PARIS, Oct. 15 -- A cloud on Mars that computer models could not reproduce for years may owe its existence to a process no one has watched happen in a planetary atmosphere, according to a study published October 7 in Nature Geoscience.

Every spring and summer in Mars's southern hemisphere, a white wisp of water ice called the Arsia Mons Elongated Cloud forms downwind of a volcano 20 kilometers tall, stretches up to 1,800 kilometers, then evaporates, and repeats the cycle each morning for several months. The European Space Agency's Mars Express orbiter first revealed it in 2018.

Jorge Hernandez-Bernal, the lead author, at France's LMD, CNRS and Sorbonne Universite, said his team's simulations produced the cloud only after they added what...