Using images obtained by two orbiting spacecraft, NASA’s Mars Reconnaissance Orbiter and ESA’s Mars Express, planetary researchers analyzed textured dust storms that develop at the northern polar cap boundary on Mars springtime.
A spiral-shaped dust storm is visible in this image taken by the High Resolution Stereo Camera (HRSC) imager on ESA’s Mars Express spacecraft. This image, taken on May 26, 2019, depicts the formation of a spiral shape whose main arm is approximately 2,000 km in length. A granular pattern of dust cells is visible in the storm, formed by a process called closed-cell convection. This type of convection is also responsible for some cloud patterns on Earth. Image credit: ESA / DLR / FU Berlin.
“Springtime season in Mars’ northern hemisphere is characterized by rich atmospheric dynamical activity that takes place at the edge of the polar cap, often revealed by the presence of local dust storms,” said lead author Dr. Agustín Sánchez-Levaga from the Universidad del País Vasco and colleagues.
“Disturbances are triggered by the temperature gradient between the receding polar ice layer and the surrounding terrain, enhanced by the orographic properties of the northern hemisphere.”
“These dynamical instabilities show a variety of morphologies in images from orbit spacecraft, variously organized in cyclonic spirals, arc shapes, bands and commas, sometimes accompanied by water ice clouds.”
“We focus in a particular case of textured dust storms characterized by the presence of large areas of organized granular or cellular patterns, mixed with other forms denoted as puffy and pebbled, and ruffled elongated features.”
The researchers analyzed images from the Visual Monitoring Camera (VMC) and the High Resolution Stereo Camera (HRSC) onboard ESA’s Mars Express spacecraft and the MARCI camera on NASA’s Mars Reconnaissance Orbiter covering the period from March 3 to July 17, 2019.
This period corresponds to the early springtime season in the northern hemisphere, about one Earth year after the global dust storm of 2018.
“The sequence of VMC images shows that the storms appear to grow and disappear in repeated cycles over a period of days, exhibiting common features and shapes,” the scientists said in a statement.
“Spiral shapes are notably visible in the wider views of the HRSC images.”
“The spirals are between 1,000 and 2,000 km in length, and their origin is the same as that of the extratropical cyclones observed in Earth’s mid-latitudes and polar latitudes.”
“The images reveal a particular phenomenon on Mars,” they added.
“They show that the Martian dust storms are made up of regularly spaced smaller cloud cells, arranged like grains or pebbles. The texture is also seen in clouds in Earth’s atmosphere.”
Comparing the cloud patterns in dust storms on Mars with closed convection cells in Earth’s atmosphere. The image on the left depicts a storm at the Martian North Pole in May 2019, as seen from the Visual Monitoring Camera (VMC) on ESA’s Mars Express spacecraft. The repeated pattern of small cloud cells is caused by convection currents, when dusty air is heated by the Sun and rises to forms a small cell of between 20-40 km in horizontal size. The cells are surrounded by cooler air which descends, forming a boundary which creates the granular pattern. On the right, Eumetsat/ESA Meteosat-8 mission captured closed-cell convection on Earth. This image was taken on March 20, 2020, over the Azores. The cellular pattern is created in a similar process to the pattern on Mars, with similar sizes of 20-50 km. However, instead of dust, the air columns contain water vapor which is heated and rises to form clouds. Image credit: ESA / GCP / UPV / EHU Bilbao / Eumetsat.
On Earth, the rising air contains water which condenses to form clouds. The dust clouds imaged by Mars Express show the same process, but on Mars the rising air columns 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 which have 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 sequence of images, the wind speed can be measured. Wind blows over the cloud features at speeds of up to 140 km/h, causing the shape of the cells to elongate in the direction of the wind.
Despite the chaotic and dynamic atmospheres of Mars and Earth, nature creates these orderly patterns.
“When thinking of a Mars-like atmosphere on Earth, one might easily think of a dry desert or polar region,” said Mars Express project scientist Dr. Colin Wilson.
“It is quite unexpected then, that through tracking the chaotic movement of dust storms, that parallels can be drawn with the processes that occur in Earth’s moist, hot, and decidedly very un-Mars-like tropical regions.”
One key insight made possible with the VMC images is the measurement of the altitude of dust clouds.
The length of the shadows they cast are measured and combined with knowledge of the Sun’s position to measure the height of the clouds above the Martian surface.
The results revealed that dust can reach approximately 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 within their complexity, organized structures such as fronts and cellular convection patterns can emerge,” Dr. Sánchez-Levaga said.
“Such organized cellular convection is not unique to Earth and Mars; observations of the Venusian atmosphere by ESA’s Venus Express spacecraft arguably show similar patterns.”
“Our work on Mars dry convection is a further example of the value of comparative studies of similar phenomena occurring in planetary atmospheres in order to better understand the mechanisms underlying them under different conditions and environments.”
The findings were published in the journal Icarus.
_____
A. Sánchez-Lavega et al. 2022. Cellular patterns and dry convection in textured dust storms at the edge of Mars North Polar Cap. Icarus 387: 115183; doi: 10.1016/j.icarus.2022.115183
