The Arsia Mons Elongated Cloud is a wisp of water ice that appears downwind of the 20-km- (12.4-mile) tall Arsia Mons volcano — the southernmost in a trio of giant Martian shield volcanoes known collectively as Tharsis Montes — in the Martian southern hemisphere’s spring and summer.
This image from ESA’s Mars Express shows the Arsia Mons Elongated Cloud. Image credit: ESA / DLR / FU Berlin / J. Cowart.
The Arsia Mons Elongated Cloud (AMEC) forms, stretches and fades within a single Martian day, growing up to 1,800 km (1,118 miles) long before evaporating. The cycle repeats every morning for several months.
ESA’s Mars Express orbiter first imaged the cloud in 2018 and has been watching it ever since.
Planetary researchers had long identified it as an orographic cloud, a type also seen on Earth, which forms when wind flows over mountains or volcanoes.
But computer simulations built on that assumption could never reproduce what Mars Express actually saw.
“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,” said lead author Dr. Jorge Hernández-Bernal, a researcher at Sorbonne Université and CNRS.
“It certainly hasn’t been seen in action before.”
“Once we included this physics in our simulations, the AMEC emerged just as we hoped.”
On Earth, clouds usually form through heterogeneous nucleation. Water vapor condenses onto tiny particles such as dust, salt, pollen or soot.
The scientists believed the same process, using dust, operates on Mars.
“For the AMEC, it seems that cloud formation takes place without needing any of this ‘stuff’,” Dr. Hernández-Bernal said.
“Water vapor turns directly into icy cloud particles without any middle step.”
“It’s akin to droplets of condensation appearing in the middle of a room, rather than on a window.”
“We call this homogeneous nucleation, and we’ve never seen it before in a planetary atmosphere. It’s wholly unexpected.”
Theorists had suggested this might occur in the upper atmospheres of Earth and Venus, but it has never been confirmed.
The process demands extreme supersaturation, with relative humidity more than 100,000 times what we normally experience on Earth.
The team’s simulations suggest Arsia Mons can deliver such conditions.
As wind hits the volcano, its bulk generates a powerful wave that lifts pockets of moist air several kilometers upward within minutes.
The air cools quickly, with temperatures dropping by around 30 degrees in just 10 minutes, and humidity spikes.
Water vapor then freezes spontaneously into the cloud’s ice particles.
“We’ve not seen these conditions on Mars before, but our finding now strongly suggests that the planet’s humidity can indeed reach these extreme levels,” Dr. Hernández-Bernal said.
The team’s work appears today in the journal Nature Geoscience.
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J. Hernández-Bernal et al. 2026. Homogeneous ice nucleation from water vapour suggested by elongated clouds on Mars. Nat. Geosci 19, 1213-1217; doi: 10.1038/s41561-026-02089-9
