Mars cloud forms via spontaneous ice nucleation in thin air
The Arsia Mons Elongated Cloud, a seasonal Martian phenomenon, forms through gravity wave-induced cooling that triggers homogeneous nucleation, a process not previously observed in nature.
A mysterious cloud on Mars forms through a previously unseen atmospheric mechanism. The Arsia Mons Elongated Cloud appears daily during the planet's spring, with its formation now attributed to homogeneous nucleation rather than dust-seeded processes.
The cloud appears every morning for a few months during Mars's dusty season. It forms just before dawn near the Martian equator above the 20-kilometre-high Arsia Mons shield volcano. The structure grows for a few hours, drifts west, and can reach spans of up to 1800 kilometres before evaporating and disappearing by midday. Visible from orbiting spacecraft, the cloud was first imaged by NASA's Viking Orbiter 2 in 1977 and later by the European Space Agency's Mars Express. Until recently, atmospheric models could not explain its formation. Jorge Hernández-Bernal, a postdoc at Sorbonne University in Paris, and colleagues demonstrated the cloud's formation by including homogeneous nucleation in their models.
Gravity waves drive initial cooling
Intense winds at Arsia Mons generate atmospheric gravity waves that rapidly cool air to enable cloud formation. Strong winds hitting the volcano's summit create localized atmospheric gravity waves that propagate several kilometres into the lower mesosphere. These waves force moist air upward and cause it to cool drastically, dropping up to 30 K in just 10 minutes.
Homogeneous nucleation enables ice crystal formation
When temperatures drop below 130 K, water vapor undergoes homogeneous nucleation to form pure ice crystals without dust particles. At that extreme cold, water vapour reaches levels 105 times higher than the air can sustain. The kinetic barrier to spontaneous freezing collapses, allowing water vapour molecules to self-assemble into pure ice crystals. This mechanism, homogeneous nucleation, was previously regarded as unlikely to occur in planetary atmospheres. Heterogeneous nucleation, which requires dust particles as seeds, was thought to be the only cloud-forming process on Mars.
Model validation through combined observations
Researchers validated the mechanism by integrating Mars Express camera data with simulations incorporating homogeneous nucleation. The team combined observations from the spacecraft's Visual Monitoring Camera, High Resolution Stereo Camera, and OMEGA instrument with simulations based on classical nucleation theory and the Mars Planetary Climate Model. Hernández-Bernal, who led the research, stated they began investigating the cloud during his PhD. He said, "Then we started experiments introducing homogeneous nucleation in actual models and the results showed a pretty good reproduction of the cloud." The work explains a cold pocket of air at 45 km altitude west of the volcano that has sufficient supersaturation for homogeneous nucleation despite low dust density. Mars's dusty season occurs when the planet is closest to the Sun, causing increased solar radiation, dust storms, and higher atmospheric water vapor.
Future work will study submicron aerosols on Mars and conduct more detailed observations of the elongated cloud, though satellite observations are challenging due to the cloud's vast size and the fast-moving orbits of satellites. The cloud formation process is also sensitive to climate changes, according to Hernández-Bernal.
