Mars's weirdest cloud forms in a way textbooks said was impossible in nature

The Arsia Mons Elongated Cloud — a wisp of water ice up to 1,800 km (1,100 miles) long that grows in about 3 hours each morning and then vanishes — was first imaged by ESA's Mars Express in 2018. A new study in Nature Geoscience by Jorge Hernández-Bernal's team at Sorbonne Université and CNRS in Paris shows it forms by homogeneous nucleation: water vapor freezing directly into ice with no dust as seeds, requiring humidity about 100,000 times normal — the first evidence of the process in any planetary atmosphere.