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Stop 6 at Hellas

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Heads of canyon systems Dao, Niger and Harmakhis Vallis, Mars
Heads of canyon systems Dao, Niger and Harmakhis Vallis, Mars
Image: Portion of HRSC nadir image H0528_0000_ND4
Location: Southern highlands northeast of the Hellas impact structure
Scale: Width of the image is approximately 160 kilometers

Dao (and its tributary, Niger Vallis; ~1200 km long) and Harmakhis Vallis (~800 km long) are a pair of canyons that extend through the cratered highlands in the eastern Hellas region toward Hellas Planitia. These features have been classified in the past as outflow channels.

What is an outflow channel? Outflow channels are very large scale erosional features that are formed by the sudden release of enormous volumes of water. Aside from carving a deep channel, this type of catastrophic event leaves behind features such as fluvial mega- ripples (similar to ripples on a beach from moving water, but hundreds of time bigger), streamlined islands (seen on Earth too), and scour depressions in its path. A prime example of this type of channel on Earth are the channeled scablands in eastern Washington state where it is thought that ancient glacial meltwater ponded behind an ice dam was suddenly released, sending hundreds of cubic kilometers of water from western Montana into eastern Washington.


Satellite image of eastern Washington showing flood-carved channels (dark braided areas)
Satellite image of eastern Washington showing flood-carved channels (dark braided areas)

Research on the Hellas region by PSI planetary scientists David Crown and Leslie Bleamaster suggests that the canyons were formed not by catastrophic floods, but rather by collapse of plateau materials. This interpretation is based on the evidence that the canyon floors do not have scour marks, mega-ripples, or the other features typically associated with a fluvial-eroded channel. Instead, they are characterized by steep-walled depressions along the adjacent sides, zones of subsided plains, fractures parallel to the canyon margins, pit chains, and collapsed debris masses (see the images below). These features suggest that growth of the canyons has been dominated by collapse and sapping processes.

Fractures and collapse blocks along the margins of Dao Vallis
Fractures and collapse blocks along the margins of Dao Vallis
Image: Portion of THEMIS daytime IR image I01870002
Location: Along the middle reaches of Dao Vallis
Scale: Image width is approximately 32.4 kilometers

Pit chains and collapsed plains in and around Niger Vallis
Pit chains and collapsed plains in and around Niger Vallis
Image: Portion of THEMIS daytime IR image I01433002
Location: Along the upper reaches of Niger Vallis
Scale: Image width is approximately 32.4 kilometers

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