In 2018, a camera aboard the Mars Express mission caught a strangely long and wispy cloud blowing across the red planet’s surface.
From a distance, the 1,000-mile trail of fog almost resembled a plume of smoke and appeared to emerge from the top of a long-dead volcano.
Looking back at archived images, researchers quickly realized that this had been going on for a while. Every few years, in the spring or summer, this curious cloud would return before disappearing. The fleeting plume was captured on camera in 2009, 2012, 2015, 2018 and again in 2020.
A newly published study has now outlined the reasons why this inscrutably long cloud keeps coming and going on Mars. To do this, researchers compared high-resolution observations of the 2018 plume with other archived observations, some of which date all the way 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 Arsia Mons Elongated Cloud begins to take shape.
At dawn, the dense sky starts to climb up the western slope from the base of the Arsia Mons volcano. As the temperature drops, this wind expands and the moisture in it condenses around dust particles, creating what we call an orographic cloud here on Earth.
Every morning, over the course of several months of observations, researchers saw this process repeat itself. At about 45 kilometers altitude, the sky begins to expand, and for the next 2.5 hours or so the cloud is pulled westward by the wind, at a speed of 600 kilometers per hour (380 mph), before finally breaking away from the volcano.
At its largest, the plume can reach 1,800 kilometers long (over 1,100 miles) and 150 kilometers wide (nearly 100 miles). By noon, when the sun is at its peak, the cloud has completely evaporated.
Ice clouds are not exactly uncommon on Mars, but the clouds over Arsia Mons continue to form in summer, when most of the others disappear. In fact, this particular volcano usually has a cloud on it, while others around it don’t, but only under certain conditions it spreads in a long sequence. (Every year, at the beginning of winter, this cloud can also spiral.)
Profile of the Arsia Mons elongated cloud. (ESA)
So if this long plume occurs daily for a time every year, why do we only have sporadic observations of it?
Researchers say this is because many of the cameras orbiting Mars only occasionally fly over this area in the morning, and observations are usually scheduled, meaning that we often take pictures of this cloud by chance.
Fortunately, an old camera still onboard 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 have.
“Although [the camera] has a low spatial resolution, a wide field of view – essential for seeing the big picture at different local times of the day – and is great for tracking the evolution of an element over a long period 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 countless life cycles.”
The study represents the first detailed exploration of the Arsia Mons cloud, and although scientists say it has similar properties to Earth’s orographic clouds, its size is enormous and its dynamics are quite vivid compared to what we see on our home planet.
“Understanding this cloud gives us the great opportunity to try to simulate the formation of the cloud with models – models that will improve our understanding of climate systems on both Mars and Earth,” said astronomer Agustin Sánchez-Lavega, also from the University of the Basque Country. .
Now that we know when the cloud occurs, it also allows us to send other, stronger cameras in orbit to the right place at the right time, giving us a better picture. It may not take too long until the following pictures
The study is published in the Journal of Geophysical Research