Plate Nº 46 · recorded October 13, 2026
Space & AstronomyReported finding
Mars' 920-Mile Cloud Forms via Physics Never Before Seen in Action
A 920-mile cloud near Arsia Mons forms via 'homogeneous nucleation' — a process never before documented in any planetary atmosphere, per a peer-reviewed study in Nature Geoscience on October 7, 2026.
By Nathan Brooks3 min read561 words
In brief
- The AMEC stretches up to 920 miles (1,500 km) and reappears every Martian spring and summer near Arsia Mons.
- Lead author Jorge Hernández-Bernal and colleagues at Sorbonne University published their findings October 7, 2026 in Nature Geoscience.
- The new model requires relative humidity more than 100,000 times Earth's typical levels, with temperatures dropping roughly 30°C in 10 minutes.
- ESA's Mars Express first spotted the cloud in 2018 and has monitored it for nearly a decade using three different cameras.
- The proposed 'homogeneous nucleation' process had never before been observed in any planetary atmosphere.
A Sorbonne University team says 'homogeneous nucleation' explains the Arsia Mons cloud.
A cloud stretching up to 920 miles across Mars every spring appears to form only through a rare process scientists have never documented in any planetary atmosphere. That's the conclusion of a peer-reviewed study published October 7, 2026 in Nature Geoscience.
The feature, called the Arsia Mons Elongated Cloud or AMEC, reappears each Martian year. It billows daily from the downwind side of the towering Arsia Mons volcano. For years, no simulation could explain how it forms — or why it can stretch about as far as the distance from Los Angeles to Seattle.
What makes this cloud so weird?
Most clouds need a "seed" — a speck of dust, salt or soot — for water vapor to condense onto. The AMEC seems to need none of that. Lead author Jorge Hernández-Bernal at Sorbonne University in Paris told the European Space Agency:
"To create the AMEC in our modeling, 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."
In plainer terms: water vapor freezes directly into ice crystals with nothing to grab onto. Scientists call this "homogeneous nucleation." Until now, researchers had only theorized it could occur somewhere in the solar system.
How extreme do conditions need to get?
Very extreme. Hernández-Bernal's team calculated that relative humidity must spike to levels more than 100,000 times what you'd experience on a typical Earth day. The towering Arsia Mons volcano helps trigger the chain of events.
Winds rushing past the peak lift moist air parcels several kilometers in minutes. Temperatures then plunge roughly 30 degrees Celsius in just 10 minutes. That rapid cooling pushes humidity past the threshold where water vapor freezes on its own — no particles required.
How did Mars Express crack the mystery?
ESA's Mars Express orbiter, which first spotted the AMEC in 2018, supplied the data. Three instruments aboard — the Visual Monitoring Camera, the High Resolution Stereo Camera and the OMEGA spectrometer — can image the cloud repeatedly within a single Martian day.
Mars Express Project Scientist Colin Wilson said the orbiter gives researchers "an unrivaled view of short-lived phenomena on the planet." That time resolution matters because the AMEC grows and fades every sol, the Martian day that runs about 24 hours and 37 minutes.
What does the model explain — and what doesn't?
It reproduces the cloud's daily rhythm and roughly 920-mile reach. But the match isn't perfect. The team hasn't measured humidity directly at the cloud's altitude, so conditions are inferred rather than observed.
Mars Express is also one of only two Mars orbiters — alongside ESA's ExoMars Trace Gas Orbiter — capable of morning observations. That exclusivity limits how often scientists can collect matching data.
Why does this matter beyond Mars?
If Mars really does reach humidity levels 100,000 times Earth's outdoor norm, the same physics might happen in extreme pockets of Earth's upper atmosphere, or on early Mars when the planet was wetter. Either way, the AMEC is the first observational evidence that homogeneous nucleation actually occurs somewhere today.
The model still awaits confirmation from missions with humidity-measuring instruments. For now, a textbook-only process has just become a real one.
via nature.com (Original)
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