of an ongoing study on the mysterious phenomenon, in a European Space Agency (ESA) statement.

“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. However, we don’t know yet if the clouds are always quite this impressive.”

A recurring feature, the curious cloud forms every year around the southern solstice, when the Sun is in the southernmost position in the Martian skies, like it is in December on Earth. Because a Martian day, or sol, is an extra 39 minutes and 35 seconds longer than Earth's, however, a year on the Red Planet lasts 668 sols, or 687 days, making the season twice as long, which is why the cloud turns up now.

The elongated cloud is likely an orographic cloud, influenced by Mars' topography. ESA/GCP/UPV/EHU Bilbao

The elongated cloud formation can reach 1,800 kilometers (1,120 miles) in length, and hovers above the 20-kilometer-high (12.4-mile) Arsia Mons volcano, close to the planet's equator. However, in spite of its location, it's not a volcanic plume or linked to any volcanic activity at all, but a cloud of water ice. It's thought the cloud stream forms due to the volcano's leeward side, the side that doesn't face the wind. These are called orographic or lee clouds, and just like on Earth, form in response to air being forcibly lifted up due to topographical features, like a mountain or volcano. 

The cloud pictured here was snapped on July 17 (above) and July 19 (top) by the Visual Monitoring Camera (VMC) on ESA's Mars Express, which has been orbiting Mars for 16 years.

Curiously, the cloud only seems to form in the early mornings, grows for around three hours, and then quickly disappears, repeating this daily cycle for around 80 days. Most spacecraft orbiting Mars observe it in the afternoon, but the Mars Express' highly elliptical orbit offers a morning front-row seat to the atmospheric feature.

“The extent of this huge cloud can't be seen if your camera only has a narrow field of view, or if you're only observing in the afternoon,” explained Eleni Ravanis, who works on the VMC instrument as a Young Graduate Trainee for the Mars Express mission.

“Luckily for Mars Express, the highly elliptical orbit of the spacecraft, coupled with the wide field of view of the VMC instrument, lets us take pictures covering a wide area of the planet in the early morning. That means we can catch it!”

The team has now named the cloud the Arsia Mons Elongated Cloud, or AMEC, and is determined to answer some of the questions it throws up every year: why does it only appear in the morning, why does it appear and disappear so quickly, and is it usually so impressive, or has it, like many other phenomena this year, looked around and thought: 2020, huh? Hold my beer.

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