At first glance, it looks like brown crocodile skin photographed at close range, but these images actually show the rough, rugged terrain of Aurorae Chaos. The data used to create the images were acquired by the High Resolution Stereo Camera (HRSC) operated by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) on board ESA’s Mars Express spacecraft, which has been orbiting Mars since 2003.
Aurorae Chaos is one of many areas of 'chaotic terrain' on Mars. Such terrain typically features a random – and thus chaotic – arrangement of table mountains and isolated boulders of various sizes. Aurorae Chaos is located in the highland region of Margaritifer Terra, a few hundred kilometres from the eastern end of the Valles Marineris canyon system. Many of these chaotic areas are found here, near the equator, and at the transition from the southern highlands to the northern lowlands. No geological formations of this type exist on Earth.
There is a morphologically comparable region on Mercury, and some chaotic areas have also been identified on Jupiter's natural satellite Europa, but in both cases, their geological origins are very different to the chaotic areas on Mars. In the case of Europa, the crust is being pulled apart and then compressed again by tidal forces, resulting in complex configurations of broken floes, fissures and ridges. On Mercury, the chaotic areas were caused by seismic movements in the planet's crust following a huge asteroid impact.
A very large chaotic area
Aurorae Chaos is part of an extensive system of fissures, grabens and hills to the northeast of the vast Valles Marineris canyon system. The bottom of the irregularly shaped depression, which measures up to 600 kilometres across, is approximately four kilometres lower than the surrounding plains. The entire chaos region within Margaritifer Terra covers around 1.6 million square kilometres, making it an extremely imposing landscape. Elsewhere on Mars, the chaotic terrains are significantly smaller, barely exceeding the size of an impact crater.
An escarpment runs from northeast to southwest through the centre (images 1, 4 and 5) and connects the high plateau in the south with the lower-lying areas of Aurorae Chaos in the north. The transition zone features fractures and grabens that run parallel with or diagonal to the escarpment. These were caused by tension in the Martian crust due to tectonic forces that stretched and tore apart the brittle crust in this region. In addition, smaller chaos regions can be made out to the south of the escarpment (left in images 1, 4 and 5 ).
Evidence of the complex history of water on Mars
Such chaotic areas bear witness to the complex transport, storage and release of large amounts of water ice and liquid water on Mars in the past.
The theory most widely held by scientists is that chaotic terrain is formed when underground ice reservoirs melt due to heating, suddenly releasing large quantities of water. The heat may have come from nearby volcanoes or been released as the result of major asteroid impacts. Once the water has drained away, the surface collapses into the newly-formed cavities.
Scientists have used crater size and frequency measurements to estimate the age of the bottom of the huge Aurorae Chaos depression at 3.5 billion years. The collapse process occurred a very long time ago. Groundwater and magma may also have escaped during the collapse, in addition to meltwater.
There are numerous mineralogical indications of the early presence of water, which would have flowed in and around Mars' chaotic terrains. Sulphate-containing layers of sediment that have been identified in some of the basins with chaotic structures indicate the formation and accumulation of this mineral due to the evaporation of relatively acidic water. Clay mineral deposits on the plateau areas of Margaritifer Terra, which predate the sulphate-containing deposits, may be related to fissures and groundwater run-off. The formation of clay minerals would have required the presence of standing, pH-neutral water.
