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The dichotomy boundary – the transition between the Martian highlands and the lowlands of the northern hemisphere was worn away by erosion over time, resulting in the landscape that we see today, with its strikingly varied and deeply scored surface. The highland plateau, at around 2000 to 3000 metres high, was altered by ice, water and wind, and in places by volcanic and tectonic activity – forces from inside the planet. Water flowing towards the north eroded the edge of the terrain and created the free-standing inselbergs or outliers, as well as the hills found in the foreland. Wind carried volcanic sand and dust into the resulting valleys and depressions in the northern foreland (north is to the right in this image). The contrast has been increased in some of the colour channels to allow us to better make out the details on the Martian surface, so that the dark dust and sand sediments appear dark blue in places.

Credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO.

The Nili Fossae region and the highly diverse transition zone from the Martian highlands to the northern lowlands is significant, both geologically and mineralogically. In the centre of the image it is possible to make out an old alluvial fan, presumably created by water flowing from a higher valley and the deposition of the material carried along with it into the wide lower valley. Spectroscopy experiments on board several orbiters have identified water-rich clay minerals at the edges and on the bottom of some valleys, which were probably produced by the weathering of volcanic basalt rocks and deposited in the watery environment there. However, Nili Fossae is also one of the few places on Mars where carbonates have been discovered. The most common carbonates on Earth are limestone (calcium carbonate or calcium magnesium carbonate, also known as dolomite) and the result of the sedimentation of calcified skeletons of dead microorganisms. The carbonate magnesite, or magnesium carbonate, has been found in Nili Fossae. The magnesium is thought to come from the silicate mineral olivine, the most common constituent of basalt.

Credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO.

Nili Fossae is a 30-kilometre-wide tectonic system of grabens on the boundary between the Martian highlands and the northern lowlands. The transition between these two major topographical Martian landscapes, known as the dichotomy boundary, is particularly typical here: the smooth highland plains form a terrain edge several thousand metres high, breaking away to the north. The foothills are home to some standalone mesas that have survived erosion, while large expanses of hilly terrain stretch out further to the north. DLR’s High Resolution Stereo Camera (HRSC) on board the ESA Mars Express spacecraft surveyed the area on 26 February 2018 from an altitude of around 400 kilometres, during orbit 17,916.

Credit: NASA/USGS (MOLA); FU Berlin.

The image strips acquired from different angles by the HRSC camera system were used to generate digital terrain models of the Martian surface, containing elevation information for each recorded pixel. The reference level to which the altitude readings refer is an imaginary surface that represents the same gravitational pull as that of sea level on Earth. Known as an equipotential surface, this is shaped like a biaxial ellipsoid and is called the Areoid, a name derived from Ares, the Greek word for Mars. In this image, North is to the right.

The colour coding of the digital terrain model (top right) clearly indicates the elevation differences: the topographical profile of the region has an altitude of approximately 4000 metres; the difference between the highest regions of the Martian highlands in the south (beige and light brown) and a number of shallow depressions in the lowlands towards the top (azure blue) is equivalent to that of the difference in altitude between the summit of Mont Blanc and the Rhône Valley in Lyon. The mesa-like outliers in the eroded transition area between highland and lowlands tower around 2000 metres over the plain.

Credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO.

So-called anaglyph images can be produced from the nadir channel (oriented vertically onto the surface of Mars) of the HRSC camera system operated by the DLR on board the ESA Mars Express space probe and one of the four oblique-view stereo channels. When viewed with red-blue or red-green glasses, these images give a realistic, three-dimensional view of the landscape.

The rugged landscape on the northern edge of the Martian highlands, with its countless mesas, hills and valleys extending from the highlands, is highly suited to 3D viewing. The topographical profile, which covers a 4000-metre difference in altitude, suggests that many geological and erosive processes helped to shape the landscape that we see today. The dark surfaces are covered in volcanic sand and dust transported here by winds on Mars.

Credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO.