The direct view of the crater rim of Hellas shows a landscape with varied structures caused by various geological processes. In the raised rim of the giant crater there were enormous tectonic movements during, and immediately after, the impact, which extend in the form of steps into the interior of the impact basin. Later, material was repeatedly transported into the crater. This can be seen from the valleys, which are reminiscent of river beds, in which ice flows moved downwards and transported and deposited debris and boulders. The lower regions are covered with frost.
Perspective view of the lower lying regions within the crater rim of Hellas, which are covered with frost and ice. In the foreground, a valley makes its way through the landscape, through which water may have flowed from the raised terraces of the crater rim into the interior of the Hellas basin.
Topographical context map of the western rim of the Hellas impact basin
The Hellas impact basin is located in the highlands of the Martian southern hemisphere. The region described in the article is located in the smaller of the two rectangles marked in white. The HRSC (High Resolution Stereo Camera) images were acquired on 6 December 2015 during Mars Express orbit 15,127. The image is centred at 48 degrees East and 45 degrees South. With a diameter of 2200 kilometres, Hellas basin is the largest impact structure on Mars. The altitude difference from the highest mountain peaks in the raised rim of the crater to the lowland plain in the centre of the crater is nine kilometres.
A diameter of 2200 kilometres and a depth of up to nine kilometres: these are the dimensions of the largest impact crater on Mars – Hellas. Only the Moon's South Pole-Aitken Basin and the Valhalla structure on Jupiter's moon, Callisto, have a similar size. The latest images acquired 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 show a part of the western rim of the crater.
In the rim of the Hellas basin, enormous tectonic movements took place during, and immediately after, the impact. The terraces of the rim faults show a spectacular stair-stepped morphology. This impressive form can be seen best in images 1 (vertical view) and 3 (3D image). The low-lying regions within the image are covered by frost and ice: at a latitude of 50 degrees south, the effects of the southern polar winter extending northwards from the South Pole are already visible.
Six-kilometre height difference
The features of the landscape here show traces of flow-like activity, as well as the transportation of material through valleys that extend to the floor of the impact crater, where the material was deposited at the topographically lowest point. The altitude difference in the region shown in the images is about 6.1 kilometres – measured from the highest point in the centre of the left section of the vertical view (image 1) to the lowest point in the bottom left section of the image. Comparable altitude differences within such a short horizontal difference are rare on Earth.
The ice flow of a rock glacier – in which the ice is covered by a large quantity of boulders and rock debris, which has slipped from the adjacent slopes of the mountain onto the glacier – has carved a narrow valley whose end is delimited by steep terrace slopes at the rim of the crater, as can be seen in the centre of the image. Glaciers can carry large quantities of material, which is deposited in the crater. This transportation and depositing mechanism results in a fan-shaped structure in the plain at the front end of the rock glacier.
Sediment transport on the back of rock glaciers
A close look into the channel reveals parallel structures on the surface indicating the flow of the masses. The sediment structures left behind by rock glaciers are known as 'lineated valley fill'. The traces of movements of masses can be found almost everywhere in the region. This can be seen best in the centre left sections of images 1, 3 and 4. Here, a smaller impact crater was filled to the brim so that it overflowed, and surplus material fell downhill. In the immediate vicinity, numerous gullies cover the terraced slopes.
In the north of this region (right of the image) several impact craters can be seen in the lowland plain, some of which have severely eroded rims and depressions that are filled with sediments. Above the centre of the image a fault can be found, which crosscuts an older, filled impact crater. This fault has slightly displaced the rims of the crater relative to each other. The fault runs from south to north and creates a further step in the landscape, which is clearly apparent in the anaglyph image (image 3). This is interesting, because this fault must be more recent than the crater that it cuts through. This gives rise to the assumption of later activity of the terraced fault zone – maybe due to subsequent subsidence of the terraces.
In July 2014, images of the varied landscape in the vicinity of the Hellespontus Montes at the western rim of Hellas were published here. The region in the present images is about 300 kilometres to the southeast and is located somewhat deeper in the impact basin. Many of the morphological features found in this region are similar to those in the Hellespontus Montes.
Image processing
The image data were acquired with the High Resolution Stereo Camera (HRSC) during Mars Express Orbit 15,127 on 6 December 2015. The region is located at 45 degrees south and 48 degrees east. The image resolution is approximately 52 metres per pixel. The colour plan view (image 1) was created using the nadir channel, which is directed vertically down onto the surface of Mars and the colour channels; the oblique perspective view (image 2) was calculated from data acquired by the HRSC stereo channels. The anaglyph (image 3), 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 one stereo channel. The colour-coded plan view (image 4) 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.
