Mars' Mile-Spanning Morning Cloud May Rely on Physics Never Seen in a Planetary Atmosphere

A narrow, wispy cloud that forms each morning near the Martian volcano Arsia Mons, stretches across more than 1,000 miles and then vanishes may have been explained at last. A new modeling study suggests the formation depends on a physical process that has never been directly observed in any planet's atmosphere.

The feature, known as the Arsia Mons Elongated Cloud, or AMEC, appears during the Martian spring and summer. It grows gradually longer through the morning until it reaches a length comparable to the distance from New York to Kansas City in a straight line, then dissipates. The cycle repeats over several months.

A probe first recorded the cloud in 2018, and scientists have since struggled to find a plausible explanation for it. The new work, published in Nature Geoscience, is the first to successfully reproduce the cloud's formation in a model.

According to the researchers, the simulations only worked once they included what lead author Jorge Hernández-Bernal called "exotic physics." Hernández-Bernal, a planetary scientist at Sorbonne Université in Paris, said in a press release that the physics appears in textbooks but is treated as theoretical and usually thought not to occur in nature. "Once we included this physics in our simulations, the AMEC emerged just as we hoped," he said.

On Earth, clouds typically form when water vapor condenses into droplets or ice crystals around tiny particles such as dust or salt. The study instead proposes that the Martian cloud forms through homogeneous nucleation, in which vapor turns directly into ice crystals without any debris to seed them. Hernández-Bernal likened it to droplets of condensation appearing in the middle of a room rather than on a window, calling it "wholly unexpected."

The process demands extreme conditions, including a temperature drop of 30 degrees Celsius within 10 minutes and high relative humidity. The volcano itself is central to creating them. When wind meets the 65,000-foot Arsia Mons, it generates a powerful wave that lifts moist air high into the atmosphere within minutes.

Scientists have long assumed that similar nucleation could occur in the upper atmospheres of other planets, including Venus and Earth, but it has never been observed. The humidity it requires may explain why: the levels needed are more than 100,000 times greater than any value measured on Earth.

Researchers also did not previously know that such humidity existed on Mars. Hernández-Bernal noted that far less is known about the Martian atmosphere than Earth's, which makes reproducing the cloud to this degree a significant achievement. "The result is remarkable," he said.

The story originally appeared on WIRED Czech Republic and Slovakia and was translated from Czech.