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

A giant Martian cloud returns every spring.  Scientists now know why.
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A cloud longer than California is moving in Mars‘red cheek. It looks as if an impressionist painter loaded his palette knife with white and scraped a line across the canvas as far as the oil paint would go.

This is not what astrophysicist Jorge Hernández Bernal first saw on the Mars Express in 2018 Visual surveillance camera(Opens in a new window) — affectionately referred to as the by the European Space Agency Mars webcam(Opens in a new window) — posted a new picture. To the average eye it was grainy and unreadable, with the resolution of a standard computer camera some 20 years ago. But Bernal, who studied Martian meteorology at the University of the Basque Country in Spain, immediately recognized the shadow as something else: a mysterious weather phenomenon that happens on the Red Planet.

It wasn’t until researchers viewed the cloud with better equipment that Mars revealed the cloud in all its sprawling glory. The team dug deeper into photo archives and found it had been there frequently. It was there all along, and it was there even during OUR‘with Viking 2 mission(Opens in a new window) in the 1970s.

Mars webcam capturing Arsia Mons Elongated Cloud

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

The secret had been knowing when to look for it.

“There were people who thought 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 and named them Arsia Mons Elongated Cloud, or AMEC for short. At 1,100 miles across, scientists believe the cloud may be the longest of its kind in the solar system. This work was followed by a second report, recently released(Opens in a new window) in which Journal of Geophysical Research: Planetsshowing the volcano producing this extraordinary cloud alone in an otherwise cloudless southern Mars at this time of year.

“There were people who thought ESA was faking it.”

How Scientists Discovered the Long Cloud of Mars

For decades, the ice cloud arrived on the west slope at sunrise Arsia Mons(Opens in a new window), an extinct volcano. The once lava spewing ancient mountain is about 270 miles wide at the base and towers 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 why it has eluded attention for so long. But some of the spacecraft around Mars, like NASA’s Mars Reconnaissance Orbiter, are in sun-synchronous orbits, meaning their cameras won’t be able to take pictures until the afternoon. By this time, the fleeting cloud, which lasts only about three hours in the morning, has already disappeared.

The Mars webcam was not originally intended for science. Its purpose was to confirm this visually ESA’s Beagle 2 lander(Opens in a new window) separated from the Mars Express spacecraft in 2003. In retrospect, the space agency is glad it made the decision to do so Turn the base camera back on(Opens in a new window).

The Mars Express spacecraft orbits Mars

A simple, non-scientific camera on the European Space Agency’s Mars Express spacecraft photographed the giant cloud.
Photo credit: ESA

Just as southern Mars is experiencing spring, the cloud grows and stretches, forming a thin tail like a steam engine over the mountain’s peak. Then, within a few hours, the cloud fades completely in the warm sunlight.

For a young doctoral scientist, the natural wonder became a kind of muse. While the realist in him said that recreational space travel was impractical — perhaps even unethical given the world’s climate problems — he couldn’t help but try to plot what the cloud might look like from the ground.

“I keep imagining what it would be like for a small civilization to have this huge cloud at the same time every year, as if maybe the solstice was something of a cloak for them,” he said, smiling. “That’s the introduction part.”

Why Arsia Mons from Mars makes the gigantic cloud

So what constitutes this strange, threadlike cloud?

for starters, it doesn’t smoke blown up by a volcanic eruption. Scientists have known this for a long time volcanoes of the red planet(Opens in a new window) are dead. Rather, it’s the so-called “orographic effect”: the physics of air rising over a mountain or volcano.

Researchers performed a high-resolution computer simulation of the effect of Arsia Mons on the atmosphere. Strong winds whip at its feet, creating gravitational waves. Humid air is then temporarily compressed and forced up the mountainside. These drafts blow up to 45 miles per hour, forcing the temperature to drop more than 54 degrees Fahrenheit. This allows water to condense and freeze about 28 miles above the volcano’s summit.

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

The Arsia Mons cloud returns year after year

The giant Arsia-Mons cloud returns for about 80 days each year in the Martian spring.
Photo credit: ESA

About five to ten percent of the Martian year the atmosphere is just right(Opens in a new window) to make the cloud, with the dusty sky helping to trap moisture in 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 climate would be too warm for water condensation.

But while the scientists’ simulation was successful in forming the cloud under the unique conditions of Arsia Mons, it failed to recreate the cloud’s long tail. Scientists say this is the biggest question right now — a mystery that could be solved with spectrometers, devices on spacecraft that identify the types of particles in a substance. A closer look at the cloud’s water ice could give researchers more clues.

“I’d like to see that cloud with my eyes, but I know where my place is,” Bernal said. “Sometimes we think of space as a utopia. I’m happy when I look at him [Earth, through] my spaceship.”

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