June 2, 2023 marks the 20th anniversary of the launch of the European space probe Mars Express. Mars Express is considered one of the most successful space missions ever sent to our neighboring planet. On board is the German "High Resolution Stereo Camera" (HRSC), which has been continuously recording images from Mars orbit ever since. The data stream from the HRSC has yielded a multitude of new insights in recent years. Gerhard Neukum (1944-2014) led the development of the HRSC, which is designed to perform global mapping of Mars at 12m resolution, providing simultaneously colour images of the surface and its topography. The HRSC was first installed on the Russian Mars 96 probe, which unfortunately failed to leave Earth's orbit after launch and burned up in the atmosphere. But Gerhard Neukum kept going, as director of the DLR Institute for Planetary Research in Berlin Adlershof, he became the key proponent for the first ESA research probe to be sent to another planet. In 2002, Gerhard Neukum moved to Freie Universität as a professor, where he established the Department of Planetary Sciences and Remote Sensing.

For its usual surface images, the HRSC normally photographs Mars from an altitude of about 300 kilometers, at about the point from which the elliptical orbit of the Mars Express satellite is closest to the planet. The resulting views of the Martian surface have a spatial resolution of up to 12.5 meters per pixel and cover areas about 50 kilometers wide. Thanks to its four colour channels (red, green, blue, infrared) and five panchromatic nadir, stereo, and photometric channels, the stereo camera can visualize Mars not only in three dimensions, but also in color.

For the global data product presented here, however, 90 individual images were used, taken from higher altitudes (roughly between 4000 and 10,000 km) above the Martian surface and thus covering areas around 2500 kilometers wide on average at lower spatial resolution (between 200 and 800 m/px) (image 3). Such large-scale images are typically obtained to observe weather patterns on Mars. If no clouds or other atmospheric phenomena are visible on the images, they are excellent for creating global views of the Martian surface.

The ever-changing transparency of the Martian atmosphere makes it difficult to determine accurate surface colours from orbit. Scattering and reflection of light off atmospheric dust causes colour shifts between images, making a mosaic of many together show a colour patchwork effect. Up until now, suppressing this effect has had the consequence of diminishing colour variations over distances greater than the scale of a single image. Here, we made use of a new high-altitude observation campaign to construct a global colour model, which was then used to colour-reference each image of the mosaic, enabling long range colour variations to be maintained and revealing a much richer colour view of the planet than has been seen before.

The global view of Mars shown here has a spatial resolution of 2 kilometers per pixel, although higher resolution data products are quite possible and are already in the works. This contrast-enhanced mosaic (image 1) reveals an unprecedented variety and detail of colors across the Martian surface, which at the same time provides information about its composition. It is well known that most of Mars is reddish in color, due to the high amount of oxidized iron in the dust on its surface, earning it the nickname "Red Planet." But it is also immediately noticeable that a not small part of Mars is rather dark, on image 1 bluish, colored. In fact, these are grayish-blackish sands, which are of volcanic origin and form far-reaching dark sand layers on Mars, but above all also were piled up by the wind to imposing sand dunes or enormous dune fields on the floor of impact craters. These unweathered sands consist of dark, basaltic minerals, of which also volcanic lava on earth is composed.

Material weathered under the influence of water, on the other hand, tends to take on lighter shades over time. For example, clay and sulfate minerals, the two most common water-weathered minerals on Mars, appear particularly bright on such color composites and are relatively easy to recognize on closer views. One of the largest clay mineral deposit on Mars around the former outflow channel Mawrth Vallis, is not shown in this view but has been observed by HRSC earlier and attest to a long-term existence of liquid water on Mars, with the original, basaltic source rock being altered at neutral pH and relatively warm temperatures weathered to clay minerals. Large occurrences of the likewise light-colored sulfate minerals can be seen in this view of Mars within the Valles Marineris canyon system (image 4). Here, they are covered by a thin veneer of dark sand and thus show their impressive colour variations only on closer look by HRSC. Sulphate minerals indicate less life-friendly environmental conditions at low pH values. 

Faint, bright to light blue areas actually show clouds in the atmosphere (image 4), as images containing clouds could not be avoided so far in the creation of this first version of the global mosaic. The depths of Valles Marineris are also covered by atmospheric phenomena. However, these are fogs and hazes (image 4), which are particularly fond of forming in depressions at certain times of the day and year.