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Home , Valley network (Mars)

15. CHANNELSAND VALLEY NETWORKS

VICTORR. BAKER Universityof Arizon1

MICHAEL H, CARR U.S. GeologicalSunet

VIRGINIA C. GULICK, CAMERON R. WILLIAMS UniversirJof Arizo a

and

MARK S. MARLEY NASAIAmes Research C enter

ChaMets, tuttels, dnd rctatetiJeat"res oJ oqueousotiqia on Mdts üe oJpro- louü inpoftance in conparattueplanetoloS!. Manian oufiow channeßJamed by laryeacale fuid ou$o|| ftuh subsurlacesources. Elede4ß oJ catacbsnic foodiks, debis aktl ice fowase, os nodifed t4 rottdnßh oad vihd .ction, seetubest to explain obsened norpholögies. Maftian nlle! ketworks shov thzir grcatestsiniLatit! to tefresttial netwotksformed br sappinsos woter etua- tuted lron seeps.nd sprinss. Exoseretic sourcesof re.haße (P4.ipitation) or endogeneticcJ.linE (hrdtothzttul stst.fts ) vould be rcquired to achievethe ißlica@.]efienstue talley derelopwnt during the heary bonbardneat ptu'se oJ Ma/tian hßtory. Recentstutlies oJ valle! network derelopnent associdte.leith nlcaün sugqest thot ehdoqeneticcy.ling is likeb an.I that it hq not be necessaryto inwkc exte.sire epo.hs oJ eam, eet atnospheric corditions to eVlainüt er her,otkJomation on Mdrs.

t 4931 494 V R. BAK-ERET AL

I. INTRODUCTION is the only planet besidesEarth known to manifest the dynamic workingsof a hldrologicalclcle. The channelsand !"llels of Mar; dre a bold testamentto the pasi operationof that cycle and its Fofound implica- tions for environmentalchange on thar ptanet. Howevet m understandenvi_ ronmental change on Maft, one must have an exact ünderstandinqof the genesisof channelsand vallels. The ;mponanceol lhis probtemFa;es back to JamesHutton, the acknowledgedfounaler of geology, who ascribedthe origin of valleyson Earahto the protongeddenudational action of riveß (Hu! ton 1788,1795). To ascertainthe origin of charmelsand valleys, genesisis first inferred from detailed study of lüdforms imagedby remoresensing devices on spaceclaft.Hypotheses are geneüted by the reasoningprocess of abduc_ tion (Engelhardrand Zimmerman 1988), whereby local details of momhol_ ogy areused Lo infer probablemechanical processes. geologic conrrols. and ultimately the exogeneaicand endogeneticenvironments of formation. This is a nontrivial exerciseinvolving a rype of reasoning(Cilbert 1886)that lies at the heart of geology.Application of the methodologyin planehry studies is discussedby Mutch(1979) and Baker (1984,1985). Alrhough mosr geneBlly recogniz€dftorD imagery of the l9?2 9 mission(Masursky 1973; Mccauley et al. 1972;Milron 19?3),the nuvial forms on Mars were also infeüed from studiesof Mariner 6 and 7 imases showingsigh of cmler dissectionby valleys(Schültz and hgerson tail. Becauseof the incompatibility of the implied liquid water wirh the modem Martian environment, it was quickly hypothesizedthat the forms must be relica irom an ancient warm, densearmospherc ( et al. 1973a,.Sharp andMalin 1975;Mutch et at. 1976c).Indeed, rhe valtey networks in partic, ularhave become a princrpalelement ofevidence rhat rhe Marian atmorphere evolvedfrom an early volatite-rich stateto its presentcondition (Walker l9?g: Pollack1979; Cess et al. 1980;Squlres 1984; Kahn 1985; Carr 198?; et al. 1987;see chapter 32). Becauseof the importanceof this inference,the presentreview will examinerhe evidence in somedetail.

II. Martian outflow channels are la.rge-scalecomplexes of fluid_eroded troughs. The flows which formed ahesechannels app€ar to have emanated ftom discrerecollapse zones known as chaolic rerrai; rFig. t).Thechannels arc immenseby teresrrial standards,as much as 100 kln wide and 2000 km in length. Gradienrsof channelfl oors lange from neady zero to 2.5 m km- 1. Many reachesof outflow channelsappear ro includeclosed deEessions. Mor_ phologicalattributes of rhe channetsare extensivelydiscussed by Ba}er (1982), whoseconclusions are briefly ßviewed and updaredhere. 15.CHANNELS AND VALLEY NETWORKS 495

Fig. 1 Chaolic te. at HydaspisChaos (H) The ldge oudoe channelexlending nofthwüd fron rhe chaoszon€ is Iiü Vallis (1). (Irt inag€s m a poftion oi JPL Viking Olbner Mosaic 2ll-5556,)

Macroscale Features The major elementsof outflow-channelpattems are the la4e-scale flow features(Table I). A regional anastomosingpaltem is noted for many of the outflow channelsincluding Arcs, Simud, Tiu (Bakerand Milton 1974),Kasei and (Baker and Kochel 1979). Other examplesof anastomosing or braidingof outflowchannels are described by Carr (1974d,1979),Mars ChannelWorking Group (1983), Komar (1980), SharP and Malin (1975)and Hartnann(1974). The issuanceof channelsfrom discretesource regions is a featurecom- mon to almost all outflow channels.These discrete sourceregions can take the form of fracturesor elongatedepressions (Schultz et aI 1979;Wise 1984; 496 V R. BAKERET AL.

TABLE I Mscm- and M6o€{al€ F€lturer of Ourfow-ChannelMorphotogy and their Consistency(x) or hconslstency(-) wirh Hr"othejtftd cenettcM€chanisns.

MACROSCALE MESOSCAII

l. Amsbnosing panem o,ciEnnels 2. Discele $we eß, sucnas chaodc

3. R€sidualupldds epeating chelelg 3. Rec4sionat headcurs(carders) 4. May upl&& paaly sm@rled dd sftamlined by nuid nows, esFcially on ü.ir uPsr@ erds 5. Pbnourced iow expansionsdd 5. Scout nErks n rff@ obshles

6, Distirct upperelev.tjonal lnn !o 6. lenddl foms (snall-*al€ sirednin d hilh, which bay bc eiurd ß6idul or

7. TMse.t d divides üd harging vatteys ?. Expdsion br conpl€r.s, d f& deftas 8. Eosion of diveft rck .ypes thousmds of kjlometeß fron probabb nuid

10. ttigh piruldepth rario 11. DifeE ial qosion oftedain conaolled by structüE md litlrolo8y 12. Indislincr chamel r€mihu. includinc the lacl ofobvioüs larre-sale fms a;rl delt4

MoQhologic.lFearm Wind MudFlow clacier Flood X X X X X X X ? X x x X x X 2 ? ? X ? ? X 'I X x x Inck of solidified nuid at X x X lrcalized sourcere8ion ? x X X x Flow for thousandsof X X x X ; I x Pronouncedupp€r limit X x X Consistentdownhill fluid ?XXX X 2XXX x High width-depthrario XX X- X Headcuts -? X_ X 'T.ble Eodified äon Baler (1982) 15.CHANNELS AND VALLEYNETWORKS 49'7

Christiansen1985: Can 19744,1979)or theycan b€ areasofjumbled blocks on the floors of large vaguely circular dePressionstermed chaotic ternin (Shaml9?3d). Chaoticterrain consistsof slump andcollapse blocks in steep- walled arcuatedepressions at channelheads lt is generallyassumed that re- movalof fluid from below the surfacecauses loss of support,and the material collapsesunder its own weight, leaving iregular blocks of varying sizeson the deFessionffoor. fesidual uplands separatingchannels and panially to fuly strean ined uoland remnantsare common in oütflow channels(Fig 2) These residual upma" t"t" two forms: one roughly similar to a single lemniscateloop, dua y wider towad the upst eam end and tap€ringdownstream narrowing to a ooint ßaker and Kochel l9?8ü). The other commonform is ftombic or shaDed.Other macroscalefeatües that hav€been desüibed include di;ond q74i exoansionsä;d constrictionsto 8owI Baler andM ilton I Baker and Ko- ctrettglta. tglSr Baler lq82), upperelevarional limils on erodedterrain'

\ok fis.2. sftdmlineduplMds \A at t lhemouh oi AEr vallisin chrvs amtua lheopffed röouihm\e0d' ;om!üem lailrC) dd $ee6'ion ol ele.rablanle'' fom mtF at up'hm o! tle uplmds. A! urcoded cnler can be @npaFd at (D). The eosive nuid noved 'orib in nassi;qudrities, $owirg be

Fig. 3. A portion of Maja Vallis i. Chrys Planüa. nuid flow followed the bpognphic Cradient sloping to the est (neh on the pho!o). Fluid pondeduPstr4d (rest) of the n@lik€ ndge (A) dd spilled thrcugh Saps(B) as it overnowedlow Poinß in the ridge tt then spill€d mund theciatei (c), $ouing chanieh dd grcoves(D) ifto the ej@tahldket (vikine oöitd rme 20467.)

someregions. Typical terrestrial lava flows do not produceobserved channel bedforms,but low-viscosity flows (Carr 1974d;Schonfeld 1976) could move turbulendyand erodeinto underlying and surroundingmaterial by heatingit and entrainingit in the flow In theory, low-viscosity lava flows might mimic fluvially producedfeatues. The most obvious advantagesof this model in- clude the abundanceof volcanic featureson Mars, the stability of la!€ in the current Martian environment,and local channeloccuüences in volcanic ter- nins and even on the flanks of volcanic constructs.Disadvantages of this nodel include: (1) dre lack of large lava depositsat channelterminations; (2) V R. BAKERET AL- the fact that lava channelsusually have distributariesat their distal reaches which are tacking in outflow channels;and (3) regional anastomosisof the channelsis not seen in terrestrial la\ä channels,since flowing tava, once channelized,tends not to move laterally, Another proposatis that winds carrying saltatinggrains may havepro- ducedthe outflow channels(Cutts and Blasius1981). Advantages of this a€olian hypo6esis include documerted aeolianactivity in the cunent envi- ronment and the lack of sedimentsourcehirk Foblerns (Nummedalet al. 1983;Baker 1982; Cutts and Blasiusl98l; Murty et al. 1984).The source for saltating grains could presumablyhave beenübiquitous, although at the two Viking Lander sites there was a markedabsence of grains in the proper range for saltation (Nummedalet al. 1983). The sinlß for the wind-blown sedimentscoüld be the geat polar ergs, or the material could have been widely dispeßedby üe tansporting winds. Streamlinedhills could be anal- ogoirsto terrestrialyardangs, and longitudinal grooving similar to that within the channelsis observedon Ea h in desertregions like the Sahara.Howevet lhere are many problernswith this model. Winds on Earth do not cüt chan- nels, and it is difrcult to envision how Martian winds could be localized enough to cut a channel and yet leave the surroundingterrain unmodified (Nummedaletal. 1983;Mürtyet al. 1984).Also, winds donot have to follow topographicgradients, yet the outnow channelsall indicate flow dire€tions down topogmphic gradient. (Baker l9'78a,b,1982;Lucchitta and Ferguson 1983).Finally, fealureslike trim lines and indicatedfluid spillageinto cmters documentthat the eroding agenthad a free upper surface(Baker and Kochel 1978d,1979),which is difrcult to reconcilewith an aeolianorigin (Murtyet al. 1984;Baker 1982; Nummedal et al. 1983).Although wind is highlyun- likely as an originator of outflow channels,secondary modifications to al- ready existing channel featuresby aeolianprocesses are much more aenable flucchit a 1982drBaker 1982). A fluid which doeshave a fte€ upperboundary and rnaybe locally stable under curent Ma ian conditionsis ice. Lucchitta and Anderson(1980) PIo- posedthat the outflow channelswere producedvia glacier or ice-streamerc- sion, and subsequentpapers extend this model (Lucchitta 1982di Lucchitta and Ferguson1983; Lucchitla et al. l98l). Theseauthors demonstrated that glaciers rnay be stable and move on the suface of Ma$, but flow under cürent conditions might be extremely slow Additional efiects, like the ad- dition of brine (Lucchitta et al. 1981),frictional heatingdue to travel down a steeptopo$aphic gradient(Lucchitta 1982a)or subglacialwater lubrication (Lucchitta 1982u; Lucchitta et al- 1981) are required for the increased- glacial-mobility hypothesis. A change in the Manian could also changeüe velocity at which glaciers could tmvel down the channels.The ad\antagesof the glaciavice-steammodel ,re the surfacestability of ice for presentMadan conditions, the similar scalesof terrestdal glacial features and Martian channelfeatures. and similarities of channelbedforms and gla- 15. CHANNETJAND VALLEY NETWORKS 5OI cial featules. The latter include similarities among streamlinedhills and ¨ins or Antarctic subglacialforms, longibdinal grooving similar to that seen in arcas of Canada, anasaomosingpattems, scouring, and U_shaped channelcross sections (Lucchitta 1982?; Lucchitta et al. 1981).The difrcul- ties with an Earthlike glacial model include the slow movementof hvpothe- sizedglaciers on Mars given cunent conditions,a lack of cirquesat the heads of the channels,the lack of glacial depositsat channelends, andthe difrculty of getting precipitationto produ€ethe glacier on the surface.Lucchitta et al. (1981) and Lucchitta (1982d) suggestedthat lhe glaciers could have been gown from subsurfacewater feedingup onto the surface,and lhat this might also produ€ethe chaotic termin at the channelheads. Allhough not stricdy glaciers by dennition, such seepage-fedice massesmight, in theory' move downgradientand be intimately associatedwith impounded and released waterflows (floods). Numrn€dal(1978) and Nummedal and Prior (1981)proposed that mud flows or debdsflows could havecut the Martian outflow chännels'suggesting that the high-fluid contentin Martran sudacerocks may lead to inducedliq_ uefaction (loss of cohesion) via shock or strain-inducedpore-Fessure in- creasefollowed by Iapid floq as observedterestrially in Scandinavianquick clays. The chaotic terrain is seen as the equivalentto quick-clay collapse depressions(Nummedal l9?8: Nummedaland Prior l98l). Groovingis ob- served in areaswhere quick clays flow (Thompson 1979). An interesnng related model was proposed(Komar 1979,1980)suggesting an analogybe- tweenoutflow channelsand featuresobserved in submarinelurbidity currents and debris flows due to the similar etrectsof Mars' lessersurface gmvity and buoyancyin terestdal submarinesettings. by tuftidity currentson Ea h producesfeatures of similar scaleto outflow chännelsas well as many simild bedforms. Debds flows begin their movementas Iaminar flows. but indease in turbulenceas they tavel do*ichannel and lose tnnsPortedmale- paitial rial, Foducing fearuresconsistent with regional anastomosisand to fully streamlinedresiduals downchannel from the sources ln debrisflows and mud flows, longitudinal roller Yorticascan b€comestable and could Produce longitualinalgrooves consistent with those obse ed in €hannels The prob- lems with this mechanismare the small size of soürcercgions comparedto the amountof mud or debris requir€dto haveeroded hundreds to thousands of kilometers, and the apparentlask of vast debris deposits(Baker 1982). Moreovet the high turbulencesuggested by scabland-strippedzones' scouF ing aroundflow obstacles,and rccessionalinner channelheadcuts are dificult to reconcilewith flows satumtedor supersaturatedby debris and mud (Baker 1979.1982).

The Cataclysmic Flood Model Baker and Mitton (1974) pointed out the many similarities betweenthe Madan outflow channelsand the ChanneledScabland in WashingtonState 502 V R. BAKERET AL. formed by breakout flooding irom Fehistoric Lake Missoüla. By analogy they proposedthat the outflow channelswere the productsof catastophic floods of immenseproportions. Numercus other workers hayesince analyzed and expandedupon this hypothesis.Analytic studieshave been mäde of ero- sion and hyd.aulics for channelized flows, chaotic terrain, longitudinal gooving, scouringand strean ining of rcsidual terrain. Highly turbulentcat astrophicflows arc extemely efective erosionagents (Bal@r 1982). The en- tire suite of outflow-channellandforms is also obseNed in the Channeled Scablad, and the scaleofthe features,although not equal, is ce ainly closer than that of usual tenes$ial fluvial features.Catastrophic floods are charac- terized by high-velocity, high-density,low-viscosity water flows with large discharges(Baker 1973,1982; Baker and Kornar 1987). Usually, catastrophic floods producechannels that are not sinuous,with high width-to-depth ratios. Featuresthat areproduced by catastrophicfloods include anastomosis, streamlinedrellmants, longitudinal grooving, inner channelswith recessional headcuts,scouring arcund flow obstacles,scabland-plucked erosional scaß, and many other features similar to those obsened in the Manian outflow channels.Komar (1983,1984) argues that somestreamlining within the out- flow channelscould only be producedby water flow. The advantageof the catastrophicflood model is that aI of the Madian channelfonns are also seenin analogoustenestrial catasEophicflood regions (Bakerand Milton 1974;Baker 1982;Komar 1979).The majordifficulties associatedwith this model include: 1 The genenl instability of ' sudaceunder current climatic condilions; 2. The lack of obvious fluvial depositionalareas at channeltermini; 3. The sourceareas seem to be too small to accountfor the amountof water requiredto fill and erodethe outflow channels. In relation to dificulty (3), Baker (1982) proposedthat the chaotic termin zonesprobably representonly the final stagesof progressivechannel growth by headwardgrowth as rnore and more te.rain collapses.Each subsequent collapse releasesnew water which erodesthe downchannelchaotic terain froin an eärlier phaseof the flood, so that evidencefor the early sourcesof the channelwater areoblitemted by subsequentoutflows down the sarnechan- nel. Associatedice and debris must have also contributedto the genesisof the evolving channel system. Low-sinuosity channelsheaded by chaotic- collapsetermin havebeen modeled by ManlGr and Johnson(1982) who con- structedscaled-down physical models of Martian near-surfacernaterials in- tusedwith groundice. When tilted andheated from below,the modelsyielded collapse-headedchannels of low sinuositysimilar to observedchaotic terrain. Carr (1979) proposeda nechanismfor outflow in which confinedground water is releasedwhen an is breachedby sornerupturing or fmcturing event. Masurskyet al. (1986r) proposedthat the water stored as ground ice 15.CHANNELS AND VALLEYNETWORKS 503 in Martian regolith might be nelted bv localized heating due to intusive masrnaempla;ement and ma) escapecalaslrophically creäling lhe channels B.i-uu,e,eLu,e pro..sre. occuned in lhe Manian'ubsurfäce lhese and olher mechanisms(Clirk 1978;Milton 1974u;Peale et al. 1975)are very dificult to confirm with presentinformation. The Droblemof maintaining surface-waterflows on Mars under Fesent condition; do€snot se€mparticularly seriousfor short-dura'ionfloods Even Drolonsedflows could be marntainedas ice-covereddvers (Lingenfelteret al' 1968;Wanaceand Sägan 1979) or seePageflows (CaIr 1983) Freezing-point deoressantscould be present(Ingersoll 1970;Brass 1980) Mol€ovel the low at;spheric pressurewould actuallv facilitate the etrective flood erosional Droc;s of;vitation (Baker1979,1982; Bater andCotta 1987) Cavitation occu." I'hen dynamic-pressureväriations in a flow Foduce vapor bubbles whosesubsequent collapse shatters channel-bed materials which are then re moved by thi flow (Baker 19?9) Scablandetching is probably producedby cavitatio; and the plücking action of kolks and is conrrolledby inherentbed' rock structure(Baler 1979) Kornar (1979) examin€dMartian outflow'channelhy'kaulics by com oarisonto similar scalealterestrial features,concluding that outflow channels ;d the ChanneledScablands were hydraulically similar to terrestrial turbid- ity currentsin terms of scale, velocity, discharge,stress and sedimenttrans- oärt capacilv.Blocks over one meterin diametermay be canied in susPension in th"r" nä.lv., demonstrating their enormous erosion potentials Larye amountsof sedimentcan be transpo ed as wash load in the Martian floods (Komar 1980) allowing very rapid erosion The paucity of depositionalfea- turesnear outflow-channelmouths and the greatextent of channelscompared to soürce-regionsizes may both be as a rcsult of extensivewashload trans- oort (Komar1980). Longitudinal grooving se€nin Martian outflow channelswas analyzed bv Thon;son (1979)and Baker (19?9) who conclude that it wasmost likely D;ducedbv lonrirudinrlrotlel vorlicei whichare generäted in high-relocit) h"*r fgri;r toist or nows trith vedicallystralrfied vilco'ilies rThompson 1979).Scouring around flow obstacleswas studiod bv Konar (1985)in flune exoerinents.Siour mark*produced around lemnr\cate{hdp€d flo$ obslacle' in'rtreflume comoare taro;ablv lo \courmark' ob

dangsand fluvial tear drop islands. In geneml, drun ins are more elliptical than riverine lemniscates,whereas yardangs may havemuch greaterlengrh- to-width ratios (although these mnge widely) comparedto rhoseFoduc€d fluvially. Cornparisonsbelween Channeled Scabland and leümiscatefoms in Martian outflow channelsshow good agreement,suggesting that streamlined residualswithin Maitian outflow channelswere producedby a high-velocity, very turbulentwater flow Becauseof the difrculries in explaining all the complexitiesof outflow channelswith a single, Earth-basedanalogy model, it may be that the unique Ma-rtianen!ironment modjfied flow processe.. Condiljons of enhancedwa.h- load sedimenttansport (Komar 1980) plus ice formarion, wirh ice flowase {Lucchinal9821lr. mdy haveacte.d in complexcombinarion. These evenrs could even have producedinterlayered ice-rich debris flows in troughs sub- sequentlyinvaded by lava and modified by wind deflation. Fat€ of Flood Water and Sediment The large channelsaround Chryse and nortbwestof Etysium deboüch onto the low-lying northem plains, and all traces of the channelsare lost betweenlatitudes 45'N ro 65'N. The plains in rheseareas display a l"riety of distinctive featuresthat have been attributed to the presenceof grounatice (Caü andSchaber 1977; Rossbacher and Judson 1981; Lucchitra l98l). The most pervasiveand striking characterisricsare a widespreadpolygonal pattem of ftactures and a mottled appeaftncacaused by bdght qater ejecrasuper- imposedon a darksurface. Mccill (1985)and Lucchitta et. al.(1986)suggest that the distinctive featuresof these aieasare causedby the presenceof se- dimentary depositsfrom the large floods. Parker et al. (1989) describethe regionalextent of northem plains featuresthat night reflect the accumulated pondingof outflow sedimentand water discharges. It seemsclear thar the floodwatersthat cut aheoutflow channelsmust have pooled in low arcas at the ends of the channels.If. when the floods occurred, climatic conditions were the sameas at presenr,(hen the pooled waterwould hdve immedialely frozen over to lorm anice-covered Iak;. Sed- iment would have settledout and freezing would havecontinued until an ice depositwas left over frozen sediments.The amountof water that would have sublimed into the atmosphereduring the flood änd while the r,erninal lake was freezing was probably tdvial comparedto ihe size of the flood (Carr 1983). Recentmodeling (Carr 1990) indicatesthat the ultimate fate of the water from thesehighlatitude ice depositswould dependtargety on wherher the deposit becamecovered with debris. ff the ice were €ontinuallv sweDt freeor debrislhen the ice $ouid \lowty subl;meinto rhe arrnosphere, and rhe water would be trappedat the poles. On the orher hand, if the ice becane coveredwith a few meters of material, rhen ar .hes€ high latitudes the ice would be permanentlystable ard the ice depositwould remain for the life of 15.CHANNELS AND VALLEYNETWORKS 505 the planet (Farmerand Doms 1979;Z€nt et al 1986) The recurrenceof dust storms,the lilelihood of a sediment-lagaccumulation on the ice, and photo- geologicevidence of the accumulationof debris at high latitudes(Soderblom et al. l9?4), all suggestthat the ice depositswould have quickly become covercdwith debnsand stabilized.A similar reasoningapplies to the deposits at the endsof the large channelsin Hellas ln contrast,the terminal ice de- posits from those channels,such as MangalaVallis, that end at low latitudes woulalhave been permanentlyunstable. The ice {iom these dePositswould have continuedto sublime into the atmosphereuntil all the water had been

III. MARTIAN VALLEYS Martian valeys are distinguishedftom channelsby the absenceof b€d- forms which are direct indicatorsof fluid flow (Mars ChannelWorking Group 1983).Although the valleys may containchannels, only in mre instancescan rhe lauer be detectedwith the resolutionof the existing imagery Most (Per- haps 989o) of the valleys are located in the old cmtered-teüainregions Youngervalleys are locatedon the southwall of Ius Chasmaand on the flanks of some Martian volcanoes.Valleys arc concenträt€din the 65"5 to 65"N latitudebelt, and deffeasein nunber towardthe high latitudes Becausemost of the \älley networksare locatedonly in the oldest tenains, the \"lley net- works themselvesare thought.o be otd (Pieri 1976; Carr and CIow 1981), with formation ceasingjust alter the end of heavy bombadment, approxi- mately 3.8 to 3.9 Gyr. Altemative explanationsfor the alrnostexclusive for- mation of valleys in the older terains alrethat theseregions were mor€ easily eroded by valleys and that water was Preferentiallylocated in the ancient terains (CarI 1984).A notablegroup of young Martian valleys is locatedon Alba Patem(Gulick and Baker 1989, l99O) These\alleys formed well after the period of heavybombardment and thus haveimportant paleoclimaticim' plications for Mars. Many origins which are similar to thoseproposed for the outflow channelshave also beenconsidered for the formation of the Martian valleys, including wind, water, lava and volcanic density curents As in the case of the outflow channels,erosion by running water is consideredto be the primary mechanismresponsible for the formation of valley networks (Märs Channelworking Group 1983).

Gercral Morphology The Ma ian valleys havewidths ßnging from < I km to nearlv 10 km and lengthsranging fron < 5 km to neady 1000kn (Mars ChannelWorking Croup 1983). Unlike the channels,valleys exhibit a wide varietv of drainage pattemsßnging fmm well-integratedto mono-filamentnetworks Cross\al- ley profrles range fron V-shapedto U-shaPedto valleys with steep, nearlv V R. BAKERET AL. vertical walls and broad, flat flooß. Although thereis a wide vadety of valley twes, most individual valleys have some characteristicsin common. Upper reachesftequen ly appeardegraded and have higher drainagedensities üan the lower rcaches(Baker and Partridse 1986). In general,drainage densities for the Martian valley networks are much lower than those for terrestrial networks(Pieri 1980a).Notable exceptions are the fluvial valleys locatedon the Martian volcanoesCeraunius . HecatesTholus and Alba Patera. Thesevalleys are morphologicallysimilar and havesimilar dminagedensities to those formed on tbe Hawaiian volcanoes(Gulick and Baker 1986). An- other characteristiccommon to most of the valley netwo*s is that tributaries comrnonlyhave blunt, theater-headedterminations. Netwo*s havenumerous s.ubby first-order tributaries. Dninage pattems often are stuctually con- trolled (Schultzet al. 1982;Gulick 1986), with \alleys commonly following fractures and other linear features. Interfluves remain largely undissected, reflecting the inemciencyof lrlley networksal filling spacewithin their own drainagebasin (Pieri l980a,bi Bal(er 1982).Unlike most tenestrial riler val- leys, tributariesoften appearto be as deepas the trunk. Most Martian valleys have steep sidewalls with a relatively constant downvalley width. Tributariesjoin main trunk valleys at low meanjungtion angleswhen com- paredto terrestrial&ainages (Pieri 1980l'). Madan valleys have b€en classifiedin many ditrerent ways: by plani- rnetric pattem (Pieri 1980a), by size and generallocation (Baker 1982) and by cornbined planimetdc pattems/crossvalley profiles (Brakenridgeet al. 1985). For the sake of discussion.we will use a modified version of the classificationscherne Fesented in Baker (1982) Valley lYpes Longitudiwl Uallej Slster'lr. Longitudnd or elongate\älley systems (Fig. 4) are by far the largestof the l"lley networks. Examplesof this type are Nirgal, Nanedi, Ba})ram,Ma'adim and Al Qahira Valles. Thesevalloys are typically severalhundrcds of kilometeß in length and severaltens of kilometeß in width. Upper reachesusually have short theater-headedtribu- taries with the exceptionof Ma'adim Vallis which has long branchingtribü- taries. Inwer reachesare usually sinuousand havebroad, flat floors. Valley walls in the lower reacheshave a scallopedappearance which is attributedto subsequentmodification by landsliding, undercuttingand other mass-wasting processes(Baker 1982). There is a general lack of dissectionof adjacent uplandsby thesevalley systernswhich are usually local€d in the old cratered uplands region. lt is thought that these valleys may have been initia.ed as srnall valleys and then becameenlarged by wall retreatas lower coursesb€- came deeply incised (Baker 1982). The undiss€€tednature of the inteduves and the short accordanttributaries with wid.h-.o-depthratios equalling that of the main trunl are thoughtto provide strongarguments against a formation by direct minfall and surfacerunotr relationships(Pieri 1980r; Baker 1982). 15.CHANNELS AND VALLEY NETWORKS 507

!ig, 4. DoNnstrean porrio. ol Nngal V.llis, !. cxmple of a Idge longiludital vauey slsten' Noteüe shorl. stübbytibütdies upstean (norlhwso md lhe ißgultr sidening dow'slr€m Landsli.le d€posnslL) md a hanging valle, (lt) also ate preenr' (viki'g Orbiter fFnes 466A61 ed 466-464.)

508 V R. EA\FR[T AL

Snall uallq S)"st.ns. Sma valleysysrems are ubiquilous in rheheav rlv cratercdterrain. However, an unusrilly largenuürber ot snall valleytypes arcooncenlrxted in thc MargaririierSinus region (Cr.tnt t987) (Fig. 5). Many of thcsevalle)'s afe locatedin thc hilly andcrarered rcrrain, which formed during the periodof heavybombardment (Cltn 1984),änd drain into thc crateredplateau region. The crateredpiateau consjsts ol smoothinrcrcraier

!i! 5. Sniallvalleys ,n the hcrvitl crarf lerain Mono_til,Jhenrvaltcls radixte.ul ftom rhe craü nn. lm€sratcdslnenN arel)carcd ndjacenr r. crarer1n lhe rtcr.rller reüari. (lhc rmaecsfe apon(in ot IPL Vikin.cOrbitefNloraic 2]l 520t.) 15.CHANNELS AND VALLEY NETWORKS plains which postdatethe hilly and crateredterrain. The intercrater plains haveburied or pardy buried somecnters. Linear valleys, also rctbrred to as endogenicdeFessions (Baker 1982), are concentmtedalong fractures and drain into linear troughs. Small parallel valleys are concentratedalong crater rims suggestingoutward akainage of fluids. Thesevalleys are common Äround large craters and clearly postdatethe period of heavy bombardment(Baker 1982).Snall valleysinthe heavilycntered teüain, in general,exhibit a wide r€riety of norphologic pattems(Pieri 1980d). Valleys forming rectängula( drainagepattems are structurally controlled, probabtyby underlyingfracture systems.Tbese valleys generallyhave low tributary junction anglesand very low drainagedensities. Monofilament parallel valleysare concentratedalong craterdms and haveliltle or no tibutary devetopmentDigitate to transitional dendritic\dleys, which are more similär to terrestrialriver välleys, haYethe highestdrainage densities in the heavily crateredtermin. Smallvalley systems also dissect some Martian volcanoes (Fig. 6) Both lava(Can l9?4d) and volcanic density-flow (Reimersand Komar 1979)ori- gins havebeen proposed for the volcano valleys. Recentstudies (Mouginis- MaIket al. 1982b,1988;Wilson and Mouginis-Mark 1987; Gulick and BaLer 1990,1991) show that some valleys are probabjy fluvial. The morphology of mostvalleys presenton lhe volcanoesHecates Tholus, CerauniusTholus and Alba Patera is compatible with a fluvial origin (Gulick and Baker 1989, 1990). These valleys forned in regions of subduedläva-flow moryhology where the surfaceappears to be composedof fine_grainedsedinents, prob- ably ash. They are inset into the surounding land surface,widen slightly in

Fig. 6. Snall nuvial valleys dissectingth€ volcmo HecatesTholus. Healy bla.k line denotes reas of valley enlargen€nr. piobably by sapping.The 6güre is latcn frcn Cdick rnd Balier ( 1989d).Ghe inaeesm a ponionof tPL vikine OrbilerMosaics 2l I 5601and 2l I 578?) 510 V R. BAKERET At_.

the downstreamdirection, form integated ributary networks, and have clrainage-densiryvalues which arc comparableto those of Ruvial valleys on the Hawaiianvolcanoes (culick and Bak€r 1989).Valleys are atsopresent on the volcano€sHadriaca Patera, Apollinaris pateraand Tynhena piten. The morphology of valleys on theseparticular volcanoessuggests that a combi, nation of sevemlgenetic processes(watet volcanic densitv flows and lava) mr, havebeen imporlanr in rheirformalion,Cutick and B;er lSq0).How- evet extensivesubsequenr modification of rhesevalleys by one or more of theseprocesses bas obscuredthe original moryhology. Other small \"lleys, referred to as slope systemsby Baker (1982), formed along the walls of VallesMarineris in rhe Ius Chasmaresion. These valle). dilpiay primitire. angutar,dendriric drainage pauem.. T-he apparent structuralcontrol of thesepattems in addiaionto the rheater-headedtr$utffy qapping j. lerminationsprovide a \trong argumenrtor a origin lKocheler 1985).

Local G€neaicProcesses Although it is widely acceptedrhat rhe Martian valleysformed by fluviat processes(Ma Channel Working Goup t9g3), the way in which fluvial processesform mlleys has a profound impact on the resulting morphology. Runofi \.?lleys form from the rransportof flowing water acrossthe surfaii. while sappingldleys form from rho outflow of ground water inro the surface environment.Both processesexhibit uniquemorphologic charactedstics. Be_ low is a brief sumrnaryof how ldleys are formed by surface runof and Srouno-wa@rsappmg.

SurfaceRunot. Fluvial \.?lleys and channets,resutting hom surface_ ßrnoffprocesses. form in tocalion\where $ater runofl concentrares and flows with enoughforce ro erodethe land surface.In order to havewater alailable at the surfacefor channel cuning, the raaeof runotr over the surfacemust exceedthe mte of infiltration. Once these conditions arc met, warer from dispersedsouce areas migrates across the surface and collects in topo_ graphicallylow areas.Thi. acrionconcenüares rhe flow of warermovinp down\lope.rhereby increa,rng rhe abrtiry ot walerro erodethe rurfaceand establisha channel. Once established,channels provide a locus for the con_ vergenceof surfaceflow leading to further incision. This processis oarticu_ laJl) $ell describedby Knighron(t984r. As drainageoi the land surface becomesmore efficient, channelsintegrate and eventually form a complex

Fluvial runof valleys and channelsare erosional features which areinset rnto lhe land surface.Tributaries aro ptesentexcept during the early stageof developmenawhere channeisform simple. rrough-shap€dpattems (Macdon_ aldel al. 1983).In planview, these dminages have tapered tributary heads, a 15. CHANNELS AND VALLEY NETWORKS 5 continuousvalley form and an increasing valley width in the downstream direction. Cround-waterSappitle. Sappingvalleys are formed by the undermin- ing of rock and sedimentby ground-wateroutflow ln the model of sapping formation propos€d by Dunne (1980.), subsurfacewate. ernergingfrom valley sides and headwallsdisrupts ground-waterflow pat- tems. Ground water convergesand concentratesalong weatnessplanes rc- moving material which supportsthe overlying surface.This rernovalof sup- port evenlually results in the collapseof overburdeninto the valley, causing !älley sidesto widen and the headwallto retreat. The ratesof headwardero- sion in thesevalleys are faster than the mtes of valley widening becausethe headwall is the region of greatestflow convergence.The Focess is self- enhancingin that the greaterthe amountof emsionthat takesplace, the more flow convergenceis producedand the higher the rate of retreatof the head- wall. This processeventually halts when the flow of ground water becomes insufrcient to maintain tuther sapping. Sapping\aIeys have6eater-headed tributaries (Baker 1982)and anom- alously large \alley width-to-depthratios when comparedto runotr \alleys. Becausegiound-water flow tendsto exploit planesof weaknessin the terain, sappingvalleys will often form along fracturesand joints in the bedrock, resultingin a rectilinear or structwally controlledpaltem. Sappingpmcesses will often modify the mor?hologyof runotr valeys which havedowncut into the ground-watersystem. This modification resuLsin valley enlargenentin which the V-shaped, cross-valleyFofiles t'?i€al of runofi-dominatedsys- temson volcanic landscapesale erodedinto broad U-shapedor flatfloored valleys with steep walls. The overdll efiect of sapping modification to an existingdrainage system is to changethe network panem to one that is sim- pler and less inte$ated. Bal(er (1990) rcviews the origin of valleys by sap- ping. Numemuscompamtive planetology studies (Laity and Malin 1985;Ko- chel and Piper 1986; Gulick and Baker 198?r,1989, 1990) have recogniz.ed the importanceof both surfacerunofi and ground-watersapping processes in the lalley developrnentand havedistinguished the morphologiccharacteris- tics resulting from each of theseprocesses. In addition to these field-based studies,a fairly extensive series of experimentalflume studies (see, e.9., Kochel and Piper 1986;Kochel et al. 1985,1988)of drainagenetworks formed by sappingprocesses have be€n conductedin soft and weally con- solidated,layered sediments. These flume studieshave been able to replicate much of the sapping morphologic characteristicsand con$ols in drainage networks,including, theater-headedtributaries, joint control of ground-water flow, subsequentorientation of drainagesand the importanceof lithology (i.e-, Fesence of permeableand imp€rmeablestmta) in conüolling ground- water sappingprocesses. The rcsults of theseexperimental studies are being 5t2 V R. BAKERET AL. usedto developnumerical simulation models of the sappingprocess (Howard 1988;Howard and McLane 1988). Regional Genetic Conditions While the morphologyof the large, longihrdinall"lleys is genenlly con- sistent with a sapping origin, rhe genesisof the small valleys is less clear. Valleysin the heavily crateredterrain exhibit a complex moryhology (Baker and Partridge 1986). These valley systemshave rclarively fresh,appearing, low-densityn€twork segmentsin their lower reachesand higher,densitynet- work segmentsin their upperreaches. Bater and Partridge(1986) concluded that the denseupper networks formed during th€ period of heavy bombard, ment and the pdstine lower reachesformed afrer the emplacementof the intercrater plains. The pristine valley segmentssubsequently evolved by headwardgrowth into the regions occupiedby the degradedvalleys. Baker and Partddgesuggested that the relatively higher-networkdensiry of the de- gaded valleys may imply a greaterinfluence of surfacerunof processesin their gercsis. A similar complexmor?hologic patlem is exhibitedby the fluvial valleys developedon older Manian volcanoes(culick and Baker 1990).Fluvial val- leys presenton volcanoesCeraunius and HecatesTholi, which form€d during the p€riod of heavybombardment (Barlow 1988c;Neukun and Hiller 1981). exhibit a parallel drainagepattem of inregratedvalley systems.Some ulleys on Cemuniusappear pdstine and lessintegrated while othersappear degraded and betterintegrated. The pristine valleys appearre-activated. Valleys on He- catesalso exhibit a parallel drainagepatrem, but the valley systemsare better inl,egrated*lan those on Ceraunius.Unlike rhe valleys on Cemunius,how- evet only the lower reachesof some valleys appearpristine and enlarged. Fluvial valleys developedon Alba Patera,a volcano which formed well after the period of heavybombadment (Barlow 1988c; and Hiller 1981), exhibit only the better-integratedupper-rcach mor?hology (Fig. ?). Indeed, thesevalleys are the most integratedand mosr tenestrial-like fluvial ldleys on the suface of Mars. The Alba \dleys have taperedtributary headsthat blend in gadually with the surroundinglandscape and V-shapedcross-r"lley profiles. The Alba valleys appearmorphologically similar to thoseformed by rainfall-runotr Focesseson the Hawaiian volcanoes.In addirion, this same complex morphologic pattem of better-integratedupper reachesand less- integmtedlower reachesis also apparcnton the Hawaiianvolcanoes. Recent morphologicsnrdies of the Hawaüanvalleys (Guli€k 1987;culick and Baker 1987ä;Kochel and Piper 1986)concluded that Hawaiian!€[eys forned ini- tially by surfacerunotr and were subsequendyenlarged by ground-watersap- ping. Basedon comparisonmorphologic srudieswith the Hawaiian r"lleys, valleys on the Maraianvolcanoes probably evolvedin a similar manner(cul- ick and Baker 1990). 15. CHANNELS AND VALLEY NETWORKS 513

lis. ?. Fluvial valleys on the noihem nank of Alba Patera.Valleys exh,bir a well-i egaied drainagepatlcm shich n notpbologically sinild !o fluvial vall€ys fo.med bv surlacerunof or lhe Easaiian {lcanoes (€.s. , Mauna Kea lolcdo, Havaii). The Alba valley syslemsd tn€ best developedmd lhe most tereslriaUike nuvial netsorks on the surfaceor Mß The fiCu@is laten frcm Guli.k a.d Balcr (1990). The inages m frcn viking Orbiter Mosaics MrM45107.)

IV RELAIED LANDFORMS Erosionalprocesses modified all varietiesof Martian landforms,includ- ing chamels and valleys. Mass wasting, aeolian,thermokarst and periglacial processeshave subsequently acted to modify the walls and flools of channels and valleys originally carvedby fluvial processes.The most extrememodili- cation processesmay have formed the fretted channelsand ftetled terrain (Fig. 8). Thealteration ofthe originallandfonns has been so complete, how ever, that it is not known if the fretted channelsformed by modifying pre- existing valleys or not. In this section, we fißt examinethe fretted channels and terrain and then considerthe other secondaryprocesses which havemod- ified the channelsand valleys. 5i4 V R. BAKERET AI.

Fig, 8. lEned tenain ih ihe Nilos}flis rgion ncd 34.N. 282T 1he isolatedüDtdd mssifs re lJmunded by vr.le)\ rhd appeero be i ed $rLr debn\dsveJ rrcmddr,;nr .toF5 Sub- pddlerndse\üa groo\e.rmnb Rotr,Ce or üe oebn. ,tpt \,.ins Orbre,Mo,aic ,tl 52!?.J

Fr€tted lbrrain and Channels Fretted teüain is defined as a complex of smoorh, flar lowland areas ::Pu."t"g !y abrupt escarpmentsfrom relarively heavily cratered uplands (Sharp 1973d).The scarpsare typically I to 2 kn high and nay indic;te rhe depih of a geologic unit, perhapsthe megaregolithor rhe transirion to ice, nch perrnaftosi(Sharp 1973d; Davis and colombek 1989: chaDter 16). The 'erraint ben de!elopedatong rhe crarercd uptand and nonhem ptains bound_ ary (Mutchei al. 1976c).Fretted channels are steep-walled vaü;vs with wide flat floor, andrndfnled xall\- Both the frefted reüain and channelsexhibit complex planjmetric pat_ temswith isolatedbutte and mesa ou(liers (Baler 1982).The channels exhibit structumlcontrol but no fluid flow bedforms(Baker 1982).The frettedchan_ nels are generallylocated in the region ofthe fretted ierain bur occasionallv e\tendhundreds ol kilomerer,back from lhe mäinscarp. inlo lheotder hea; ily crateredterrain (Baker 1982).The g€onorphotogy;f the fretre{ttenain is discu\qedin detailby Kocheland Peale (t984J A conmon aspectof both the floors of the ftened channetsand the low_ lands of the ftetted tenain is the presenceof debris ffows (Carr and Schaber 1977; Squyres 1978). In the fre(ed terrain, debris flows emanatefrom the scarpsand flow onto the surroundinglowlands, often forming apronsaround isolaaedmesas. The lobatedebris flows characreristicof the ftefted teffain are found in two planer,wide 25' larirude swarhscentered at 40oN and 45oS 15, CHANNELS AND VAILEY NETWORKS 515

(Squyres1979d). When unconfined,these flows may reachlengths of 10 to 20 km- Therc may be a correlationbetween scarp face orientationand apron widü (Eppler and Malin 1982;Kochel and Peale 1984) The floors of some of the fretted channelsaI€ also covercdby lineateddeposits which appearto be debris flows. The debns apronsgenerally have a convex profile (Squyres 1978), although some charinel and outlier escarpmentsexhibit a noatlike depressionor swale on the aPronat the baseof the escarpment(Weiss and Fagan 1982). ln a crater-countingsludy, Squyres(1978) found that both li- neatedand unlineateddebris are youngerthan the uplands,escarpments and sürounding lowland. This may imply that the debris-formingprocess may still be operatingtoday. It is unknownwhether mass wasting and debnsflow- age are secondary-modificationproc€sses acting on featuresformed by an_ other mechanism,or whether they are the Fimary mechanismthat formed the . The best evidencethat flow of debris indeedtook place is the presence of numerousstriae in the debris deposits.In the fretted terrain the stiae are mierted at nght anglesto scarpfaces, and they divergeand convergearound obstacles.These characteristics are generallyaccepted as implying flow from the scarpfaces. In the fretted channels,where the striaetypically run paüllel to the ahannellength (Fig. 8), the interpretationof the striaeis more problem- atic. Squyr€s(1978) interyretedthe striae orientationas inplying flow from the channelwalls meetingin the centet the resulting comFession-producing stdae paratlel with the walls. Squyresstates that litde or no down\alley flow has taken place. Lucchitta (1983d) arguedagainst this interpretation,noting that side-canyondebris bendsdownvalley, that striae split at obstacles'änd that striae are always oriented downvalley,even at the baseof mesaswhere right-angle-orientedflow would be expected.The main problem with Luc- chitta's interpretationof considerabledowNalley flow is the lack of deposi tional featuresat the mouthsof most channels. Fretted channelstypically extend into plateausand have snrbby tribu- taries. They are best developedalong the boundariesof the northem tenain and other older elevatedterrains. There is typically a $adation from channels cutting plateauto ftetted terrainsto plains with piateauoutliers (Carr 1981). As distancefrom the cmtered uplands and northem plains boundary scarp increases,the numberand sizeof the outliers b€comessmaller while the areal volume of the plains materialincreases (Kochel and Peake1984) The sirnF larity of the channelsand terrain led Baker (1982)to concludelhat both were formed by the sameerosional proc€ss Under such a scenariothe channels would erode back into escarpmentsalong zonesof leasl resistance As the channelswidened and dissectedthe highländs,the landscapewould eventu- ally take the form of the fretted terrain. Kochel and Perle (1984) also con- cluded that progressivedegradation and retreat of lhe boundary scarp was responsiblefor the fretted terain. While an appealingconcept, no thorough study of lhis fretted'lerrain degradationprocess bas yet be€nmade. 516 V R- BAKERET AL

Beyond the questionsof do\'r'n\,?lleytranspo( and frettedlerrain evolu- don is the simple question of what causedrhe scarp-formingand debris- tansport processin the lirst place. Mass-wastingFocesses probably under- mined escarymentsand provid€d rnaterials which flowed down onro the sunounding lowlands. Shary (1973d) proposeddry sappingas the principle mass-wastingprocess. Dry sapping proc€€dsas exposedice is sublimated ftom scarpfaces. As ice is lost, supportis removedfrom overlying material which then collapses. Squyres (1978) Foposed more conventionalmass- wastingprocesses in which material was suppliedfrom eroding angle of re- pose scarps.Both processesrequire an initial clitr or valley ro provide the face from which the ice sublimatesor masswasting occurs. Wet sappingis anotherpossibility that cannot be ruled out (Sharp and Malin l9?5; Baker 1982). Resolution of this question will probably require much higher- resolutionimagery and topographt or acrualfield reconnaissance. Once the debris material has beenwasted otr the sunoundingscarps, ir must then be fiansportedaway fiom the scarp and perhapsdown the valley. This lransport removesthe talus and allows masswasting from the scarp to continne (Carr 1984). Most authoß agre€ that ice facilitates the floq but there is less agreementon the sourceof the ice. Squyres(19?8) favors sea, sonal frost depositsbeing coveredover by subsequentmass wasting. This Focess would result in interstitial ice similar ro that in terestrial rock glaciers (for discussionsof terrestrialrock glaciers, seerhe cotlectionedited by ciar- dino et al- t19871).Lucchitta and Persky (1982) argue instead that the ice containedin the gound over which the debris flows is sufficientto lubricate the transport.The problem with this mechanismis that, in similar situations on Earth, a basalshear of approximately1 bar is requitd to initiate the flow On Mars this requiresflows 2.ro 5 km thick (Lucchiua1983d). which is thicker than observed.Howevet the renestrial criteria may be inapplicable b€€auseof the long time scaleneeded to maintain Martian flows (Lucchitta 1983l'). Another possiblemechanism for removing debris from scarp bases is weatheringfollowed by aeoliandeflation (Squyres1978). Fina[y, .he most provocative mechanismsuggested for escarpmentformation and debris re- moval is wave action at the shoresof an ancientMarrian oceanGarker et al. 1989). The frettedchannels and terrain may representimportanr examples of an ongoingMartian geologic process.Lucchitta (1983a)notes that, in thosela! itudes where fretted termin is locared,the current ctimate allo\rs for th€ re- tention of ic€ in the ground. Thus, ground ice is presenaro be exposedar scarpsfor &y sapping and to lubricate movementof ice-rich materials. At high latitudes with colder tempemtures,it may be too cold for rock glaciers to be mobilized. Since the observedfretted rerrain coüespondswith those regionswhere presentconditions allow ice borh ro be rerainedin the ground and to lubricate the flow of material, ftefting may srilt be occurring_ 15. CHANNELSAND VALLEYNETWORKS 5I'7

Sincethe channelsseem to be nodified at low temperaturesby a process which is probably associatedwith ice, the fretting processmay be an arche- typical Martian mechanism.As such, the processand the associatedfeatures deservemore study.Specifically, the mass{ransportprocess and the question of whether downvalley transport is opemting should be rosolved. If these processesb€come better understood,they may help provide a record of the rccent Martian climate. High-resolutionimagery of the e$rarpments'tribu- tary junctions, %lley mourhsand topographicinformation is neededin order to answersuch questions.

Other Secondaryhocess€s once a \älley or channelhas been formed, it rcpresentsa systemof steep walts and valley floors which other secondarygeologic processes can exploit Th€ resultsof secondäryprocesses can be found in and along most channels and valleys. Wind, for example,can act as an erosiveagent by transporting saltating padcles. While inadequateto actually carve channelsor \alleys' aeolian processescan subsequentlymodify lhe landfonns Valleys can also Fovide shelteredareas for depositionby wind and the formation of dunes. In the mid to low latitudes,dunes are present mainly in the protectedareas of canyonsand \alleys (Carr 1984). Another se€ondaryprocess, mass washng' rcmovesmaterial from scarpsand slopesunder the influenc€of graviry Most dopes sunounding channelsand \alleys have probably been modified by these processes,as they have been on Eartb. SapPingand hillsloPe-reftat processesmay have eilarged the width of the outflow channels(Baker and Kochel 1979). Such processesarc impoltant, for if they are not taken into accountwhen considedngchannel volume, anomalouslylarge quantitiesof water may be derivedas necessaryfor channelformation (Baker 1982).Mass wastingis also important in the formation or modificationof the fretted land- forms. Processeswhich dependon the melting of ground ice can also modify channelsand valleys. The very large landslidesfound aroundthe marginsof someof the outflow channelswere probablylubricated by groundice-derived water (Ma$ ChannelWorking Group 1983). Thermokarsttopographv forms when melting of ground ice pmduceslocal collaPsedepressions which should not be confusedwith channelsor valleys.Chapter 16 discusses'thesepro- cessesin moredetail.

V. IMPLICATIONS FOR PALEOENVIRONMENTAL CHANGE

Global Martian llhter Budget The channelsprovide a crude wäy of estimating the total amount of wateroutgassed ftom the planet(Cärr 1986,1987).The calculationutilizes 518 V R. BAKER ET AL the volumes of material removedto form the variouscanyons, channels and chaotic teffain around the Chryse basin. Ignoring the volumes of Ius and Copmtescanyons, in which faulting is most evident, then there is a negative regolith volume of roughly 5 x 105km3 in the renaining canyon, channels and chaotic terrain. If this volume was all rcmoved by watet and the water caried its maximum sedimentload, or 40% by volume (Komar 1980),then ?.5 x lff km3 of water would be requircd, which is the equivalentof 50-m spreadover the entire planet. Clearly thereis considerableuncertainty in this number since it is unlnown precisely how much of the negativevolume of the canyonsis due aoerosion and how much is due to tectonic forces. More- over, it is unlil@ly that all the water caried the maximumsediment load- Further assumptionsare requiredto estimatehow much water has out- gassedfrom the planet. The planelwide distributionof valley rctworks (Pieri 19?6;Carr andClow 1981)suggests that, eady in theplanet's history waaer wasdistributed globally and not concentratedin the local areassuch as around . The concen$ationof floods aroundChryse Fobably repre- sontslater ground-waü9raccumulation in the region as a result of slow migra- tion of ground water from the sunounding higher tenains (Caü 1979). The drainagebasin aroundCkyse Planitia can be roughly outlined from drainage pattemsand topography.It constitutesroüghly one sixth to one eighth of the planet's surface. Thus, assumingthat water was initially distributedevenly over the planet, and that the Chrysedüinage basincontained at least50 m of watet the indicaled global inventory is at least 300 to 400 m. This is a very rough esdmat€-It would b€ too high if the samewater passedseveral times thmugh the circum-Chrysechannels. However, most of the circum-Chryse outflow channels fom€d after the period of intense %lley formation for which wamer climatic conditions have been proposed-Climatic conditions during formation of Oreoutflow channelsare likely to have been sirnilar to modem conditions, which cause a thick, planelwide permaftost. As de- scribedabove, the water that erodedthe channelsprobably formed permanent ice depositsin the lowlying no hem plains, and was not rccycled through some global aquifer syslem. The estimateof 300 to 400 m may also be too low becauseia ignoresthe groundwater within the Chrysedrainage basin that failed to rcach rhe surface.For comparison,the Earth is estimatedto have outgassed3 km of water (Turekianand 1975). From the volumeof volcanicrocks that haveaccumulated on the surface, Greetey(1987) estimatedlhat the planet has outgassedapproximately 50 m of water in the last 3.8 Gyr. This numberis entirely €onsistentwith the 500 m estimated(Carr 1987)for the total outgassedwater since most of the out- gassingis believedto have occurredvery early in the history of the planet, before the geologic record was retained.Mast geochemicalestimates (sum- marizedin Pepin 1987d)are lower than both the Greeleyand Carr estimaaes but can b€ reconcilod if Mars tost part of its early atrnosphereby impact erosion(Carneron 1983; Melosh and Vickery 1989)or hydrodynamicescape 15.CHANNELS AND VALLEYNETWORKS 5I9

(Hunten et al. 1989). The volatile invenlory and evolution is reviewed in chaplers4, 6 and 32.

Paleoclimatic Implications The lalley networks have been uridely cited as evidenceof a fbrmer thicker atmospherealrd surfacetemperature substantially warmer than thos€ al present(see, e.g., Saganet al. 1973d;Masursky 1973; Pollack 1979;Toon et al. 1980;Pollack et al. 1987). However the climatic conditionsrequired to form channelsand valleys are unclear' Outflow channelscould probably form uder presentclimaaic conditions They involve floods of such magni- tude that the anount of fteezing ünder presentclimatic conditionswould be trivial (Lingenfelter et al. 1968; Carr 1979). Cold climatic conditions with temperaturewell below freezing may even be required for the fonnation of those floods causedby eruption of ground water. A thick permafrostmay havebeen needed to containthe gmund watet therebyallowing artesianples- surcsto build alrd eventuallycause massive outflows (Car 1979). The valley rctworks are much smaller than the ffood outflow channels and are presumedto have fomed from correspondinglysmaller discharges. Moreover, the valleys divide into smaller valleys upstream lf temperatures were well below freezing, it is rcasonableto assumethat, inespectiveof the sourceof the watet small streamsin the distal parts of the networks would freeze, thereby cutting otr flow into the laryer channelsdownstream and ar- resting further developmentof the valleys. The presum€dwalmer conditions arc believedto have occüred mainly ve.y early in the planet'shistory The valley networks are almost entirely restnctedto lhe oldest terrainson Ma$' in contast to the outflow channelswhich cut into materialswith a wide range of ages(Carr and Clow 1981; Baker and Partridge 1986). The simplestex- phn;tion of the alnost comPleterestdction of vatleysto the oldesttenains is ihat the valleys thenselves are old, and that conditions required for \äIey formation werc commonly met early in the planet'shistory but only rarely throughoutthe planea'ssubsequent history. Thesetwo inferences'that warm conilitions are required for lalley formation and that the valleys ale mostly old, have led to a perceptionahat early Mars was warm and wet' and thal climatic conditions then changedsuch that conditions similar to those pre- vailing today were maintainedfor much of the later planetary history We cautionthat theseconclusions are by no meanspmven. The climatic conditionsrcquired for valley formation arc not clear The assumptionof warm wet conditions is basedon the premisethat the valleys formed by slow ercsion of running water. The argumentsfor water as the erosiveagent arc basedon analogy with terrestdal ulleys änd the abundant conobo€tive evidence,in addition to dle valleys, for the preserceof water at the Mafüan surface.Agents other than water or ice, such as wind' carbon- dioxide ice and lal" arevery unlikely for a numberof reasons(for summades, seeBaker 19821CarI 1981). However,as describedabove, many of the net_ 52O V R. BAKERET Al_

ltorks havea dr.tincrlydiflerenl appedrance from rerresnialriver la els. Be{ause ot rhesediferencel and lhe dimcutLvot mainlainrngtiquid ware; al the ,and, given the abundanievidence for waterand ice, the possibilil) shouldbe hfr op€nrhar many. perhap. mosr va els formednol b' bdnh-lrke lluvial processes.bLrr by ,ome orhermechanism, ruch as masr waslrng,that involves water or ice_ If - the valleys were fomed by running water rhen rhe water could come trom possible two sources,the ground or the atmosphere.Many \."lley net_ work. ha!e characrerisdcc \uggesli! e of ground-warer sapp ing irieri t oAOzr, barerI vb2. nrggrns lq82: Kochel el at. t985'.One requüemenr for ground_ water sappingis that liquid water is stablesufficiently close ro the surf;ce rhat rr can seep onto the sur{ace.This impliesr}al remperdturesaJe abo\e the rreeTrngpoinr al deplhscomparable to rhescale of tocaltopognphjc reliet. Thiscan be achie\edin L\rowaj\: ( I I rhesurface re.p.*,"""".,"y U "f.r. Iree-zrng:or.f2) if surfacerempera(ures are beto$ freering.rhe temperarure I,radienris largeenough io allo\r liquid waterclose to üe sunace.Sr,erp thermälgradienß are lilely on MarJ. Hearot accretionand core formarion would have resulted in heat flows more than a factor of fO farger than at duringthe.fir\t.few hundred Mlr ot üe pJanet.shislor) (;ee chaprer ),.fresenl I nr\. combrned$ idr tos conductiviueserpecred of rhebreccrared miga. haveresuked rn tiqurdqarer fg"lll,.:i, ar shatro$deplh\ earty in ;he planels hi{or} irrespecli!eot üe surfacerempemlure\. || sunacetemperatures uere belowfree,,ing and wäler reached the sur face, then it nay havebeen possible for sreams ro survive and cut the valley Ty"t. YSlirC of the freezing of small sreams (Wa ace and Sagan 1979;Can 1983) suggeststhat if a srreamt_rn deepor more can be srar;d then the_water could flow for a few 100 km, dependingon slope and other factors, before it froze solid. Howeve! such cat;kionJ." fr*irty ta"ui;; and do not tale inro accountthe garies of natural ,yst".". St "ums t om tere

Endogenetic Hydrologic Cycling Local water budgetsmust be balancedto achievethe developmentof valley rctworks. For water to flow at the surface, it must be al"ilable at sowce areas.For spring sapping,a hydraulic gradientis ultimately achieved for terreslnalexamples as solarenergy generates an aamospherichydrological cycle that transportse\äporated or transpiredwater as vapor bank to headwaters,where it pr€cipitatesand infiltrates to rechargeaquifer sys.ems. Howevor i. may also be possibleto supply energy geothemrallyto rccycle water in the subsurface. The ancient%lleys of the heavily crateredtermins are associatedwith very high cratedng rates and with widesFead volcanism. The former ässo- ciation led Bratenridse et al. (1985) to hypothesizethat someof the \alleys might have originated through the interactionof ground ice and hot sp ngs located along the semi-permeablefringes of slowly cooling impact melts. They concluded that, with widespreadoperation of impact-Ielatedhydro- thermal systems,the possibleexistence of which was proposedby Newsom (1980)and Schultzet al. (1982),it is not necessaryto infer majoratmo- sphericchange to explain valley forrnation. Gulick (1992)estimated lifetimes of hydrothermalsystems associated with impact crater and volcano forma- tion. She concludedthat hydrothermalsystems could remain active for long enough peiods to form the valley networks that are associatedwith theso features. Hydrothermalsystems are an inevitableconsequence of volcano or cra- ter formation, tectonism, sill intusion, or other plutonic intusions into per- meable, liquid water or ice-rich subsurfaceenvironments. Such g€ologic eventsgenerate local thernal anornalieswhich induce density pe urbations in the ground water as heat is dissipatedout in o the suüounding country rcck. Water near the anomaly migrates upward in responseto buoyancy forces. This action results in the flow of ground water towardsthe thermal anomaly.Hydroüermal systemscan focus, transport and reciculate large amountsof gmund water to dynamic surfacewater environmentsfor periods in excessof 10r yr (Gulick 1992).As pointedout by Gulick and Baker (1987d) water flow tansport€d to the suface, via seepsand springs, rnay initiate va eys if there is a low pemeability, erodiblesurfac€ (e.g., ash) and sufficientrunof. However, if the surfaceis permeable(e.g., basalt flows), water will infiltmte and rechargenear surface(perched) . These aqui- feß may eventuallyintersect with the surfacefarther downslopeand initiate sapping at these locations. However, some of the upward migrating water may rechargehighlevel aquiferswithout ever flowing on the suiface. Squ''res (1989,) proposesthat the higher heat flow and steeperthermal gadient that existedearly in Mars' history would haveresulted in the melting of ground ice to a minimum depth of 350 m (seealso chapter32). However in order to forrn sappingvalleys, water would haveto intersectwith the sur- V R. BAKER ET AL-

face environment and exploit a pre-existing (runotr) valley, joint, fault or ftacture systemas indicatedby severalrerrestrial sapping proc€ss studies. The remainingproblem, then, is in getting the subsurfacewater up into rhesurface environment, so that valleys can be initiated and erodeddown ro depthsof the melted-groundice reservoirs.The generationof hydrcthermalsystems, as discussedabove. may Fovide a way of rransporringand circulating rhis water through the suface environment. The complex sequencesof ancientcratered plateaus on Mars apparently include interstratifiedimpact breccia,reworked aeolian sediments, lam flows or sills, and ice (Tanaka1986; Wilhelms and Baldwin 1989r).The coinci- denc€of very heavy cratering, extensivevolcanism and regional l"lley for- mation suggestsan associationwith endogeneticenergy sourcesdriving a dynamic hyalrologicalsystem. As with the outflow channels,the subsuface characterof this hypothesizedsystem precludes its dired snrdy

VI. CONCLUSIONS The spectacularrelict channelsand valley networks of Man represent ancienthydrological conditions gready diferent ftom thoseseen to be active on the planettoday. The outflow channelsare relatively young, late or Amazonianin age (Thnaka1986). They forrned by immenseoutbursrs of flüid from subsurfacesources. Complexity in ourflow-channelmorphology was generatedby \."rying amountsof sedimentand ice in the aqueous-fluid flow systems.The overall cataclysmic-floodmorphology thus may be locally tansitional to morphologiesgenemted by ice and debris flowage.Moreover secondaryprocesses, including wind, lava ffows and massmovement, exten- sively modified somechannel systems. Although local ar€asof valley networks,such as on Alba Patem,formed coevally with outflow channel activity, regionally extensivenetworks domi nate in the heavily cmteredtemins. Thesenetworks are Noachianand early Hespedan in age. The mor?hology of many valleys suggestsgenesis by ground-watersapping; for some valleys, suface nrnof may havebeen more impo(ant. The morphologicevidence is consistentwith bur doesnot require atmosphericor climatic change for its explanation. Endog€niccycling of watet as in volcanic or impaclrelated hydrothermalsystems, Fovides an altemativeexplanaaion.

Acknowledgnents. We thank R. C. Kochel and P D. Komai for their insightful reviews of the manuscript.Partial support for the preparationof this chapterwas provided by the National Aeronauticsand SpaceAdminisfa- tion, PlanetaryGeology and Ceophysicshogram.