Catastrophic flood events left their mark

Located south of Eos Chasma, one of the eastern branches of the gigantic Valles Marineris canyon system, the Pyrrhae Regio stretches on the cratered, highland plateaus. The image presented here intriguingly displays the birth of a chaotic terrain on the northern (right in the color image) edge of the plan view.

Chaotic terrains are formed by collapse of the surface overlying (see also the scheme of origin) an ice and sediment containing reservoir. The ice melts for example triggered by lava flows, by volcanic intrusions at greater depth or by impacts. The water drains out of the sediments, often in a catastrophic and relatively quick and short-lived flood event. The small chaos shown here also possesses flood channels outside this observation in northwestern direction, visible on the context map. Research of the collapse structures in Eos Chasma and of Pyrrhae Regio shows, that these flood channels, named the Osuga Valles, hosted at least two flood events in opposite directions. First, the water drained from the Pyrrhae Regio towards Eos Chasma and later in the opposite direction. After the drainage, broken blocks remain in the cavity that hosted the ice. Presumably, groundwater upwelling also occurred during the collapse in addition to the meltwater release. The intruding groundwater then leads to slumps and breakoff at the rims and in consequence to an enlargement of the chasm. This chipping is very well visible at the rims of this small chaotic terrain. The height difference within this image is more than 4 km! The amount of material transported out of the chasm is enormous.

Sapping valleys

On the southern part (left side in the color image), sit 2 large and one smaller crater, the latter about 20 km in diameter, on a relatively undisturbed looking surface. The largest crater shows some fractures at its floor that could result from quick cooling of extruded lava after the impact. In the region between the impact craters and the chasm, two 2 km broad valleys can be spotted. These valleys, especially the upper one, show similarities to sapping valleys that are created by groundwater seepage. They may represent a further stage in the expansion of the chaotic terrain.

A little mythology at the end

The region was named after Pyrrha, the daughter of Pandora and wife of Deukalion. In Greek mythology Deukalion has the same role as Noah. He was asked by his father Prometheus to build an ark to save himself and his wife from the great flood that Zeus had summoned to destroy mankind. Pyrrha and Deukalion were the only ones who survived the catastrophe and created mankind anew by throwing stones over their shoulders, from which humans were then created.

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