Mars produces a cloud unlike anything scientists have directly observed in a planetary atmosphere. Each morning during the Martian spring and summer, a thin strand emerges near the Arsia Mons volcano, stretches across more than 1,000 miles, and then disappears.
The cycle repeats for several months, creating one of Mars’ most persistent atmospheric mysteries. A new model now suggests that the cloud may depend on ice-forming physics that has never been seen in action anywhere in nature.
A Cloud That Returns Every Morning
The formation is often called the Arsia Mons Elongated Cloud, or AMEC. It always begins near Arsia Mons before gradually elongating into a long, wispy strand. At its greatest extent, the cloud reaches over 1,000 miles—about the distance from New York to Kansas City as the crow flies—before dissipating.
A probe first recorded the cloud in 2018, but scientists could not find a plausible explanation for its existence. Its regular timing and enormous length made AMEC difficult to explain, especially because the cloud appears, grows, and vanishes within the same repeating pattern.
That mystery has now moved closer to a solution. Researchers successfully modeled the cloud’s formation for the first time, and their results point to a process called homogeneous nucleation.
Ice Crystals Without Dust or Salt
On Earth, clouds usually form when water vapor creates droplets or ice crystals around tiny particles such as dust or salt. These particles give the vapor a surface where the change can begin.
The new study, published in Nature Geoscience, suggests AMEC may form in a different way. Through homogeneous nucleation, vapor changes directly into ice crystals without help from debris.
“We found that we needed to include some exotic physics… physics that, while included in textbooks, is treated as theoretical and usually thought not to happen in nature. It certainly hasn’t been seen in action before,” said Jorge Hernández-Bernal, a planetary scientist at Paris’ Sorbonne Université and lead author of the study. “Once we included this physics in our simulations, the AMEC emerged just as we hoped.”
The process requires conditions far beyond anything measured on Earth. The model calls for a temperature drop of 30° Celsius within 10 minutes, along with extremely high relative humidity.
That humidity must exceed every value measured on Earth by more than 100,000 times. Before the modeling work, scientists did not know whether humidity levels like that existed on Mars either.
“It’s akin to droplets of condensation appearing in the middle of a room, rather than on a window,” Hernández-Bernal explained. “It’s wholly unexpected.”
How Arsia Mons Launches Moist Air
Arsia Mons does more than mark the cloud’s starting point. The 65,000-foot-tall volcano helps create the conditions that allow AMEC to form.
When a wind current hits the volcano, it creates a powerful wave that launches moist air high into the atmosphere within minutes. That sudden rise helps produce the sharp temperature drop and high humidity required for homogeneous nucleation.
The volcano’s role explains why the cloud always appears near Arsia Mons before stretching into its long atmospheric strand. The model connects the mountain, the wind, the moist air, and the extreme conditions in one formation process.
Scientists have assumed that a similar phenomenon could occur in the upper atmospheres of Venus and even Earth, but they have never observed it. AMEC therefore offers a chance to study a process that may matter beyond Mars, even though its required conditions remain far outside Earth’s measured range.
“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,” Hernández-Bernal said, adding, “the result is remarkable.”
AMEC turns a strange daily cloud into a test of atmospheric physics. Its repeated appearance during the Martian spring and summer gives researchers a natural setting for examining how vapor can become ice without dust or salt.
The cloud’s reach—over 1,000 miles—and its narrow formation window make the mystery impossible to ignore. For several months, Mars repeats the same atmospheric performance each morning, offering a planetary example of physics that textbooks describe but nature has not directly displayed before.
Based on
- A 1,000-mile cloud on Mars may reveal physics never seen in nature — arstechnica.com
- The ‘Exotic Physics’ Behind a Mysterious 1,000-Mile Cloud on Mars | WIRED — wired.com
- Tag: planetary science – Ars Technica — arstechnica.com
- Ice sheet expansion may have started 2,000 years ago, but we canceled that – Ars Technica — arstechnica.com
