A new mosaic of Mars marks 20 years since the launch of the Mars Express of ESA and reveals the planet's color and composition in stunning detail.
The mosaic was created using data from Mars Express' High Resolution Stereo Camera (HRSC).
HRSC normally photographs the Martian surface from an altitude of about 300 km—as the spacecraft approaches Mars in its elliptical orbit—with the resulting images covering areas of about 50 km. However, the mosaic shown below uses a slightly different approach. To see the planet more widely, HRSC collected 90 images at higher altitudes (from 4000 to 10 000 km), thus recording areas about 2500 km wide. These images were then assembled to form a complete overall view.
Such large-scale images are usually taken to observe the weather on Mars – but even in the absence of atmospheric effects they provide a wonderful view of the planet's surface.
Thanks to its nine imaging channels, HRSC can image Mars not only in three dimensions but also in color. However, the ever-changing opacity of the Martian atmosphere makes it difficult to accurately determine surface colors from orbit. Dust scatters and reflects light, causing colors to shift between images and creating a patchwork-like effect when assembling a mosaic.
Until now, suppressing this effect during image processing reduced color variations between different parts of Mars. But to create this mosaic, the HRSC team gave each component image a color model derived from high-altitude observations, allowing them to preserve color variations and reveal a much richer color view of Mars than ,what they had seen before.
While beautiful in its own right, the mosaic also provides fascinating information about the composition of Mars, revealing an unprecedented variety and detail of color across its surface.
Mars is famous for its reddish color, which is caused by high levels of oxidized iron. However, large parts of the planet appear to be rather dark and have blue tones. It is the gray-black basaltic sand of volcanic origin that forms extensive, dark layers all over Mars. It piles up as it moves with the wind, creating imposing dunes and sand dunes within the impact craters.
On the other hand, water-eroded material tends to appear lighter. The two most common water-derived minerals on Mars, clay and sulfate minerals, appear particularly bright in composite colors.
Their presence was detected by the Mars Express OMEGA spectrometer. The presence of these minerals indicates that there was water on Mars for a long time, which weathered the rock over time to form significant clay deposits such as Mawrth Vallis (a former outflow channel not seen in this view but previously observed from the HRSC).
Sulfite minerals are visible within the Valles Marineris canyon system, as seen more clearly in the annotated image. Here, however, they are covered by a thin veneer of dark sand, but their striking color variations can be seen on closer inspection. Unlike clay deposits, sulfate minerals indicate more acidic environmental conditions that would be less friendly to life.
