Martian ice cloud identified and studied

A 1,800-kilometer-long ice cloud, called AMEC (Arsia Mons Elongated Cloud), forms every morning west of the large Martian volcano Arsia Mons during the southern hemisphere's spring and summer. It disappears within a few hours, and the phenomenon was first observed in 2018 using images from the Mars Express probe.

Researchers reconstruct cloud’s life cycle

Scientists from the University of the Basque Country reconstructed the life cycle of the cloud three years after its discovery. However, planetary scientists initially struggled to replicate it in computer models, suggesting the process was not fully understood.

Study replicates cloud formation

A study published in the journal Nature Geoscience on October 7 by Jorge Hernandez-Bernal and three colleagues successfully simulated the cloud’s formation. The cloud appears at an altitude of about 45 kilometers, stretches westward, and forms every morning for several months during Mars’s dust season.

Different nucleation processes involved

On Earth, orographic clouds form when moist air rises over terrain, cools, and condenses. On Mars, the researchers found that the cloud forms through both heterogeneous and homogeneous nucleation. While heterogeneous nucleation—where water vapor condenses around dust particles—was previously thought to be the main process, it alone failed to reproduce the cloud in simulations.

Homogeneous nucleation requires extreme conditions

The researchers added homogeneous nucleation to their model, where water vapor directly turns into ice crystals without needing a particle to form around. This process requires relative humidity more than 100,000 times higher than on Earth, a condition that had never been observed before, though it is theoretically possible in the upper atmospheres of Earth and Venus.

Martian conditions enable the process

The thin Martian atmosphere and the height of Arsia Mons create the extreme conditions needed for this process. As wind passes over the volcano, moist air rises rapidly, causing a quick temperature drop and the formation of ice particles. While the simulated cloud doesn’t perfectly match observations from Mars Express, the results are considered a significant achievement.

Study’s findings adjusted after review

The study initially claimed homogeneous nucleation was "demonstrated" on Mars in a preprint, but the peer-reviewed version in Nature Geoscience presents it only as "strongly suggested." Milena Corcos of the NorthWest Research Associates also used the word "suggest" in her commentary on the study.

Cloud visibility depends on atmospheric balance

The cloud appears in the morning due to a delicate balance of temperature and wind conditions that allow its formation at sunrise. As temperatures rise and humidity decreases, the cloud evaporates. The streak extends downwind due to atmospheric circulation and the movement of ice particles.

Nucleation mechanisms differ

Heterogeneous nucleation involves water vapor forming around particles like dust, salts, or aerosols, acting as condensation nuclei. Homogeneous nucleation, in contrast, does not require such particles but needs extreme supersaturation. On Mars, this becomes possible due to the thin atmosphere and rapid temperature changes when air rises, creating brief conditions that allow direct ice formation.

New insight for Martian weather models

The discovery shows that both nucleation mechanisms must be considered in Martian weather models. Classical models based only on dust particles might miss phenomena like AMEC. Validating these models requires observations that can measure humidity and particle size accurately.