Scientists Model Mars's 1,000-Mile Cloud, Find Unseen Physics

A cloud stretching more than 1,000 miles across the Martian sky may be forming through a process never directly observed in nature, according to a study published in Nature Geoscience, as reported by Ars Technica and WIRED.

The formation, known as the Arsia Mons Elongated Cloud, appears each Martian spring and summer near the Arsia Mons volcano. It emerges in the morning, stretches into a long wispy band roughly the distance from New York to Kansas City, then dissipates. The cycle repeats for months. A probe first recorded the cloud in 2018. Scientists had no working model for how it formed until now.

What is the Arsia Mons Elongated Cloud?

The cloud always forms near the same 65,000-foot volcano before stretching into its elongated shape, according to the European Space Agency imagery cited by Ars Technica. Researchers led by Jorge Hernández-Bernal, a planetary scientist at Sorbonne Université in Paris, built the first model to successfully reproduce the cloud's behavior.

What physics explains the cloud's formation?

The model points to homogeneous nucleation. On Earth, clouds typically form when water vapor condenses onto dust, salt or other particles. Homogeneous nucleation skips that step. Vapor turns directly into ice crystals with no particle involved.

"It's akin to droplets of condensation appearing in the middle of a room, rather than on a window," Hernández-Bernal said in a press release cited by the outlet. "It's wholly unexpected."

Why hasn't this process been seen before?

The process requires extreme conditions: a 30-degree Celsius temperature drop within 10 minutes and relative humidity more than 100,000 times greater than any level measured on Earth, per the study. Arsia Mons supplies the trigger. Wind hitting the volcano creates a wave that launches moist air high into the atmosphere within minutes.

Scientists have suspected similar physics could occur in the upper atmospheres of other planets, including Venus and Earth, Hernández-Bernal said. It has never been confirmed. The humidity levels needed were not known to exist on Mars before this modeling work.

What happens next for researchers?

Hernández-Bernal called the result significant given how little is known about Mars's atmosphere compared with Earth's. "We don't have nearly as much information about Mars' atmosphere as we do about Earth's, so reproducing the AMEC to this degree is a big success," he said, adding that "the result is remarkable."

What to watch

• Whether follow-up observations from Mars orbiters can directly detect homogeneous nucleation in action rather than infer it from modeling.
• Whether similar extreme-humidity conditions get confirmed in the upper atmospheres of Venus or Earth.
• Whether the Nature Geoscience findings prompt new instrument proposals aimed at measuring Martian atmospheric humidity directly.
• Whether other long, repeating cloud formations on Mars get re-examined under the same model.

The study adds to a small but growing body of work using Mars's thin, dry atmosphere as a natural laboratory for processes too rare or extreme to study on Earth.