Get ready to be blown away by the most detailed images of a Martian sunset ever captured!
On January 21, 2024, a European orbiter witnessed a breathtaking sight - the curved edge of Mars, glowing with intricate layers against the vast emptiness of space. This moment was immortalized by the orbiter's advanced camera system, providing us with an unprecedented view of Mars' atmosphere.
The images, captured at a resolution of 59 feet (18 meters) per pixel, reveal a complex and fascinating structure. The orbiter, ESA's ExoMars Trace Gas Orbiter, flew approximately 250 miles (400 kilometers) above Terra Cimmeria in the southern highlands, and looked back at the planet during twilight. Its Color and Stereo Surface Imaging System, or CaSSIS, captured the Martian atmosphere from a unique perspective - the planet's shadow, where the backlit layers stood out with remarkable clarity.
The sunset on Mars is a sight to behold, with five narrow, vertical slices, each about 2.2 miles (3.5 kilometers) wide, showcasing the layered structure of the atmosphere at twilight. These layers repeat from roughly 9 to 34 miles (14 to 56 kilometers) in altitude, displaying subtle changes in brightness and color.
This research was led by Nicolas Thomas, the principal investigator of CaSSIS at the University of Bern. His expertise lies in high-precision planetary imaging and the physics of dusty sunsets on Mars. The camera is capable of discerning structures smaller than a mile and recording significant variations in color within individual haze layers, with swings of about 20% in color intensity.
The geometry of the Martian sunset plays a crucial role. During twilight, dust particles are positioned between the Sun and the camera, creating a forward scattering configuration. This configuration amplifies brightness and gives preference to blue wavelengths, which become more concentrated near the Sun compared to red wavelengths that are spread across the sky.
"When the blue light scatters off the dust, it stays closer to the direction of the Sun," explains Mark Lemmon, a scientist working with the Curiosity rover. This phenomenon is well-documented and adds to the unique beauty of Martian sunsets, often appearing in a stunning shade of blue.
The color ratios in the layers tend towards blue with increasing altitude, suggesting that aerosols, tiny suspended particles like dust or ice, become smaller as altitude increases. Above approximately 27 miles (43 kilometers), some layers turn red again, indicating shifts in composition and particle size within the higher atmosphere.
The team also identified delicate layers in the mesosphere, the cold middle layer of the atmosphere, hovering between 29 and 35 miles (46 and 56 kilometers). These detached sheets are situated above a bright bulge around 25 miles (40 kilometers), a feature that atmospheric models will need to carefully explain.
Below this height, the layers likely contain dust lifted from the surface, while the higher sheets may include tiny water ice grains. The changes in color track the average size of the particles, known as the particle radius, which decreases steadily with increasing altitude in most profiles.
To capture these incredible images, CaSSIS rotated its detector to sweep the limb while the spacecraft cruised at a speed of about 2 miles per second (3.2 kilometers per second). The geometry was near a phase angle of roughly 174 degrees, an ideal condition for bright forward scattering.
Five well-exposed datasets were collected every 124 miles (200 kilometers) along the ground track during a four-minute window over southern Terra Cimmeria. Each image spanned approximately 2.2 miles (3.5 kilometers) across the limb, providing a near-continuous sampling of the stacked atmospheric sheets.
CaSSIS is designed to deliver targeted color imaging with a resolution of about 15 feet (4.5 meters) per pixel for surface science. However, for this limb sequence, the team pushed the camera's sensitivity and timing capabilities to focus on tracking the thin atmospheric sheets rather than ground textures.
These images provide valuable insights into Mars' climate. Past profiles from the Mars Climate Sounder have shown seasonal variations in dust heights and the presence of ice caps above haze. The new layering snapshots offer modelers a more precise target for understanding how particles stack during calm periods and during stormy weeks.
Mars' atmosphere is not static; it changes with the seasons and the intensity of sunlight. This variability makes the vertical layering more than just a beautiful sight; it provides crucial data for climate models, anchoring them to real heights and colors.
Robotic explorers rely on light and temperature, so understanding how dust and ice stack at sunset is essential for operational planning. These snapshots also help bridge measurements taken from orbit with sky observations gathered by landers and rovers on the ground.
ESA plans to repeat these limb observations approximately once a month to build a long-term dataset of twilight data. A seasonal atlas of layers could reveal whether the current pattern remains consistent or undergoes changes under the influence of regional storms.
Scientists will investigate why some layers turn red at higher altitudes, whether composition shifts explain this change, and how particle sizes vary with latitude. They will track the texture from the equator to the pole to observe when the layers thicken, thin, or separate into detached sheets.
For now, we can appreciate that Mars reveals more than just a red sky at sunset. These images showcase a precise and layered climate, and with monthly views and careful modeling, we can gain even more insights into the particle sizes, textures, and movements within Mars' evening atmosphere.
The full study has been published in Science Advances, offering a deeper dive into this fascinating research.
