06 Jun 2023
Publication marks 20 years since the launch of the successful Mars Express mission.European Space Agency’s Mars Express, the High Resolution Stereo Camera (HRSC) team has produced a new global color mosaic of Mars “as never seen before”. The mosaic reveals the planet’s surface color and composition in spectacular detail, stated ESA.
Reliably determining accurate surface colors from orbit is difficult due to the variable amounts of dust in Mars’ atmosphere, leading to many mosaic images taking on a patchwork-like appearance; suppressing this effect in image processing usually reduces variations in colour between different parts of the Red Planet.
However, to create the mosaic, the HRSC team instead color-referenced each constituent image using a model derived from high altitude imagery, allowing them to preserve color variations and reveal a richer view of Mars than has been seen before.
This image is a simulated view of Mars from a vantage point 2500 km above the colossal Valles Marineris canyon system, with enhanced color and contrast; at this relatively low altitude, the planet’s polar caps are not visible. It is a composite of red, green and blue filter mosaics with the color band values stretched individually, and has a spatial resolution of 2 km per pixel. A higher resolution data products are possible and already in the works, said ESA.
Darker grey-toned areas of Mars represent grey-black basaltic sands of volcanic origin; lighter patches show clay and sulphate minerals; and the large scar across the planet's face is Valles Marineris.
The HRSC normally photographs Mars’s surface from an altitude of about 300 km – the closest the spacecraft gets to Mars in its elliptical orbit – with the resulting images covering areas about 50 km across. However, the mosaic presented here uses a slightly different approach. To view the planet more widely, HRSC gathered 90 images at higher altitudes (of 4000 to 10 000 km), thus capturing areas of around 2500 km wide. These images were then put together to form a global view.
Such large-scale images are typically obtained to observe weather patterns on Mars – but even in the absence of atmospheric phenomena they offer wonderful views of the planet’s surface. This new view highlights variation across Mars’s surface by enhancing local colour and contrast.
Thanks to its nine imaging channels, HRSC can visualise Mars not only in three dimensions but also in color. However, the ever-changing opacity of the Martian atmosphere makes it difficult to determine accurate surface colours 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 has reduced variations in colour between different parts of Mars. But to create this mosaic, the HRSC team instead colour-referenced each constituent image to a colour model derived from high-altitude observations, allowing them to preserve colour variations and reveal a far richer colour view of Mars than has been seen before.
While beautiful in its own right, the mosaic also provides fascinating information about Mars’s composition, revealing an unprecedented variety and detail of colours across its surface. Mars is famous for its reddish colour, which is caused by high levels of oxidised iron.
However, large parts of the planet appear to be rather dark and blue-toned here. These are grey-black basaltic sands of volcanic origin that form far-reaching, dark layers of sand across Mars. They pile up as they move in the wind, creating imposing sand dunes and dune fields within impact craters.
Material weathered by water, on the other hand, tends to look lighter. The two most common water-weathered minerals on Mars, clay and sulphate minerals, appear particularly bright on such colour composites; their presence was established by the OMEGA spectrometer on Mars Express.
The presence of these minerals signals that liquid water existed on Mars for a long time, weathering and altering rock over time to form significant clay deposits such as Mawrth Vallis. Sulphate minerals are visible within the Valles Marineris canyon system, as seen most clearly in the annotated image. Here, however, they are covered by a thin veneer of dark sand, but their impressive color variations can be seen on closer look. Unlike clay deposits, sulphate minerals indicate more acidic environmental conditions that would be less friendly to life.