Light-dark shadows in the Martian highlands

Originally planned to last only one Mars year (corresponding to two Earth years), the Mars Express mission, entailing seven experiments, has been orbiting Earth's neighbouring planet for almost 16 Earth years now. The HRSC will soon have captured 100 percent of the Martian surface with high-resolution stereo image data, which will be used to generate a global digital terrain model. Approximately two thirds of the planet's nearly 150 million square kilometres (roughly equivalent to the total area of all continents on Earth) have been recorded in resolutions of about 20 metres per pixel and better.

Dark Mars regions once thought to be forests

Bright and dark areas on Mars can even be seen from Earth through a telescope. In the 19th century such observations led to the assumption that Mars had extensive forest areas – that there could at least be vegetation on Mars, if not living creatures. Since the late 1960s, space probes have made it possible to observe Mars at close range. It quickly became clear that although Mars has an atmosphere, it contains almost no water and, above all, is much thinner. As a result, the average temperatures there today are far too low to support life. Even if we cannot rule out the existence of primitive organisms on Mars today or in the past, Mars has certainly never had forests on the scale of Earth's continents.

Different silicate minerals cause the different colourings

The reason for the striking shades of light and dark, which can also be seen in the pictures of the Terra Cimmeria region, are in fact due to differences in mineralogy. Bright areas are covered by dust, which contains a larger proportion of bright silicate minerals in which the eponymous element silicon as well as aluminium, calcium, potassium or sodium form the crystal structure. This is the case, for example, with feldspar minerals, which occur as rock-forming minerals in light volcanic rocks such as rhyolite or the deep rock granite. The silicate content (silicon dioxide, SiO₂) of these rocks can be up to 65 percent.

Dark areas, on the other hand, are mainly covered by material consisting of 'mafic' dark minerals such as basalt. Basalt is the most widespread product of magmatic silicate melts on Mars as well as on Earth. They melted in the rock mantle and are lighter than the surrounding rock. As a result, they rise to the surface as thin lava. The main minerals that make up basaltic rock are plagioclase, pyroxene and olivine. Like feldspars, they also belong to the group of silicate minerals, but in contrast, they contain a high proportion of iron, magnesium or titanium. This explains why these minerals are darker than others. The silicate content of basalt is between 35 and 50 percent.

Dark layers of basaltic sand, as in the northern (right) part of the colour image, are probably formed from volcanic ash, which in many places forms impressive dark dune fields on Mars. In the past, five large volcanic provinces and countless small, scattered volcanoes distributed immense amounts of ash across the planet, which was later covered with other rock material. When the dark ash layers formed in this way reappear on slopes, for example in impact craters, the ash is blown away and deposited again over large areas.

Traces of snow, ice and running water

The largest impact crater shown in the images has a diameter of 25 kilometres, but is only about 300 metres deep. Over time, the once bowl-shaped crater was gradually filled with lava, glacier material and river and wind-blown sediments, almost completely levelling out its original shape. In its current appearance it is more reminiscent of a quiche dish than an impact crater. The strongly eroded crater rim and the amount of material deposited within the crater indicate that it is very old, probably over three billion years old.

Mesas and valleys south of the impact crater are the remains of a once extensive river system in Terra Cimmeria. Research suggests that the outflowing water came from melted ice or snow and was probably released during several periods of glaciation.

The top layers that make up the mesas may have come from aeolian deposits. They are comparable to loess deposits (solidified dust sediments), which can be up to several hundred metres thick, as in China, for example. Numerous valley networks cut through these sediments, which are easily eroded by flowing water, as can be seen north of the large impact crater. They most likely flowed into a group of depressions to the north, which probably contained a larger lake called Eridania.

Dust devils reveal dark material

Small, dark dust devil tracks can also be seen in the southern (left) part of the image. These form when atmospheric vortices remove the uppermost, bright layer of dust and expose the darker surface underneath. Starting from the smallest impact craters, (in the bottom left of the picture), so-called south-easterly and erosive wind streaks can also be seen. This type of wind travel occurs when wind speeds increase behind an obstacle (in this case a crater), thereby increasing the erosive force of the wind. As with the dust devils, this removes the uppermost, bright layer of dust, revealing the dark volcanic material underneath. Dust devil tracks and wind streaks are, to some extent, the short-lived witnesses of what is probably the last geological activity on Mars today, primarily the transport of dust and sand by wind.

The name of the region Terra Cimmeria means 'Land of the Cimmerians', the Cimmerians being an equestrian people of antiquity who lived on the northern shore of the Black Sea and in the Caucasus.