Collapsed and disrupted ("chaotic") terrain within the planet's older, cratered highlands.
The Source Region of the Outflow Channel Ravi Vallis (1°S,42°W).
This mosaic of the head of Ravi Vallis shows a 300-kilometer-long portion of the channel. Like many other channels that empty into the northern plains of Mars, Ravi Vallis originates in a region of collapsed and disrupted ("chaotic") terrain within the planet's older, cratered highlands. Structures in these channels (slides #25 and #26) indicate that the channels were carved by liquid water moving at high flow rates. The abrupt beginning of the channel in this image, with no apparent tributaries, suggests that the water that carved the channel was released under great pressure from beneath a confining layer of frozen ground. As this water was released and flowed away, the overlying surface collapsed, producing the disruption and subsidence shown here. Three such regions of chaotic collapsed material are seen in this image, connected by a channel whose floor was scoured by the flowing water. The flow in this channel was from west to east (left to right). This channel ultimately links up with a system of channels that flowed northward into Chryse Basin. (From Mars Digital Image Map, image processing by Brian Fessler, Lunar and Planetary Institute.)
Outflow Channels Emptying into Northern Plains of Chryse Planitia (20°N,55°W).
The channel system in the upper half of the image is Maja Vallis, which averages 10 kilometers wide, 1 kilometer deep, and extends for about 180 kilometers. The channels appear to have been carved by flowing water released from Juventae Chasma (2°S,61°W), a 250 x 100-kilometer chaotic depression that is located several hundred kilometers to the south. The channel system in the lower half of the image is Vedra Vallis. Both channel systems start in and cut into the old cratered surface of Lunae Planum and discharge into the younger plains of Chryse Planitia. The area shown is approximately 150 kilometers across. (Viking Orbiter mosaic 211-5190.)
