Slices from the edge of Mars reveal a layered atmosphere of delicate complexity. A European spacecraft has captured a luminous mille-feuille of dust enveloping the Red Planet in unprecedented detail.
ESA’s ExoMars Trace Gas Orbiter keeps gathering information from its orbit around Mars to understand its ancient past and potential habitability. The spacecraft was cruising over the southern highlands of Terra Cimmeria, some 400 km above the martian surface, when it recorded this composite of five vertical images on 21 January 2024. Towards the bottom is Mars; at the top, space.
The kaleidoscope of light and colour is composed of the highest resolution images of the atmosphere above the limb of Mars ever obtained. Mars’s limb is the curved edge of the planet, the apparent boundary where its surface meets space. Observing a planet’s limb can reveal details about the hazy edge of its atmosphere.
The spacecraft was in the shadow of Mars, looking towards a veil of dust backlit by sunlight at dusk. From this vantage point, the Colour and Stereo Surface Imaging System (CaSSIS) aboard could reveal fine layers of cloud and dust scattered throughout the atmosphere. The five images, each covering a slice of the atmosphere 3.6 km wide, show tens of layers ranging from 15 to 55 km of altitude. Each slice pictured is 200 km apart from each other.
Subtle changes in colour indicate that particles become smaller at higher altitude. “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,” says Nicolas Thomas, CaSSIS Principal Investigator from the University of Bern and lead author of the paper published in Science Advances on 19 September 2025.
The many layers of ice particles and dust show evidence of a restless atmosphere. These particles scatter sunlight when interacting with solar and planetary radiation.
At 40 km and above Mars’s surface, the CaSSIS camera shows layers of fine particles which may include small ice grains in this cold part of the atmosphere. Below 40 km, the layers probably consist mostly of dust lifted from the surface.
The result of this atmospheric recipe can also vary depending on the season on the Red Planet. Dust storms, and hence the quantity of particles in the atmosphere, vary by region and time of year. “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,” explains Nicolas.
Images of the martian limb have been obtained previously by Mars Express, but this is the first time that researchers have received pictures with a much finer spatial resolution of 18 metres per pixel. These unique CaSSIS observations of the limb will now be carried out once a month.
The team is building an extensive database with these types of images to crack the recipe behind Mars’s atmospheric mille-feuille. “This wealth of information will support the detailed analysis ahead,” adds Nicolas.
Since 2018, the ExoMars Trace Gas Orbiter has been returning spectacular images of the surface and providing the best inventory of atmospheric gases, as well as mapping the planet’s surface for water-rich locations.