View of the western part of the upper reaches of Nirgal Vallis
View of the west­ern part of the up­per reach­es of Nir­gal Val­lis
Bild 1/5, Quelle: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO

View of the western part of the upper reaches of Nirgal Vallis

Nir­gal Val­lis stretch­es 700 kilo­me­tres across the south­ern Mar­tian High­lands; north is to the right in the im­age. The shape of the main val­ley and its re­mark­ably short side val­leys is some­what un­usu­al when com­pared to the mor­phol­o­gy of most riv­er val­leys on Earth. The cross sec­tion of the val­ley is a strik­ing U-shape, with very steep scarps sev­er­al hun­dred me­tres high at the sides and a flat, broad val­ley floor. Par­tic­u­lar­ly strik­ing are the two im­pact craters that are al­most iden­ti­cal in size, but which have erod­ed in very dif­fer­ent ways. These can be seen at the bot­tom right and up­per left edges of the im­age. Each mea­sures ap­prox­i­mate­ly 20 kilo­me­tres across. Par­tial­ly lob­u­lar ejec­ta blan­kets can be clear­ly seen around both craters. The high­land plain in which Nir­gal Val­lis was cre­at­ed by the out­flow of wa­ter con­sists of so­lid­i­fied, low-vis­cos­i­ty flood basalts.
Oblique perspective view of part of Nirgal Vallis
Oblique per­spec­tive view of part of Nir­gal Val­lis
Bild 2/5, Quelle: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO

Oblique perspective view of part of Nirgal Vallis

Nir­gal Val­lis stretch­es 700 kilo­me­tres across the South­ern High­lands of Mars, cut­ting across an­cient la­va de­posits. This view shows an ap­prox­i­mate­ly 75-kilo­me­tre-long sec­tion in the west­ern part of the up­per val­ley. The U-shaped pro­file of Nir­gal Val­lis and its steep slopes (shown to slight­ly ex­ag­ger­at­ed ef­fect here), which tow­er sev­er­al hun­dred me­tres on both sides of the val­ley, are both very strik­ing. It is al­so no­tice­able that the side val­leys are com­par­a­tive­ly short. They were formed not by the run-off of sur­face wa­ter, but rather by ret­ro­grade ero­sion caused by wa­ter seep­age from a ground­wa­ter hori­zon be­low the up­per rim of the sur­round­ing ter­rain. The da­ta used to cre­ate the im­ages of this re­gion were ac­quired by the High Res­o­lu­tion Stereo Cam­era (HRSC) on 16 Novem­ber 2018 dur­ing Mars Ex­press Or­bit 18,818. The im­age res­o­lu­tion is ap­prox­i­mate­ly 14 me­tres per pix­el. This oblique per­spec­tive view was gen­er­at­ed from a dig­i­tal ter­rain mod­el and da­ta from the nadir and colour chan­nels of HRSC.
Topographic image map showing part of Nirgal Vallis
To­po­graph­ic im­age map show­ing part of Nir­gal Val­lis
Bild 3/5, Quelle: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO

Topographic image map showing part of Nirgal Vallis

Im­age strips ac­quired from dif­fer­ent an­gles by the High Res­o­lu­tion Stereo Cam­era (HRSC) on board Mars Ex­press were used to gen­er­ate dig­i­tal ter­rain mod­els of the Mar­tian sur­face; these con­tain height in­for­ma­tion for each record­ed pix­el. The colour cod­ing of the dig­i­tal ter­rain mod­el (leg­end, top right) in­di­cates the el­e­va­tion dif­fer­ences. In this area, the val­ley depth av­er­ages be­tween 200 and 250 me­tres.
Topographic overview of the highland region to the southeast of Valles Marineris
To­po­graph­ic overview of the high­land re­gion to the south­east of Valles Mariner­is
Bild 4/5, Quelle: MOLA Science Team / FU Berlin

Topographic overview of the highland region to the southeast of Valles Marineris

The High Res­o­lu­tion Stereo Cam­era (HRSC) op­er­at­ed by DLR on board ESA’s Mars Ex­press space­craft im­aged the marked strip dur­ing or­bit 18,818. The land­scapes shown in the im­ages in this set are lo­cat­ed in the small­er rect­an­gle. In to­tal, Nir­gal Val­lis stretch­es 700 kilo­me­tres, run­ning west to east across the south­ern Mar­tian High­lands. It even­tu­al­ly opens in­to the large out­flow chan­nel Uzboi Val­lis, which drained this part of the cen­tral Mar­tian High­lands to­wards the north.
3D view of part of Nirgal Valles on Mars
3D view of part of Nir­gal Valles on Mars
Bild 5/5, Quelle: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO

3D view of part of Nirgal Valles on Mars

Anaglyph im­ages can be gen­er­at­ed from da­ta ac­quired by the nadir chan­nel of HRSC, which is ori­ent­ed ver­ti­cal­ly on­to the sur­face, 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. North is to the right.
  • In November 2018 the HRSC camera system operated by DLR on board ESA’s Mars Express spacecraft imaged a 75-kilometre section of the western upper reaches of Nirgal Vallis.
  • A river once flowed here, draining up to 4800 cubic metres of water into Uzboi Vallis each second – more than twice the volume of water that the Rhine delivers into the North Sea in the same amount of time.
  • The shape of the valley indicates that it may have been formed by seeping groundwater. Melting ground ice is thought to have been the primary source of this groundwater.
  • Focus: Space, planetary research

These images, created using data acquired by the High Resolution Stereo Camera (HRSC) instrument, show the upper reaches of Nirgal Vallis, a river valley on Mars, which extends over 700 kilometres across the Southern Highlands. In Babylonian mythology, Nirgal is the equivalent of Mars, the Roman god of war.

The shape of the main valley and its remarkably short side valleys is somewhat unusual when compared to the morphology of most river valleys on Earth. The cross section of the valley is a striking U-shape, with very steep scarps several hundred metres high at the sides and a broad, flat valley floor. In November 2018, the ESA Mars Express spacecraft imaged an approximately 75-kilometre-long section of the western part of the upper valley using the HRSC, which is operated by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR).

Approximately 400 kilometres further east, Nirgal Vallis flows into the large outflow channel Uzboi Valles, which runs from south to north. Scientists have estimated that the river that once flowed through Nirgal Vallis drained up to 4800 cubic metres of water per second into Uzboi Vallis – more than twice the volume of water that the Rhine delivers into the North Sea in the same amount of time. Many Mars researchers believe that several billion years ago these large, north-flowing masses of water filled the 150-kilometre-wide Holden Crater – which lies to the northeast – with a lake that was up to 250 metres deep.

Nirgal Vallis is the visible remains of this ancient river. However, it is thought that the river did not flow constantly, but sporadically carried large quantities of water. The steep sides and flat bottom of the valley are features that, on Earth, tend to be seen in valleys created by glacial erosion.

Valley morphology has few equivalents on Earth

The short side valleys have the semi-circular or basin-shaped entrances typical of this type of valley. These resemble the circular amphitheatres built by the Romans, and are thus referred to as such. Examples of river valleys of this type can be seen on Earth in the Atacama Desert in Chile, in the Colorado Plateau in the USA and on the islands of Hawaii. In addition to Nirgal Vallis, other well-known examples on Mars include Nanedi Valles.
Nirgal Vallis and its side valleys cut across ancient volcanic plains. Traces of former lava flows can be seen in the ridged furrows (centre of Image 1  – between the forked valley branches – and at the top centre right), which were created when these volcanic deposits cooled and thus shrank.

The flat floor of Nirgal Vallis is largely covered by transverse ripples (visible as small parallel lines), which indicate the prevailing wind direction along the course of the valley. A group of small sand dunes can be observed on the floor of the large crater to the north of Nirgal Vallis (lower-centre-right edge of the image). Unlike the transverse ripples, these dunes consist of old volcanic ash, and appear dark and blue-grey in this image.

Nirgal Vallis is similar in shape to valleys on Earth that have been formed by retrograde erosion due to seeping groundwater. This is also suggested by the lack of dendritic side valleys feeding into the main channel. A linear valley structure of this kind is often a sign that the erosion of the valley was not caused by precipitation and surface run-off.

In laboratory experiments, researchers have been able to demonstrate that valleys such as Nirgal Vallis could have been formed by seeping groundwater. The constant seepage of water just below the surface leads to the undermining of the already steep valley entrances and the eventual collapse of the valley walls. On Mars, melting ground ice is thought to have been the primary source of groundwater.

Image processing

Systematic processing of the data acquired by HRSC was carried out at the DLR Institute of Planetary Research. From these data, staff specialising in planetary science and remote sensing at the Freie Universität Berlin produced the images shown here. The image data were acquired by HRSC on 16 November 2018 during Mars Express Orbit 18,818. The image resolution is approximately 14 metres per pixel. The centre of the image is located at approximately 315 degrees east and 27 degrees south. The colour image was created using data from the nadir channel, the field of view of which is aligned perpendicular to the surface of Mars, and the colour channels of the HRSC. The colour-coded topographic view is based on a Digital Terrain Model (DTM) of the region, from which the topography of the landscape can be derived. The reference body for the HRSC-DTM is a Mars equipotential surface (Areoid). The oblique perspective view was generated from the DTM and data from the nadir and colour channels of HRSC. The anaglyph, which provides a three-dimensional view of the landscape when viewed using red-green or red-blue glasses, was derived from data acquired by the nadir channel and the stereo channels.

More images acquired by the High Resolution Stereo Camera can be found on DLR's Mars Express Flickr gallery.

Visit the Mars Express mission site.

The HRSC experiment on Mars Express

The High Resolution Stereo Camera was developed by the German Aerospace Center (Deutsches Zentrum für 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 52 co-investigators from 34 institutions in 11 countries. The camera is operated by the DLR Institute of Planetary Research in Berlin-Adlershof.

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