This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Scientists using the European Space Agency's Mars Express, in combination with a state-of-the-art meteorological model of the Red Planet, have found that there may be some exotic physics behind Mars' most curious cloud. The paper, "Homogeneous ice Nucleation from Water Vapour Suggested by Elongated Clouds on Mars," is published in Nature Geoscience.
Every spring and summer in Mars' 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 (12-mile-tall) Arsia Mons volcano. It forms, grows and fades daily, stretching out for up to 1,800 km (1,120 miles)—nearly twice the length of the U.K.—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 behavior, 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. "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," says Jorge Hernández-Bernal of LMD/CNRS/Sorbonne Université in Paris, lead author of the new study.
"Once we included this physics in our simulations, the AMEC emerged just as we hoped." On Earth and elsewhere, clouds typically form when moist air cools and water vapor 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 vapor 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 Hernández-Bernal. "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.
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