{{Short description|Vallis on Mars}} {{Infobox feature on celestial object |name = Enipeus Vallis |image = [[File:Enipeus Vallis IR.jpg|280px]] |caption = Enipeus Vallis, as seen in [[Thermal Emission Imaging System|THEMIS]] daytime [[infrared|IR]] mosaic. |type = [[Valley networks (Mars)|valley network]]<ref name=Carr1>Carr, M.H. (1995). The Martian Drainage System and the Origin of Valley Networks and Fretted Channels. ''J. Geophys. Res.,'' '''100'''(E4), p. 7491, Fig. 9a.</ref> |location = [[Arcadia quadrangle]] |coordinates = {{coord|36.68|N|266.9|E|globe:mars_type:landmark|display=inline,title}} }} '''Enipeus Vallis''' is a valley in the northern hemisphere of the planet [[Mars]]. It is centered at lat. 37°N, long. 267°E in the [[Arcadia quadrangle]] (MC-3) between the large volcano [[Alba Mons]] and the [[Tempe Terra]] plateau. The valley follows a gently [[sinuous]], north–south path for a distance of about 357 km (222 mi).<ref name=GPN>USGS Gazetteer of Planetary Nomenclature. https://planetarynames.wr.usgs.gov/Feature/1803.</ref> It is likely an ancient [[watercourse]] that formed during the early [[Hesperian]] (or late [[Noachian]]) period,<ref name =Moore01>Moore, H.J. (2001). Geologic Map of the Tempe-Mareotis Region of Mars. USGS Geologic Investigations Series I-2727. https://geopubs.wr.usgs.gov/i-map/i2727/.</ref> around 3.7 billion years ago.<ref>Hartmann, W.K. (2005). Martian Cratering 8: Isochron Refinement and the Chronology of Mars. ''Icarus,'' '''174''', p. 317, Tbl. 3. {{doi|10.1016/j.icarus.2004.11.023}}.</ref>
The valley is named after a river in [[Thessaly]], [[Greece]]. [[Enipeus (mythology)|Enipeus]] is also the name of a river god in [[classical mythology]].<ref>Simpson, D.P. (1968). ''Cassell's New Latin Dictionary;'' Funk & Wagnalls: New York, p. 215.</ref> The [[International Astronomical Union]] (IAU) formally adopted the name Enipeus Vallis in 1991.<ref name=GPN /> ''Vallis'' is the Latin word for valley.<ref>USGS Gazetteer of Planetary Nomenclature. Descriptor Terms. https://planetarynames.wr.usgs.gov/DescriptorTerms.</ref>
==General description== Enipeus Vallis is mapped as a [[valley networks (Mars)|valley network]].<ref name=Carr1 /><ref>Hynek, B.M.; Beach, M.; Hoke, M.R.T. (2010). "Updated Global Map of Martian Valley Networks and Implications for Climate and Hydrologic Processes". ''J. Geophys. Res.,'' '''115''', E09008, {{doi|10.1029/2009JE003548}}. Cited in Andrews‐Hanna, J. C.; Lewis, K.W. (2011). "Early Mars Hydrology: 2. Hydrological Evolution in the Noachian and Hesperian Epochs". ''J. Geophys. Res.,'' '''116''', E02007, Fig. 1. {{doi|10.1029/2010JE003709}}.</ref><ref>Carr, M. H., (2002). "Elevations of Water-Worn Features on Mars: Implications for Circulation of Groundwater". ''J. Geophys. Res.'', '''107''' (E12), p. 14–15, Fig. 3, {{doi|10.1029/2002JE001845}}.</ref> Valley networks are branching systems of valleys on Mars that superficially resemble terrestrial river [[drainage basin]]s. They are abundant in the equatorial and southern highlands of the planet but less common in the northern hemisphere.<ref>Carr, M.H.; Clow, G.D. (1981). "Martian Channels and Valleys: Their Characteristics, Distribution, and Age". ''Icarus,'' '''48''', p. 93.</ref> Most valley networks are thought to have formed by flowing water, although the primary source of the water (melting ice, rainfall, springs) is still debated.<ref name=Carr06>Carr, M.H. (2006). ''The Surface of Mars''; Cambridge University Press: Cambridge, UK, p. 113. {{ISBN|978-0-521-87201-0}}.</ref>
Enipeus Vallis is a single trunk valley,<ref>Boyce, J.M. (2008). ''The Smithsonian Book of Mars;'' Konecky & Konecky: Old Saybrook, CT, p. 163. {{ISBN|1-56852-714-4}}.</ref> with no large tributaries. The valley is widest (about {{cvt|10|km|mi|disp=or}}) at its southernmost reach near Lat. 33.6°N and rapidly tapers northward, maintaining a regular width of {{cvt|3|to|5|km}} throughout most of its course.<ref>JMARS distance tool.</ref> Above lat. 39.7°N the valley opens up, and drainage features become poorly integrated and ill-defined. North of this area, a somewhat subdued valley segment continues to the northwest from 40.5°N to about 42.5°N (northwest of the crater Lowbury) where it is locally buried by impact crater ejecta and young ([[Amazonian (Mars)|Amazonian]]-aged) plains material.<ref name=Moore01 /> This northern valley segment gradually loses its identity amid the narrow [[graben]]s of [[Tantalus Fossae]].
The elevation of the valley floor ranges from {{cvt|885|m}} above [[datum (geodesy)|datum]] ([[Areoid|Mars "sea" level]]) in the south to {{cvt|100|m}} or less in the north, where the northern segment of the valley begins to lose its definition. In cross-sectional profile, the valley is [[U-shaped valley|U-shaped]] to rectangular, with depths ranging from {{cvt|20|to|50|m}}.<ref name=JMOLA>JMARS gridded [[Mars Orbital Laser Altimeter|MOLA]] elevation dataset.</ref> In the south, Enipeus Vallis has [[levee]]-like ridges along its edges that are up to {{cvt|20|m}} high.<ref name=JMOLA /> The levees are locally breached by lava flows from the surrounding plains.<ref name=Moore01 />
In places, inner channels are visible along the valley floor. At around 36°N, the channels form an [[anastomosing]] (braided) pattern. Near the middle section of the southern valley segment, the valley divides into several branches, which then rejoin to form diamond-shaped islands.<ref name=Moore01 /> Faint longitudinal [[striation (geology)|striations]] and streamlined erosional [[bedform]]s are common all along the floor of the valley. (See Photo Gallery.)
In [[fluvial]] [[geomorphology]], a distinction is made between the terms [[river valley|stream valley]] and [[stream channel]]. Stream valleys are wide, elongate depressions. The stream itself flows within the confines of a channel in the center of the valley. A stream valley usually contains many channels. Channels are always much narrower and shallower than the valley that contains them, and water in the stream channel never comes close to filling the entire valley.<ref name=Carr96>Carr, M.H. (1996). ''Water on Mars;'' Oxford, p. 47. {{ISBN|0-19-509938-9}}.</ref>
The distinction is important when discussing features of probable fluvial origin on Mars. Although Enipeus Vallis is described here as a valley, it has many features typical of a channel, including fluvial bedforms, streamlined features, a U-shaped profile,<ref name=Baker92>Baker, V.R.; Carr, M.H.; Gulick, V.C.; Williams, C.R.; Marley, M.S. (1992). "Channels and Valley Networks" in ''Mars,'' H.H. Kieffer et al. Eds.; University of Arizona Press: Tucson, 493–522.</ref> and levee deposits, which are always associated with channels not valleys. Except for its small width and general [[sinuosity]], Enipeus Vallis has characteristics resembling the immense Martian [[outflow channels]],<ref>Carr, M.H. (2006). ''The Surface of Mars;'' Cambridge University Press: Cambridge, UK, p. 122.</ref> which are true channels formed by the catastrophic release of large volumes of water.<ref name=Carr96 /> Fluvial landforms like Enipeus Vallis that show characteristics of both valley networks and outflow channels are relatively common on Mars ([[Ma'adim Vallis]], for example).<ref name=Carr06 /> and indicate that fluvial erosion on Mars has a distinctive genesis and evolution compared to Earth.<ref>Head, J.W. (2007). "The Geology of Mars: New Insights and Outstanding Questions" in ''The Geology of Mars: Evidence from Earth-Based Analogs,'' M. Chapman, Ed.; Cambridge University Press: Cambridge: UK, p. 23. {{ISBN|978-0-521-83292-2}}.</ref>
==Regional geology== Enipeus Vallis is located at the western edge of the [[Tempe Terra]] plateau in the Tempe-Mareotis region.<ref name="Moore01"/> The region lies on a broad [[topography|topographic]] shelf bounded by the higher elevations of Alba Mons to the west and Tempe Terra to the east.<ref>Frey, H.; Roark, J.; Sakimoto, S; McGovern, P. (1999) The Crustal Dichotomy Boundary West of Tempe Terra: Speculation on Where It Lies Beneath Alba Patera Based on MOLA Topography. 30th Lunar and Planetary Science Conference, Abstract #1798. http://www.lpi.usra.edu/meetings/LPSC99/pdf/1798.pdf.</ref> The region slopes gently to the north with a gradient of 0.3° or less.<ref name= JMOLA /> The area is characterized by abundant lava flows, low volcanic [[shield volcano|shields]], and small domes that are part of the Tempe Volcanic Province, a northeastern extension of the [[Tharsis Montes]] chain of volcanoes. Although spatially related to the large Tharsis volcanoes, volcanism in the Tempe Volcanic Province has a different eruptive style and has been likened to the [[basalt]]ic plains volcanism seen in [[Idaho]]’s [[Snake River Plain]].<ref>Greeley, R. (1977) Basaltic "Plains" Volcanism, Volcanism of the Eastern Snake River Plain, Idaho. NASA Contract. Report, CR-154621, 23–43 cited in Greeley, R.; Spudis, P. (1981). Volcanism on Mars. ''Rev. Geophys. Space Phys.,'' '''19'''(1), 13–41.</ref><ref name =Plescia81>Plescia, J.B. (1981). The Tempe Volcanic Province of Mars and Comparisons with the Snake River Plains of Idaho. ''Icarus,'' '''45''', 586–601.</ref> This category of volcanic activity is intermediate between [[Flood basalt|flood (plateau) basalts]] and [[Hawaiian eruption|Hawaiian volcanism]].<ref name=Moore95>Moore, H.J. (1995). Geology of the Tempe-Mareotis Region, Mars. 26th Lunar and Planetary Science Conference, Abstract #1497. http://www.lpi.usra.edu/meetings/lpsc1995/pdf/1497.pdf.</ref><ref>Greeley, R. (1982). The Snake River Plain, Idaho: Representative of a New Category of Volcanism. ''J. Geophys. Res.,'' '''87'''(B4), 2705-2712, http://www.agu.org/pubs/crossref/1982/JB087iB04p02705.shtml {{Webarchive|url=https://web.archive.org/web/20121003052151/http://www.agu.org/pubs/crossref/1982/JB087iB04p02705.shtml |date=2012-10-03 }}.</ref>
The region is transected by numerous northeast-trending [[graben]]s that form parts of the [[Tempe Fossae|Tempe]], [[Mareotis Fossae|Mareotis]], and [[Tantalus Fossae]]; these are fracture systems of varying ages that are oriented radially from the [[Tharsis]] bulge. The region west of Enipeus Vallis is characterized by Hesperian-aged volcanic flows from [[Alba Mons]]. The area to the east consists of fractured and terraced highland plateaus of [[Noachian]] age.<ref name= Moore01 /> At the end of the Noachian period, these highland materials underwent an episode of [[fluvial]] erosion and resurfacing. Enipeus Vallis probably formed during this fluvial episode.<ref name= Moore95 />
==Photo gallery==
<gallery class="center" widths="190px" heights="180px" > Image:Southern Enipeus Vallis.jpg|Southern end of Enipeus Vallis is about 10 km wide and broadly U-shaped in profile. The western wall of the valley has a straight NE orientation similar to that of fractures in the Noachian plateau materials to the east. This suggests that the valley formed along pre-existing faults. ([[Thermal Emission Imaging System|THEMIS]] VIS image) Image:Enipeus Vallis.JPG|[[Anastomosis|Anastomosing]] channels in Enipeus Vallis at 35.9°N, 267.0E, as seen in [[HiRISE]] image. Image:Enipeus Central Channel V29345011.jpg|Enipeus Vallis (right). Note inner channel, levee-like deposits, and streamlined bedforms. ([[Thermal Emission Imaging System|THEMIS]] VIS image) Image:Enipeus anabranches.jpg|Middle portion of Enipeus Vallis. The valley divides into several channels, forming large central island at 37.0°N, 267.2°E. Image is [[Thermal Emission Imaging System|THEMIS]] VIS mosaic. </gallery>
==Notes== {{reflist}}
==References== *JMARS. http://jmars.asu.edu/. (See Christensen, P.; Gorelick, N.; Anwar, S.; Dickenshied, S.; Edwards, C.; Engle, E. (2007) "New Insights About Mars From the Creation and Analysis of Mars Global Datasets;" American Geophysical Union, Fall Meeting, abstract #P11E-01. http://adsabs.harvard.edu/abs/2007AGUFM.P11E..01C.)
==See also==
* [[Geology of Mars]] * [[HiRISE]] * [[Outflow channels]] * [[Vallis (planetary geology)]] * [[Valley network (Mars)]] * [[Water on Mars]]
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[[Category:Geology of Mars]] [[Category:Valleys and canyons on Mars]] [[Category:Arcadia quadrangle]]