ESA’s Mars Express showed that Mars forms surprisingly Earth-like cloud structures reminiscent of clouds in our planet’s tropical regions. Earth and Mars have very different atmospheres. The dry and cold atmosphere of Mars is made up almost entirely of carbon dioxide, while Earth’s atmosphere is rich in nitrogen and oxygen. The density of its atmosphere is less than one-fifth of the Earth’s atmosphere, which is equivalent to the density at an altitude of about 35 km from the Earth’s surface.

Despite being very different, the cloud structures turned out to be surprisingly similar to Earth’s, suggesting similar formation processes. A new study delves deeper into two dust storms that occurred near Mars’ North Pole in 2019. Storm monitoring was conducted in the Arctic in the spring, when local storms typically form around the retreating ice cover.

Two cameras aboard Mars Express – the Visual Monitoring Camera (VMC) and the High-Definition Stereo Camera (HRSC) – captured storms from orbit, along with the MARCI camera aboard NASA’s Mars Reconnaissance Orbiter.

A series of VMC images show storms growing and decreasing in repeated cycles over several days, showing common features and shapes. Spiral shapes are clearly visible at wider angles of HRSC images. The spirals are between 1,000 and 2,000 km long, and their origin is the same as that of non-tropical cyclones observed in Earth’s middle and polar latitudes.

The images show a particular phenomenon on Mars. They show that dust storms on Mars are composed of regularly spaced smaller cloud cells, such as grains or pebbles. The texture can also be seen in clouds in Earth’s atmosphere.

Familiar textures are produced by convection, where warm air rises because it is less dense than the cold air around it. The type of convection seen here is called closed-cell convection, in which air rises in the center of small cloud pockets or cells. Gaps in the sky around cloud cells are pathways where cold air sinks under rising warm air.

Air rising above the earth contains water that condenses to form clouds. The dust clouds imaged by Mars Express show the same process, but the air columns rising on Mars contain dust instead of water. The sun heats the dusty air, causing it to rise and form dust cells. The cells are surrounded by zones of sinking air with less dust. This produces a grainy pattern that can also be seen in cloud images on Earth.

Wind speed can be measured by tracking the movement of cells in a series of images. Wind blows over cloud objects at up to 140 km/h, resulting in the shape of the cells being stretched downwind. Despite the chaotic and dynamic atmospheres of Mars and Earth, nature creates these regular patterns.

“When you think of a Mars-like atmosphere on Earth, it’s easy to think of a dry desert or polar region. It is therefore quite unexpected that by tracking the chaotic movement of dust storms, parallels could be drawn with processes occurring in tropical regions of Earth that are humid, hot, and clearly very different from Mars,” says the Mars Express ESA project.

One of the most important insights VMC images enable is the measurement of dust cloud heights. The length of the shadow they cast is measured and combined with the sun’s position information to measure the height of the clouds above the Martian surface. The results showed that the dust can reach a height of about 6-11 km above the ground and the typical horizontal dimensions of the cells are 20-40 km.

“Despite the unpredictable behavior of dust storms on Mars and the high winds that accompany them, we have seen that organized structures such as facades and cellular convection patterns can emerge within their complexities,” explains Agustin Sánchez-Levaga of the University of País. Vasco UPV/EHU (Spain), leading the VMC scientific group and lead author of the paper presenting the new analysis.