Vallis orvalles/ˈvælɪs/ (pluralvalles/ˈvæliːz/)^[1] is theLatin word forvalley. It is used inplanetary geology to namelandform features on other planets.

Scientists usedvallis for old river valleys they discovered when they sent the first probes toMars. The Viking Orbiters caused a revolution in our ideas aboutwater 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.^[2][3][4] 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 orchaotic terrain. It has been suggested that massive amounts of water were trapped under pressure beneath a thickcryosphere (layer of frozen ground), then the water was suddenly released, perhaps when the cryosphere was broken by a fault.^[5][6]

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 issapping.^[7] 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 theCoprates quadrangle of Nigal Vallis taken withMars Odyssey'sTHEMIS.

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  Nirgal Vallis that runs in two quadrangles has features looking like those caused bysapping. Picture taken withTHEMIS.
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  Nirgal Vallis, as seen by THEMIS.
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  Nirgal Vallis Close-up, as seen by 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 inEchus Chasma, nearValles Marineris, and empties intoChryse Planitia, not far from whereViking 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.^[8]

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  Map of Lunae Palus with labels.Kasei Valles is a very large ancient river valley.
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  Area around northern Kasei Valles, showing relationships amongKasei Valles,Bahram Vallis,Vedra Valles,Maumee Valles, andMaja Valles. Map location is inLunae Palus quadrangle and includes parts of Lunae Planum andChryse Planitia.
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  Kasei Valles, as seen byTHEMIS.

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  Hypanis Vallis in theLunae Palus quadrangle of Mars, as seen byHiRISE. Scale bar is 500 meters long.
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  Streamlined islands seen by Viking showed that large floods occurred on Mars. Image is of a small part ofMaja Valles and is located inLunae Palus quadrangle.
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  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 inLunae Palus quadrangle and was taken by Viking Orbiter.
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  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 inLunae Palus quadrangle.
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  Vedra Valles,Maumee Valles, and Maja Valles flow from Lunae Planum on the left toChryse Planitia on the right. Image is located inLunae Palus quadrangle and was taken by Viking Orbiter.
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  Bahram Vallis, as seen by Viking. Valley is located in Northern Lunae Planum and theLunae Palus quadrangle. It lies nearly midway betweenVedra Valles and lowerKasei Valles
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  Bahram Vallis in theLunae Palus quadrangle of Mars, as seen byHiRISE. Rotational landslides (slumps) are visible at the base of north wall.
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  Maja Valles streamlined island in theLunae Palus quadrangle of Mars, as seen byHiRISE. Island formed behind the impact crater at the lower right.
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  Nanedi Valles, as seen by THEMIS. Click on image to see more details.
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  Nanedi Valles close-up, as seen by THEMIS. Click on image to see more details.

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  Auqakuh Vallis. At one time a dark layer covered the whole area, now only a few pieces remain asbuttes. Image was taken withTHEMIS.
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  Huo Hsing Vallis in Syrtis Major, as seen byTHEMIS. Straight ridges may bedikes in which liquid rock once flowed.
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  Huo Hsing Vallis Ridges, as seen byHiRISE. Ridges may be caused by water moving along faults.
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  Arnus Vallis layers, as seen byHiRISE.

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  Mad Vallis, as seen byHiRISE. Picture on right is an enlargement of part of the other picture.

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 tolobate debris aprons, which have been proven to contain large amounts of ice. Reull Vallis, as pictured below, displays these deposits.^[9]

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  Drainage features inReull Vallis, as seen byTHEMIS. Click on image to see relationship of Reull Vallis to other features.
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  Reull Vallis with lineated floor deposits, as seen byTHEMIS. Click on image to see relationship to other features.
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  Layers inReull Vallis, as seen byTHEMIS.
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  Niger Vallis with features typical of this latitude, as seen byHiRISE. Chevon pattern results from movement of ice-rich material. Click on image to see chevron pattern and mantle.

Dao Vallis begins near a large volcano, called Hadriaca Patera, so it is thought to have received water when hotmagma 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.^[10]

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  Hellas quadrangle map showing Dao Vallis in the upper middle of the map.
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  Dao Vallis, as seen byTHEMIS. Click on image to see relationship of Dao Vallis to other nearby features.

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  Wide view ofIberus Vallis, as seen by HiRISE. Imagine taking a walk in these canyons and looking up at the layers.
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  Detail from the center of the previous image, as seen by HiRISE.
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  Patapsco Vallis, as seen byHiRISE. Patapsco Vallis is a valley in theElysium quadrangle of Mars. Scale bar is 1000 meters long.
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  Lethe Vallis, as seen byHiRISE. Flow was from southwest to northeast. Wider part of Lethe Vallis had less erosive power, so mesas are left behind from pre-existing material. Lethe Vallis is a valley in theElysium quadrangle. Scale bar is 500 meters long.
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  Athabasca Valles, showing source of its water, Cerberus Fossae. Note streamlined islands that show direction of flow to south. Athabasca Valles is in theElysium quadrangle.
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  Athabasca Valles Streamlined Form, as seen byHiRISE. Athabasca Valles is in theElysium quadrangle.
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  Ituxi Vallis, as seen byTHEMIS. Ituxi Vallis is a lava channel that lies east ofElysium Mons.

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  Close-up ofSimud Valles, as seen by HiRISE. Simud Valles is found in theOxia Palus quadrangle.
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  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.
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  Ares Vallis, as seen by HiRISE. Ares Valles is found in theOxia Palus quadrangle.
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  Channels in Ares Vallis region (Oxia Palus quadrangle), as seen by HiRISE.
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  Tear-drop shaped islands caused by flood waters from Maja Valles, as seen by Viking Orbiter. Image is located inOxia Palus quadrangle.

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  Samara Vallis in theMemnonia quadrangle, as seen byHiRISE. Scale bar is 500 meters long.
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  Padus Vallis, as seen by THEMIS. Padus Vallis is in theMemnonia quadrangle.
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  Close-up of Padus Vallis, as seen by THEMIS. Padus Vallis is in theMemnonia quadrangle.

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  Frento Vallis inNoachis quadrangle, as seen byHiRISE. Click on image to see better view ofDust Devil Tracks. In theArcadia 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.^[11]
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  Enipeus Vallis in theArcadia quadrangle, as seen byHiRISE. Scale bar is 500 meters long.
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  Scamander Vallis, as seen byHiRISE. Click on image to seedark slope streaks in a variety of shades. The darker the streak the younger it is. is an ancient river valley in theAmenthes quadrangle. Scale bar is 500 meters long.
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  Her Desher Vallis, as seen by HiRISE. Her Desher Vallis is found in theCoprates quadrangle.
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  Her Desher Vallis close-up, as seen by HiRISE. Her Desher Vallis is found in the Coprates quadrangle.
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  Branched channels inThaumasia quadrangle, as seen by Viking Orbiter. Networks of channels like this are strong evidence for rain on Mars in the past.
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  Ravi Vallis, as seen by Viking Orbiter. Ravi Vallis was probably formed when catastrophic floods came out of the ground to the right (chaotic terrain). Image located inMargaritifer Sinus quadrangle.
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  Small meandering channel in the stream bed of a larger channel. Water probably eroded the two channels at different times. Image from HiRISE under theHiWish program. Image located inMare Tyrrhenum quadrangle.

• List of valles on Mars – Valley-like topographical features
• List of geological features on Mercury
• Outflow channels – Long, wide swathes of scoured ground on Mars
• Peace Vallis – Martian valley
• Valley network (Mars) – Branching networks of valleys on Mars
• Vallis Alpes – Lunar valley

• Planetary geology
• Valleys and canyons on Mars

• Articles with short description
• Short description is different from Wikidata