ESA’s Mars Express has revealed that Mars produces strikingly Earth-like cloud patterns reminiscent of those in the tropical regions of our planet.
Earth and Mars have very different atmospheres. Mars’ dry and 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 of the Earth’s atmosphere, which is equivalent to the density found at about 35 km above the Earth’s surface.
Although extremely different, their cloud patterns were found to be strikingly similar to Earth’s, indicating similar formation processes.
A new study dives deeper into two dust storms that occurred near the Martian North Pole in 2019. The storms were monitored in spring at the North Pole, a time when local storms typically brew around the receding ice sheet .
Two cameras aboard Mars Express – the Visual Surveillance Camera (VMC) and the High-Resolution Stereo Camera (HRSC) – as well as the MARCI camera aboard NASA’s Mars Reconnaissance Orbiter, imaged the storms from orbit.
The VMC image sequence shows that the storms appear to grow and die in repeated cycles over a period of days, exhibiting common characteristics and shapes. The spiral shapes are especially visible in the wider views of the HRSC images. The spirals are between 1,000 and 2,000 km long and their origin is the same as that of extratropical cyclones observed in the middle and polar latitudes of the Earth.
The images reveal a peculiar phenomenon on Mars. They show that Martian dust storms are made up of small, evenly spaced cloud cells, arranged like grains or pebbles. The texture is also visible in clouds in Earth’s atmosphere.
The familiar textures are formed by convection, whereby warm air rises because it is less dense than the cooler air around it. The type of convection seen here is called closed-cell convection, when air rises in the center of small cloud pockets, or cells. Gaps in the sky around cloud cells are the pathways through which cooler air descends below rising warm air.
On Earth, the rising air contains water which condenses to form clouds. Dust clouds imaged by Mars Express show the same process, but on Mars, rising columns of air contain dust rather than water. The sun heats the dust-laden air, causing it to rise and form dusty cells. The cells are surrounded by areas of sinking air which contain less dust. This gives rise to the granular pattern also seen in the image of clouds on Earth.
By tracking the movement of cells in the image sequence, wind speed can be measured. The wind blows over cloud features at speeds up to 140 km/h, causing the cell shape to elongate in the direction of the wind. Despite the chaotic and dynamic atmospheres of Mars and Earth, nature creates these orderly patterns.
“When we think of a Mars-like atmosphere on Earth, we could easily think of a dry desert or a polar region. It is therefore quite unexpected that in following the chaotic movement of dust storms, parallels can be established with the processes that occur in the humid, hot and decidedly very different tropical regions of Mars,” comments Colin Wilson, ESA’s Mars Express Project Scientist.
A key insight made possible by VMC images is measuring the altitude of dust clouds. The length of the cast shadows is measured and combined with knowledge of the position of the Sun to measure the height of the clouds above the Martian surface. The results revealed that the 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 gusts of wind that accompany them, we have seen that in their complexity, organized structures such as fronts and cellular convection patterns can emerge”, explains Agustín Sánchez- Levaga of the Universidad del País. Vasco UPV/EHU (Spain), who leads the scientific team of the VMC and is the main author of a paper presenting the new analysis.
Such organized cellular convection is not unique to Earth and Mars; observations of the Venusian atmosphere by Venus Express likely show similar patterns. “Our work on Mars dry convection is another example of the value of comparative studies of similar phenomena occurring in planetary atmospheres to better understand the mechanisms underlying them under different conditions and environments,” adds Agustín.
In addition to learning more about how planetary atmospheres “work”, understanding dust storms is relevant for future missions to Mars. In extreme cases, dust storms can block much of the Sun’s light from reaching rover solar cells on the Red Planet’s surface. In 2018, a planet-wide dust storm not only blocked sunlight reaching the surface, but also covered the solar panels of NASA’s Opportunity rover with dust. These two factors caused the rover to lose electrical power, ending the mission.
Monitoring the evolution of dust storms is crucial to help protect future solar-powered missions – and possibly crewed missions to the planet – from such powerful phenomena.
‘Cellular patterns and dry convection in textured dust storms at the edge of Mars’ north polar cap‘ by A. Sánchez-Lavega et al. is published in the November 15, 2022 issue of the journal Icarus.
The spectacular images of the VMC appear on the cover of the newspaper.
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