Channels and Valleys on Mars

Channels and valleys on Mars

MARS CHANNEL WORKING GROUP*

ABSTRACT INTRODUCTION

The discovery of channels, valleys, and related features of The discovery of channels, valleys, and related features of aqueous origin on Mars is of profound importance in comparative aqueous origin on Mars is probably the single most significant planetology. Models of the evolution of planetary surfaces and result of the Mariner 9 and Viking orbital-imaging missions. atmospheres must be reconciled with the diversity, abundance, and Indeed, it was one of the most surprising discoveries in the emerging origins of channels and valleys on Mars. The term "channel" is science of planetary geology. The importance of these landforms properly restricted to those Martian troughs that display at least can be appreciated by recalling that the fundamentals of terrestrial some evidence of large-scale fluid flow on their floors. Outflow geomorphology were elucidated by studies of the Earth's rivers and channels show evidence of flows emanating from zones of chaotic their valleys by James Hutton (1788, 1795). terrain. The term "valley" applies to those elongate Martian The study of landforms on planetary surfaces is accomplished troughs, or systems of troughs, that also appear to have formed by mainly by the use of images acquired by orbiting spacecraft. The fluid flow, but which lack a suite of bed forms on their floors. The interpreter of those images relies on reasoning by analogy to recon- Martian valleys of greatest interest consist of interconnected, dig- struct the complex interaction of processes responsible for the itate networks that dissect extensive areas of heavily cratered observed features. The difficulties with this approach are twofold uplands on the planet. The diversity of Martian channels and val- (Mutch, 1979): (I) Investigators often assume a unique correlation leys is nearly as great as that of their terrestrial counterparts. Even between landforms and the processes responsible for their genesis. though polygenetic and highly modified features abound, water was Actually, some landforms may be generated by different combina- a necessary ingredient in the various channel- and valley-forming tions of processes converging on the same result. This dilemma of processes. The outflow channels involved large-scale fluid flow, "equifinality" is a limitation on any geomorphic analysis. (2) The entailing as yet unresolved percentages of liquid and solid phases, photo interpreter is artificially constrained in his hypotheses by his entrained sediment, and debris flowage. The formation of valley range of familiarity with natural landscapes. For this reason, any networks required ground water or ground ice, contributing to sap- proposed analogues must be exhaustively pressed for their limita- ping and various other hillslope phenomena. Channels and valley tions as explanations for phenomena under study. Methodologic networks probably require an ancient epoch with surface tempera- uniformitarianism may not apply in such studies, because processes tures and pressures higher than at present. The aqueous formation comparable to those on other planets may not be within the realm of channels is release-limited, requiring short-duration floods of of experience of the investigators. immense volumes. The origin of valley networks is perseverance- Owing to the above problems, the origins of landforms on limited, requiring the maintenance of prolonged seepage and sur- other planets are resolved not so much by the individual study of face flow. Both phenomena are consistent with a thick, ice-rich analogues as they are by a consensus among active investigators Martian permafrost formed either during a volatile-rich early epoch (Mutch, 1979). After the images have been studied for many years, or by very effective recycling of planetary water. one hypothesis remains that explains the majority of terrain fea-

*The Mars Channel Working Group is an informal consortium of investigators supported by the Planetary Geology Program, National Aeronautics and Space Administration. This paper is a compilation of contributions by the individual working group members, arranged by V. R. Baker, Department of Geosciences, The University of Arizona, Tucson, Arizona 85721. Other members of the working group are J. C. Boothroyd, Department of Geology, University of Rhode Island, Kingston, Rhode Island 02881; M. H. Carr, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025; J. A. Cutts, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109; P. D. Komar, School of Oceanography, Oregon State University, Corvallis, Oregon 97331; J. E. Laity, Jet Propulsion Laboratory, Mail Station 183-501, 4800 Oak Grove Drive, Pasadena, California 91109; B. K. Lucchitta, U.S. Geological Survey, 2255 North Gemini Drive, Flagstaff, Arizona 86001; M. C. Malin, Department of Geology, Arizona State University, Tempe, Arizona 85281; H. Masursky, U.S. Geological Survey, 2255 North Gemini Drive, Flagstaff, Arizona 86001; D. Nummedal, Department of Geology, Louisiana State University, Baton Rouge, Louisiana 70803; P. C. Patton, Department of Earth and Environmental Sciences, Wesleyan University, Middletown, Connecticut 06457; D. Pieri, Jet Propulsion Laboratory, Mail Station 183-501, 4800 Oak Grove Drive, Pasadena, California 91109; D. E. Thompson, Jet Propulsion Laboratory, Mail Station 183-501, 4800 Oak Grove Drive, Pasadena, California 91109.

Geological Society of America Bulletin, v. 94, p. 1035-1054, 16 figs., 3 tables, September 1983.

1035

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TABLE I. COMPARISON OF VARIOUS PUBLISHED CLASSIFICATION SCHEMES FOR MARTIAN CHANNELS AND VALLEYS

Scale

Authors Smalt in termed ta te Large

McCauley and others {1972) Fine channels Large channels Masursky (1973) Small channels Intermediate channels Large channels Milton (1973) Dendritic channels Sharp and Malin (1975)* Slope gullies/runoff channels Out flow/runoff/fretted Pieri (1976) Small channels Large channels Masursky and others (1977) Small channels Intermediate channels Large channels Carr(1980) Small valleys Intermediate runoff channels Large outflow channels Pieri (1979, 1981) Valleys Large channels

'Categories not dependent on scale but on morphology.

tures and is not incompatible with those remaining. A decade has these features were called "channels." As noted by Sharp and Malin now elapsed since Mariner 9 first revealed the channels on Mars. (1975), this term is not wholly appropriate, as a channel is an open This paper will attempt to state the consensus of a group of investi- conduit for a moving fluid. Only a few of the large, elongate depres- gators who have been intensively studying these remarkable fea- sions on Mars show direct flow-related morphology, such as linear tures. Our conclusions will be guarded, and the remaining grooves, streamlined islands, and other bed forms on their floors. controversial issues will be identified wherever appropriate. Martian channels may or may not have tributaries; they may origi- This paper will emphasize conclusions. Readers are referred to nate at a point or in a diffuse source area. other publications that provide more complete descriptive treat- Early studies found it convenient to group Martian channels ments. Additional papers will be cited in the context of providing a by scale, and size remains the most commonly invoked distinguish- background for various hypotheses concerning channel and valley ing criterion (Table 1). Thus, McCauley and others (1972) and Mil- origins. It is not our intention to present an exhaustive review of ton (1973) classified Martian channels (and valleys) as "large" and Martian geomorphology, as this has been accomplished elsewhere "small," the distinction being rather arbitrary and qualitative. Sim- (Carr, 1980, 1981; Baker, 1981a, 1982). ilarly, both Masursky (1973) and Pieri (1976) classified these features as "large," "intermediate," and "small," a scheme still used by TERMINOLOGY AND CLASSIFICATION Masursky and others (1977). The immense size of some Martian channels deserves emphasis, because it is a factor that must be The images of Mars returned by Mariner 9 in 1972 revealed a explained by any genetic scheme. Ares Vallis is typical, with a variety of elongate, narrow, sinuous to curvilinear depressions. length of 1,800 km and a maximum width of 75 km. Perhaps because of the ease with which the term came to mind, Sharp and Malin (1975) proposed a somewhat different

Figure 1. Ravi Vallis, an outflow channel extending about 250 km from its head at a zone of chaotic collapse terrain (left) to its grooved, channeled distal reaches (right). (Viking orbiter picture mosaic.)

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The largest features on Mars often display characteristics of fluid erosion, head in areas of chaotic terrain or other disturbed topography, and merge downstream with plains units. Nearly all occur in the Chryse region, but a few (for example, Mangala Vallis) occur elsewhere. This type will continue to be termed outflow channel. The network-forming features found ubiquitously in the heavily cratered portions of Mars will be called valleys or valley networks. A few features appear to fall intermediate between these two extremes; Nirgal Vallis is an example. These constitute a broad class in which the members are best described individually. Although the fretted channels will not be treated directly in this paper, the important processes of their formation are reviewed in the section entitled "Secondary Modification of Channels and Valleys." Martian geography, including the names and locations of individual channels, will pose difficulties for some readers of this review. Rather than elaborating on this topic, those readers unfa- miliar with Martian geography are urged to consult the Atlas of Mars (Batson and others, 1979), which provides localities for all the named features mentioned in this article. Commonly used terms are established by convention of the International Astronomical Union, including "vallis" (plural "valles") for "sinuous valley," "chasma" (plural "chasmata") for "large canyon," and "planitia" for Figure 2. Parana Vallis, a well-developed valley network in the "low-elevation plain." Margaritifer Sinus region of Mars (lat. 25° S, long. 26° W). Note the relatively short tributaries to long subparallel trunk valleys. (Viking OUTFLOW CHANNELS orbiter picture 084A47.) The picture shows an area 240 by 250 km.

Morphology nomenclature, based in part on terms invoking unspecified genetic processes. Channels and valleys were categorized as exogenic (ero- Outflow channels occur for the most part within the ±30° lati- sional) or endogenic (presumably formed by fracturing, collapse or tude belt of Mars. They form great composite anastomosing pat- subsidence) and subdivided into 7 main categories and 11 subcate- terns on a regional and local scale. At their origins, the channels gories. There were, however, three principal types: outflow, runoff, appear fully developed on the landscape, with somewhat indistinct and fretted. Outflow channels are generally the largest and are source relationships. Most channels clearly head at collapsed zones sometimes deep and broad at their heads, becoming somewhat nar- marked by chaotic terrain (Fig. 1), but some do not, instead display- row and shallower at their mouths. The apparent shallowing down- ing branching headwaters with irregular aspects (Sharp and Malin, stream of some channels is associated with their merging with a 1975). Where the channels emerge from the chaos, they may breach large region of plains. Outflow channels contain the richest assem- the downslope side of the chaotic area. As pointed out by Sharp blage of scour and bed-form features suggestive of fluid erosion (1973a), the volume of the excavated channel often equals the (Fig. 1). Runoff channels, on the other hand, start small and apparent volume of the released fluid from the chaotic terrain. This increase in size and depth downstream. They rarely exhibit bed suggests either that channel formation was not entirely an erosional forms or bank forms suggestive of fluid erosion. Fretted channels event, or that fluid was supplied to the channel from a much larger have steep walls, wide, flat floors, and sharp, linear reaches pre- reservoir than indicated by the surface morphology of the source sumably controlled by tectonism or lithology. Tributaries to fretted area. channels are frequently tapered. The channels generally trend northward and traverse terrains The smallest channels have often been discussed separately, of varying age and type, including heavily cratered uplands and because of their distinctive network pattern. They have been alter- cratered plateaus, such as the Lunae Planum. Channel walls com- natively called "fine channels" (McCauley and others, 1972), "small monly are steep, abrupt cliffs, especially where the channels are channels" (Pieri, 1976), "filamental channels" (Soderblom and oth- deeply incised. Many of the cliffs have steep upper slopes and more ers, 1974), "dendritic channels" (Milton, 1973), "furrows" (Mutch gentle lower slopes. Some of the cliffs have well-developed terraces and others, 1976), and "slope gullies" or "runoff channels" (Sharp (Fig. 3). For instance, in Ares Vallis, four to eight layers are and Malin, 1975). Pieri (1981) proposed that the term "valley" be exposed in the walls (Masursky and others, 1977). The terracing can applied, as there is no evidence of fluid erosion on either the floors be explained as an effect of differential erosive stripping or plucking or walls. Although no bed forms are visible, valleys display indirect of rock units and thus may suggest that rocks of different resistance evidence of having been caused by fluid erosion, including inte- to erosion form the walls. The most likely rock types displaying grated network patterns and tributary junction angles pointing such erosional morphology are layered basaltic lava flows or sedi- down the regional slope (Fig. 2). Valleys are distinctive in having mentary rocks (Baker and Kochel, 1978a). Basaltic flow units, per- long, narrow, deep reaches and multiple-branched network haps underlain by less consolidated material, are favored by many patterns. investigators (Milton, 1975; Scott and Carr, 1978). The over-all Martian channels are a heterogeneous family. As such, any morphology indicates that the channels are eroded into bedrock. classification will fail to properly treat their diversity. For conven- Residual hills of plateau material occur as "islands" between ience, two general classes of features will be treated in this work. anastomosing channel elements (Baker and Kochel, 1979). Many of

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Figure 3. Ares Vallis, incised in heavily cratered uplands. The channel is about 25 km wide and about 1 km deep. Note benches on channel walls that may delineate layers in the uplands. In contrast with the old uplands, the younger channel floor is only sparsely cratered. (Part of JPL mosaic 211-5251.)

these are probably erosional residuals of pre-channel topography (Fig. 4). The remnants exhibit various quadrilateral, rhombic, and diamond-shaped forms. Many have streamlined shapes and display horizontal terracing along their flanks. Another variety of residual channel upland is characterized by secondary channels that cut through the residual form but are not eroded as deeply as the primary channel floor. Such uplands are usually larger than nearby streamlined forms (Baker, 1978a). Some of the remnant upland islands are developed downstream from flow obstructions, such as crater rims, whereas others have no apparent obstruction of their upstream ends. The streamlined islands are also erosional remnants of the upland plateau or hilly or cratered terrain. These remnants have been shaped into fluid-dynamic forms that offer minimum resistance to the responsible flows (Baker and Kochel, 1978b). The streamlining conforms to fluid outflow directions dictated by gradient and source constraints. Streamlined hills have rounded prows on their upstream ends and tapering tails pointing downstream (Fig. 5). Some tails are probably remnant bedrock and not allu- vium, as terracelike benches are common along the downstream

Figure 4. Geomorphic map prepared from Viking orbiter pictures 3A11 through 3A16. The region occurs along the eastern margin of the Chryse Planitia, where fluid flows emanated from Ares Vallis. The large crater at the bottom (named "Shawnee") acted as an obstacle in the flow field, favoring preservation of the terraced upland (T) on its downstream margin. The resulting "island" is approximately 100 m high. A low scarp separates unmodified preflow terrain in the eastern one-third of the mapped region from the channelized zones (CHg) to the west.

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Figure S. Mouth of Ares Vallis in Chryse Planitia. The islands (A, B) have impact craters at the upstream end and long, streamlined forms extending downstream to the north (C). Except for the northernmost crater (D), ejecta blankets have been partly cut away by the eroding fluid, indicating that the craters preceded the channel. The islands have well-developed terracelike benches (E), which suggests that the islands are for the most part cut in bedrock. Scarp on central islands (A) is about 600 m high; scarp on western island (B) is about 400 m high. Also note the moatlike feature surrounding small island at F. (Part of JPL mosaic 211-498S.)

edges. Others may be partly or wholly depositional, although, for hills. These probable scour marks (Baker, 1978a) are best- channel features in general, the evidence for deposition is not over- developed in constricted reaches of the channels and absent in whelming. Baker and Kochel (1978a) mapped some forms in the lee broad, shallow reaches. of obstacles as pendant bars of fluid-transported sediments. This Tributary canyons are common in channel segments bounded type of morphology is most common in the Chryse Planitia flow by escarpments, such as Kasei Vallis (Baker and Kochel, 1979). expansion of the Maja Vallis system. In this reach, deposition may These canyons, similar to the tributaries of the Valles Marineris have been rapid and concurrent with flow expansion, developing system (Lucchitta, 1978a), typically have blunt, cirquelike heads fan deltas at high stage that subsequently were scoured by waning and smooth, V-shaped cross profiles. In most cases, the develop- flows to produce diamond-shaped remnants. ment of tributary canyons appears to be structurally controlled. Many streamlined islands developed downstream either from After coursing through the highlands, most channels cross the raised crater rims or, in some places, from remnant blocks of chao- boundary escarpment that separates the southern cratered high- tic terrain that apparently served as flow obstructions. Other lands from the northern smooth plains. This planet-wide boundary streamlined hills have no such obstructions at their upstream mar- defines the great "planetary dichotomy" (Malin and others, 1978). gins. These appear to be more common near channel margins, It is commonly marked by a dissected escarpment 1 to 2 km in whereas hills behind crater obstructions tend to occupy positions height. Beyond the boundary, the channeling becomes much more toward the center of the channels. Crescent-shaped depressions or subdued than in the headwaters. In the Chryse Basin, for example, moats occur on the channel floors upstream of some streamlined there is no evidence of ponded water, such as deltas, beach ridges,

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or bars, some of which should be visible at the image resolution channel gradually fades into obscurity. Generally, Martian chan- now available in that area. nels have relatively high width-to-depth ratios, and the channel The Martian outflow channels are long, generally 1,000 to depth is inversely related to channel width; wide channels show 1,500 km. Kasei Vallis is the longest, extending as much as 3,000 shallow incisions into adjacent terrains, and narrow channels dis- km. Channels widen and constrict as they diverge and converge play deep incisions. The sinuosity of Martian channels is very low, between islands. Expansion and contraction occur abruptly along much lower than that of most terrestrial rivers. the flow path to a far greater degree than is seen in most terrestrial Channel gradients are very difficult to estimate on Mars. rivers (Baker, 1978a). The widest parts of individual channel sys- Earth-based radar profiles of Ares-Tiu and Simud Valles yield gra- tems range from as little as 20 km in Ma'adim Vallis to 180 km in dients of 1.7 and 0.5 m/km, respectively (Masursky and others, the grooved terrain of Kasei Vallis. Channel width reaches as much 1981). Slope estimates from the U.S. Geological Survey topo- as 220 km for the area possibly affected by flooding in lower Ares graphic map (U.S. Geological Survey, 1976), range from 20 m/km Vallis (Sharp and Malin, 1975). Gorges at the entrance to con- in Maja Vallis (Carf, 1979) to 3 m/km in Mangala Vallis (Komar, stricted portions may be only a few kilometres wide. Channel 1979). The Maja values are perhaps an order of magnitude too high, depths have been variously estimated or measured on scarps. as recent Earth-based radar determinations for Maja yield a gra- Shadow measurements on scarps in Kasei Vallis yield cliff heights dient of approximately 2.5 m/km (Lucchitta and Ferguson, 1982). of as much as 2,500 m, although the general relief is about 2,000 m. The channels display numerous details of morphology on At the mouth of Ares and Tiu Valles, scarp measurements are 400 floors and walls. Channel truncations and scour marks are com- to 500 m. Sharp and Malin (1975) estimated Mangala Vallis to be mon. Baker and Milton (1974) noticed that in Mangala Vallis some 500 to 1,000 m deep, whereas Nummedal and others (1976) esti- channels were cut by later channels. Multiple channel episodes are mated 1,000 to 1,500 m depth in upper Mangala Vallis. For Ares suggested by some channel crosscutting relationships along the Vallis, Sharp and Malin (1975) estimated general depth of about 1.5 margins of the Maja canyon system (Baker and Kochel, 1979), but km, decreasing away from the chaotic terrain in the south as the it is unclear whether various small upper channels are created by

Figure 6. Maja Vallis in Chryse Planitia. Terrain slopes to the right (east). Fluid ponded west of mare ridge (A) and cut gaps (B) as it overflowed low points in the ridge and breached it. Fluid spilled around crater (C) at D. (Viking orbiter picture 20A62.)

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earlier channeling episodes, or simply by fluctuating stages asso- most common in constricted regions (Baker and Kochel, 1979), but ciated with the waxing and waning flood stages of a single event. A in Kasei Vallis they are found across nearly 200 km of channel distinct trimline that separates fluid-scoured terrain from unmodi- floor. Sharp and Malin (1975) considered the channel floors to be fied regions is found in many places. Pre-channel ridges show con- highly scoured, and Baker (1978a) maintained that the longitudinal siderable modification by the fluid flow. Some of the high ridges are grooves follow streamlines in the paleoflow field. Lucchitta and breached by narrow channels (Fig. 6). In other cases, flow was others (1981) suggested that the grooves may be flutes left by ice diverted around the ridges, and erosional processes have stream- masses that moved through the channel beds. lined the topography. Breaching probably resulted from temporary Other features found within the channels include plucked ponding of fluid flows. Various interactions occur, with some ridges zones, inner channels, and transverse headcuts (Baker and Kochel, probably totally obliterated by the channeling process. 1979). Etched or plucked terrain on channel floors in the southern One of the most noticeable features of the channel floors is the Chryse Planitia (Greeley and others, 1977) consists of zones in pronounced grooving. The grooves may appear as dark streaks on which the upper 25 to 75 m of plains has been stripped by erosive the channel floors, or they may be gigantic, with spacings of 200 to process. The floors of these etched zones are light-toned relative to 800 m and depths of more than 100 m in places. They display the surrounding channel floor. At the mouth of Tiu Vallis, the regional conformity to presumed fluid flow directions as dictated by etched terrain is organized into a distinct zone that parallels the channel boundaries and the general channel geometry (Baker and probable fluid-flow direction. Etching or plucking is also apparent Milton, 1974). The grooves occur on the lateral margin of flow in the differential exhumation or erosion of joint patterns and bur- obstructions, such as streamlined hills. They are deflected around ied craters, or other preflow topography. Inner channels may have islands and are absent in the lee of some islands. In Kasei Vallis, the distinct headcuts that are horseshoe-shaped, similar to cirques or to grooves are also well-developed upstream from inner-channel head- the inner-channel cataracts described by Baker (1973, 1978b) in the cuts (Fig. 7). The grooves converge at deeply incised channels and Channeled Scabland of eastern Washington. These cataracts may concentrate at gaps (Carr, 1979). In Maja Vallis, the grooves are have developed by headward erosion during various flow events.

% Ji Figure 7. Lower kasei Vallis near lat. 62" W, long. 25° N, showing grooves on channel k floors. Grooves ditplaj regional conformity to presumed fluid flow direction from left to right. Grooves are deflected around island (A), and occur above possible inner-channel cataract (B). (Part of JPI. mosaic 211-5371.) \ 1lit , "<\vl

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Genesis

In the decade since their initial discovery, the channels on Mars have been attributed to nearly every conceivable fluid: water, ice, clathrate, wind, lava, and mud. The proliferation of genetic models is in part due to the search for pure morphological analogues, with little attention being given to dynamic similitude, and in part to the construction of theoretical models unconstrained by terrestrial experience. For example, the eolian model of outflow channel genesis has certain theoretical attractions for operation on Mars (Cutts and Blasius, 1981). However, with one exception, the Mars Channel Working Group members find eolian processes inconsistent with the geomorphic evidence of primary channel genesis. On the other hand, eolian processes undoubtedly have played an important role in the modification of channels and valleys that initially formed by fluvial processes. Wind and water both move sediment, yet the morphological imprints of the two agents here on Earth differ greatly because: (1) water generally follows the local terrain gradient, and (2) aqueous flow exerts its maximum bottom shear stress, hence potential for erosion, at its deepest points, resulting in channelization of flow rather than its lateral spreading. Further- more, fluvial-erosion features are continuous. Given enough time, fluvial erosion will eliminate gradient discontinuities on the channel floor. On the other hand, because wind direction is generally not controlled by the local slope of the land-atmosphere interface, wind-eroded features do not parallel the terrain gradient. The boundary shear stress in a wind field is not proportional to "depth" because the wind does not have a free upper surface; therefore, wind erosion has no tendency to produce channels. Finally, wind-eroded features are generally discontinuous. The dynamic differences between fluvial and wind erosion are the same on Mars as on Earth. Therefore, wind can be ruled out as a primary agent of channel formation on Mars. Figure 8. High-resolution images of small valleys incised into heavily cratered terrain on Mars at lat. 10° S, long. 278° W. (Viking Although lava locally produces channels on Earth, it is not a orbiter pictures 754A16, 754A17, 754A18, and 754A19.) The primary erosive agent. In fact, laya effusion is primarily an accre- imaged scene is 40 km wide and is an oblique view toward the tionary constructive process. However, the depositional forms that southeast (upper right). are probable in Martian channels (bars) could not be produced by lava flows because there is no "bed-load" separation in "sediment" transport by such flows. The channel distribution also makes it cepted, causing sudden dramatic increase in the flow rate and over- highly unlikely that lava would be a primary erosive agent on Mars. burden collapse. Masursky and others (1977) envisioned jokulh- Thus, the study of the origin of channels and valleys on Mars laups of melted ground ice, released from an ice-rich permafrost becomes synonymous with fluvial studies, taken in the broadest heated by volcariism. Carr (1979) presented a detailed analysis of a sense to include investigations of ground water, debris flows, and model relating ground collapse and fluid release to outbursts from a ice processes, as well as river systems. subsurface confined aquifer, and Nummedal (1978) suggested that Among ideas on the origin of outflow channels of Mars, the liquefaction of a metastable subsurface sedimentary unit might be catastrophic flood hypothesis (Masursky, 1973; Baker and Milton; the origin of some of the chaos. Through this last mechanism, the 1974) has gained the most widespread acceptance. The Channeled released mudflow would have been the primary channel-forming Scabland of the Columbia Plateau (Washington) is generally rec- agent. ognized as the best terrestrial analogue (Baker and Nummedal, Studies of catastrophic flood features in Alaska (Thompson, 1978). The scale of these channels, the anastomosing system, the dry 1980) and Iceland (Malin, 1980) generally have strengthened the falls and cataracts, the linear channel-floor grooves, and stream- flood hypothesis for the origin of Mars channels. Yet, as demon- lined hills all have their inferred equivalents on Mars. If the chan- strated by Boothroyd and Timson (1980) in a study of Quaternary nels were of the same origin, however, the Martian discharges landscape development on the Arctic Slope of Alaska, moderate- would have been one to two orders of magnitude greater than those discharge braided streams, which frequently change their active of the Channeled Scabland. Although such discharges are possible, channels of flow, may produce erosional residuals strikingly similar the catastrophic model does create serious questions about the to the flood-generated streamlined shapes encountered in the source of the water and the mechanism for sudden release of such Channeled Scabland. Ice rises in the Antarctic also have similarities quantities. to the streamlined forms produced by catastrophic flooding (Luc- Several theories have been put forth to explain possible catas- chitta and others, 1981). Thus, the entire assemblage of related trophic release of the water on Mars. Soderblom and Wenner features must be correctly interpreted to model channel evolution. (1978) proposed that it happened by headward retreat of a scarp or Further discussion of channel genesis requires an analysis of flow small channel tributary until a subsurface fluid reservoir was inter- physics, which will be reviewed at the end of this paper.

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Distribution

More than 99.9% of the valley networks that are visible at a resolution of 125 to 300 m per picture element on Viking orbiter images occur in the old cratered terrain of Mars (Fig. 11). The main exceptions are some branching channels on the flanks of Alba Pat- era, some channels in northern Tharsis, and some deeply incised branching channels on the south wall of Ius Chasma. Although a small number of valleys are present almost everywhere in the old cratered terrain of Mars, much of that province remains undissected, with the valleys generally constituting a small fraction of the total area. That is, drainage density (total valley length per unit surface area) is extremely low in the dissected terrain. In the 65° S to 65° N latitude belt, valley density tends to decrease toward high latitudes. There are also relatively few valley networks around the Argyre and Hellas Basins, and east of Chryse Planitia, between latitudes 0° and 40° N and longitudes 20° and 340° W. The valleys tend to be preferentially located in those parts of the cratered terrain that have low albedo, low violet/red ratios, and high elevations (Pieri, 1979; Carr and Clow, 1981). The above relations may be due in part to local obscuration by surficial debris and in part to the characteristics of the cratered Figure 9. Theater-headed valleys in the Nirgal Vallis system upland rocks, on which they are located. The cratered uplands are (lat. 27.5° S, long. 47° W). These valleys are about 800 m deep and composed of an older, more rugged, plateau and younger, inter- have flat floors. Note the lack of dissection in the surrounding crater plains. Channels may be more visible in the dark areas upland. (Viking orbiter picture 466A49.) The picture shows an because these are largely swept free of surficial debris (Soderblom area 73 by 80 km. and others, 1974; Pieri, 1976). Although both the intercrater plains

MARTIAN VALLEYS

Morphology

Martian valleys are distinguished from channels by the absence of bedforms that directly indicate fluid flow. This is an operational distinction, and it remains possible that direct evidence of fluid flow is simply at too small a scale to be seen in either Mariner 9 or Viking images, although features as small as 10 m can be resolved in some ài Viking orbiter images of valleys (Fig. 8). Valley walls are typically DIGITATE STEM rugged and clifflike, sometimes displaying debris accumulations and talus deposits; valley floors are generally flat. Mantling by materials of eolian and volcanic origin is common. Some valleys display an interior topography of cliffs and benches similar in character and scale to features that exist, for example, in the Grand Canyon of the Colorado River, formed there in response to differential resistance to erosion. This type of morphology may be evidence of extensive layering of contrasting materials in the Martian heavily cratered terrain (Malin, 1975, 1976a). The most striking morphological characteristic of many valleys is the presence of PARALLEL RECTILINEAR steep-walled, cuspate terminations at the heads of the smallest tributary valleys (Fig. 9). Valley widths vary from less than 1 km to nearly 10 km. Lengths range from less than 5 km to nearly 1,000 km. Valley network morphologies vary from spatially compact digitate and parallel networks to monofilament networks with few tributaries (Fig. 10). These differences may result from contrasting modes of origin. There is one general global correlation between the type of RADIAL IV RADIAL network morphology and the terrain type: a small number of rela- RADIAL CENTRIPETAL CENTRIPETAL tively fresh-appearing, low-density valley networks tend to occur in CENTRIFUGAL (EXTERIOR) (INTERIOR) ridged-plains units (for example, upper Nirgal Vallis), whereas the more numerous degraded and somewhat more complex networks Figure 10. A partial classification of planimetrie valley patterns tend to occur in the heavily cratered terrain. observed on Mars.

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Figure 11. Map of Mars showing the distribution of valleys. Note the concentration in the heavily cratered upland regions (continuous black boundary line). The stippled pattern shows the Valles Marineris and related chaotic terrain and outflow channels. This distribution is discussed more fully by Carr and Clow (1981).

and the cratered plateau are dissected, the latter has the highest history were atmospheric conditions such that slow erosion of run- valley density. Many valleys traversing the cratered plateaus termi- ning water could occur. nate abruptly against the intercrater plains, indicating possible burial. Soderblom and others (1978) showed that the intercrater plains Genesis tend to be more blue than the cratered plateaus, and Mutch and others (1976) suggested that within the cratered terrain the more Perhaps the most important question with regard to the Mar- primitive crust is exposed at higher elevations. Thus, both direct tian valley networks is what their mode of origin implies for the observations and terrain correlations indicate preferential location history of the Martian surface environment and climate. The rami- of valleys in the older parts of the cratered terrain. fied pattern of these valley systems has provoked comparisons with The simplest explanation of the distribution just described, terrestrial drainage networks, and particularly with those formed that is, restriction of valleys to the densely cratered terrain and primarily by runoff associated with rainfall. This analogy has pro- concentration within this unit to the older parts, is that the valleys vided fuel to the controversy of whether or not it has ever rained on formed, for the most part, early in the history of the planet. Other Mars (McCauley and others, 1972; Masursky, 1973; Milton, 1973; explanations are, however, possible. The channels conceivably Sharp and Malin, 1975; Pieri, 1976, 1980). Despite the evocative could have formed throughout the history of Mars but are re- nature of hypotheses for Martian rainfall, close scrutiny of valley stricted to the old terrains because they formed by sapping (Pieri, interiors and network planimetric morphologies reveals features 1979). The sapping process may have operated most effectively in that are strikingly different from those associated with terrestrial the rock materials underlying the densely cratered terrain. Most rainfall-runoff drainage networks. A closer affinity in morphology investigators favor the hypothesis that only very early in Martian appears to exist between the Martian valleys and those formed in

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the terrestrial environment by basal sapping or seepage-fed runoff way. Lubowe (1974) showed that tributary junction angles in- (Pieri, 1979, 1980, 1981). creased downstream as the size of recipient streams increased. A Martian valley networks are distinctive in the notable absence quantitative model for the systematics of junction angles in surface of the dendritic pattern so common to terrestrial streams (Pieri, drainage networks has been tested on terrestrial dendritic networks 1979, 1980). This pattern is characterized by a nearly uniform dis- mapped at various scales (Pieri, 1979), showing a statistically signif- tribution of tributary directions and filling of the available intra- icant tendency for tributary junction angle to increase with network space. Many Martian valley systems show remarkable increased recipient stream size in a way closely predicted by parallelism and lack of tributaries in undissected intervalley terrain, Lubowe's model. Martian valleys do not follow the model; their thus appearing to be sparse relative to most terrestrial systems. A junction angles are generally narrow (< 25°) and show no statisti- few, equally sparse Martian valley networks show strong reticulate cally significant correlation with either position in the network or patterns indicating local structural controls. Whereas terrestrial relative size of intersection tributaries. Small tributaries exhibit dendritic drainage patterns are generally scale invariant, retaining a cross-sectional areas equal to those of main trunk segments, a char- dendritic character at increased magnification, Martian valley net- acteristic that, together with the narrow junction angles, is not works are scale variant. Differences of pattern with scale, along common in terrestrial drainage networks. Martian network systems with system parallelism, most probably result from the introduction do appear to coalesce in the downhill direction on regional slopes, of fluid into the system from a restricted headward source region. to the extent that this can be determined from available data. Horton (1932) and Howard (1971) established that tributary junc- Downhill coalescence is suggestive of fluid flow, but uphill-tending tion angles are proportional to the relative slopes of the intersecting headward erosional processes could produce an identical result. tributaries. It is also reasonable to expect that tributary junction The presence of steep-walled, theaterlike valley terminations angles and, therefore, network patterns, should reflect the rate of suggests that headward extension by basal undermining and wall decrease of slopes downstream in drainage networks in a systematic collapse ("sapping") may be an important process in valley forma-

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tion on Mars (Pieri, 1979, 1980, 1981). Strikingly similar theater- gests either that they are quite old or that they preferentially headed valleys on the Colorado Plateau (Fig. 12) are possible develop by a process restricted to those terrains (for example, sap- analogues t:o many Martian valleys. Valleys developed in the highly ping). Few crater counts have been performed on the small valleys permeable and transmissive Navajo Sandstone show numerous sim- themselves because the small surface areas of individual valleys do ilarities to the Martian valleys, including (1) theater-headed termi- not provide an adequate sample size. The youngest extensively dis- nations of first-order tributaries, (2) common structural control, sected intercrater plains have about 3,300 craters larger than 1 km (3) a relatively constant width from source to outlet, (4) high and in diameter per 106 km2. This is relatively heavily cratered, com- steep valley sidewalls, (5) large scale (0.5-1 km wide, 2-5 km long), pared to many other surfaces on Mars. Assuming a lunarlike flux of and (6) both qualitative and quantitative similarities in network impacting bodies (Neukum and Wise, 1976), the absolute age of the pattern (Laity, 1980; Laity and Malin, in press). dissected plains is approximately 3.5 b.y. Since the time of formation of these youngest dissected plains, it appears that conditions on Mars have been unfavorable for the widespread formation of valley networks. Channels dissect not only the old cratered terrain but also several younger terrain units. Many channels are large enough in surface area to permit crater counting on their floors. These counts form the basis for the relative age ranking of channels presented in Table 2. As implied by the crater dating, the floors of the large Martian channels range in age throughout most of geologic history, although most are quite old. Some channels might be correlative with the episode of valley formation, whereas others, like many of the big circum-Chryse channels, may date back to mid-Martian history. Ares, Vedra, and Tiu channels show relatively young ages in comparison to other geomorphic features on Mars (Masursky and others, 1980). On the other hand, the floors and walls of these and other channels have been extensively modified by postchannel resurfacing processes (Malin, 1978; Baker and Kochel, 1979). Thus, the modified channels may be significantly older than implied by the data in Table 2. By restricting their counts to fluid-scoured parts of channel floors, Carr and Clow (1981) found that the circum- Chryse channels have 200 to 2,500 craters larger than 1 km in diameter per 10() km2. This confirms Malin's (1976b) estimation that channels are relatively ancient features on Mars. The channels and valleys now visible on Mars are those that formed late enough in the geomorphic evolution of the planet's surface to survive subsequent degradational processes. The studies completed to date have not yet resolved the age of initial channel and valley development, nor have they established the relative effi- cacy of these processes as a function of time, except near the end of Figure 12. Iceberg Canyon, a tributary to the their activity. The data indicate that most valley formation ceased Glen Canyon of the Colorado River, Utah. Iceberg somewhat earlier in Martian history than did channel formation, Canyon developed in the Navajo Sandstone by a but the absolute temporal relationships have not yet been sapping process initiated by ground-water seepage at ascertained. the contact between the permeable sandstone and an underlying less permeable unit. The picture shows an SECONDARY MODIFICATION OF area 4.5 by 7 km. CHANNELS AND VALLEYS

A major difficulty in assessing the correct outflow channel AGE RELATIONSHIPS genesis is that the channels have experienced extensive modification of floors and walls after the primary fluid-flow events. The modifi- Determination of the temporal evolution of the different chan- cation processes included impact cratering, spring sapping, volcanic nel and valley systems on Mars utilizes both the standard terrestrial processes, thermokarst phenomena, eolian processes, slumping, ril- stratigraphic principles of superposition and a technique unique to ling, pedimentation, debris fan development, talus production, extraterrestrial planets: relative age dating based on crater frequen- debris flowage, and other hillslope phenomena. Indeed, such a var- cies. Studies by Pieri (1979), Carr and Clow (1981), and Masursky iety of processes have modified many of the channels that the uni- and others (1980) summarized the state of knowledge concerning que signature of any one has proven difficult to isolate. Moreover, the temporal evolution of channels and valleys. Nearly all Mars the channels may have served as conduits for more than one large- valleys are found in the densely cratered terrain, both in heavily scale flow, leading to a composite of erosional and depositional cratered areas and in intercrater plains. The valley morphology landforms. ranges from barely visible depressions to some that are quite crisp The most important modification processes can be broadly in appearance. Their occurrence on densely cratered surfaces sug- classed as follows: (1) mass-wasting phenomena, (2) eolian proc-

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TABLE 2. AGES FOR SOME MARTIAN CHANNEL SURFACES subsidence and collapse of large blocks (Fig. 1). These blocks are as much as 30 km wide, have flat tops, and are mapped variously as Channel Cumulative number of Approximate age (IO9 yr) chaotic or cratered plateau material, depending on present eleva- name craters ^ 1 km diameter (Neukum and Wise, 1976) per IO6 km2 tion in relation to the cratered uplands (Wilhelms, 1976). They are surrounded by a vast array of transported and buried blocks that Reuil 200+ 100 0.5 Vedra 360 ± 120 1.0 predate and interact with the latest of multiple flows. Hrad 500 ± 200 2.0 Mangala 600 ± 250 2.5 The walls and floors of chasmata, channels, and larger valleys Tiu 650 ± 300 2.5 all exhibit prominent features that imply significant backwasting Ares (Young) 750 ± 250 3.0 Maja 750 ± 250 3.0 since the time of original formation and the cessation of flow Viking 1 site 2,000 ± 400 3.7 Bahram 3,000 ± 800 3.8 events. The morphology of chasmata walls has been discussed in Ma'adim 3.400 ± 200 3.8 great detail by Lucchitta (1978a, 1978b, 1979), and her landform types apply to outflow channels and the larger sinuous valleys, as Note: Data from Masurskyan ;d others (1980). well. Large landslides are the most spectacular wall modification features but are generally confined to chasmata or deeply incised esses, (3) cratering, and (4) mantling by lava flows. Mass-wasting outflow channels, where wall heights of several kilometres exist. phenomena include both "dry" and "wet" processes, although these The larger slides leave broad, curved scars 10 to 20 km wide, where- are not always possible to distinguish. It is also useful to keep in as the smaller slides may leave no major re-entrant. The slides show mind the difference between process or event (for example, debris a transition from rotated slump blocks at the head to upright or flow) and product, that is, the resulting deposit (such as debris tilted blocks resulting from block gliding farther downslide to a apron, debris fan). lobate apron of debris-flow material on the chasma or channel floor. Intermediate-scale slump blocks and either debris or solifluc- Mass-wasting, Periglacial, and Thermokarst Features tion lobes and fans are present in the larger sinuous valleys, such as Nirgal, Nanedi, and Bahram Valles. The scalloped appearance of Mass-wasting processes and deposits described by various the slide scars gives these valleys a "pseudo-meander" pattern authors for Martian chasmata (canyons), channels, and valleys are (Fig. 13). shown in Table 3. It is implied that the melting of ground ice, or Small scarp-face valleys, as wide as 1 km, are eroded into the water from melted ice, played an important role in the development cliff faces of the chasmata and the deeper outflow channels (Patton of many of the landforms and deposits listed in Table 3, particularly and Baker, 1980). These features merge with spur-and-gully topo- chaotic terrain (Sharp, 1973a; Carr and Schaber, 1977) and large graphy described by Lucchitta (1978a). Spur-and-gully topography landslides (Lucchitta, 1978b, 1979). These features are classed as is also well-displayed in the deeper outflow channels (Baker and "periglacial" on the basis of analogy to terrestrial landforms. Kochel, 1979). Talus slopes or debris aprons accumulated at the Ground ice in the Martian permafrost is a key factor in proposed base of the wall features, and these encroach on and bury older flow genetic processes, and it has long been assumed to exist beneath the features (Fig. 14). Some slope gullies lead directly to fan-shaped Martian surface, on the basis of surface temperature, regolith prop- aprons of material on channel or valley floors. erties (Smoluchowski, 1968), outgassing history (Fanale, 1976), and The above processes may have occurred (1) by oversteepening morphological features (Sharp, 1973a, 1974). Disequilibrium in ter- in dry regolith and bedrock, (2) by melting or sublimation of restrial ground ice results in various thermokarst features (Czudek ground ice, or (3) through active lubrication by water escaping from and Demek, 1970; Washburn, 1980). a subpermafrost aquifer. The backwasting of high scarps by slab The arcuate scarps and theater-headed incipient canyon tribu- failure and slumping can be achieved by an excess overburden taries of some outflow channels suggest a thermocirque analogy as height of exposed canyon walls in the absence of water or ground proposed by Baker and Kochel (1979). The chaotic terrain within ice (Carson and Kirkby, 1972). However, the morphology and areal and adjacent to the large Martian outflow channels exhibits fea- coverage of debris flow deposits on the channel floors suggest that tures that suggest a removal of underlying material, and in-place some incorporated water was involved in the depositional process

TABLE 3. POSSIBLE PERIGLACIAL AND PERMAFROST FEATURES ON MARS

Process Feature Size on Mars Size of terrestrial analogues Reference

Patterned ground processes Polygons 20-km-diam 1 -lOOdiam Carrand Schaber(1977) Stripes 1 -2 km wide; 1 -2 km spacing 0.1 -1.5 m wide; 3-4 m spacing Carrand Schaber(1977)

Thermokarst Alases 10-km-diam Several kilometres in diameter Theitig and Greeley (1979) Coalescing alascs (alas valleys) 10-100 km long Several tens of kilometres long Carr and Schaber (1977) Fretted terrain Scarps 1-3 km high Relief of 10-100 m Gatto and Anderson (1975) Chaotic terrain 1 00-km diatn Several kilometres in diameter Sharp(1973a) Sapping and ground-ice collapse Several kilometres high; tens of Variable, but smaller than Sharp (1973a) features kilometres long Martian counterpart

Hiilslope processes Spur-and-gully topography Several kilometres high As much as 1 km high Lucchitta (1978b) Hiilslope chutes Several kilometres high As much as 1 km high Sharp(1973b)

Mass movement Debris lobes 10-50 km long Variable, as much as several Squyres (1979) kilometres long Landslides 1-100 km wide: 1-180 km long 1 - 10 km wide; 1 -20 km long Lucchitta (1978b, 1979) Solifluction Tens of kilometres long Variable, as much as several Lucchitta (1978a) kilometres long

Noie: Dala from Baker (1982).

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km wide are visible as dark patches. High-resolution images obtained during the Viking extended mission illustrate the eolian mantling of valley floors (Fig. 15). Such a modification of small craters on valley floors contributes to the problem of crater-age determinations. Wind erosion resulting in deflation and selective transport has been invoked to accomplish the removal of material generated by other channel-modification processes (Sharp, 1973c; Sharp and Malin, 1975; Blasius and others, 1977; Baker and Kochel, 1979). This "etching" process is defined for Mars as either (1) a large-scale stripping resulting in irregular, structurally controlled, closed depressions (Sharp, 1973c; Greeley and others, 1977); or (2) an erosional enhancement of pre-existing flutes, grooves, and fracture patterns (Milton, 1973; Baker and Kochel, 1979; Carr and Evans, 1980). Evidence for probable eolian deflation on Mars is the presence of well-preserved, complex, exhumed landforms, such as the sinuous channel and crater shown in Figure 16. This complex is within Mangala Vallis and appears to represent the stripping of main channel-floor material and the subsequent exposure of an inner channel filled with an erosionally resistant material. The result is an inverted topography in which the sinuous channel fill now constitutes a sinuous ridge. Yardangs, defined as elongate, streamlined, wind-eroded Figure 13. Meandering reach of Nirgal Vallis showing the steep ridges (Hedin, 1903), are abundantly distributed on the Martian walls and flat floor of this valley. The width in this reach is approx- surface (Ward, 1979). Features that may be yardangs are as much as imately S km. (Viking orbiter picture 466A60.) a few tens of kilometres long and 100 m high, may occur over broad swaths of terrain, may be in part structurally controlled, and are (Lucchitta, 1978b). Wasting of the high scarps of the Capri elongate in the direction of strong unidirectional winds. The prob- Chasma-Simud Vallis area and perhaps in central Kasei Vallis may lem with identifying yardangs in Martian channels and valleys is have tapped a subpermafrost aquifer, as discussed by Kaufman and that they are similar in form and dimension to flood-formed longi- Lucchitta (1980) and proposed by Carr (1979) as a major source of tudinal grooves and small, streamlined erosional remnants. Com- water for primary channel genesis. pounding this problem is that wind direction, known from wind Although no one mass-wasting landform can be shown streaks, may be topographically controlled and blow up or down unequivocably to result from periglacial processes, the complete the axis of the channel or valley. suite of features suggests that melting of ground ice or flow of subpermafrost water played at least some role. Summary

Eolian Features This review of a range of processes of secondary modification may leave the reader with the impression that little is left within the Eolian modifications of chasmata, outflow channels, and val- channels that conveys information regarding the original channel- leys can result in the following features: (1) wind streaks, (2) dunes, forming event(s). The degree to which various alterations have ob- (3) eolian mantles, (4) deflation-produced exhumed landforms, and scured the primary morphology is a problem subject to active (5) etched terrain and yardangs. current research. The safest statement that can be made at present is It is assumed that the bright and dark streaks associated with that the larger the scale of the feature being considered, the stronger craters and other elevated topography formed by wind action in the the likelihood that it formed by the primary channelization event. lee of the obstacle. By terrestrial analogue and experiment, dark and bright streaks are the result of eolian erosion and deposition PHYSICS OF FLOWS (Greeley and others, 1974; El-Baz and others, 1979). Wind direction, and thus streak azimuth, are controlled both by global sea- In order to account for the immense volumes of Martian out- sonal changes and by topography (Thomas and Ververka, 1979). flow channels, one may assume that fluid flows repeatedly invaded Such streaks commonly develop in channeled areas, but they have and enlarged pre-existing depressions on the Martian surface. orientations that are inconsistent with the channel-forming fluid- Many channels probably grew headward as chaotic terrain was flow directions (Nummedal, 1981). These features may have formed created at points of subsurface fluid release. Owing to this and to in response to winds that have been confined by the antecedent various secondary modification processes, the cross section of an channel topography. outflow channel is not necessarily the cross section of the responsi- Dune fields composed of massed transverse or crescentic eolian ble fluid flows. Although various terrestrial analogues have proven dunes are common in the north-polar and high-latitude areas of useful in the study of Martian channels and valleys, no one ana- Mars on plains and on the floors of large craters (Breed and others, logue has fully accounted for the suite of features observed in the 1979). Transverse dunes appear to be less common on the floors of Martian examples. This has led to theoretical studies of the basic outflow channels and chasmata, but there are examples on the floor flow physics and how various flow systems might induce unique of eastern Gangis Chasma, where two dune fields about 15 and 25 landforms that can be recognized on Mars.

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Figure 14. Talus slopes showing talus cone and fan development in northern Kasei Vallis (lat 27° N, long 66° W). (Viking orbiter picture 655A28.)

Any consideration of the physical processes that may have extremes from high-velocity winds, to catastrophic water flows, to been involved in the development of the Martian channels must, of slow-moving ice. The purpose of this section is not to fully review course, rely on Earth-based investigations of those flows and their the physics of these various flows, but to consider what their behav- abilities to erode and transport sediments. However, applications to ior would be like under Martian conditions and to assess whether Mars, with its lower gravitational acceleration and present-day this behavior would be consistent with the observed channel and rarefied atmosphere, stretch this Earth-based knowledge to its valley morphology. extreme. Moreover, surficial sediments on Mars may have a paucity of sand-sized grains (Mutch and others, 1976; Moore and others, Channelized Water Flows 1977), in distinct contrast with Earth. In spite of these differences, the physical processes of various flow types on Mars must be ana- The flow of water as rivers and streams is the most important lyzed with the objectives of estimating erosion rates and times geomorphic agent on the landmasses of Earth. The same cannot be required for channel development, the geomorphology of channels said with certainty for Martian channels and valleys. Many of the and channel networks, and the expected scales of erosional and features related to the Martian channels do bear marked similarities depositional bed forms. In turn, attempts to understand flow proc- to those produced by fluvial action, the large outflow channels in esses on Mars have noted deficiencies in the basic understanding of particular having a remarkable similarity to catastrophic flood ter- fluvial hydraulics and sedimentation. Planetary geology has many rains on Earth (Baker and Milton, 1974). The channels and ero- such positive feedbacks into terrestrial geologic research. sional remnants of the Channeled Scabland in eastern Washington, The types of flow processes that might have played a role in produced by release of water from Pleistocene Lake Missoula carving or modifying the Martian channels and valleys range in (Bretz, 1969), have commonly served as possible analogues to the

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JL X

Figure 16. A portion of Mangala Vallis showing a sinuous Figure 15. Transverse dunes (arrow) developed on the floors of ridge (arrow) and filled crater (C). Presumably, these represent a valley complex dissecting a plateau area. The scene measures 12 depressions that were filled with a resistant material and subse- km across. (Viking orbiter picture 763A16.) quently exhumed by erosional processes. (Viking orbiter pictures 452S19, 452S20, 4S2S21, and 4S2S22). The ridge is approximately 1.5 km wide.

outflow channels. Analyses of the erosional forms and flow processes of the Missoula floods by Baker (1973, 1981b) and Baker and the aquifer, the depth of burial, the permeability, and other factors. Nummedal (1978) served as a point of comparison for hypothesized His calculations indicated that discharges in the range 105 to 107 floods on Mars by Baker (1978a) and Baker and Kochel (1978a, m3/sec are plausible, a range required for a catastrophic-flood 1979). genesis of the channels. One scenario of floodwater erosion of the outflow channels Estimates of the discharges can also be made by employing the envisions a sudden release of water from an underground reservoir equations utilized by engineers and hydrologists to calculate mean of permafrost, the ice being converted to water by meteorite impact flow velocities of terrestrial rivers (Carr, 1979; Komar, 1979). This (Maxwell and others, 1973) or by the underground emplacement of approach also provides an example of how the all-too-common an igneous intrusion (McCauley and others, 1972; Masursky and reliance on empiricism has been unwise and how Earth-based rela- others, 1977). As already discussed, the outflow channels generally tionships must be modified to make them "universal" and thus originate in one or a few areas of chaotic terrain, and there are applicable on Mars. examples of smaller channels originating in craters. Once released, The most commonly employed equation used to predict the the water would have flowed in a direction governed by the regional mean flow velocity u of a river is the Manning equation (Chow, slope and local barriers, carving the channels much as the release of 1959, p. 98-99), Lake Missoula water carved the Channeled Scabland. Carr (1979) developed a somewhat different scenario, one that u= 1/n R2/3Sl/2[mks units], (1) is based on the existence of more clement conditions early in the history of Mars, permitting surface flows (which formed the valley where R is the hydraulic radius of the river (approximately equal to networks of the old, cratered terrain). Subsequent global cooling the mean flow depth), S is the water surface slope, and n is the trapped ground water under a developing layer of permafrost, Manning-n. The Manning-n acts as a drag coefficient, and exten- forming a system of confined aquifers. High pore pressures could sive tables of its values have been developed, based on measure- have developed in the aquifers by the thickening permafrost and by ments in terrestrial rivers (Chow, 1959, p. 108-123). The value of n warping of the surface. Final breakout could have occurred if pore depends on such factors as the numbers and sizes of boulders in the pressure attained lithostatic pressure, or it could have been trig- channel and the vegetation growth, including tree trunks extending gered by impact or faulting. Carr (1979) calculated the expected into the water. Can equation 1 be applied to the analyses of flows outflows. The rate of flow would have depended on the thickness of on Mars? It is seen that it does not include the acceleration of

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gravity, g, as a dependent parameter, implying that the reduced One major difficulty in any hypothesis of water-flow origin of gravity of Mars would have no effect. This is a mistaken interpreta- the Martian channels is that liquid water cannot persist for any tion but one easily made (by Weihaupt, 1974, for example). Equa- length of time under the present environmental conditions. If the tion 1 is empirical and based only on Earth data. Thus, the values of same conditions of low atmospheric pressure and temperature the Manning-n coefficients have gravity built into them. One can existed at the time of channel formation, the water would have instead go directly to the fundamental equations of fluid flow and tended to boil and evaporate away (or, more correctly, would have derive a similar expression (Komar, 1979), experienced surface cavitation) and at the same time would have tended to freeze due to the low temperatures. Lingenfelter and oth- 2 U = [l/CrgRS]'/ , (2) ers (1968) and Schubert and others (1970) suggested that ice might have formed on the flow surface, permitting higher pressures to

where Cr is a dimensionless drag coefficient applicable on Mars, as develop beneath, thus preventing boiling and the rapid loss of well as Earth. The acceleration of gravity is now included, showing volume to evaporation. Wallace and Sagan (1979) performed a that, other factors being identical, the resulting velocity u on Mars quantitative assessment of such a model, concluding that an ice will be less than on Earth because of the lower g. A further compar- cover could have provided sufficient protection. However, Ma- l/3 1 2 ison between equations 1 and 2 gives n = [CrR /g] '' , revealing sursky and others (1977) questioned whether such an ice layer could 2 3 the inclusion of g in n, or, in reverse, Cr = gn /R'^ , which permits form on a highly turbulent catastrophic flood. Baker (1979) pointed

the conversion of our Earth-restricted n values to equivalent Cr out the importance that floating ice and ice jams would have had in values that are usable on Mars. Komar (1979) employed such an channel-bank erosion, similar to occurrences in large terrestrial riv- analysis to compare presumed water flows in the Martian outflow ers flowing into the Arctic Ocean. channels and the resulting sediment transport rates to the Lake Numerous erosional remnants or islands are found within the Missoula floods and other large-scale flows on Earth. At present, outflow channels, residuals of the preflow surface. Patton and the limitations of such an analysis involve the large uncertainties in Baker (1978) recognized three classes of islands that have close our values for channel slopes (S) and flow depths of the several analogues to those found in the Channeled Scabland. Of these, the Martian outflow channels. It is expected that in the near future, most interesting are those that have a high degree of streamlining further reduction of the Viking data and Earth-based radar obser- (Fig. 5). Baker (1979) and Komar (1981) demonstrated by analogy vations will supply better evaluations of S as well as over-all chan- to idealized streamlined bodies that many Martian streamlined nel dimensions. Problems also exist in estimating drag coefficients islands have a shape that provides a minimum resistance to flow. for flows of water much larger in scale than any in terrestrial Resistance is defined as total drag, the sum of the pressure drag (or experience. form resistance), the shear drag (or skin resistance), and the drag Although the reduced gravity of Mars would tend to lower produced by any waves generated on the flow surface. Baker also water-flow velocities, it would at the same time enhance the rate of compared the geometries of the streamlined forms found in Maja sediment transport. A grain on Mars weighs less than it does on Vallis, Kasei Vallis, and the Channeled Scabland. Comparisons Earth and so can be transported by lower flow velocities. The among island lengths, widths, and areas all followed the same dimensionless threshold graph of Shields (1936) can be employed trends. In many cases, large, crescentic depressions occur imme- on Mars as well as on Earth because it correctly includes g as well as diately in front of the islands and other flow obstructions. These other environmental parameters, whereas the standard Hjulstrom probably represent scour holes formed by a horseshoe-vortex sys- (1935) curve that relates the threshold u directly to the grain diame- tem (Richardson, 1968). Such scours provide strong evidence that ter is restricted to use on Earth (Miller and others, 1977; Miller and the causative flows achieved high Reynolds numbers and a high Komar, 1977). Once in motion, a sediment grain in a water flow on degree of turbulence (Baker, 1978a). Mars will have a much lower settling velocity than on Earth, again, Attempts have been made to analyze the patterns of Martian due to the reduced gravity. The consequence of this is that it might channel morphology and channel networks, usually comparing the be transported at high rates as suspended load, rather than at results with terrestrial river systems. Trevena and Picard (1978) slower rates as bed load. Komar (1980a) employed mathematical compared braided Martian outflow channels with braided terres- criteria for distinguishing grain sizes that are predominantly in bed trial rivers, the comparisons involving the numbers and dimensions load versus suspension and concluded that, in the flows that carved of the islands and various braiding and channel width indices. They the outflow channels, gravel and cobbles could have been trans- concluded that the braiding in the Martian channels was probably ported in suspension and sediment of sand size and finer would of fluvial origin, the analysis ruling out tectonic fractures as sug- have been in the wash load, permitting it to be carried at very high gested by Schumm (1974). Weihaupt (1974) analyzed the assumed concentrations. meandering of Nirgal Vallis, measuring the meander wavelength Much of the erosion would have been into bedrock, just as and then employing a wavelength versus flow discharge empirical occurred in the Channeled Scabland. Baker (1979) in particular relationship of Dury (1965) to estimate the causative discharge, analyzed the processes of bedrock erosion by channelized water arriving at a volume comparable to that of the Mississippi River. In flows on Mars, including the development of macroturbulence that addition to the probable importance of nonfluvial processes in the forms longitudinal grooves, inner channels, and cataracts. Baker Nirgal meanders, it is questionable whether an empirical equation also examined the potential for cavitation, a process that can based on terrestrial river meandering can be applied to the mean- rapidly erode rock. He concluded that the combination of lower dering of Martian channels. Dimensional analysis and the various gravity and reduced atmospheric pressure would have allowed flu- theories of channel meander generation indicate that gravity will vial cavitation to occur at much lower flow velocities than are have an effect on the meander geometry resulting from a certain required in terrestrial rivers. flow discharge (Komar, 1980b).

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Debris Flows and Mudflows steep energy gradients whereby kolks and localized cavitation processes can concentrate scour into high-intensity plucking of bedrock Several investigators have indicated from different lines of obstacles. Second, a much lower-energy vortex can be developed analysis that selected Martian channels may have been scoured or transverse to the mainstream flow direction, and this vorticity gives otherwise formed by fluids containing a high concentration of rise to transverse bed forms both in river channels or in rapidly debris or mud (Nummedal, 1978; Thompson, 1979; Nummedal and draining lakes. At higher energy levels, this transverse vortex can be Prior, 1981). Fluid flows with high sediment concentration are gen- stretched or bent around large bedrock obstacles. Although no new erally turbulent down a steep energy gradient,- but if enough sedi- vorticity is created, the stretching tends to intensify the transverse ment or debris is entrained, as may be possible under reduced- vortex, and this intensification gives rise to horseshoe-vortex scour gravity flows on Mars, the flows may become laminar or pluglike, features observed around large bedrock obstacles. Third, certain with very high debris capacity but little erosive power. Many of the hydraulic criteria allow the growth of stable secondary vortices in channels emanating from well-defined source regions in the Mars the sense of longitudinal rolls. This alignment of vorticity may be chaotic terrain are envisioned by some to have been analogous to responsible for longitudinal grooving and terracing in channel bot- submarine debris flows on Earth (Nummedal and Prior, 1981), with toms as well as relate to the initiation of meandering or braiding of sediment being dropped along the channel course until a final the channel along a given reach. For Mars, it is important to debris-depleted fluid anastomoses and erodes the downstream develop the stability criteria that dictate the potential growth of reaches of the channels. these three types of vorticity and to identify in particular how the Sediment-transport relations or even concentration distribu- concentration of sediment or debris alters this stability either by tions are not available for flooding and debris-flow events on Earth enhancing flow characteristics or by revising the effective rheology because generally large-scale catastrophic events do not erode or and erosion potential of the fluid. transport debris in the same way as does an alluvial stream flowing under equilibrium. Furthermore, no sediment-transport relations Glaciers and Ice Sheets are well enough defined to allow full prediction of river behavior, let alone that of debris flows. Hence, it is an even more difficult The flow and erosion by glaciers or ice sheets on Mars has only problem to hypothesize realistic flow and erosion characteristics on recently been addressed formally (Lucchitta and others, 1981; Luc- Mars due to additional unknowns of debris particle size, bedrock, chitta, 1982). The reasons for this lack of attention are numerous. lithology, fluid depth, surface gradient, and fluid "lifetime" in a Glaciers, thick enough to be capable of internal deformation and low-pressure environment. flow, of slip at the bed, and of significant erosion, do not appear Thompson (1979) analytically treated the flow stability of a possible under current Martian environmental conditions of very highly sediment-laden fluid, modeled as having a continuously cold surface-temperature, no free source of water, and paucity of stratified viscosity profile. The resulting secondary flow pattern was atmosphere (Sharp, 1974). These constraints are most severe in particularly interesting. For a model of Tiu Vallis, he found that polar areas. For the equatorial areas, where most outflow channels mudflows and debris flows have a much greater chance of develop- occur, some of the restrictions are considered less severe by Luc- ing longitudinal rolls to cause grooving than do clear-water flows, chitta (1982). Catastrophic flooding models survive criticism pri- glacier ice, or lava flows (also represented by different but variable marily because the suite of features observed in Mars channels is viscosity profiles). The analysis points out the need for better consistent with a flooding model. Many Mars channel features are understanding of sediment-transport mechanics under Martian also consistent with glacier or ice-sheet flow as it is understood on conditions. The answers also seem to be in line with Nummedal's Earth, but prominent transverse ridges indicative of end moraines (1978) mudflow model. are absent on Mars, even though large sets of longitudinal grooves, Nummedal (1980) approached the problem of origin of the similar to morainal glacial flutes, are present. In addition, assuming Chryse channels from the idea that the fluid itself has an identifiable current Mars heat flow, an ice sheet thick enough to become tem- rheology, not explicitly tied to any sediment distribution. The perate at the bed and thereby flow (under reduced gravity) and rheology of quick clays (represented as a Bingham plastic) provides erode effectively would be thicker than allowed by the current total a representative model flow for subaqueous Mississippi Delta mass balance of water on Mars. If ice were in equilibrium with debris flows that leave remnant features strikingly similar to those brines, however, ice sheets need not have been excessively thick in observed in many Martian outflow channels. The slide scars and order to be temperate at the base. Of course, conditions might have flow chutes of the Mississippi Delta front are juxtaposed in patterns been different during the Martian past, in which case the flow of ice similar to those of the chaotic terrains and outflow channels on on Mars would have been facilitated. Mars. Moreover, a similar disparity between source and channel Claims of introductory textbooks notwithstanding, the actual volume is found on the Mississippi Delta and on Mars. In the mechanics of erosion and debris transport by glaciers is still poorly terrestrial submarine setting, this occurs because the initial failure understood. Several recent lines of evidence point to the develop- scar expands upslope by retrogressive flow sliding, and downslope ment of structured, secondary flow patterns at the base of glaciers by undrained loading mechanisms (Nummedal and Prior, 1981). or ice sheets that may be responsible for erosion or till deposition The size of the channel, therefore, actually bears no relationship to and deformation; but these patterns relate to glacier erosion or the "scour potential" of the fluid going through it. The delta chan- ice-stream instability on Earth, where some aspects of intraglacial nels are essentially the product of endogenic processes, in the termi- water flow and shear heating in temperate ice are understood. Such nology of Sharp and Malin (1975). analysis is not understood for subpolar or polar ice sheets, wherein Scour from catastrophic flooding or from more structured tur- temperature distributions radically affect the dynamics of the ice bid flow with high sediment concentration appears to be able to mass and its erosive capability. If an ice sheet is frozen to the bed originate from three different distributions of flow vorticity. First, (the expected case on Mars), its internal deformation does not cause vertical vortices may develop under very high flow regimes down scour and erosion. Dynamics of glaciers under Martian conditions

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have not been addressed and, in fact, better valley profiles and have been either perseverance-limited or release-limited. The gradients are needed before any of the even simplified dynamics former case includes the valley networks for which relatively slow models would be meaningful. Whether ice could have played a release from the Martian ground-ice reservoirs poses few problems. significant role in the sculpturing of Martian channels will, there- Temperature changes, volcanic heating, and other phenomena can fore, remain a question to be answered in future investigations. release the water; the problem is in maintaining prolonged flows on the planet's surface. Prolonged surface flow requires such assump- Discussion tions as a warmer, denser ancient atmosphere (Pollack, 1979), ice- covered rivers (Wallace and Sagan, 1979), or freezing-point Perhaps the ultimate test of various physical flow systems pro- depressants in the water. For the outflow channels, the formative posed for outflow channel genesis is the test of rationality. Granted mechanism is release-limited. Short-duration floods are less that a variety of fluid-flow models can be stretched to explain some severely constrained even by the present Martian atmosphere. The of the erosional and depositional features in the channels, investiga- major problem is in yielding the immense quantities of water tors nonetheless should remember that explanation in science is not implied by the morphology of the outflow channels. The proposed synonymous with truth. Comfort in a scientific hypothesis derives release mechanisms described above all involve an extensive planet- not from what it explains but rather from the lack of an alternative wide permafrost system rich in ground ice (Soderblom and Wenner, hypothesis that better explains all the unknown phenomena. To 1978). Nevertheless, the precise release mechanism for fluids to date, no such alternative has emerged for the catastrophic-flood form outflow channels remains highly speculative. model with its as-yet-unknown components of debris and ice The channels and valleys of Mars probably indicate an ancient transport. epoch of Martian atmospheric environment with higher temperatures and pressures than at present. If a ground-water flow system SOME IMPLICATIONS was involved in channel and network formation, the cessation of that system indicates that major changes occurred in the hydrologic On the basis of our present knowledge of the solar system, cycle and therefore in the hydroclimate on Mars. However, the Mars is the only planet besides Earth that displays surface geologic range of possible states in the responsible fluid flows does not allow evidence of past fluvial activity. This fact implies unique histories an exact estimate of atmospheric conditions to be derived solely for the geologic evolution of the third and fourth planets from the from the geomorphological considerations presented in this paper. sun. Either volatile outgassing produced more water on these than Channels, valleys, and related geomorphic features are consistent on other planets, or climatic conditions and reservoir characteristics with the hypothesis that Mars possessed or retains a thick, ice-rich permitted its sustained retention. On Earth, world-wide surface permafrost. The data suggest that either Mars was volatile-rich reservoirs and recirculating ground water and atmospheric cycles during its early history, especially the period of valley network form the hydrologic system. This system, in turn, continuously formation, or that the planet was extremely effective at recycling its drives most surface geological processes and sustains the operations limited inventory of water. of the biosphere. The hydrologic system has operated as far back as we can trace geologic history and continues with unabated effi- ACKNOWLEDGMENTS ciency. On Mars, a frozen surface-water reservoir occurs at the north polar cap, and there is strong geomorphic evidence for wide- The research reported here was supported by various grants spread permafrost and discontinuous episodes of surface fluid flow. through the Planetary Geology Program, Mars Data Analysis Pro- These episodes appear to have occurred throughout most of the gram, and the Viking Mission of the National Aeronautics and history of Mars but with decreasing frequency over time. Space Administration. Joseph M. Boyce and Stephen E. Dwornik A central theme in the evolutionary history of any planet is its were the key program officials who encouraged and facilitated our budget of volatiles and the history of that budget. Do the fluvial efforts. We thank R. L. Hooke, R. P. Sharp, and R. L. Shreve for studies provide estimates for the amounts of water once present? helpful reviews of the manuscript. Do they identify the temporal variations in the water budget? Do they demonstrate the existence of a (past) hydrologic cycle? Do they identify the role of meteoric water? REFERENCES CITED Some answers have emerged. Large quantities of liquid water must have existed at the time of primary channel formation. The Baker, V. 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MANUSCRIPT ACCEPTED SEPTEMBER 17, 1982

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