Vallis (planetary geology)

Vallis (plural valles) is the Latin word for valley. It is used in planetary geology to name landform features on other planets.

Scientists used 'vallis for old river valleys they discovered when they sent the first probes to Mars. The Viking Orbiters caused a revolution in our ideas about water on Mars; finding huge river valleys in many areas. Space craft cameras showed that floods of water broke through dams, carved deep valleys, eroded grooves into bedrock, and traveled thousands of kilometers. Some valles on Mars (Mangala Vallis, Athabasca Vallis, Granicus Vallis, and Tinjar Valles) clearly begin at graben. On the other hand, some of the large outflow channels begin in rubble-filled low areas called chaos or chaotic terrain. It has been suggested that massive amounts of water were trapped under pressure beneath a thick cryosphere (layer of frozen ground), then the water was suddenly released, perhaps when the cryosphere was broken by a fault.

Nirgal Vallis is one of the longest valley networks on Mars. It is so large that it is found on more than one quadrangle. Scientists are not sure about how all the ancient river valleys were formed. There is evidence that instead of rain or snow, the water that formed the valleys originated underground. 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 from the Coprates quadrangle 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.

Nirgal Vallis, as seen by THEMIS.

Nirgal Vallis Close-up, as seen by THEMIS.

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