The mystery of the giant cloud that appears and disappears on Mars within hours - and the secret revealed
An ice cloud up to 1,800 km long appears every morning on Mars and disappears within hours. A new study offers an explanation: ice crystals form without dust, in a theoretical process never before observed in a planetary atmosphere.
A giant ice cloud up to 1,800 kilometers long appears every morning for several months a year near the Arsia Mons volcano on Mars, and disappears within hours. The Mars Express spacecraft has documented it repeatedly, but existing cloud formation models could not replicate its development. Now a team of researchers from France, led by Dr. Jorge Hernandez-Bernal from Sorbonne University, offers a new explanation in a study published in Nature Geoscience. According to them, in the upper layers of the atmosphere a process of "homogeneous nucleation" occurs – ice crystals form directly from water vapor without the need for dust grains. The process is known in theory but has never been actually observed in a planetary atmosphere. The combination of Mars' thin atmosphere and the mountain's immense height creates a rapid rise of moist air, a sharp cooling of 30 degrees within ten minutes, and extreme supersaturation. Adding the process to a computer model made it possible to simulate a cloud similar to the photographs, although differences still exist between the simulation and observations. The researchers emphasize that the findings strengthen the explanation but are not direct proof. The discovery may also have implications for understanding cloud formation on Earth and Venus.