Colour view of the conspicuous crater north of the Hellas basin
The direct view of the crater reveals the multiple layers of material that was ejected during impact. The ‘fluid’ appearance as well as the southward river valleys suggest that, at the time of impact, groundwater was present near the surface. North is to the right in the image.
So-called anaglyph images can be created from the nadir channel of the HRSC camera system operated by DLR on board the ESA Mars Express spacecraft, which is directed vertically onto the surface of Mars, and the oblique view from one of the four stereo channels. When using red-blue or red-green glasses, one can see a realistic, three-dimensional view of the landscape.
In this view, the rampart of the large crater in the centre of the image is particularly prominent.
False colour image of the topography of the crater
Digital terrain models of the Martian surface can be generated with an accuracy of up to 10 metres per pixel from the nadir channel 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 Areoid (derived from Ares, the Greek word for Mars), are well depicted. These elevation values can be read based on the colour scale at the top right of the image. The colour differences clearly show the height difference between the ramparts and the crater floor.
The ‘fluid’ appearance as well as the southward river valleys suggest that, at the time of impact, groundwater was present near the surface.
Focus: Space exploration
Over billions of years, meteorite impacts have altered the surface of Mars. Current images from the High Resolution Stereo Camera (HRSC) operated by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) on board the ESA Mars Express spacecraft show an impact crater over 30 kilometres in size with a prominent ejecta blanket. It is located to the north of the largest impact basin on Mars, Hellas Planitia, where some scientists believe there was once a large lake.
When a meteorite impacts, material from below the surface is uncovered. The ejected rock masses settle around the crater mostly radially in what is known as an ejecta blanket. Depending on the size and speed of the impact, this material can originate from different depths and settle kilometres away from the crater. The properties of these deposits depend on the size of the crater, the composition of the substrate and the presence of an atmosphere. The deposits may consist of finely dispersed, disconnected loose material, for example, or a coherent mass, which then produces a more compact ejecta blanket. Scientists can therefore learn about the composition of the subsurface of Mars through analysis of the different ejecta blankets.
Many impact craters on Mars exhibit such lobe-shaped ejecta blankets as can be seen in this crater in the centre of images 1, 2, 3 and 5. They have clear, wall-shaped rims called ramparts, meaning steep embankment or wall. In this rampart crater 32 kilometres in size, up to three layers of lobe-shaped ejecta blanket can be seen. They run radially from the crater and have tongue-shaped offshoots, some of which end in a steep wall. If a crater has such a lobe-shaped ejecta blanket, it indicates the presence of water or ice in the subsurface at the time of impact. This melting subsurface ice or water is mixed with the released material to form a free-flowing mass that is ejected from the point of impact, thus forming the typical lobe shape. The ejecta blanket obtains its flow structure from this mixture of water and rock. By determining the age of the ejecta blanket, the time of the meteorite impact can be calculated, which gives an indication of when water or ice was present in the Martian soil.
Indications of a prehistoric lake
The impact crater with the prominent ejecta blanket is located north of the extensive impact basin Hellas Planitia, in a region that may have once belonged to the hollow of a lake within the basin in Mars' early history. Some scientists believe old coastlines can be retraced that appear to show the former bank line of the lake. However, conclusive evidence has yet to be found. Nonetheless, a number of indications of glacial processes can be seen in and around Hellas, regarding ice in the ground and glacial deposits. Old dried up river valleys can also be seen to the south of the ejecta blanket on closer inspection (in the images to the left and further) and have been cut off and covered by the blanket. All of these surface features substantiate the assumption that at the time of impact of the small crater described here, ice or groundwater was present near the surface, which led to the ‘molten’ appearance of its ejecta blanket.
Image processing
The images were acquired with the HRSC (High Resolution Stereo Camera) during Mars Express Orbit 16,890 on 3 May 2017. The image resolution is 21 metres per picture element (pixel). The centre of the image is located at approximately 70 degrees east and 22 degrees south. The colour plan view was created from data acquired by the nadir channel, which is oriented perpendicular to the surface of Mars and the HRSC colour channels; the perspective view was computed from data acquired by the HRSC stereo channels. The anaglyph image, which provides a three-dimensional impression of the landscape when viewed through red and blue or red and green glasses, was derived from data acquired by the nadir channel and one stereo channel. The colour-coded overview 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 sphere. Staff from the Planetary Sciences and Remote Sensing Department at Freie Universität Berlin produced the images shown here. Systematic processing of the data was undertaken at the DLR Institute of Planetary Research in Berlin-Adlershof.
The HRSC experiment
The High Resolution Stereo Camera was developed at 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 over 40 co-investigators from 33 institutions and 10 countries. The camera is operated by the DLR Institute of Planetary Research in Berlin-Adlershof. The camera has been delivering images of the Red Planet since 2004.
