Aurorae Chaos – catastrophic collapse by water masses?

Aurorae Chaos is part of a large-scale system of depressions east of the Valles Marineris canyon. The up to 600 kilometers wide irregular shaped basin has a depth of about four kilometers with respect to the surrounding plains. The random pattern of mesas, buttes and knobs on the basin floor is characteristic of so-called 'chaotic terrain', a landscape special to Mars without an Earth analog. In the old highland region of Margaritifer Terra around the Martian equator, numerous chaotic terrains crosscut the highland plateau.

In the image center, a prominent scarp runs through the scene from northeast to south-west, and connects the southern plateau to the northern low-lying region of Aurorae Chaos. The transition area shows fractures and graben resulting from crustal stretching, which are oriented parallel or inclined to the scarp. In addition, smaller chaos regions can be made out to the south of the escarpment (left in image).

Chaotic terrains visible on the Martian surface today indicate a complex history of transport, storage, and release of large amounts of water in the past. Scientists believe these areas formed by collapse of a surface overlying a reservoir containing ice and sediments, when the ice melted due to, e.g., a volcanic or impact heat source. After drainage of the water masses, broken surface blocks remained in the cavity that previously hosted ice and sediments. Possibly, groundwater as well as magma upwelling could have occurred during collapse in addition to the release of meltwater. For Aurorae Chaos, crater size-frequency distribution measurements yielded a model age of 3.5 billion years for the basin floor, indicating collapse happened long time ago.

There are numerous mineralogical references of the early presence of liquid water in and around chaotic terrains on Mars. Sulphate-containing layers of sediment identified in some of the basins that host chaotic terrains point to evaporative processes. Clay-bearing deposits on the plateau units of Margaritifer Terra, predating the sulfate deposits in the chaotic terrains, probably resulted from fracturing and groundwater outflow. The formation of clay minerals would have required the presence of standing, pH-neutral water.

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