Daily cloud forms on Mars

A cloud measuring up to 1,600 kilometers appears almost every day on Mars near Arsia Mons, a volcano located near the equator of the red planet, and disappears within a few hours.

Cloud's movement and composition

This phenomenon, known as the Arsia Mons Elongated Cloud (AMEC), forms at dawn, stretches westward at more than 600 kilometers per hour, and disappears around noon. The cloud, composed of water ice, forms during 80 days during the spring and summer of the southern hemisphere of Mars and does not originate from an eruption of the volcano.

Volcano's role in cloud formation

The volcano's height, approximately 20 kilometers, contributes to the cloud's consistent location by lifting moist air rapidly, causing temperatures to drop by 30 degrees in ten minutes. The air becomes so cold that it contains much more water vapor than it can normally store in this form.

Earth's cloud formation differs

On Earth, water vapor typically requires dust, salt, or soot to form ice crystals, but Mars has a lot of dust, and previous models using this mechanism failed to accurately reproduce the size, appearance time, and evolution of the clouds.

New model explains cloud formation

A new model suggests that water molecules can form ice crystals directly without a dust grain, a process called homogeneous nucleation. This mechanism, well described in textbooks but never directly observed on Earth, Mars, or any other planet, requires extreme conditions, with local humidity levels more than 100,000 times higher than those near the ground on Earth.

Conditions on Mars support this process

These conditions occur briefly above Arsia Mons when the air rises rapidly and cools sharply. The discovery indicates that water ice exists on several volcanoes near the equator of Mars, including Arsia Mons, with approximately 150,000 tons of water freezing and evaporating daily during cold seasons.

Observations and historical context

Observations from Mars Express and the ExoMars Trace Gas Orbiter have detected this cloud and its evolution. In the 1970s, Viking 2 photographed part of this phenomenon, but its significance was not understood at the time.

Mars Express's unique view

Mars Express's orbit allows it to observe the cloud earlier in the morning compared to other probes. This finding could interest researchers studying other atmospheres, as physicists have considered ice formation in Earth's high atmosphere and on Venus, but without evidence until now.

Future research directions

The next step is to determine whether the same mechanism could cause clouds in the atmospheres of other planets.