True colour plan view of the western part of Arda Valles
This view of Arda Valles and its surroundings shows a diverse landscape, shaped by various geological processes. Particularly striking is the branching network of small valleys shown in the left half of the image, which is characteristic of the run-off of surface water. On the surface of Mars, such ‘dendritic’ run-off systems, like the branches of a tree, were principally formed in the planet’s early days. If one zooms into the high-resolution image, light layered deposits can be seen, formed by clay minerals. These are also characteristic of the early period of Mars, during which the minerals in volcanic rock were converted into clay minerals by contact with water.
At the top centre of the image, there is a 2000-metre-high mountain range, from which sediments were borne into the surroundings. These were also then deposited in craters. In the right half of the image, the southwest portion of an enormous, unnamed impact basin can be seen. This basin is also filled with sediments; tectonic faults were the result of settlement movements.
Oblique perspective view of a crater filled with sediment
The concave depression of this crater, with a diameter of around 25 kilometres, is filled with sediments that were transported into the crater from a mountain range to the west. The once smooth surface now features deep fissures as a result of settlement movements, so that it has formed a ‘chaotic terrain’. On the internal slopes of the crater, numerous uneven areas can be seen instead of the topographic features usually found on such craters, rather more terraced, curved characteristics. These structures may indicate the original level of the sediment layers in the crater interior.
Colour-coded image of the topography of the western part of Arda Valles
Digital terrain models of the Martian surface with a resolution down to 10 metres per pixel can be derived from data acquired by the nadir channel, which is directed vertically onto the Martian surface, and the stereo channels of the High Resolution Stereo Camera (HRSC) on Mars Express. In this colour-coded image, the absolute elevations above a reference level, the aeroid (derived from Ares, the Greek name for Mars), are depicted. These elevation values can be read based on the colour key at the lower right of the image.
The maximum differences in elevation values exceed 3500 metres. The colour distribution gives a clear picture of the topography in this area. A very old, heavily eroded impact basin, filled to the brim with sediments is seen in the northeast – lower right in the image. Arda Valles is located in the former crater rim and apparently drained into this basin. In the basin edge, there is a younger crater, also filled with sediment that has been broken up to produce ‘chaotic terrain’. At the top centre of the image, there is a mountain range from which material was transported to lower-lying areas.
When looking at the large-scale topography along the equator of Mars, what truly stands out are the many, extremely wide drainage channels leading north without many lateral inflows. The smaller valley systems are not so noticeable in these images. They often have multiple branches and meander across the terrain. Such valley systems can be found on Earth. Arda Valles in the Martian highlands is a good example of such a drainage system.
Arda Valles, named after a river in ancient Thrace, lies approximately 260 kilometres north of Holden Crater. A flow of water originating from the south flowed through Holden Crater and proceeded to drain out via Ladon Valles to the north, in the direction of a large, unnamed and heavily-weathered impact basin. The water that flowed through Arda Valles streamed through the southeastern edge into this large, sediment-filled basin (see overview map). The images of the western section of Arda Valles shown here were acquired on 20 July 2015 by the High Resolution Stereo Camera (HRSC), operated by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), on board the European Mars Express spacecraft, which has been orbiting Earth’s neighbouring planet since 25 December 2002.
A closely branched drainage network of wide valleys
In the left (southern) half of images 1, 4 and 5 it is apparent that water flowed through a drainage network approximately 175 kilometres long and 50 to 80 kilometres wide. More than three billion years ago, water flowed through this network, draining in the direction of an ancient impact basin. In hydrology, such a river course pattern is referred to a 'dendritic', a term derived from dendron, the Greek word for tree. The main valleys are up to two kilometres across, which is exceptionally wide for valleys in a dendritic network on Mars. It is possible that, in the planet's earlier history, large quantities of water flowed through this relatively narrow, confined area; however, this cannot be verified with complete certainty based on the images.
A large mountain range, some 20 kilometres wide and a little over 2000 metres high, is visible at the top centre of the images. It might be the remains of the crater rim of a very old impact structure. At the foot of the eastern side of this mountain range is a smaller, approximately 8.5-kilometre-wide crater with a strikingly flat interior. The crater was filled with sediment that came from the mountain ridge. This can be determined by the fan-like shape of the deposits.
To the right of the image centre, a 25-kilometre-diameter crater is visible; it, too, has been filled by sediment. The once smooth surface, however, now features deep fissures as a result of settlement movements, so that it has formed 'chaotic terrain'. A very similar structure was discovered just a few kilometres further east in this region by HRSC in 2012, in the Sigli and Shambe craters . The images presented here reveal an unusual number of uneven areas on the slopes of the crater interior, instead of what is usually seen in craters of this size, specifically more terrace-like, curved topographic features. These structures may indicate the original level of the sediment layers in the crater interior.
Clay minerals in Arda Valles indicate water in Mars' early history
Upon closer inspection, layered, significantly lighter deposits are found in some places between this crater and the lower courses of the dendritic valleys. Measurements with spectrometers suggest that this is clay. Clay minerals are found in numerous places on Mars; they are a typical weathering product of dark, iron- and magnesium-rich volcanic rock that has been eroded by water. This also points to water having flowed through the dendritic valley network a very long time ago, probably more than three-and-a-half billion years ago. The clay minerals identified on Mars are located almost exclusively on landscapes dating to the early days of Mars. These hydrous clay minerals contrast with salt deposits in other locations that are not as old.
The region to the north of the images (right in the plan views) is dominated by the southwestern part of the surface of an unnamed impact basin approximately 350 kilometres across. This surface was also formed by sediments that were transported by the dendritic valleys to the crater rim and to the interior of the basin, and also by larger tributaries from the south, such as Ladon Valles. The prominent linear structures indicate that the surface was exposed to tension due to settlement movements and that this resulted in tectonic faults, the structural process of which can be easily understood. The cause of the settlement movements could have been the evaporation of water and soil moisture, resulting in the formation of cavities that caved in on themselves, reduced the volume, and triggered tectonic fractures.
Image processing
The images were created using data acquired by HRSC during Mars Express orbit 14,649 on 20 July 2015. The centre of the images is located at 327 degrees east and 19 degrees south. The colour plan view (image 1) was created using data acquired by the HRSC nadir channel, which is directed vertically down onto the surface of Mars, and the colour channels. The oblique perspective view (image 2) was computed from the data acquired by the HRSC stereo channels. The anaglyph image (image 4), which creates 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 one stereo channel. The colour-coded plan view (image 5) is based on a digital terrain model of the region, from which the topography of the landscape can be derived.
The HRSC experiment
The High Resolution Stereo Camera (HRSC) was developed at DLR and built in collaboration with industrial partners (EADS Astrium, Lewicki Microelectronic GmbH and Jena-Optronik GmbH). The science team, which is headed by Principal Investigator (PI) Ralf Jaumann, consists of 52 co-investigators from 34 institutions in 11 countries. The camera is operated by the DLR Institute of Planetary Research in Berlin-Adlershof.
