Each morning for several months, a long streak of water ice forms under the wind of the Arsia Mons volcano on Mars, which rises to a height of 20 km—more than twice the height of Mount Everest. This cloud can grow up to 1,800 km in length, nearly twice the size of the United Kingdom, before quickly evaporating. A study published on October 7, 2026, in Nature Geoscience suggests that this cloud forms through a mechanism that had never been observed in nature before. Researchers led by Jorge Hernández-Bernal from the Laboratoire de Météorologie Dynamique in Paris used a Mars meteorological model that included a theoretical process called homogeneous nucleation. This process allows water vapor to turn directly into ice without needing a dust grain or other particle to start the process. The team compared their simulations with images from the ESA's Mars Express probe, confirming that the cloud forms through this unique process. The cloud stretches approximately 612 km/h and reaches its maximum length of 1,800 km, though this is not a constant size. Observations from the VMC camera on Mars Express, which has been monitoring the cloud since September 13, 2018, show variations in its length. For example, an image from October 10, 2018, showed a cloud of 1,500 km, while a stereo HRSC camera image from June 24, 2024, showed about 980 km. The phenomenon has been named AMEC, short for Arsia Mons Elongated Cloud. It develops on the western side of the volcano after sunrise and its tail stretches for several hours before evaporating by the afternoon. The study highlights that the cloud forms when the temperature drops by 30 degrees in 10 minutes above the volcano—comparable to a rapid drop from 20 °C to -10 °C in the time it takes to cook pasta. This rapid cooling creates extreme supersaturation, allowing water vapor to freeze directly into ice particles without needing a nucleus. This process, known as homogeneous nucleation, was previously thought to be too extreme to occur in natural conditions. The model that includes this process successfully reproduces the observed structure of the cloud, which had previously been impossible to simulate with conventional methods. The study notes that while the model aligns with observations, some discrepancies remain. The researchers did not directly measure homogeneous nucleation within the cloud; their conclusion is based on comparing simulations with images. This means the cloud most likely forms this way, though it has not been definitively proven. The discovery challenges common assumptions about cloud formation and could have implications for understanding clouds on Earth and other planets. However, the study does not yet determine how frequently this phenomenon occurs on Mars or its role in the planet's water cycle. The cloud appears every Martian year during spring and southern summer, and it is observed by a few probes, including Mars Express and the ExoMars Trace Gas Orbiter, which can capture it at dawn.