Each morning, an immense white cloud can appear near a Martian volcano, stretch for nearly 2,000 km, and then vanish within a few hours.
This phenomenon occurs near Arsia Mons, one of Mars's major volcanoes. The cloud appears mainly during a particular period of the Martian year, around the summer solstice in the Southern Hemisphere. Space probes have tracked its growth: a long trail of water ice stretches above the planet before disappearing as the morning progresses.
The white cloud of Arsia Mons stretches west of the volcano in this view from Mars Express.
Credit: ESA/Mars Express, via Earth.com.
The presence of clouds on Mars is not surprising. Despite its very thin atmosphere, the planet contains water vapor. When conditions become cold enough, it can form tiny ice crystals. But the shape and duration of this particular cloud resisted the usual explanations.
On Earth as on Mars, ice generally forms around particles already present in the air, such as dust. These particles provide a starting point for the crystals. However, models using this mechanism alone could not reproduce the Arsia Mons cloud and its long trail.
The researchers therefore studied what happens when wind meets the volcano. The terrain disrupts the air and creates a region where the temperature drops sharply. For a brief moment, the atmosphere becomes so rich in water vapor relative to its temperature that crystals could appear directly, without dust.
The elongated cloud of Arsia Mons observed by Mars Express on June 24, 2024. The water-ice trail stretches downwind of the volcano.
Credit: ESA/DLR/FU Berlin — CC BY-SA 3.0 IGO.
This spontaneous formation of ice is known as “homogeneous nucleation.” The phenomenon is known in theory, but it usually requires such unusual conditions that it had not been demonstrated in a genuine atmospheric cloud. The research team therefore incorporated this possibility into a weather model of Mars.
This time, the simulation produced an elongated cloud with characteristics similar to those observed from orbit. Small crystals formed in the cooling zone and were then carried away by the wind. Their evolution allowed a long visible trail to persist behind the volcano.
The result is strong evidence, but not a direct observation of the mechanism. The probes see the cloud and track its changes without individually observing the birth of each crystal. The researchers therefore propose this new formation pathway because it reproduces the phenomenon better than the usual methods.
The implications extend beyond Mars. If ice can truly form directly from water vapor in a natural atmosphere, some cloud models will need to be reconsidered. The researchers particularly want to determine whether comparable conditions exist in the upper atmospheres of Earth or Venus, where this mechanism has already been considered in theory.