One of the most striking features of a global view of Mars is the dichotomy between the topographically diverse highlands in the southern hemisphere and the flat, expansive lowland plains of its northern counterpart. Many landscape features shaped by erosion can be observed in the transitional zone between the highlands and lowlands, which explains the particular geological interest in this region. The images presented here show mesas in Aeolis Mensae alongside formations carved by intense wind erosion as the terrain transitions gradually into the lowlands.
The image data was acquired by the High Resolution Camera System (HRSC), developed and operated by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) on board the European Mars Express spacecraft. The region is situated roughly 400 kilometres east of Gale Crater, where NASA’s Mars Science Laboratory touched down in 2012 carrying the rover Curiosity. The image resolution is approximately 15 metres per pixel.
Wind and water erode the landscape
Numerous fracture zones known as tectonic grabens cross the region. Expansion of the Martian crust produced a large number of freestanding mesas (sometimes called witness buttes), the plateaus of which indicate that the highlands of Mars once extended further to the north. A large expansion fracture extends diagonally through the image in the south of this scene, on the left-hand side of the image (north is on the right in these images). The large mesa in the centre of the image, which measures approximately 40 by 40 kilometres and rises some 2500 metres above the surrounding terrain, is a remnant of these expansion processes. Its top is at the same elevation as the adjacent Hesperia Planum highland region further to the south.
Wind, and possibly also water and ice, have eroded the landslides and weathered the rock over millions of years, so the mesas become smaller and will disappear in the distant future. In the north, in the right-hand section of images 1, 3 and 4 it becomes apparent that wind is the ‘shaping force’ on Mars; the landscape there shows a characteristic pattern of small ribs, referred to as yardangs. These are produced when the wind blows from a prevailing direction over a long period of time, carrying sand and particles of dust that scour ‘wind gullies’ into the rocky surface. The word yardang is taken from the Uyghur language, which describes steep walls of sand and wind gullies in the desert around Lop Nor.
Most of the yardangs run in a southeasterly to northwesterly direction, which is indicative of the prevailing winds in this region. A small, steep-sided rib in the terrain, extending for around 7.5 kilometres perpendicular to the main direction of the yardangs, is an eye-catching feature of this area. Quite evidently, the rib is made of harder and far more resistant rock that allows it to withstand the erosive power of the wind. It is possible that a minor expansion fracture formed transversely to the principal tectonic orientation and was then filled with molten volcanic rock that became harder than the surrounding material after solidification.
At Christmas, Mars Express will have spent 13 years in orbit
In just a few days, Mars Express will celebrate the thirteenth anniversary of its arrival at Mars. When the ESA spacecraft entered orbit around the Red Planet on Christmas Day 2002, its mission was designed to last just two Earth years (or one Martian year). But the extensive scientific yield prompted its extension on multiple occasions, most recently to the end of 2018. ESA’s first planetary mission is therefore also the longest to date. HRSC has mapped approximately 90 percent of Mars’ surface – equivalent to 150 million square kilometres, or approximately the area of all Earth’s continents combined – with high-resolution stereo images and in colour. Image data representing around two thirds of the surface have already been rendered to produce a digital terrain model with a precision of 40 metres per pixel or better. The objective of this camera experiment is to compute a digital terrain model covering the entire surface of Mars.