ESA’s Mars Express has shown that Mars stirs up surprisingly Earth-like cloud patterns reminiscent of those found in our planet’s tropical regions.
Earth and Mars have very different atmospheres. Mars’ dry, cold atmosphere is composed almost entirely of carbon dioxide, while Earth’s is rich in nitrogen and oxygen. Its atmospheric density is less than one-fiftieth that of the Earth’s atmosphere – which corresponds to the density at an altitude of about 35 km above the Earth’s surface.
Although they are very different, their cloud patterns have been found to be surprisingly Earth-like, indicating similar formation processes.
A new study delves deeper into two dust storms that struck near Mars’ north pole in 2019. The storms were monitored in the spring at the North Pole, a time when local storms often brew around the receding ice cap.
Two cameras aboard Mars Express — the Visual Monitoring Camera (VMC) and the High Resolution Stereo Camera (HRSC) — along with the MARCI camera aboard NASA’s Mars Reconnaissance Orbiter, captured the storms from orbit.
The sequence of VMC imagery shows that the storms appear to wax and wane in repeated cycles over a period of days, sharing common features and shapes. Spiral shapes are particularly visible in the broader views of the HRSC images. The spirals are between 1000 and 2000 km long and have the same origin as the extratropical cyclones observed in the middle and polar latitudes of the earth.
The images show a special phenomenon on Mars. They show that the dust storms on Mars consist of regularly spaced smaller cloud cells, arranged like grains or pebbles. The texture can also be seen in clouds in Earth’s atmosphere.
The familiar textures are created by convection, where hot air rises because it is less dense than the cooler air around it. The type of convection observed here is called closed-cell convection, when air rises in the middle of small cloud pockets, or cells. The gaps in the sky around the cloud cells are the pathways for cooler air to sink below the hot rising air.
On Earth, the rising air contains water that condenses into clouds. The dust clouds imaged by Mars Express show the same process, but on Mars the rising columns of air contain dust rather than water. The sun heats dust-laden air, causing it to rise and form dusty cells. The cells are surrounded by areas of sinking air that contain less dust. This creates the grainy pattern seen in the image of clouds on Earth.
By following the movement of the cells in the image sequence, the wind speed can be measured. Wind blows over the cloud structures at speeds of up to 140 km/h, causing the shape of the cells to elongate downwind. Despite the chaotic and dynamic atmospheres of Mars and Earth, nature creates these ordered patterns.
“When you think of a Mars-like atmosphere on Earth, you could easily think of an arid desert or polar region. It is therefore quite unexpected that by tracing the chaotic motion of dust storms, parallels can be drawn with the processes taking place in the humid, hot and clearly non-Martian tropical regions of Earth,” comments Colin Wilson, Mars at ESA Express project scientist.
An important finding made possible with the VMC images is the measurement of the height of dust clouds. The length of the shadows they cast is measured and combined with knowledge of the position of the sun to measure the clouds’ height above the Martian surface. The results showed that dust can reach about 6-11 km above the ground and the cells have typical horizontal sizes of 20-40 km.
“Despite the unpredictable behavior of dust storms on Mars and the strong wind gusts that accompany them, we have seen that organized structures such as fronts and cellular convection patterns can emerge within their complexity,” explains Agustín Sánchez-Levaga of the Universidad del País Vasco UPV/EHU ( Spain), who leads the VMC science team and is lead author of a paper presenting the new analysis.
Such organized cellular convection is not unique to Earth and Mars; Observations of Venus’ atmosphere by Venus Express appear to show similar patterns. “Our work on Mars dry convection is another example of the value of comparative studies of similar phenomena that occur in planetary atmospheres to better understand the mechanisms underlying them in different conditions and environments,” adds Agustín.
Understanding dust storms is not only relevant for future missions to Mars, but also for learning more about how planetary atmospheres “work”. In extreme cases, dust storms can block much of the sunlight from reaching the rovers’ solar arrays on the Red Planet’s surface. In 2018, a planetary dust storm not only blocked sunlight reaching the surface, but also blanketed the solar panels of NASA’s Opportunity rover in dust. Both of these factors caused the rover to lose power and the mission was terminated.
Monitoring the development of dust storms is crucial to protect future solar-powered missions – and eventual manned missions to the planet – from such powerful phenomena.
‘Cellular patterns and dry convection in structured dust storms at the edge of Mars’ north polar cap‘ from A. Sánchez-Lavega et al. appears in the magazine from November 15, 2022 Icarus.
The spectacular VMC images can be seen on the cover of the magazine.
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