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+--- Thread: Mars’s oddest cloud may be even odder than we thought (Arean weather) (/thread-21528.html)
Mars’s oddest cloud may be even odder than we thought (Arean weather) - C C - Oct 8, 2026
https://www.esa.int/Science_Exploration/Space_Science/Mars_Express/Mars_s_oddest_cloud_may_be_even_odder_than_we_thought
EXCERPTS: Every spring and summer in Mars’s southern hemisphere, during the martian dusty season, we see a spectacular cloud emerge: the Arsia Mons Elongated Cloud (AMEC), the most visually striking cloud on the Red Planet.
This white wisp of water ice emerges downwind of the 20-km-tall Arsia Mons volcano. It forms, grows, and fades on a daily basis, stretching out for up to 1800 km – nearly twice the length of the UK – before quickly evaporating. This cycle repeats every morning for several months.
Mars Express first revealed the AMEC in 2018, and has viewed the recurrent cloud repeatedly since. Researchers have explored its evolution, vivid dynamics and intriguing behaviour, determining it to be an orographic cloud: a type of cloud also seen on Earth that forms as wind flows past the craggy topography of a volcano or mountain.
However, when trying to model exactly how the cloud springs to life, our simulations simply didn't reproduce what we see in the images – until now.
[...] On Earth and elsewhere, clouds typically form when moist air cools and water vapour condenses into liquid droplets or icy crystals. This usually happens via a process known as heterogeneous nucleation, which requires specks of other ‘stuff’ to be present in the atmosphere for vapour to cling and condense onto – salt, pollen, soot, or dust, for instance. On Mars, it’s thought to be dust.
“For the AMEC, it seems that cloud formation takes place without needing any of this ‘stuff’,” adds Jorge. “Water vapour 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.”
Scientists had suggested that this process might take place in the upper atmospheres of Earth and Venus, but it hasn’t been spotted. Its rarity is due to it requiring exceptional circumstances, with extreme relative humidity levels of over 100 000 times those usually experienced in our daily life on Earth. “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,” says Jorge.
Jorge and colleagues found that the AMEC sits in a unique position where Mars’s thin atmosphere and the towering height of the nearby Arsia Mons volcano come together to create the conditions needed to see this rare process in action.
As winds flow past Arsia Mons, the volcano’s bulk triggers a powerful wave that lifts moist parcels of air several kilometres up into the air in just a few minutes. This cools the atmosphere rapidly, causing temperatures to drop by 30 degrees in just 10 minutes and relative humidity levels to spike. Water vapour then spontaneously freezes directly into cloud particles, forming the AMEC. (MORE - missing details)