A huge Martian cloud returns every spring. Scientists now know why.

A huge Martian cloud returns every spring.  Scientists now know why.
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A cloud longer than California crosses Marsruddy cheek. It looks as if an impressionist painter loaded his palette knife with white and drew a line across the canvas as far as the oily paint would go.

This is not what astrophysicist Jorge Hernández Bernal first saw in 2018 when the Mars Express visual surveillance camera(Opens in a new window) — affectionately known to the European Space Agency as the Mars webcam(Opens in a new window) — posted a new photo. To the average eye, it was grainy and inscrutable, with the resolution of a standard computer camera from about 20 years ago. But Bernal, who was studying Martian meteorology at the University of the Basque Country in Spain, immediately recognized the shadow as something else: a mysterious weather phenomenon occurring on the Red Planet.

It wasn’t until researchers looked at the cloud with better equipment that Mars revealed the cloud in all its glory. The team delved into the photo archives and discovered that it had been there frequently. He was there through the years, and he was even there for POT‘s viking mission 2(Opens in a new window) in the 1970s.

Mars webcam capturing the elongated cloud of Arsia Mons

A low-resolution camera on the European Space Agency’s Mars Express probe first captured the massive cloud in 2018.
Credit: ESA

The secret had been knowing when to look for it.

“There were people thinking that ESA was faking it,” Bernal told Mashable. “It was a bit difficult because I was very young at the time. [of the discovery]and I was on Twitter trying to talk to people.”

Bernal and his team published their observations in 2020, calling it the Arsia Mons Elongated Cloud, or AMEC for short. With the cloud spanning 1,100 miles, scientists believe it could be the longest of its kind in the solar system. That work was followed by a second report, recently posted(Opens in a new window) in it Geophysical Research Journal: Planetswhich reveals how the volcano creates this extraordinary cloud, only on a cloudless southern Mars at that time of year.

“There were people thinking that ESA was faking it.”

How Scientists Discovered the Long Cloud on Mars

For decades, the ice cloud reached the western slope of the arsia mons(Opens in a new window), an extinct volcano. The ancient mountain that once spewed lava is about 270 miles wide at the base and rises 11 miles into the sky. It dwarfs Mauna Loa, the largest volcano on Earth, which is about half its height.

The curious case of the gigantic cloud is how it went unnoticed for so long. But some of the spacecraft around Mars, like NASA’s Mars Reconnaissance Orbiter, are in sun-synchronous orbits, meaning their cameras can’t take pictures until late afternoon. At that time, the fleeting cloud, which lasts only about three hours in the morning, is already gone.

The Mars Webcam was not originally designed for science. Its purpose was to provide visual confirmation that ESA’s Beagle 2 lander(Opens in a new window) had been separated from the Mars Express spacecraft in 2003. In hindsight, the space agency is glad it decided turn basic camera back on(Opens in a new window).

Mars Express spacecraft orbiting Mars

A simple camera not even intended for science on the European Space Agency’s Mars Express spacecraft photographed the huge cloud.
Credit: ESA

Just as southern Mars experiences spring, the cloud grows and stretches, forming a wispy tail like a steam locomotive, over the top of the mountain. Then, in a matter of hours, the cloud completely vanishes in the warm sunlight.

For a young scientist working on his PhD, the natural wonder became something of a muse. While the realist in him said that recreational space travel is impractical, perhaps even unethical given the world’s climate problems, he couldn’t help but try to draw what the cloud would look like from the ground.

“I keep imagining what it would be like for a small civilization to have this huge cloud every year at the same time, as if the solstice was something like a coat for them,” he said, smiling. “This is the imagination part.”

Why does Arsia Mons of Mars make the giant cloud?

So what makes this weird, stringy cloud?

For starters, it’s not smoke coming out of a volcanic eruption. Scientists have long known about the Volcanoes of the Red Planet(Opens in a new window) they are dead. Rather, it is the so-called “orographic effect”: the physics of air rising over a mountain or volcano.

The researchers performed a high-resolution computer simulation of the effect of Arsia Mons on the atmosphere. Strong winds whip at your feet, forming gravity waves. The moist air is then temporarily squeezed out and blown up the mountainside. Those air currents blow up to 45 mph, forcing the temperature to drop more than 54 degrees Fahrenheit. This allows the water to condense and freeze about 28 miles above the volcano’s peak.

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“I keep imagining what it would be like for a small civilization to have this huge cloud every year at the same time, as if the solstice was something for them like a coat.”

The Arsia Mons cloud returns year after year

The huge Arsia Mons cloud returns year after year in the spring of Mars for about 80 days.
Credit: ESA

For about five to ten percent of the Martian year, the atmosphere is perfect(Opens in a new window) to make the cloud, with the dusty sky helping moisture to cling to the air. Too early in the year and the air would be too dry, according to the team’s model. Too late in the year and the weather would be too warm for water condensation.

But while the scientists’ simulation succeeded in forming the cloud in the unique conditions of Arsia Mons, it was unable to replicate the cloud’s long tail. Scientists say that’s the biggest question at the moment — a mystery that could be solved with spectrometers, devices on spacecraft that identify the types of particles in a substance. A more detailed study of the cloud’s water ice could give researchers more clues.

“I would like to see this cloud with my eyes, but I know where my place is,” Bernal said. “Sometimes we think of space as a utopia. I am happy looking at it from [Earth, through] my spaceship.”

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