Paris — Scientists Explain Mars' Strange Elongated Cloud

A team of planetary scientists reported that Mars' Arsia Mons Elongated Cloud, long considered the planet's most visually distinctive weather feature, may form through a process that textbooks describe but researchers had not confirmed occurring naturally, according to a study published in Nature Geoscience and reported by the New York Post via SWNS.

What is the Arsia Mons Elongated Cloud?

The cloud, known by researchers as AMEC, stretches up to 1,800 kilometers — nearly twice the length of the United Kingdom — and appears each spring and summer in Mars' southern hemisphere, during the planet's dusty season, the Post reported. It forms downwind of the 20-kilometer-tall Arsia Mons volcano, growing through the morning before evaporating, a cycle that repeats daily for several months.

How did researchers study the cloud's formation?

The team combined observations from the European Space Agency's Mars Express spacecraft with a meteorological model to simulate conditions around Arsia Mons. Lead author Jorge Hernández-Bernal of LMD/CNRS/Sorbonne Université in Paris said the standard physics used in prior models could not reproduce the cloud. "To create the AMEC in our modelling, 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," Hernández-Bernal said. "It certainly hasn't been seen in action before. Once we included this physics in our simulations, the AMEC emerged just as we hoped."

What did the study find?

The researchers found that winds striking Arsia Mons generate a wave that lifts moist air several kilometers upward within minutes. According to the study, that lift cools the air by roughly 30 degrees in about ten minutes, sending relative humidity sharply upward. Rather than condensing onto dust grains — the typical path for cloud formation on both Mars and Earth — the water vapor appeared to freeze directly into ice particles, producing the AMEC.

What is homogeneous ice nucleation?

The study attributes this direct freezing to a process called homogeneous ice nucleation, in which water vapor turns into ice without first attaching to dust or another particle to serve as a nucleus. The process is described in atmospheric physics textbooks but is rarely observed because it typically requires conditions — extreme cold reached very quickly — that are difficult to produce outside a laboratory, according to the findings reported by the Post.

Where does the uncertainty stand?

The published findings rely on matching simulated atmospheric behavior to spacecraft observations rather than direct in-situ sampling of the cloud's ice particles, so confirmation that homogeneous nucleation — rather than some other mechanism — is actually occurring will likely depend on future observation campaigns timed to the cloud's daily cycle.

What to watch

• Follow-up Mars Express or future orbiter observations timed to AMEC's spring-summer appearance season
• Whether other Martian volcanoes with similar wind-wave dynamics produce comparable clouds
• Lab or modeling work testing whether homogeneous nucleation occurs under similarly rapid cooling on Earth
• Peer response within the planetary science community to the Nature Geoscience findings

Artiglio is A coming-soon iPhone chief of staff for briefings and drafts. Not on the App Store yet.