Coprates Quadrangle - Nirgal Vallis and Sapping

Nirgal Vallis and Sapping

Nirgal Vallis is one of the longest valley network on Mars. It is so large that it is found on more than one quadrangle. Scientists do not know how all the ancient river valleys were formed. There is evidence that instead of rain or snow, the water that formed the valleys originated under ground. One mechanism that has been advanced is sapping. In sapping, the ground just gives away as water comes out. Sapping is common in some desert areas in America's Southwest. Sapping forms alcoves and stubby tributaries. These features are visible in the picture below of Nigal Vallis taken with Mars Odyssey's THEMIS.

• Nirgal Vallis that runs in two quadrangles has features looking like those caused by sapping. Picture taken with THEMIS.