We finally know what’s going on with that strange, long, recurring cloud on Mars


In 2018, a camera aboard the Mars Express mission saw an eerily long and wispy cloud, billowing over the surface of the red planet.

From a distance, the 1,500-kilometer (930-mile) trail of fog almost resembled a plume of smoke, appearing to be emerging from the top of a long-dead volcano.

Looking back at the archived footage, the researchers soon realized that this had been going on for a while. Every few years in spring or summer, this curious cloud would return, before disappearing once more. The fleeting feather was captured on camera in 2009, 2012, 2015, 2018, and again in 2020.

A recently published study has now detailed the reasons behind why this unfathomably long cloud keeps coming and going on Mars. To do this, the researchers compared the high-resolution observations in the 2018 column with other archived observations, some of which date back to the 1970s.

Here’s the story of the cloud. Every year, around the beginning of spring or summer in the southern hemisphere of Mars, the elongated cloud of Arsia Mons begins to take shape.

At dawn, the dense air at the base of the Arsia Mons volcano begins to rise up the western slope. As temperatures drop, this wind expands and the moisture it contains condenses around dust particles, creating what here on Earth we call an orographic cloud.

Each morning, over several months of observations, the researchers observed that this process was repeated. At about 45 kilometers altitude, the air begins to expand and for the next 2.5 hours or so, the cloud is blown westward by the wind, at a speed of 600 kilometers per hour (380 mph), before finally breaking away. of the volcano.

At its largest point, the plume can reach 1,800 kilometers long (more than 1,100 miles) and 150 kilometers wide (almost 100 miles). At noon, when the Sun is at its peak, the cloud will have completely evaporated.

Ice clouds aren’t exactly unusual on Mars, but clouds over Arsia Mons continue to form in summer when most of the others disappear. In fact, most of the time, this specific volcano has a cloud on top of it when others around it don’t, but only under some conditions does it spread out in a long streak. (Every year at the beginning of winter, this cloud can also form a spiral.)

Arsia Mons Elongated Cloud Article ProfileProfile of the elongated cloud of Arsia Mons. (ESA)

So if this long column occurs every day for a period of time every year, why do we only have sporadic observations of it?

The researchers say that is because many of the cameras orbiting Mars only occasionally fly over this region in the morning, and observations are generally planned, meaning that we often take pictures of this cloud simply by chance.

Fortunately, an old camera that is still aboard the Mars Express mission, the Visual Monitoring Camera (VMC) that has the power of a 2003 webcam, has an advantage that newer technology does not.

“Though [the camera] it has a low spatial resolution, has a wide field of view, essential for seeing the big picture at different local times of the day, and is wonderful for tracking the evolution of a feature over a long period of time and in a small interval of time. explains astronomer Jorge Hernández Bernal from the University of the Basque Country in Bilbao, Spain.

“As a result, we were able to study the entire cloud over numerous life cycles.”

The study represents the first detailed exploration of the Arsia Mons cloud, and although scientists say it has properties similar to orographic clouds on Earth, its size is huge and its dynamics quite vivid compared to what we see on our own planet.

“Understanding this cloud gives us the exciting opportunity to try to replicate the formation of the cloud with models, models that will improve our knowledge of climate systems both on Mars and on Earth,” says astronomer Agustín Sánchez-Lavega, also from the University of the Basque Country. .

Now that we know when the cloud occurs, it also allows us to direct other more powerful cameras in orbit to the right place at the right time, giving us a closer view. It may not be long until the next photos.

The study was published in the Geophysical Research Journal.

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