Plate Nº 54 · recorded October 10, 2026
Space & AstronomyReported finding
Mars' 1,800-km cloud forms through rare 'exotic' physics
ESA's Mars Express and new simulations show that Mars' 1,800-km Arsia Mons Elongated Cloud forms via homogeneous nucleation—a rare process requiring 100,000× Earth's humidity.
By Nathan Brooks3 min read646 words
In brief
- The AMEC stretches up to 1,800 km (1,120 miles), nearly twice the length of the United Kingdom.
- Arsia Mons volcano stands 20 km (12 miles) tall and drives the cloud's daily formation cycle.
- Homogeneous nucleation demands relative humidity over 100,000 times Earth's typical surface levels.
- Air temperatures around the cloud drop by 30°C in just 10 minutes during formation.
- ESA's Mars Express first imaged the AMEC in 2018 and has tracked it repeatedly since.
Mars' 1,800-kilometer Arsia Mons Elongated Cloud (AMEC) forms through a rare atmospheric process never before observed in action on any planet, scientists report in Nature Geoscience.
What is the Arsia Mons Elongated Cloud?
The AMEC is a white wisp of water ice that emerges downwind of Arsia Mons, a 20-kilometer-tall Martian volcano. Every Martian spring and summer, during the dusty season in the planet's southern hemisphere, the cloud forms, grows, and evaporates on a daily cycle.
It stretches up to 1,800 km—nearly twice the length of the United Kingdom—before dissipating. This sequence repeats every morning for several months. ESA's Mars Express first imaged the cloud in 2018 and has monitored it repeatedly since, making it the most visually striking cloud feature on the Red Planet.
Researchers had classified the AMEC as an orographic cloud, the same type that forms when wind flows past a mountain on Earth. But early simulations of its behavior fell short of the observations.
Why does this cloud form differently?
On Earth and Mars, clouds usually form through heterogeneous nucleation. Water vapor condenses onto tiny particles—dust, soot, pollen, or salt—before growing into droplets or ice crystals. Scientists assumed dust played that role on Mars.
The new study argues the cloud needs no such scaffold. Water vapor turns directly into icy cloud particles without any intermediate step. Physicists call this homogeneous nucleation. Researchers had proposed it might occur high in Earth's or Venus's upper atmosphere, but no one had caught it in action.
The process demands extreme conditions: relative humidity over 100,000 times what humans experience on Earth's surface. Lead author Jorge Hernández-Bernal of LMD/CNRS/Sorbonne Université in Paris described the result as wholly unexpected.
How did researchers confirm the mechanism?
Hernández-Bernal's team combined Mars Express data with a state-of-the-art meteorological model of Mars. Simulations failed to reproduce the AMEC until researchers added homogeneous nucleation to the physics.
"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. It certainly hasn't been seen in action before," Hernández-Bernal said.
The mechanism works like this. As winds sweep past Arsia Mons, the volcano's bulk generates a powerful wave. That wave lifts moist air parcels several kilometers within minutes. Temperatures drop by 30°C in 10 minutes, and humidity spikes dramatically. Water vapor then spontaneously freezes into ice crystals, building the AMEC.
What data supported the modeling?
The team drew on three Mars Express cameras:
- Visual Monitoring Camera (VMC)
- High Resolution Stereo Camera (HRSC)
- OMEGA spectrometer
Mars Express is one of only two spacecraft orbiting Mars—the other being ESA's ExoMars Trace Gas Orbiter—that can image the planet during morning hours, when the AMEC appears. The orbiter can also resolve changes on timescales of mere hours.
ESA Mars Express project scientist Colin Wilson called the tracking capability "an unrivaled view of short-lived phenomena on the planet."
What does this mean for planetary science?
The researchers caution that not every modeled feature lines up perfectly with observations. Hernández-Bernal called the reproduction "remarkable" given how little scientists know about Mars' atmosphere compared with Earth's.
The finding carries a broader lesson. "While clouds on Earth and Mars seem to be governed by the same 'rules,' understanding this exotic Martian cloud required exotic physics—and this may be true elsewhere in the cosmos," Wilson said.
Scientists should not discount unlikely processes when studying planets, including exoplanets. Conditions that seem impossible on Earth can arise elsewhere in the universe.
Reference: Hernández-Bernal et al., "Homogeneous ice Nucleation from Water Vapour Suggested by Elongated Clouds on Mars," Nature Geoscience (2026). DOI: 10.1038/s41561-026-02089-9.
via Phys.org Space & Astronomy (Source)