The data were obtained on January 21, 2024, while the spacecraft passed 400 km above Terra Cimmeria in Mars's southern highlands. From within the planet's shadow at dusk, the CaSSIS camera imaged the sunlit limb of Mars in five vertical slices, each covering a 3.6 km-wide section of atmosphere separated by 200 km. The resulting mosaic shows tens of distinct layers at altitudes between 15 and 55 km.
The colour variations indicate changes in particle size with altitude: fine grains appear higher up, while lower layers contain mostly dust lifted from the surface. At altitudes above 40 km, researchers believe the layers may include small ice particles in the colder atmosphere. Below this, dust dominates the structure.
"Our observations, especially the colour, provide unique insight into the particle radius at each altitude in the atmosphere. Shape and composition could also play a role. This stuff is wild," said Nicolas Thomas, CaSSIS Principal Investigator at the University of Bern and lead author of the new study.
Dust storms, seasonal effects, and regional weather contribute to the variability of the atmospheric recipe. "The lack of understanding of the vertical distribution of particles in the atmosphere is one of the key questions about the climate of present-day Mars," Thomas explained.
Previous limb observations by Mars Express offered coarser resolution, but CaSSIS has now achieved unprecedented clarity at 18 metres per pixel. The team plans to repeat these observations monthly, building a long-term dataset to better understand Martian atmospheric dynamics.
Since beginning operations in 2018, the ExoMars Trace Gas Orbiter has mapped water-rich sites, catalogued atmospheric gases, and delivered striking surface images. These new limb observations add another dimension to the spacecraft's contributions, offering a path toward decoding the layered complexity of Mars's sky.
Research Report:Millefeuille: The layering of the Martian atmosphere observed in forward scattering geometry
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