Every spring and summer on Mars, a thin, wispy cloud forms near the volcano Arsia Mons and stretches to more than 1,000 miles long — roughly the distance from New York to Kansas City — before dissipating, only to repeat the cycle for months. Scientists have struggled to explain it since a European probe first recorded it in 2018. Now, researchers say they have finally modeled how it forms, and the answer involves physics that has never actually been observed happening on any planet.
"We found that we needed to include some exotic physics … physics that, while included in textbooks, is treated as theoretical and usually thought not to happen in nature," said Jorge Hernández-Bernal, a planetary scientist at Sorbonne Université in Paris and lead author of the study, published in Nature Geoscience. "It certainly hasn't been seen in action before."
Ice forming out of thin air
On Earth, clouds typically form when water vapor condenses onto tiny particles like dust or salt. The new study suggests the Martian cloud, known as the Arsia Mons Elongated Cloud, instead forms through "homogeneous nucleation," in which vapor turns directly into ice crystals with no particles to help it along. "It's akin to droplets of condensation appearing in the middle of a room, rather than on a window," Hernández-Bernal said. "It's wholly unexpected."
The process requires extreme conditions: a temperature drop of 30 degrees Celsius within just 10 minutes, and humidity more than 100,000 times greater than any level ever measured on Earth. The 65,000-foot volcano itself drives the effect, generating a powerful atmospheric wave when wind hits its slopes, launching moist air high into the Martian sky within minutes.
Why it matters beyond Mars
Scientists have long suspected similar nucleation could occur in the upper atmospheres of other planets, including Venus and even Earth, but the phenomenon requires humidity levels so extreme that it had never been confirmed anywhere. The modeling also revealed something unexpected about Mars itself: researchers did not previously know such humidity levels existed there at all. "We don't have nearly as much information about Mars' atmosphere as we do about Earth's, so reproducing [the cloud] to this degree is a big success," Hernández-Bernal said. "The result is remarkable."