An enormous Martian cloud returns every spring. Scientists now know why.

A cloud longer than California lines stretches across Marsred cheek It appears as if an Impressionist painter loaded his palette knife in white and scraped a line across the canvas as far as the oil paint would go.

This was not what astrophysicist Jorge Hernandez Bernal saw for the first time in 2018 when the Mars Express Visual surveillance camera(opens in a new window) – affectionately known by the European Space Agency as Mars webcam(opens in a new window) A new photo has been posted. To the average eye, it was grainy and fuzzy, with the standard computer camera resolution of 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 sprawling glory. The team dug deeper into the image archives, and discovered that they were there quite often. There have been through periods, and there has been even during NASA‘s Viking mission 2(opens in a new window) In the 1970s.

Mars webcam captures Arsia Mons Elongated Cloud

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

The secret was knowing when to look for it.

“There were people who thought the ESA was faking it,” Bernal told Mashable. “It was a little difficult because I was really 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 think it may be the longest of its kind in the solar system. This work was followed by a second report, recently published(opens in a new window) In the Journal of Geophysical Research: Planetsrevealing just how the volcano made this unusual cloud, alone in the clear south of Mars at that time of year.

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“There were people who thought the European Space Agency was faking it.”

How scientists discovered the Martian long cloud

For decades, the icy cloud has reached sunrise on the western slope of Arcia Mons(opens in a new window)Dormant volcano. The ancient lava-spewing mountain is about 270 miles wide at the base and rises 11 miles into the sky. It dwarfs Mauna Loa, Earth’s largest volcano, which is almost half its height.

The strange case of the giant cloud is how it has eluded notice for so long. But some spacecraft around Mars, like NASA’s Mars Reconnaissance Orbiter, are in sun-synchronous orbits, which means their cameras can’t snap pictures until well into the afternoon. By that time the passing cloud, which lasts only about three hours in the morning, had already cleared.

The Mars Webcam was not originally intended for science. It was intended to provide visual confirmation The European Space Agency’s Beagle 2 lander(opens in a new window) It broke away from the Mars Express spacecraft in 2003. In hindsight, the space agency is glad it decided to Restart the primary camera(opens in a new window).

Mars Express spacecraft orbiting Mars

A simple camera not even dedicated to science aboard the European Space Agency’s Mars Express spacecraft has photographed the massive cloud.
Credit: ESA

Just as southern Mars experiences spring, the cloud grows and stretches, trailing a crisp tail like a steam locomotive, over the mountaintop. Then, within hours, the cloud fades completely into the warm sunlight.

For a young scientist working on his PhD, natural wonders have become a kind of inspiration. While the realist in it argued that recreational space travel was impractical—and perhaps unethical given the world’s climate problems—he couldn’t help but try to sketch what a cloud might look like from Earth.

“I still imagine what it would be like for a small civilization to have this huge cloud every year at the same time, maybe the solstice is something for them like a coat,” he said with a smile. This is the imaginary part.

Why does Mars make the giant cloud Arsia Mons

So what makes this strange, tense cloud?

for beginners, It’s not smoke Rising from a volcanic eruption. Scientists have known it for a long time Volcanoes of the Red Planet(opens in a new window) They died. Rather, it is what is called the “orographic effect”: the physics of air rising above a mountain or volcano.

The researchers ran a high-resolution computer simulation of the effect of Arsia Mons on the atmosphere. Strong winds blow at its feet, creating gravitational waves. The moist air is then temporarily squeezed out and forced up the mountainside. Those drafts blast at speeds of up to 45 miles per hour, dropping the temperature by more than 54 degrees Fahrenheit. This allows water to condense and freeze 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 every year at the same time, maybe the solstice would be something for them like a coat.”

Arsia Mons cloud come back year after year

The massive Arsia Mons cloud returns year after year in the Martian spring for about 80 days.
Credit: ESA

For about five to ten percent of a Martian year, The atmosphere is just right(opens in a new window) To form the cloud, as the sky full of dust helps the adhesion of moisture to the air. So early in the year, the air will be very dry, according to the team’s model. Very late in the year, the climate will be too warm for the water to condense.

But although the scientists’ simulations succeeded in cloud formation under the unique conditions of Arcia Mons, they couldn’t replicate the cloud’s long tail. That’s the biggest question right now, scientists say — a puzzle that can be solved using spectrometers, which are instruments on spacecraft that identify the types of particles in matter. A closer study of the cloud’s water ice may give more clues.

“I would like to see this cloud with my own eyes, but I know where I am,” said Bernal. “Sometimes we think of space as a utopia. I’m happy to look at it through it [Earth, through] My spacecraft.

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