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.
One of the most significant features of the Lunae Palus region, Kasei Valles, is one of the largest outflow channels on Mars. Like other outflow channels, it was carved by liquid water, probably during gigantic floods.
Kasei is about 2,400 kilometers (1,500 mi) long. Some sections of Kasei Valles are 300 kilometers (190 mi) wide. It begins in Echus Chasma, near Valles Marineris, and empties into Chryse Planitia, not far from where Viking 1 landed. Sacra Mensa, a large tableland divides Kasei into northern and southern channels.
Scientists suggest it was formed by several episodes of flooding and maybe by some glacial activity.
Map of Lunae Palus with labels. Kasei Valles is a very large ancient river valley.
Scour patterns were produced by flowing water from Maja Valles, which lies just to the left of this mosaic. Detail of flow around Dromore crater is shown on next image. Image is located in Lunae Palus quadrangle and was taken by Viking Orbiter.
Great amounts of water were required to carry out the erosion shown in this Viking image of a small part of Maja Valles. Image is located in Lunae Palus quadrangle.
Nanedi Valles, as seen by THEMIS. Click on image to see more details.
The floors of some channels have features called lineated floor deposits. They are ridged and grooved materials that seem to deflect around obstacles. Scientists believe they are ice-rich. Some glaciers on the Earth show such features. Lineated floor deposits may be related to lobate debris aprons, which have been proven to contain large amounts of ice. Reull Vallis, as pictured below, displays these deposits.
Dao Vallis begins near a large volcano, called Hadriaca Patera, so it is thought to have received water when hot magma melted huge amounts of ice in the frozen ground. The partially circular depressions on the left side of the channel in the image below suggests that groundwater sapping also contributed water.
Hellas quadrangle map showing Dao Vallis in the upper middle of the map.
Wide view of Iberus Vallis, as seen by HiRISE. Imagine taking a walk in these canyons and looking up at the layers.
Ares Vallis, as seen by Viking. The channel is 25 km wide and about 1 km deep. It would be quite spectacular to walk down this valley.
Channels in Ares Vallis region (Oxia Palus quadrangle), as seen by HiRISE.
Tear-drop shaped islands caused by flood waters from Maja Valles, as seen by Viking Orbiter. Image is located in Oxia Palus quadrangle.
Close-up of Padus Vallis, as seen by THEMIS. Padus Vallis is in the Memnonia quadrangle.
Frento Vallis in Noachis quadrangle, as seen by HiRISE. Click on image to see better view of Dust Devil Tracks. In the Arcadia quadrangle on Mars, its exact location is centered at 37 degrees north latitude and 93.1 degrees west longitude. It is 357 km long and was named after a classical river.
Branched channels in Thaumasia quadrangle, as seen by Viking Orbiter. Networks of channels like this are strong evidence for rain on Mars in the past.