PARIS, France, Oct. 7, 2026 - Researchers suspect that a rare process called homogeneous nucleation creates the Arsia Mons Elongated Cloud on Mars, the European Space Agency said on 7 October.^[1] In homogeneous nucleation, water vapour freezes directly into ice cloud particles without dust, salt or other seed particles.^[2][3] The process had been described in theory but never confirmed in any planetary atmosphere.^[4] Wind flowing past the 20-kilometre-tall Arsia Mons volcano sets up an atmospheric wave that cools the air by 30 degrees Celsius in 10 minutes, producing the extreme humidity the process needs.^[5] The cloud forms, grows and fades each day^[6] and can reach up to 1,800 kilometres in length.^[7] Clouds on Earth almost always need a speck of dust or salt to form around, which is why a cloud that grows without one is unusual. Impact Insight. Clouds usually form when water vapour condenses on tiny grains of dust; homogeneous nucleation is the rarer case in which vapour freezes into ice without any seed. There is no direct planetary-defense link: this is Mars weather science, included for readers following the wider solar system.