Clouds on Earth need something to grow on, and water vapor settles onto specks of dust, salt, or soot before it turns into droplets or ice.

Ice can theoretically form directly from water vapor without dust or other particles, but the conditions required are so extreme that scientists have never confirmed the process in a planetary atmosphere.

On Mars, one cloud appears to be the exception. A white streak forms every morning beside a giant volcano, and its ice may have formed directly from water vapor with nothing to freeze onto, according to a new modeling study. If the model is right, it’s the first time that process has been found in any planet’s atmosphere.

Jorge Hernández-Bernal of the Laboratoire de Météorologie Dynamique in Paris, a lab shared by CNRS and Sorbonne Université, led the work. His team’s conclusion rests on a computer model of the Martian atmosphere checked against years of spacecraft images, not on a direct measurement of the ice itself. “It certainly hasn’t been seen in action before,” Hernández-Bernal said.

The cloud appears beside Arsia Mons, a volcano about 12 miles (20 kilometers) tall. For several months of the dusty southern spring and summer it comes back every morning. It starts forming at sunrise, spreads sideways for about three hours, and then breaks away from the volcano and fades. At its longest it stretches about 1,100 miles (1,800 kilometers), farther than the straight-line distance from Chicago to Houston. It forms roughly 28 miles (45 kilometers) above the surface.

The European Space Agency’s Mars Express orbiter first spotted the cloud in 2018. Few spacecraft at Mars can take pictures in the morning, when the cloud exists, and the mission’s cloud atlas holds two decades of such images. Researchers already knew the cloud forms where wind flows over the volcano, as mountain clouds do on Earth. No simulation had managed to reproduce its long tail. With ice allowed to grow only on dust grains, the standard rule, the simulated cloud never looked like the streak in the photos.