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Every spring, a huge Martian cloud returns. Scientists now know why.

A cloud taller than California crosses the blushing cheek of Mars. It looks like an Impressionist painter loaded his palette knife with white and scraped a line across the canvas as far as the oily paint could reach.

This is not what astrophysicist Jorge Hernández Bernal first saw in 2018 when the Mars Express Visual security camera(Opens in a new window) — affectionately known by 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 studied Martian meteorology at the University of the Basque Country in Spain, immediately recognized the shadow as something else: a mysterious weather phenomenon playing out on the Red Planet.

It wasn’t until researchers looked at the cloud with better equipment that Mars revealed the cloud in all its expansive glory. The team dug deeper into photo archives and found it had been there many times. It was there through the years, and it was even there during NASA’s Viking 2 mission(Opens in a new window) In the seventies.

Mars webcam capturing 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 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 called it the Arsia Mons Elongated Cloud, or AMEC for short. With a cloud stretching 1,800 kilometers, scientists believe it could be the longest of its kind in the solar system. That work was followed up with a second report, recently published(Opens in a new window) in the Journal of Geophysical Research: Planetsrevealing how the volcano makes this extraordinary cloud, only found in otherwise cloudless southern Mars at that time of year.

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

How scientists discovered the long cloud of Mars

For decades, the icy cloud arrived at dawn on the western slope of Arsia Mountains(Opens in a new window), an extinct volcano. The once lava spewing ancient mountain is about 270 miles wide at its base and soars 11 miles into the air. It dwarfs Mauna Loa, the largest volcano on Earth, which is about half its height.

The curious case of the giant cloud is how it escaped notice for so long. But some of the spacecraft orbiting Mars, such as NASA’s Mars Reconnaissance Orbiter, are in orbits synchronized with the sun, meaning their cameras can’t take pictures until midday. By this time, the volatile cloud, which lasts only about three hours in the morning, has already dissipated.

The Mars Webcam was not originally intended for science. The goal was to confirm that visually ESA’s Beagle 2 lander(Opens in a new window) was separated from the Mars Express spacecraft in 2003. In retrospect, the space agency is glad it decided to do so turn the base camera back on(Opens in a new window).

Mars Express spacecraft in Mars orbit

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 the south of Mars experiences spring, the cloud grows and stretches, making a wispy tail like a steam locomotive, over the top of the mountain. Then, within a few hours, the cloud disappears completely in the warm sunlight.

For a young scientist working on his doctorate, 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 from the ground what the cloud might look like.

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

Why Arsia Mons on Mars Makes the Giant Cloud

So what makes this strange, stringy cloud?

For starters, it’s not smoke rising from a volcanic eruption. Scientists have known for a long time volcanoes of the Red Planet(Opens in a new window) 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 Arsia Mons’ effect on the atmosphere. Strong winds whip at its base, creating gravitational waves. Moist air is then temporarily compressed and forced up the mountainside. That draft blows up to 45 mph, dropping temperatures by more than 54 degrees Fahrenheit. This allows water to condense and freeze at 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, like maybe the solstice is like a coat to them.”

Arsia Mons cloud that returns year after year

The massive 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 just right(Opens in a new window) to make the cloud, with the dusty air helping to attach moisture 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 climate would be too warm for water condensation.

But while the scientists’ simulation was successful in forming the cloud under Arsia Mons’ unique conditions, it failed to replicate 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 study of the cloud’s water ice may give researchers more clues.

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

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