Astronomers may have uncovered a wild reason for how one Martian cloud grows hundreds of miles longer than California, possibly making it the largest of its kind in the solar system.
Astrophysicist Jorge Hernández Bernal first spotted the cloud in an otherwise clear sky in 2018. Images by the European Space Agency's Mars Express Visual Monitoring Camera, aka the Mars webcam, captured the strange phenomenon as the Red Planet's southern hemisphere experienced spring.
When he looked through archival Mars images, he found the cloud as far back as NASA's Viking 2 mission in the 1970s. But it largely escaped notice because of its ephemeral nature: It forms daily at sunrise around the summit of Arsia Mons, a mountain that soars 11 miles high, and vanishes within three hours. Many space cameras snapping photos of that region can't take pictures until the afternoon, due to orbits timed to the sun.
For years, Bernal and colleagues have continued investigating what makes this extraterrestrial wonder — and, no, it's not steam spewing from the volcano. Arsia Mons has not been active for ages. Instead, the answer may be physics that was only theoretical until now. Scientists have never seen this happen in any planet's atmosphere before.
The team's shocking findings, published in Nature Geoscience, are a reminder to scientists that they should never rule out obscure or unfamiliar processes when studying other worlds.
The key to the cloud could be something called "homogeneous nucleation," according to the new paper, a process wherein water vapor seems to freeze straight into ice particles in the air.
On Earth, clouds tend to form when water vapor cools and clings to airborne particles, like salt, pollen, or dust. Originally, researchers believed Mars' copious dust was the catalyst. But when they used computer models to try to simulate the cloud, dubbed the Arsia Mons Elongated Cloud, or AMEC, they could never reproduce its long tail.
Something was missing from their cloud recipe, which urged Bernal, now at Sorbonne Université in Paris, to continue looking for missing steps or ingredients.
"The actual answer seems obvious now, but at first we were quite skeptical of ourselves," Bernal told Mashable. "The point is that homogeneous nucleation is another pathway to form cloud particles without requiring dust, and that happens precisely when humidity is very high."
Researchers liken this unusual phenomenon to droplets of condensation appearing in the middle of a room, rather than on a window. For this cloud process to work, the air has to be extremely moist — over 100,000 times more humid than anything we experience on Earth.
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So how does a dry, dusty planet get that soggy? Blame the volcano. As wind rushes past Arsia Mons, the mountainside kicks up a powerful gravity wave in the atmosphere. That wave shoves moist air several miles higher in just a few minutes. Up there, the air chills fast, dropping about 54 degrees Fahrenheit in only 10 minutes. Cold air can't hold much water vapor, so the humidity shoots up until the vapor freezes on the spot.
Mars may be famous for its dust, but the researchers found that altitude makes all the difference. The cloud forms about 28 miles above the ground, where far less dust floats around.
With homogeneous nucleation added to the cloud recipe, the team's computer model finally produced the long tail.
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"It feels good to find a long-sought explanation like this, especially when the explanation turns out to be something really special that has not been observed in nature before," Bernal said.
Confirming the result will require new observations that won't be easy to get. But the researchers hope the paper will spark broader scientific interest in dust-free cloud formation, not just in the upper atmosphere of Mars, but Venus and even Earth.
Bernal, who has devoted his whole career to studying this exotic cloud, worries that humans could one day destroy it, as they have other wondrous features on our home planet. He's an advocate for studying space and other worlds from afar.
"It is an eye-catching, aesthetically sublime cloud," he said. "If anyone tried to terraform Mars, any change in its current climate could easily make the AMEC disappear."
