These images, acquired by the High Resolution Stereo Camera (HRSC), operated by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), show a region close to the Nili Fossae. The Nili Fossae are located at the border between the southern highlands and the northern lowlands of Mars. The landscape is characterised by mesas, valleys and numerous low hills, shaped by water-driven erosion processes. The HRSC experiment is on board ESA's Mars Express mission, which has been orbiting Mars since 2003.
The images show a hilly landscape scored through by a myriad of furrows. This area lies on what is known as Mars' dichotomy boundary, or sometimes as the highland-lowland boundary. The pronounced dichotomy between the crater-strewn highlands south of the equator and the smooth northern lowlands, is one of Mars' most striking topographical features. In most places, the transition between the two is marked by a steep terrain edge, with numerous inselbergs or outlier mountains at the foothill, which represent the remnants of the highlands and were able to withstand the processes of erosion for a longer period of time. Traces of glacier activity and ice can often be seen along the dichotomy boundary, indicating the presence of ice in the past.
The southern region, with its plateau-like rocks (on the left in images 1, 4 and 5), is significantly higher than areas further north (on the right), which mainly consist of smaller mesas and hills. These are interspersed with depressions that formed through erosion processes during the planet’s past. Water and ice transported the rocky material downslope into areas of lower elevation millions of years ago. The narrow valleys bear particularly clear traces of these flow processes, with line-like structures that show where material was transported by water or ice. The elevations in the adjacent northern lowlands are testament to the former level of the terrain and are thought to consist of more resistant material.
One process still at play on the Martian surface is the transport of sand and dust by wind. Visible traces of this can be found in the depressions, which are filled with dark material. The dark sand is of volcanic origin and forms wide-ranging expanses of sand and dunes.
Clay minerals and carbonates indicate earlier water deposits
The Nili Fossae area is particularly interesting to those researching Mars because of its diverse mineralogy. In the course of the planet’s geological development, the minerals of the rocks located here were once increasingly altered due to the effects of the water flowing over the surface, but also by water circulating through fissures and cracks in the rocky crust, presumably warmed by volcanic heat, in the form of hydrothermal solutions.
Traces of these minerals transformed by influence of water are found in several places. Instruments for investigating the mineralogical composition of the Martian rock, such as the OMEGA spectrometer on board Mars Express, have discovered substances including clay minerals with water molecules embedded into their crystal structure. The presence of these water-rich minerals allows us to draw conclusions about the conditions under which they formed and the climate of Mars in the past.
Moreover, the discoveries in Nili Fossae also include carbonates that occur primarily as limestones (calcium carbonate) or dolomite (calcium magnesium carbonate) on Earth. In parts of Nili Fossae, the CRISM spectrometer on NASA's Mars Reconnaissance Orbiter discovered the magnesium carbonate magnesite, which was probably formed there from olivine, a mineral contained in the volcanic rock.