Every morning during the Martian spring and summer, a thin, extremely long cloud appears near the volcano Arsia Mons, stretches to more than 1,000 miles—roughly the distance from New York to Kansas City as the crow flies—and then fades away. The pattern repeats for several months before the cloud vanishes again.
First spotted by a probe in 2018, the formation, known as the Arsia Mons Elongated Cloud (AMEC), has long defied explanation. Now a team of researchers has produced the first successful model of its creation, suggesting that a rarely observed atmospheric process may be responsible.
“We discovered that we had to include some exotic physics… physics that appears in textbooks as theoretical and is generally thought not to occur in nature. It has never been seen in action before,” said Jorge Hernández‑Bernal, a planetary scientist at Sorbonne Université and lead author of the study, in a press release. “When we added this physics to our simulations, the AMEC appeared exactly as we expected.”
On Earth, clouds form when water vapor condenses onto tiny particles such as dust or salt, creating droplets or ice crystals. The new study, published in Nature Geoscience, proposes that the AMEC forms through homogeneous nucleation, a process where vapor turns directly into ice crystals without any particulate assistance.
“It is like droplets of condensation forming in the middle of a room instead of on a window,” Hernández‑Bernal explained. “It is completely unexpected.”
Homogeneous nucleation requires extreme conditions: a rapid temperature drop of about 30 °C within ten minutes and exceptionally high relative humidity. The towering Arsia Mons volcano, which rises roughly 65,000 feet, contributes by generating a strong atmospheric wave that lifts moist air high into the sky in minutes.
Scientists have long suspected that similar mechanisms could exist in the upper atmospheres of other planets, including Venus and even Earth, but they have never been observed. The required humidity exceeds Earth’s measured values by more than 100,000 times, making detection difficult.
Before this modeling effort, researchers did not know that such humidity levels were present on Mars either. “We have far fewer data about Mars’ atmosphere than we do about Earth’s, so reproducing the AMEC to this degree is a major achievement,” Hernández‑Bernal said, adding that the result is remarkable.