When NASA's Mariner 9 spacecraft reached Mars more than 50 years ago at the end of 1971 and began to explore the planet from orbit, there was initially great disappointment at the control centre: virtually nothing could be seen in the images sent to Earth. At that time, a global dust storm was raging on Mars, which made it impossible to see the surface. Only the peaks of the highest volcanoes stood out from the monotonous grey. In early 1972, the weather improved, the dust settled and the mission began to take a global survey of Mars.

One of the most impressive structures observed was a rift valley up to 10 kilometres deep and roughly 3800 kilometres long - as long as the distance from the United States' east to west coast or from North Cape to Sicily. In honour of the mission, the extensive system of this structure, torn open by tectonic forces, was named Valles Marineris - 'the valleys of Mariner'. The images shown here, acquired using the DLR-developed High Resolution Stereo Camera (HRSC) on board ESA's Mars Express spacecraft, show sections of two elongated valleys in the west of Valles Marineris: Ius Chasma and Tithonium Chasma.

Tithonium Chasma measures approximately 800 kilometres from east to west and Ius Chasma 840 kilometres. To the west, the rugged region of Noctis Labyrinthus adjoins the two valleys, and to the east, the central depressions of Melas and Candor Chasma, which open further to the north. The term chasma has been designated by the International Astronomical Union to describe an elongated depression with steep slopes on the sides.

In the images presented here, the greatest differences in elevation from the plateau to the lowest regions within the chasmata reach more than 7000 metres. The plan view colour image shows Ius Chasma on the left (south) and Tithonium Chasma on the right (north). The plateau regions around and between the chasmata clearly show the linear, large fractures that were involved in the formation of Valles Marineris.

Dunes of black volcanic sand
There are clear differences between the two valleys. For example, the bottom of Tithonium Chasma is much darker. On closer inspection, dark dunes can be seen at the top of the image (northwest, roughly in the middle of the gorge), and the surrounding areas are also covered by a thin layer of dark sands(shown in bluish in these contrast-enhanced images). Due to the proximity of the Tharsis volcanic region to the west, many of the rocks present in this area consist of layers of lava flows and volcanic ash, which may be the source of the dark coloured sands that make up the dunes. Mineralogical studies of the dune material confirmed their volcanic origin.

Another very interesting feature is the two lighter-coloured mountains - one directly opposite the dark dune field and cut from the edge of the picture and the other in the middle of this part of Tithonium Chasma. They are over 3000 metres high and their surfaces have been heavily eroded by the wind. These 'erosion channels' are called yardangs and indicate that the material forming these dome-shaped hills is less resistant to erosion compared to the surrounding rocks, and so the wind has been able to carve these landforms out of the rock with the sand and dust particles it carries.

These furrows even reveal that the wind direction of this sand blast was from north-east to south-west (bottom right to top centre in the picture). The soil between the two lighter deposits shows peculiar small nodules, probably made of the same material as the mounds. Spectroscopic studies indicate high concentrations of hydrous sulphate minerals in these layers. Many scientists believe that these deposits were formed by evaporation at a time when the chasmata were filled with water. However, this formation scenario is still the subject of intense debate in the scientific community.

Landslides of enormous magnitude
In the north-east of this wind-swept, light-coloured mountain, enormous deposits of landslides can be seen that had detached themselves from the steep slopes. The larger one is from the break-off of the canyon wall to the northeast (below the hill) and looks relatively young as it has divergent striations and apron-like lobes at its outlet.

It is also overlain by smaller landslides in the lower part of the image. This is best seen in the colour-coded digital terrain model. Another large landslide is located to the south (left) of the central hill. However, this looks much more eroded and may therefore be older. Landslides also occur in topographically higher regions, as shown by the 'small' 15-kilometre-wide deposit near the breached crater rim in the central part of the image.

The valley floor in Ius Chasma is just as spectacular. In the undulating and rutted surface, large boulders tilted backwards seem to trace the direction of movement of landslides from the slopes towards the centre of the rift valley. Interestingly, the southern (left) flank of Ius Chasma shows several parallel scarps in a similar east-west orientation as the chasmata and faults themselves. Thus, the north-south orientation of the extensional tectonics that caused the rift to break open is documented in the deposits on the Chasma floor.

Image processing
These HRSC images were taken on 21 April 2022 during orbit 23,123 of ESA's Mars Express spacecraft around Mars. The image resolution is 25 metres per pixel. The centre of the image is at approximately 272 degrees east and 6 degrees south. The colour view (image 1) was created from the nadir channel directed perpendicular to the surface of Mars and the colour channels of the HRSC, the perspective oblique views (images 2, 4) were calculated from the stereo channels of the HRSC. The anaglyph image (image 5), which gives a three-dimensional impression of the landscape when viewed with red-blue or red-green glasses, was derived from the nadir channel and a stereo channel.

The top view encoded in rainbow colours (Figure 6) 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 an equipotential surface of Mars (areoid). The systematic processing of the camera data was carried out at the DLR Institute of Planetary Research. Staff from the Department of Planetary Sciences and Remote Sensing at the Freie Universitat Berlin used the data to create the image products shown here.

Related Links
High Resolution Stereo Camera at DLR
Mars News and Information at MarsDaily.com
Lunar Dreams and more

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