The horst and graben landscape of Ascuris Planum

These images show a landscape deformed by strong tectonic activity in the area north of Labeatis Fossae in the Tempe Terra region of Mars. They were acquired by the High Resolution Stereo Camera (HRSC), operated by the German Aerospace Center (Deutsches Zentrum fur Luft- und Raumfahrt; DLR), on board ESA's Mars Express spacecraft. Here, the results of the enormous forces that once affected the Martian crust as large magma pockets rose from below can be seen. These pockets lifted the crust upwards and triggered volcanic and tectonic activity. HRSC has been mapping the Red Planet since 2004, as part of ESA's Mars Express mission. It was developed and is operated by DLR. The area depicted is located northeast of the large volcanic region of Tharsis, where there are many other similar geological structures. The Tharsis region has a diameter of several thousand kilometres, making it almost as large as Europe. Tharsis is a magmatic bulge, approximately five kilometres tall, and was formed over the course of several billion years. During the gradual upwelling and stressing of the lithosphere by volcanic and plutonic rocks, enormous tensile stresses occurred in the Martian crust, transforming large regions into 'horst and graben' landscapes. Tempe Terra is the northernmost highland region on Mars. The landscape is characterised by numerous tectonic expansion structures, shield volcanoes and solidified lava flows. The fracture structures shown here are in the south-western foothills of the Tempe Fossae troughs, which are over 1000 kilometres long and whose characteristics can be compared to that of the Kenya Rift on Earth, which is a part of the East African Rift.

How do horst and graben structures develop?
The region shown in these HRSC images is a textbook example of horst and graben tectonics. If a rigid, brittle rock crust is stretched, for example when the ground below it is raised, the surface experiences tension. If the tensile stress rises above the tolerable limit for the rock, the crust breaks up along somewhat steeply sloping fracture surfaces and a 'fault zone' is created.

If the crust continues to expand, large blocks of rock slide down along the fracture surfaces for hundreds of metres, and even up to 1000 or 2000 metres in places. Over many millions of years, tectonic grabens develop. The regions left standing on both sides now tower above the landscape and form the corresponding horsts. The word pair 'horst and graben' have their origins in early medieval miners' German and were incorporated into many languages following geology's establishment as an 'Earth science'.

A change in the stress regime
Tempe Terra is a part of the Martian crust that must have experienced high tectonic stress over a very long period of Martian history. The grabens run mostly parallel from northeast to southwest. However, there are also grabens that cut across this primary direction. This indicates a change in the orientation of the stress field. Particularly in the south (to the left of Image 1), some fractures run almost perpendicular to the prevailing direction of the faults.

Varied landscape
In the north (to the right of image 1), the landscape has a much smoother profile. The grabens are partly filled by debris-covered glaciers, which are characteristic of all steeper slopes at these latitudes. What are referred to as 'wrinkle ridges' can be seen at the top of the image. These were formed in the Tempe Terra region by compressive stress and form a concentric ring around the entirety of Tharsis.

Erosion processes have also shaped this northern part of the region. The ejected material of a small crater (on the right of Image 1 and the upper right of the perspective view) rises like a platform above the surrounding landscape. These types of crater are formed whenever the ejecta are significantly more resistant to erosion processes than the surface rock. They form an erosion-resistant layer which, after the surrounding material has been removed, creates a plateau around the crater.

Image processing
The images were acquired by the High Resolution Stereo Camera (HRSC) on 30 September 2019 during Mars Express orbit 19,913. The image resolution is approximately 15 metres per pixel. The image centre is located at 279 degrees east and 36 degrees north. The perpendicular colour view was generated from the data acquired by the HRSC nadir channel, which is directed perpendicular to the Martian surface, and the colour channels.

The oblique perspective view was computed using a Digital Terrain Model (DTM) and data from the nadir and colour channels of HRSC. The anaglyph image, which gives a three-dimensional impression of the landscape when viewed with red-blue or red-green glasses, was derived from data acquired by the nadir channel and the stereo channels. The colour-coded image map is based on a DTM of the region, from which the topography of the landscape can be derived. The reference body for the HRSC DTM is an equipotential surface of Mars (areoid).

HRSC was developed and is operated by the German Aerospace Center (DLR). The systematic processing of the camera data was performed at the DLR Institute of Planetary Research in Berlin-Adlershof. Personnel at the Department of Planetary Sciences and Remote Sensing at Freie Universitat Berlin used these data to create the image products shown here.

The HRSC experiment on Mars Express
The High Resolution Stereo Camera (HRSC) was developed by the German Aerospace Center (Deutsches Zentrum fur Luft- und Raumfahrt; DLR) and built in collaboration with partners in industry (EADS Astrium, Lewicki Microelectronic GmbH and Jena-Optronik GmbH). The science team, which is headed by Principal Investigator (PI) Ralf Jaumann, consists of 50 co-investigators from 35 institutions in 11 countries. The camera is operated by the DLR Institute of Planetary Research in Berlin-Adlershof.

Shaping the surface of Mars with water, wind and ice

ESA's Mars Express has imaged an intriguing part of the Red Planet's surface: a rocky, fragmented, furrowed escarpment lying at the boundary of the northern and southern hemisphere. This region is an impressive example of past activity on the planet and shows signs of where flowing wind, water and ice once moved material from place to place, carving out distinctive patterns and landforms as it did so. Mars is a planet of two halves. In places, the northern hemisphere of the planet sits a full few kilometres lower than the southern; this clear topographic split is known as the martian dichotomy, and is an especially distinctive feature on the Red Planet's surface. Northern Mars also displays large areas of smooth land, whereas the planet's southern regions are heavily pockmarked and scattered with craters. This is thought to be the result of past volcanic activity, which has resurfaced parts of Mars to create smooth plains in the north - and left other regions ancient and untouched. The star of this Mars Express image, a furrowed, rock-filled escarpment known as Nili Fossae, sits at the boundary of this north-south divide. This region is filled with rocky valleys, small hills, and clusters of flat-topped landforms (known as mesas in geological terms), with some chunks of crustal rock appearing to be depressed down into the surface creating a number of ditch-like features known as graben.

As with much of the surrounding environment, and despite Mars' reputation as a dry, arid world today, water is believed to have played a key role in sculpting Nili Fossae via ongoing erosion. In addition to visual cues, signs of past interaction with water have been spotted in the western (upper) part of this image - instruments such as Mars Express' OMEGA spectrometer have spotted clay minerals here, which are key indicators that water was once present.

The elevation of Nili Fossae and surroundings, shown in the topographic view above, is somewhat varied; regions to the left and lower left (south) sit higher than those to the other side of the frame (north), illustrating the aforementioned dichotomy. This higher-altitude terrain appears to consist mostly of rocky plateaus, while lower terrain comprises smaller rocks, mesas, hills, and more, with the two sections roughly separated by erosion channels and valleys.

This split is thought to be the result of material moving around on Mars hundreds of millions of years ago. Similar to glaciers on Earth, flows of water and ice cut through the martian terrain and slowly sculpted and eroded it over time, also carrying material along with them. In the case of Nili Fossae, this was carried from higher areas to lower ones, with chunks of resistant rock and hardy material remaining largely intact but shifting downslope to form the mesas and landforms seen today.

The shapes and structures scattered throughout this image are thought to have been shaped over time by flows of not only water and ice, but also wind. Examples can be seen in this image in patches of the surface that appear to be notably dark against the ochre background, as if smudged with charcoal or ink.

These are areas of darker volcanic sand, which have been transported and deposited by present-day martian winds. Wind moves sand and dust around often on Mars' surface, creating rippling dune fields across the planet and forming multi-coloured, patchy terrain like Nili Fossae.

The data comprising this image were gathered by Mars Express' High Resolution Stereo Camera (HRSC) on 26 February 2018.

ESA's Mars Express was launched in 2003. As well as producing striking views of the martian surface such as this, the mission has shed light on many of the planet's biggest mysteries - and helped to build the picture of Mars as a planet that was once warmer, wetter and potentially habitable. Read more about the past 15 years of Mars Express, and what the mission has discovered so far, here.