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Channels in the Solar System
Pkunrf. Space Sci., Vol. 44. No. 8. pp. 801-815, 1996 Copyright !Q 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0032-0633/96 $15.00+0.00
Pergamon
PII: S0032-0633(96)00010-4
Channels in the Solar System Goro Komatsu’*2*3 and Victor R. Baker1,4 ’ Lunar and Planetary Laboratory, University of Arizona, Tucson. AZ 85721, U.S.A. ’ Geological Institute. University of Tokyo, Tokyo 113, Japan ’ School of Sciences and Applications, International Space University, 67400 Illkirch, France ’ Department of Geosciences, University of Arizona, Tucson, AZ 85721, U.S.A. Received 17 July 1995; accepted 13 December 1995
Abstract. Among the major discoveries made by planetary missions, channel structures. formed by a wide range of fluids such as water and lavas, have proven to be surprisingly common. Recent identification of gigantic flood channels in continental regions and submarine channels on the continental shelves provide new channel categories to an already rich inventory of terrestrial channels. Sinuous rifles on the Moon, once hypothesized to have formed by water action, now constitute an important lava channel type. Mars has abundant evidence of an ancient warm--wet climate recorded in the channels, including an outflow channel which is the widest (z 200 km) known in the Solar System. Some lava channels observed on the flanks of the huge Martian shield volcanoes seem to have been modified by water. The Magellan radar mission revealed a wide range of volcanism on Venus. Most unusual is the great variety of channels produced by large-scale, low-viscosity lava eruptions. This includes potentially the longest channel in the Solar System (6800 km). Venusian atmospheric conditions alone do not produce sufficient differences in the lavas’ cooling rates to explain the formation of many Venusian channels. The origin of these channels must be explained by eruption parameters, probable high discharge rates, high temperatures, long duration, and the possible involvement of very fluid silicates or lavas of exotic composition. Copyright t’i 1996 Elsevier Science Ltd
Introduction We define channels as sinuous troughs formed by a wide range of fluids on planetary surfaces. Rivers have been the only well-known channel structures to human beings
Correspondenc~eto : G. Komatsu
for most of their history, but there are still many mysteries about how they were formed and how they behave. In the last 40years planetary exploration and the development of new technologies, such as radar and sonar, contributed to the discovery of rich inventory of channels in the Solar System. The morphology and scale of planetary channels provide key indicators for genetic flow processes. These indicators are used to estimate the scale of the phenomena, including flow rates, fluid properties. and erosional/ depositional history. In this paper, we will use indicators from recently discovered channels to illustrate these phenomena. Fluvial channels Rivers display a remarkable variety of channel patterns that are especially amenable to study using airborne and spaceborne remote sensing systems. The patterns relate to large-scale conditions of climate and tectonism that can only be appreciated on a global perspective. It is remarkable that, despite the geologic dominance of “big rivers” (Potter, 1978). it is precisely those rivers that have received the least study. Experimental work by Schumm ( 1977) has done much to increase our understanding of channel patterns. Pattern adjustments, measured as sinuosity variation, are closely related to the type, size, and amount of sediment load. A variety of pattern classifications can be proposed (Schumm, 1981). However, the immense complexity of natural fluvial systems appears to defy our present understanding (Baker, 1978a; Hickin, 1983). For this reason, the classification employed in Table I must be considered tentative. Meandering pattern
Meandering meandering
is the most common river pattern, and rivers develop alternating bends with an
G. Komatsu and V. R. Baker: Channels in the solar system
802 Table 1. Simple classification of terrestrial channel patterns
Channel type
Examples
Meandering Braided
Mississippi River, U.S.A. Colville River, Alaska, U.S.A. Yukon River, Alaska, U.S.A. Brahmaputra River, India Burke and Hamilton Rivers, Australia Cooper Creek. Australia Yangtze River, China Kosi Fan. India Niger River, Mali Pantanal, Brazil
Anastomosed
Distributary
irregular spacing along the valley trend. Such rivers tend to have relatively narrow, deep channels and stable banks. The system adjusts to varying discharge by vertical accretion on its floodplain and/or by lateral migration of its channel. A vast complex of floodplain depositional features is associated with such rivers, as illustrated by the Mississippi River.
patterns can be considered to be composed of multiple interconnecting channels separated by relatively stable areas of floodplain (in the case of alluvial streams) or bedrock (in the case of bedrock streams). In contrast, braided patterns are single-channel, multiple-thalweg systems with bars of sediment or vegetated islands around which flow is diverted in the channel.
Distributary
pattern
Distributary patterns occur where fluvial systems are spreading water and sediment across depositional basins. Two varieties are fans and deltas. Fans (Bull, 1977) develop in Piedmont areas under the influence of both tectonic and climatic controls. Arid-region alluvial fans are constructed by infrequent depositional events that include both debris flows and water flows. The Kosi Fan of Nepal and India (Fig. 1) is an excellent example of an immense humid-region alluvial fan related to the Himalayan tectonism.
Braided pattern Braided rivers have channels divided into multiple thalwegs by alluvial islands. Braided rivers tend to have steeper gradients, more variable discharge, coarser sediment loads, and lower sinuosity than meandering streams. Their channels tend to be relatively wide, and shallow. Braided patterns are “. . . developed depositionally within a channel in which the flow obstructions are sand and gravel deposited by the water moving around them” (Garner, 1974, p. 435). Midchannel bars are emplaced because of local flow incompetence. The resulting braid channels formed by splitting the flow are more competent than the original channel for conveying the load downstream (Leopold et al., 1964). Another way of describing braiding is that it is caused by channel widening that increases the boundary resistance of rivers with noncohesive banks (Church, 1972, p. 74). To maintain enough velocity for sediment transport in a wide, shallow cross section, the channel must divide and form relatively narrow and deep secondary channels through incision. Excellent examples of braiding occur in gravel-transporting rivers, such as Yukon, Colville, and upper Kosi.
Anastomosedpattern Many multichannel rivers have relatively low gradients, deep and narrow channels, and stable banks. Such river systems have been termed “anastomosed” (Smith and Smith, 1980). The terminology is a bit confused because “anastomosis” is a general designation for interconnected channelways whether in alluvial or in bedrock rivers. Thus, Garner (1974) (p. 435), following Bretz (1923), defined an anastomosing channel system as “. . an erosionally developed network of channels in which the insular flow obstructions represent relict topographic highs and often consist of bedrock”. Therefore, anastomosing
Catastrophicflood
channels
Earth. The Channeled Scabland of the north-western United States consists of a spectacular complex of anastomosing channels, cataracts, loess “islands”, and immense gravel bars created by the catastrophic fluvial erosion of the loess and basalt of the Columbia Plateau (Fig. 2) (Baker, 1978b). Breakout floods from the glacierdammed Lake Missoula occurred repetitively during the late Pleistocene. The resulting peak discharge was as great as 17 x 106m3 s-’ (O’Connor and Baker, 1992). Recent investigations on the Eurasian Continent revealed that the catastrophic flooding has not been restricted to the North American Continent. Baker et al. (1993) studied flood features in Altai Mountains and concluded that water levels had to exceed 1900m in order to explain some landforms in some constricted reaches, and the peak discharge may have exceeded 18 x 106m3 s-’ which is the largest known on Earth. Mars. Martian outflow channels are large-scale complexes of fluid-eroded troughs (Baker, 1982). The flows which formed these channels appear to have emanated from discrete collapse zones known as chaotic terrain (Fig. 3a). The channels are immense by terrestrial standards, as much as 200 km wide and 2000 km in length (Fig. 3b). Residual uplands separating channels and partially to fully streamlined upland remnants are common in outflow channels (Fig. 3~). The morphology and scale of the outflow channels clearly indicates genesis by fluid flows of immense magnitude. It is most likely that outflow channels were curved by catastrophic water floods (Baker, l982), although secondary modifications by other fluids such as glaciers and wind are possible. The peak discharge could have been greater than 109m3 for the Kasei Valles, the largest outflow channels on Mars (Robinson and Tanaka, 1990).
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Fig. 1. The Kosi Fan to the south of Himalaya Mountains. This is an excellent example of azimmense humid-region alluvial fan forming a distributary pattern. Landsat image 2751-03455-7. Image width is about 180 km Fig. 2. The Channeled Scabland in the eastern Washington State. The greatest flood in the geological record of river activity was caused by the catastrophic failures of the glacier-dammed Lake Missoula during the Pleistocene. The channels appear dark on the Landsat scene because the loess cover was locally eroded. exposing the dark basalt. Landsat image 10624-l 13. Image width is about IX0 km
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Fig. 3. An example of outflow channels on Mars. This outflow channel called Ares Vallis is ch; Lractel -ized by chaotic source area (a), very wide reaches (b), and streamlined hills (c). Viking Orbit er imag ;e (a) 689A03, (b) 689A02. (c) 827A23
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0 II
in the solar system
50 Km
Fig. 3. (Conrind) Fig. 4. An example of paleochannels discovered in SIR-A imagery in the eastern Sahara region. This scene is a strip of SIR-A radar image placed over a Landsat image. The radar penetration up to several meters reveals paleoriver systems under extremely dry drift sands
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Fig. 5. Submarine channels discovered on the submarine fan off the Amazon River by the sidelooking sonar system GLORIA. Submarine channels are considered to have formed through the action of turbidity currents. Normark et a/. (1993), copyright by the American Geophysical Union Fig. 6. Lava channels (arrows) on the flank of Mauna Loa, Hawaii. Image width is about 2800 m
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Fig. 7. (a) Examples of lunar sinuous rilles. Sinuous rilles originate from single or multiple collapsed pits and tend to narrow and shallow downstream. Lunar Orbiter image V-190M. Image width is about 60 km. (b) A comparison of a sinuous rille (A), and typical terrestrial-type lava channels (B) on the Moon. In contrast to typical terrestrial-type lava channels, many sinuous rilles do not have clear association with flow units. Sinuous rilles tend to have much larger depth-to-width ratio than typical terrestrial-type lava channels. The meanders of sinuous rilles tend to have high width-towavelength ratio compared with typical lava channels. Apollo 15 Metric Camera image 1702. Image width is about 165 km
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Fig. 8. Lava channels with clearly observable levees/lateral flow deposits on Venus. These lava channels tend to shallow and narrow downstream and it is rare that they have an observable depth. Magellan image Cl_6OS347 Fig. 9. Venusian sinuous rilles. Venusian sinuous rilles have similar morphologies as lunar sinuous rilles. Sinuous rilles’ depths range up to several hundred meters. Magellan image Cl_158095 Fig. 10. Widest reach of the Kallistos Vallis on Venus. The Kallistos Vallis was originally named “the Outflow Channel” because of its morphological similarity with Martian outflow channels curved by catastrophic floodings of water. They share commonalty such scale, collapsed pit sources, wide main reaches, anastomosing reaches, and streamlined hills. However it is more likely that Kallistos Vallis has volcanic origin due to the fact that it has radar bright channel levees/lateral flow deposits near the terminus, which is characteristic of lava channels. Magellan image Cl_ ._458032. Image width is about 270 km
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Fig. 11. A canali-type channel on Venus. Canali morphology is unique in the Solar System. They are characterized by nearly constant width for hundreds of thousands of kilometers. shallow depth, Ruvial-like features (e.g. oxbows. lateral migration patterns. point bars). fan-shaped terminal deposits and no clear association with flow units. Magellan image ClL3OS153 Fig. 12. Channels on the flank of Hecates Tholus. a shield volcano on Mars. These channels are characterized by the dendritic patterns. It has been suggested that these channels may have a fluvial origin. Another possibility is that they may have originated as lava channels but later modified by fluvial processes. Viking image 65 1A I8
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G. Komatsu and V. R. Baker: Channels in the solar system Paleochannels discovered b)l radar observation Discovery of ancient river paleochannels in the Sahara desert (Fig. 4) (McCauley et al., 1982, 1986; Schaber et al., 1986) by Space Shuttle imaging radar systems is an excellent example of how new remote sensing technology contributes to identification of channels on the planetary surfaces. Radar responses from bedrock and gravel surfaces beneath windblown sand several centimeters to possibly meters thick delineate sand and alluvium filled valleys, some nearly as wide as the Nile Valley and perhaps as old as middle Tertiary (McCauley et al., 1982).
Submarine channels are relatively recent discoveries (Fig. 5) (Flood and Damuth, 1987 ; Mienert et al., 1993). They were discerned by side-looking sonar systems, such as GLORIA. Submarine channels occur in close association with submarine fans (Flood and Damuth, 1987), and are considered to have formed through the action of turbidity currents (Normark et al., 1993). Their meandering characteristics are similar to those of terrestrial fluvial channels (Clark et al.. 1992).
Lava channels Earth Present-day terrestrial volcanism produces only a limited range of lava channel morphologies (Fig. 6). Although some lava tube systems exhibit complex morphologies (e.g. Hatheway and Herring, 1970), they are generally simple and short (e.g. Mauna Loa lava channels (Greeley, 1974)). Terrestrial lava channels are generally constructional in origin. Thermal erosion has more rarely been observed in some terrestrial lava flows. Swanson (1973) inferred that as much as 15 m of material was eroded in the master tubes around Mauna Ulu, Hawaii, during a 9 month period. Terrestrial flood basaltic volcanism forms extensive lava tubes and channels, as illustrated by the Snake River Plains (Cattermole, 1989). On Earth, weathering processes could possibly have modified ancient, very large lava channels beyond recognition. Any channelized structure is subject to intense secondary erosion, particularly by water. Burial by sediments is another way of making channels unrecognizable. Komatiite is a lava very rich in MgO, with a very high melting temperature. and a low viscosity. Komatiite normally occurs in association with Archean shield regions, such as in western Australia and Canada. Based on the high eruption temperature of komatiite, it has been argued that the Archean Earth had higher interior temperatures than present. Examples of lava channels formed by komatiite have been recognized on the western Australian Archean shield (Barnes et al., 1988a. b ; Huppert and Sparks. 1985). Komatiite lavas can erode ground materials of lower melting temperatures quite efficiently
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(Huppert et al., 1984; Huppert and Sparks. 1985). Locally, komatiite flows have thermally eroded felsic ground rocks to depths in excess of 1OOm (Barnes et al.. 1988a) in the Yilgarn Block, a 1000 x 700 km quasicircular Archean shield consisting of granitic intrusive rocks and greenstone belts.
Moon The sinuous rilles (Fig. 7a) are one of the most interesting volcanic landforms on the Moon. A fluvial origin was suggested by Lingenfelter et al. (1968). However. the common association of sinuous rilles with volcanic plains, and their morphological similarities to terrestrial lava channels leads most investigators to interpret them as lava drainage channels (e.g. Strom, 1966). The lunar sinuous rilles have their source areas either on the perimeter of the mare surface or close by in the highlands, and the emanating lavas flowed consistently toward the inner parts of maria (Guest and Murray. 1976). Sinuous rilles narrow and shallow downstream, originate from collapsed pits. These are characteristics often observed for terrestrial lava channels. Nevertheless, there are important differences between sinuous rilles and typical terrestrial-type lava channels (Figs 6 and 7a and b). In contrast to typical terrestrialtype lava channels, many sinuous rilles do not have clear association with flow units. In general, sinuous rilles have much larger depth-to-width ratios. The meanders of sinuous rilles have high width-to-wavelength ratios compared with typical terrestrial-type lava channels (Komatsu and Baker, 1994a). Oberbeck et al. (1969) and Greeley ( 1971) thought sinuous rilles may have originated as lava tubes based on the observation that there are chains of pits which may evolve to sinuous rilles. Hulme (1973, 1982) and Carr (1974) proposed thermal erosion as an important formation mechanism for the sinuous rilles. Many lunar basalt rocks are enriched in Ti and Fe, and this makes the basalt a high temperature, low viscosity lava. These are factors favorable for thermal erosion. In addition, the size dimensions of the sinuous rilles are very large compared with those of terrestrial lava channels. This may mean that the eruption rates were very high, which would also contribute to high thermal erosion rates.
Venus The first Venusian channel was discovered on the radar images of the Venera 15/ 16 mission. However, this channel was mapped as a tectonic feature (Kotelnikov et al., 1989) or as a narrow depression (Sukhanov et ul., 1989). and it was named “Hildr Fossa”. Among the major discoveries made by the Magellan radar mapping mission. the channels and valleys are one of the most unexpected. The very first channel identified in Magellan imagery. named later a canali-type channel, was sinuous, had a constant width, and no tributaries. It had no clear association with flow units, and was very long ( > 500 km) (Head et NI., 1991). This unique combination of characteristics has never been seen in terrestrial or Martian fluvial chan-
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nels, nor in lava channels on the terrestrial planets. These unique morphologies attracted the attention of researchers, leading to hypotheses concerning the channel’s origins, considered as a type of lava channel (Komatsu et al., 1992; Baker et al., 1992; Gregg and Greeley, 1993 ; Komatsu et al., 1993; Kargel et al., 1994). Later in the mission, researchers discovered a wide range of channel morphologies (Komatsu et al., 1993). Lava channels on Venus have the widest range of morphologies in the Solar System. These channels probably represent a large spectrum of lava emplacement processes. Eruption factors, such as chemical composition, eruption temperature, eruption rate and duration, in addition to environmental conditions such as ambient temperature, atmospheric pressure, and topography, must have varied to form the remarkable range of morphologies. There are a number of Venusian channels similar to typical terrestrial-type lava channels (Fig. 8). They have clear association with flow units (levees/lateral flow deposits) and tend to narrow and shallow downstream. Although their scales are often an order or two of magnitude larger than those of terrestrial counterparts, these Venusian lava channels are considered to have a similar formation mechanism to terrestrial ones. Sinuous rilles (Fig. 9) were so named because of their close resemblance to lunar sinuous rilles. They narrow and shallow downstream, they originate from collapsed pits, have deep sections (up to several hundred meters), and normally do not have a clear association with flow units. Meander characteristics separate many of the Venusian and lunar sinuous rilles from Venusian canali and from lava channels on Earth (Komatsu and Baker, 1994a; Kargel et al., 1994). This was attributed to the possible involvement of erosional processes, particularly thermal erosion, in sinuous rille formation. Their close association with coronae, which are possible mantle upwelling or mantle diapir related features (Stofan et al., 1992) indicates that the channel-forming lavas may have come form mantle. Kallistos Vallis (Fig. lo), originally named “the Outflow Channel”, deserves special attention (Baker et al., 1992 ; Parker et al., 1996). This channel is wide, originates from a collapsed pit, and has braided reaches. These characteristics are typical of Martian outflow channels, and this implies the channel’s origin by high discharge flows. Kallistos Vallis forms radar bright levees/lateral flow deposits at its terminus, which indicates that the channel was formed by lava. Canali-type channels (Fig. 11) remain a mystery. It has been found that mafic lava, such as tholeiitic lava would not retain turbulence, which is probably required for a channel’s erosional origin, over the observed channel length (Komatsu et al., 1992). Ultramafic lavas have the same problem. However, mafic and ultramafic lava can flow the required distance with a thickened crust (Komatsu et al., 1992). Increased capacity of Venusian atmosphere to remove heat helps thickening of a crust which may promote longer flow length (Gregg and Greeley, 1993). Calculations suggest that canali-type channels may have formed by constructional processes, and some of the canali may have roofed (Komatsu et al., 1992 ; Gregg and Greeley, 1993). However, a lack of clear association with flow units (absence of levees/lateral flow deposits) has to
G. Komatsu and V. R. Baker: Channels in the solar system
be explained (Komatsu et al., 1992, 1993). An alternative explanation is that canali were formed by lavas that are exotic, by terrestrial standards (Komatsu et al., 1992; Baker et al., 1992 ; Kargel et al., 1994 ; Treiman, 1994 ; Gregg and Greeley, 1994). Sulfur would retain its fluidity and carbonatite may also stay fluid for a long time under Venusian conditions. Both have very low viscosities at their melting temperatures. Some canali have deformed longitudinal profiles (Baker et al., 1992 ; Komatsu and Baker, 1994b). These profiles are evidence of the regional tectonic movement and provide clues to understand the tectonic history of the region after the channel formation.
Mars
Martian lava channels are observed on the flanks of shield volcanoes such as Alba Patera (e.g. Cattermole, 1987). They are simple to complex, and mostly associated with clearly observable flow margins. They are thought to have formed by tube-fed flows (Cattermole, 1987). There are anastomosing channel networks on some shield volcanoes, such as Alba Patera (Mouginis-Mark et al., 1988 ; Gulick and Baker, 1990) and Hecates Tholus (Fig. 12) (Mouginis-Mark et al., 1982). These may originate as lava channels but Mouginis-Mark and co-workers considered fluvial origin instead based on their morphology and the fact that they are incised into the relatively unconsolidated materials compared with lava rocks. Gulick and Baker (1990) discussed fluvial modification of lava channels on the flank of Alba Patera by citing similarities between these channels and valleys on Hawaiian volcanoes.
Discussion
Other types of channels formed by water include meltwater channels on glaciers (Ewing, 1972) and channels formed by the solution of limestone by carbon dioxide rich water (Sweeting, 1972). These are orders of magnitude smaller than other fluvial channels, and not particularly significant in planetary landscapes. Based on preliminary mapping of Venus, Earth, Mars, and the Moon, we know that lava channels are a common phenomenon on the terrestrial planets. However, their morphologies vary widely. Venus has the widest range of lava channel morphologies in the Solar System (Tables 2 and 3), almost comparable to that for terrestrial fluvial systems (Baker et al., 1992; Komatsu et al., 1993). Atmospheric conditions (environmental factors) for Venus, particularly its high ambient temperatures, are important factors. As Head and Wilson (1986) estimated, the average flow length on Venus would be about 10% longer than its counterpart on Earth, if other conditions are held constant. Venus’ atmosphere actually increases the cooling rate because of efficient heat removal from flow surface in the dense atmosphere, resulting in enhanced crustal growth (Head and Wilson, 1986). Diverse eruption conditions, driven by the catastrophic resurfacing (Schaber et
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Table 2. Lava channel morphology
Meandering Braided Oxbows Streamlined hills Distributary Tributary 0. observed
Venus
Moon
Mars
Earth (modern)
Earth (komatiite) ~~_~~ NK NK NK NK
Earth (fuvial)
0 0 0 0 0
0 0 NK 0 0
0 NK NK NK NK
0 0 NK NK NK
NK
NK
NK
NK
NK NK
0 0
Earth (fluvial)
0 0 0 0
; NK. not known.
Table 3. Maximum
dimensions
Length
Width Depth
of lava channels
Venus
Moon
Mars
Earth (modern)
Earth (komatiite)
-6800km
-3OOkm
NK -5OOm
-5km -3Om
NK -5OOm
- 6700 km -- 20 km” - 150kmh
NK
NK
- 1OOm
-_ 60 m” - 100mh
-3Okm -3OOm
-1Okm -3OOm
NK. not known. i’ River. ‘Catastrophic floods. al., 1992) and subsequent mantle plume volcanism, are probably the most important factors responsible for the diversity of channel morphologies on Venus. Are there channels on other terrestrial planets? Mercury has extensive evidence of volcanism (Strom, 1984), but there are no lava channels observed on the half of Mercury observed by Mariner 10. This is important because the maximum resolution in Mariner 10 images should allow recognition of lava channels at a scale equivalent to some of the largest lunar sinuous rilles (R. Storm, personal communication). This may be due to the possible difference in the volcanic style. In particular, the Mercurian surface is considered to be low in Ti and Fe compared with the Moon (Hapke et al., 1975). Other potential objects which may have channel structures in the Solar System are outer Solar System satellites such as IO and Triton, and asteroids such as Vesta. These objects are known to have experienced volcanism on their surfaces. The volcanism on outer Solar System satellites is quite exotic. IO is erupting sulfur-rich lava, and lavas on icy satellites are mixtures of a variety of ice compositions. Whether these objects have channel structures or not has to wait for future planetary missions.
Conclusions
Channels are common features in the Solar System. Fluvial channels are found on Earth and Mars. Recent studies have focused on the paleochannels. In the late Pleistocene. catastrophic outburst of water from the glacier-dammed lakes scoured bedrock and formed channels on the North American and Eurasian continents. Use of spaceborne radar in the Sahara desert revealed a number of paleochannels under the sand. Submarine channels are considered to be part of turbidite systems, but their formation mechanism is not well understood.
Lava channels are known to exist on the Moon and all terrestrial planets except for Mercury. In particular, lava channels on Venus are comparable to the fluvial channels on Earth in their morphology and scale. This suggests that volcanism can produce the range of channels formed by water. Acknowle~yements. This project was supported by National Aeronautics and Space Administration through the Venus Data Analysis Project, grant NAGW-3515, and the Planetary Geology and Geophysics Program. grant NAGW-285. Goro Komatsu is a Research Fellow of Japan Society for the Promotion of Science for Young Scientists, Ministry of Education.
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Petrol. Geologists Mern. 5, 21 l-230. 1978a. Baker, V. R., The Spokane Flood controversy and the Martian outflow channels. Science 202, 1249-1156. 1978b. Baker, V. R., The Churznels uj’Mar.s, 198 pp. Llniversity of Texas Press, Austin, Texas. 1982. Baker, V. R., Komatsu, G., Parker, T. J., Gulick, V. C., Kargel, J. S. and Lewis, J. S., Channels and valleys on Venus : preliminary analysis of Magellan data. J. G’roph,rs. Res. 97, 13421.-13444. 1992. Baker, V. R., Benito, G. and Rudoy, A. N., Paleohydrology of late Pleistocene superflooding Altay Mountains. Siberia. Science 259, 348-350, 1993. Barnes, S. J., Gole, M. J. and Hill, R. E. T.. The Agnew nickel deposit, Western Australia: Part I. Structure and stratigraphy. Econ. Gtd. 83,524-536, 1988a. Barnes, S. J., Hill, R. E. T. and Gale, M. J., The Perseverance ultramafic complex, Western Australia the product of a komatiite lava river. J. Petdog>, 29, 305. 331. 1988b. Bretz, J H., The Channeled Scabland of the Columbia Plateau. J. Geol. 31, 617-649. 1923.
814 Bull, W. B., The alluvial-fan environment. Prog. Phys. Geogr. 1, 222-270, 1977. Carr, M. H., The role of lava erosion in the formation of Lunar rilles and Martian channels. Icarus 22, 3243, 1974. Cattermole, P. J., Sequence, rheological properties and effusion rates of volcanic flows at Alba Patera, Mars. J. Geophys. Res. 92,553-560, 1987. Cattermole, P. J., Planetary Volcanism, 443 pp. Ellis Horwood. Chichester, 1989. Church, M., Baffin Island sandurs: a study of Arctic fluvial processes. Geol. Surv. Can. Bull. 215,208, 1972. Clark, J. D., Kenyon, N. H. and Pickering, K. T., Quantitative analysis of the geometry of submarine channels : implications for the classification of submarine fans. GeologJl20,633-636, 1992. Ewing, K. J., Supreglacial streams of the Kaskawulsh Glacier, Icejeld Ranges Research Project ScientiJic Results, Vol. 3, pp. 153-162, 1972. Flood, R. D. and Damuth, J. E., Quantitative characteristics of sinuous distributary channels on the Amazon deep-sea fan. Geol. Sec. Am. Bull. 98, 728-738, 1987. Garner, H. F., The Origin qf Land.wapes, 734 pp. Oxford University Press, New York, 1974. Greeley, R., Lunar Hadley Rille: consideration of its origin. Science 172,722%725, 197 1. Greeley, R. (ed.), Geologic Guide to the Island @‘Hawaii, 257 pp. National Aero. and Space Admin., Washington, DC, 1974. Gregg, T. P. and Greeley, R., Formation of canali: considerations of lava types and their thermal behaviors. J. Geophys. Res. 98, 10873-10X82, 1993. Gregg, T. K. P. and Greeley, R., Reply. J. Geophys. Res. 99, 17165-17167, 1994. Guest, J. E. and Murray, J. B., Volcanic features of the nearside equatorial lunar maria. J. Geol. Sot. Lond. 132, 251-258, 1976. Gulick, V. C. and Baker, V. R., Origin and evolution of valleys on Martian volcanoes. J. Geophys. Res. 95, 14325-14344, 1990. Hapke, B., Danileson, G. E., Klassen, K. and Wilson, L., Photometric observations of Mercury from Mariner 10. J. Geoplzys. Res. 80, 2431, 1975. Hatheway, A. W. and Herring, A. K., Bandera lava tubes of New Mexico, and lunar implications. CO~WWZ. Lunar Planet. Lab. 8,132-159,197o. Head, J. W. and Wilson, L., Volcanic processes and landforms on Venus : theory, predictions, and observations. J. Geoph_w. Res. 91,9407-9446, 1986. Head, J. W., Campbell, D. B., Elachi, C., Guest, J. E., McKenzie, D. P., Saunders, R. S., Schaber, G. G. and Schubert, G., Venus volcanism : initial analysis from Magellan data. Science 252, 276-299. 199 1. Hickin, E. J., River channel changes : retrospect and prospect, ht. Ass. Sedimentologists Special Publication No. 6, pp. 61x3. 19x3. Hulme, G., Turbulent lava flow and the formation of lunar sinuous rilles. Modern Geol. 4, 107-l 17, 1973. Hulme, G., A review of lava flow processes related to the formation of lunar sinuous rilles. Geophys. Surv. 5, 245-279, 1982. Huppert, H. E. and Sparks, R. S. J., Komatiites I : eruption and flow. J. Petrol. 26, 694-725, 1985. Huppert, H. E., Sparks, R. S. J., Turner, J. S. and Arndt, N. T., Emplacement and cooling of komatiite lavas. Nature 309, 19-22, 1984. Kargel, J. S., Fegley Jr, B., Treiman, A. and Kirk, R. L., Carbonatite-sulfate volcanism on Venus. Icarus 112, 219-252, 1994. Komatsu, G. and Baker, V. R., Meander properties of Venusian channels. Geology 22,67-70, 1994a. Komatsu, G. and Baker, V. R., Plains tectonism on Venus : infer-
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ences from Canali longitudinal profiles. Icarus 110, 275-2X6. 199413. Komatsu, G., Kargel, J. S. and Baker, V. R., Canali-type channels on Venus : some genetic constraints. Geophys. Res. Lett. 19, 1415-1418. 1992. Komatsu, G., Baker, V. R., Gulick, V. C. and Parker, T. J., Venusian channels and valleys: distribution and volcanological implications. Icurus 102, l-25, 1993. Kotelnikov, V. A., Yashenko, V. R. and Zolotov, A. F., Atlas qf’the Surface of‘ Venus. Main Board of Geodesy and Cartography, Minsters of the U.S.S.R., Moscow, 1989. Leopold, L. B., Wolman, M. G. and Miller, J. P., Fluvial Processes in Geornorphology, 522 pp. W. H. Freeman. San Francisco, 1964. Lingenfelter, R. E., Peale, S. J. and Schubert, G., Lunar rivers. Science 161, 266-269, 1968. McCauley, J. F., Schaber, G. G., Breed, C. S., Grolier, M. J., Hynes, C. V., Issawi, B,, Elachi, C. and Blom, R., Subsurface valleys and geoarcheology of the eastern Sahara revealed by Shuttle radar. Science 218, 1004-1020, 1982. McCauley, J. F., Breed, C. S., Schaber, G. G., Mchugh, W. P., Issawi, B., Hyues, C. V., Grolier, M. J. and el Kilani, A., Paleodrainage of the eastern Sahara--the radar rivers revisited (SIR-A/B implications for a Mid-Tertiary transAfrican drainage system). IEEE Trans. Geosci. Remote Sensing GE-24, 624-648. 1986. Mienert, J., Kenyon, N. H., Thiede, J. and Hollender, F.-J., Polar continental margins : studies off east Greenland. EOS Truns. 74,225-236, 1993. Mouginis-Mark, P. J., Wilson, L. and Head, J. W., Explosive volcanism on Hecates Tholus. Mars : investigation of eruption conditions. J. Geophys. Res. 87, 989&9904, 1982. Mouginis-Mark, P. J., Wilson, L. and Zimbelman, J. R., Polygenie eruptions on Alba Patera. Mars : evidence of channel erosion on pyroclastic flows. Bull. Vol. 50, 361-379, 1988. Normark, W. R., Posamentier, H. and Mutti, E., Turbidite systems : state of the art and future directions. Rev. Geophys. 31, 91-116, 1993. Oberbeck, V. R., Quaide, W. L. and Greeley, R., On the origin of lunar sinuous rilles. Modern Geol. 1, 75580, 1969. O’Connor, J. E. and Baker, V. R., Magnitudes and implications of peak discharges from glacial Lake Missoula. Geol. Sot. Am. Bull. 104, 267-279, 1992. Parker, T. J., Komatsu, G. and Baker, V. R., Kallistos Vallis, Venus: geology of a catastrophic flood channel. In preparation. 1996. Potter, P. E., Significance and origin of big rivers. J. Geol. 86, 13-33, 1978. Robinson, M. S. and Tanaka, K. L., Magnitude of a catastrophic flood event at Kasei Valles, Mars. Geology, 18,902Z905. 1990. Schaber, G. G., McCauley, J. F., Breed, C. S. and Olhoeft, G. R., Shuttle imaging radar: physical controls on signal penetration and subsurface scattering in the eastern Sahara. IEEE Trurls. Geosci. Remote Sensing GE-24, 60462 1, 1986. Schaber, G. G., Strom, R. G., Moore, H. J., Soderblom, L. A., Kirk, R. L., Chadwick, D. J., Dawson, D. D., Gaddis, L. R., Boyce, J. M. and Russell, J., Geology and distribution of impact craters on Venus : what are they telling us? J. GeophJx. Res. 97, 13257-I 3302, 1992. Schumm, S. A., The Fluvial System. 338 pp. Wiley, New York, 1977. Schumm, S. A., Evolution and response of the fluvial system: sedimentologic implications, Sot. Economic Paleontologists and Mineralogists Special Publication, Vol. 31, pp. 19.-29, 1981. Smith, D. G. and Smith, N. D., Sedimentation in anastomosed river systems: examples from alluvial valleys near Banff, Alberta. J. Sedimentary Petrol. 50, 157-164, 1980. Stofan, E. R., Sharpton, V. L., Schubert, G., Baer, G., Bindschadler, D. L., Janes, D. M. and Squyres, S. W., Global
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distribution and characteristics of coronae and related features on Venus : implications for origin and relation to mantle processes. J. Geophys. Res. 97, 13347713378, 1992. Strom, R. G., Sinuous rilles, in Ranger VIII and IX, part II. Experimenters .4nalyses and Interpretations (edited by R. L. Heacock, G. P. Kuiper. E. M. Shoemaker, H. C. Urey and E. A. Whitaker), JPL Tech. Rept. 32-800, pp. 199-210, 1966. Strom, R. G., Mercury, in The Geology ofthe Terrestrial Planets, 3 17 pp. National Aero. and Space Admin., Washington, DC, 1984. Sukhanov, A. L., Pronin, A. A., Burba, G. A., Nikishin, A. M., Kryuchkov, V. P., Basilievshy, A. T., Markov, M. S., Kuzmin, R. O., Bobnia, N. Iv., Shashkina, V. P., Slyuta, E. N. and
815 Chernaya, 1. M., GeomorphiclGeobgic Map qf’ Part of‘ thr Northern Hemisphere of Venus. U.S. Geological Survey Misc. Invest. Ser., Map I-2059, 1989. Swanson, D. A., Pahoehoe flows from the 196991971 Mauna Ulu eruption, Kilaeuea Volcano. Hawaii. Grol. Sot. Anr. Bull. 84, 615-626, 1973. Sweeting, M. M., Karst Lanc$‘kwm.s,362 pp. Macmillan. London, 1972. Treiman, A. H., Comment on “Formation of Venusian canali : considerations of lava types and their thermal behaviors” by T. K. P. Gregg and R. Greeley. J. Gcop,pl~~x. RCS. 99, 17163 17164, 1994.
Pergamon
PII: S0032-0633(96)00010-4
Channels in the Solar System Goro Komatsu’*2*3 and Victor R. Baker1,4 ’ Lunar and Planetary Laboratory, University of Arizona, Tucson. AZ 85721, U.S.A. ’ Geological Institute. University of Tokyo, Tokyo 113, Japan ’ School of Sciences and Applications, International Space University, 67400 Illkirch, France ’ Department of Geosciences, University of Arizona, Tucson, AZ 85721, U.S.A. Received 17 July 1995; accepted 13 December 1995
Abstract. Among the major discoveries made by planetary missions, channel structures. formed by a wide range of fluids such as water and lavas, have proven to be surprisingly common. Recent identification of gigantic flood channels in continental regions and submarine channels on the continental shelves provide new channel categories to an already rich inventory of terrestrial channels. Sinuous rifles on the Moon, once hypothesized to have formed by water action, now constitute an important lava channel type. Mars has abundant evidence of an ancient warm--wet climate recorded in the channels, including an outflow channel which is the widest (z 200 km) known in the Solar System. Some lava channels observed on the flanks of the huge Martian shield volcanoes seem to have been modified by water. The Magellan radar mission revealed a wide range of volcanism on Venus. Most unusual is the great variety of channels produced by large-scale, low-viscosity lava eruptions. This includes potentially the longest channel in the Solar System (6800 km). Venusian atmospheric conditions alone do not produce sufficient differences in the lavas’ cooling rates to explain the formation of many Venusian channels. The origin of these channels must be explained by eruption parameters, probable high discharge rates, high temperatures, long duration, and the possible involvement of very fluid silicates or lavas of exotic composition. Copyright t’i 1996 Elsevier Science Ltd
Introduction We define channels as sinuous troughs formed by a wide range of fluids on planetary surfaces. Rivers have been the only well-known channel structures to human beings
Correspondenc~eto : G. Komatsu
for most of their history, but there are still many mysteries about how they were formed and how they behave. In the last 40years planetary exploration and the development of new technologies, such as radar and sonar, contributed to the discovery of rich inventory of channels in the Solar System. The morphology and scale of planetary channels provide key indicators for genetic flow processes. These indicators are used to estimate the scale of the phenomena, including flow rates, fluid properties. and erosional/ depositional history. In this paper, we will use indicators from recently discovered channels to illustrate these phenomena. Fluvial channels Rivers display a remarkable variety of channel patterns that are especially amenable to study using airborne and spaceborne remote sensing systems. The patterns relate to large-scale conditions of climate and tectonism that can only be appreciated on a global perspective. It is remarkable that, despite the geologic dominance of “big rivers” (Potter, 1978). it is precisely those rivers that have received the least study. Experimental work by Schumm ( 1977) has done much to increase our understanding of channel patterns. Pattern adjustments, measured as sinuosity variation, are closely related to the type, size, and amount of sediment load. A variety of pattern classifications can be proposed (Schumm, 1981). However, the immense complexity of natural fluvial systems appears to defy our present understanding (Baker, 1978a; Hickin, 1983). For this reason, the classification employed in Table I must be considered tentative. Meandering pattern
Meandering meandering
is the most common river pattern, and rivers develop alternating bends with an
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802 Table 1. Simple classification of terrestrial channel patterns
Channel type
Examples
Meandering Braided
Mississippi River, U.S.A. Colville River, Alaska, U.S.A. Yukon River, Alaska, U.S.A. Brahmaputra River, India Burke and Hamilton Rivers, Australia Cooper Creek. Australia Yangtze River, China Kosi Fan. India Niger River, Mali Pantanal, Brazil
Anastomosed
Distributary
irregular spacing along the valley trend. Such rivers tend to have relatively narrow, deep channels and stable banks. The system adjusts to varying discharge by vertical accretion on its floodplain and/or by lateral migration of its channel. A vast complex of floodplain depositional features is associated with such rivers, as illustrated by the Mississippi River.
patterns can be considered to be composed of multiple interconnecting channels separated by relatively stable areas of floodplain (in the case of alluvial streams) or bedrock (in the case of bedrock streams). In contrast, braided patterns are single-channel, multiple-thalweg systems with bars of sediment or vegetated islands around which flow is diverted in the channel.
Distributary
pattern
Distributary patterns occur where fluvial systems are spreading water and sediment across depositional basins. Two varieties are fans and deltas. Fans (Bull, 1977) develop in Piedmont areas under the influence of both tectonic and climatic controls. Arid-region alluvial fans are constructed by infrequent depositional events that include both debris flows and water flows. The Kosi Fan of Nepal and India (Fig. 1) is an excellent example of an immense humid-region alluvial fan related to the Himalayan tectonism.
Braided pattern Braided rivers have channels divided into multiple thalwegs by alluvial islands. Braided rivers tend to have steeper gradients, more variable discharge, coarser sediment loads, and lower sinuosity than meandering streams. Their channels tend to be relatively wide, and shallow. Braided patterns are “. . . developed depositionally within a channel in which the flow obstructions are sand and gravel deposited by the water moving around them” (Garner, 1974, p. 435). Midchannel bars are emplaced because of local flow incompetence. The resulting braid channels formed by splitting the flow are more competent than the original channel for conveying the load downstream (Leopold et al., 1964). Another way of describing braiding is that it is caused by channel widening that increases the boundary resistance of rivers with noncohesive banks (Church, 1972, p. 74). To maintain enough velocity for sediment transport in a wide, shallow cross section, the channel must divide and form relatively narrow and deep secondary channels through incision. Excellent examples of braiding occur in gravel-transporting rivers, such as Yukon, Colville, and upper Kosi.
Anastomosedpattern Many multichannel rivers have relatively low gradients, deep and narrow channels, and stable banks. Such river systems have been termed “anastomosed” (Smith and Smith, 1980). The terminology is a bit confused because “anastomosis” is a general designation for interconnected channelways whether in alluvial or in bedrock rivers. Thus, Garner (1974) (p. 435), following Bretz (1923), defined an anastomosing channel system as “. . an erosionally developed network of channels in which the insular flow obstructions represent relict topographic highs and often consist of bedrock”. Therefore, anastomosing
Catastrophicflood
channels
Earth. The Channeled Scabland of the north-western United States consists of a spectacular complex of anastomosing channels, cataracts, loess “islands”, and immense gravel bars created by the catastrophic fluvial erosion of the loess and basalt of the Columbia Plateau (Fig. 2) (Baker, 1978b). Breakout floods from the glacierdammed Lake Missoula occurred repetitively during the late Pleistocene. The resulting peak discharge was as great as 17 x 106m3 s-’ (O’Connor and Baker, 1992). Recent investigations on the Eurasian Continent revealed that the catastrophic flooding has not been restricted to the North American Continent. Baker et al. (1993) studied flood features in Altai Mountains and concluded that water levels had to exceed 1900m in order to explain some landforms in some constricted reaches, and the peak discharge may have exceeded 18 x 106m3 s-’ which is the largest known on Earth. Mars. Martian outflow channels are large-scale complexes of fluid-eroded troughs (Baker, 1982). The flows which formed these channels appear to have emanated from discrete collapse zones known as chaotic terrain (Fig. 3a). The channels are immense by terrestrial standards, as much as 200 km wide and 2000 km in length (Fig. 3b). Residual uplands separating channels and partially to fully streamlined upland remnants are common in outflow channels (Fig. 3~). The morphology and scale of the outflow channels clearly indicates genesis by fluid flows of immense magnitude. It is most likely that outflow channels were curved by catastrophic water floods (Baker, l982), although secondary modifications by other fluids such as glaciers and wind are possible. The peak discharge could have been greater than 109m3 for the Kasei Valles, the largest outflow channels on Mars (Robinson and Tanaka, 1990).
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Fig. 1. The Kosi Fan to the south of Himalaya Mountains. This is an excellent example of azimmense humid-region alluvial fan forming a distributary pattern. Landsat image 2751-03455-7. Image width is about 180 km Fig. 2. The Channeled Scabland in the eastern Washington State. The greatest flood in the geological record of river activity was caused by the catastrophic failures of the glacier-dammed Lake Missoula during the Pleistocene. The channels appear dark on the Landsat scene because the loess cover was locally eroded. exposing the dark basalt. Landsat image 10624-l 13. Image width is about IX0 km
803
804
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Fig. 3. An example of outflow channels on Mars. This outflow channel called Ares Vallis is ch; Lractel -ized by chaotic source area (a), very wide reaches (b), and streamlined hills (c). Viking Orbit er imag ;e (a) 689A03, (b) 689A02. (c) 827A23
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0 II
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50 Km
Fig. 3. (Conrind) Fig. 4. An example of paleochannels discovered in SIR-A imagery in the eastern Sahara region. This scene is a strip of SIR-A radar image placed over a Landsat image. The radar penetration up to several meters reveals paleoriver systems under extremely dry drift sands
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Fig. 5. Submarine channels discovered on the submarine fan off the Amazon River by the sidelooking sonar system GLORIA. Submarine channels are considered to have formed through the action of turbidity currents. Normark et a/. (1993), copyright by the American Geophysical Union Fig. 6. Lava channels (arrows) on the flank of Mauna Loa, Hawaii. Image width is about 2800 m
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Fig. 7. (a) Examples of lunar sinuous rilles. Sinuous rilles originate from single or multiple collapsed pits and tend to narrow and shallow downstream. Lunar Orbiter image V-190M. Image width is about 60 km. (b) A comparison of a sinuous rille (A), and typical terrestrial-type lava channels (B) on the Moon. In contrast to typical terrestrial-type lava channels, many sinuous rilles do not have clear association with flow units. Sinuous rilles tend to have much larger depth-to-width ratio than typical terrestrial-type lava channels. The meanders of sinuous rilles tend to have high width-towavelength ratio compared with typical lava channels. Apollo 15 Metric Camera image 1702. Image width is about 165 km
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Fig. 8. Lava channels with clearly observable levees/lateral flow deposits on Venus. These lava channels tend to shallow and narrow downstream and it is rare that they have an observable depth. Magellan image Cl_6OS347 Fig. 9. Venusian sinuous rilles. Venusian sinuous rilles have similar morphologies as lunar sinuous rilles. Sinuous rilles’ depths range up to several hundred meters. Magellan image Cl_158095 Fig. 10. Widest reach of the Kallistos Vallis on Venus. The Kallistos Vallis was originally named “the Outflow Channel” because of its morphological similarity with Martian outflow channels curved by catastrophic floodings of water. They share commonalty such scale, collapsed pit sources, wide main reaches, anastomosing reaches, and streamlined hills. However it is more likely that Kallistos Vallis has volcanic origin due to the fact that it has radar bright channel levees/lateral flow deposits near the terminus, which is characteristic of lava channels. Magellan image Cl_ ._458032. Image width is about 270 km
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Fig. 11. A canali-type channel on Venus. Canali morphology is unique in the Solar System. They are characterized by nearly constant width for hundreds of thousands of kilometers. shallow depth, Ruvial-like features (e.g. oxbows. lateral migration patterns. point bars). fan-shaped terminal deposits and no clear association with flow units. Magellan image ClL3OS153 Fig. 12. Channels on the flank of Hecates Tholus. a shield volcano on Mars. These channels are characterized by the dendritic patterns. It has been suggested that these channels may have a fluvial origin. Another possibility is that they may have originated as lava channels but later modified by fluvial processes. Viking image 65 1A I8
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G. Komatsu and V. R. Baker: Channels in the solar system Paleochannels discovered b)l radar observation Discovery of ancient river paleochannels in the Sahara desert (Fig. 4) (McCauley et al., 1982, 1986; Schaber et al., 1986) by Space Shuttle imaging radar systems is an excellent example of how new remote sensing technology contributes to identification of channels on the planetary surfaces. Radar responses from bedrock and gravel surfaces beneath windblown sand several centimeters to possibly meters thick delineate sand and alluvium filled valleys, some nearly as wide as the Nile Valley and perhaps as old as middle Tertiary (McCauley et al., 1982).
Submarine channels are relatively recent discoveries (Fig. 5) (Flood and Damuth, 1987 ; Mienert et al., 1993). They were discerned by side-looking sonar systems, such as GLORIA. Submarine channels occur in close association with submarine fans (Flood and Damuth, 1987), and are considered to have formed through the action of turbidity currents (Normark et al., 1993). Their meandering characteristics are similar to those of terrestrial fluvial channels (Clark et al.. 1992).
Lava channels Earth Present-day terrestrial volcanism produces only a limited range of lava channel morphologies (Fig. 6). Although some lava tube systems exhibit complex morphologies (e.g. Hatheway and Herring, 1970), they are generally simple and short (e.g. Mauna Loa lava channels (Greeley, 1974)). Terrestrial lava channels are generally constructional in origin. Thermal erosion has more rarely been observed in some terrestrial lava flows. Swanson (1973) inferred that as much as 15 m of material was eroded in the master tubes around Mauna Ulu, Hawaii, during a 9 month period. Terrestrial flood basaltic volcanism forms extensive lava tubes and channels, as illustrated by the Snake River Plains (Cattermole, 1989). On Earth, weathering processes could possibly have modified ancient, very large lava channels beyond recognition. Any channelized structure is subject to intense secondary erosion, particularly by water. Burial by sediments is another way of making channels unrecognizable. Komatiite is a lava very rich in MgO, with a very high melting temperature. and a low viscosity. Komatiite normally occurs in association with Archean shield regions, such as in western Australia and Canada. Based on the high eruption temperature of komatiite, it has been argued that the Archean Earth had higher interior temperatures than present. Examples of lava channels formed by komatiite have been recognized on the western Australian Archean shield (Barnes et al., 1988a. b ; Huppert and Sparks. 1985). Komatiite lavas can erode ground materials of lower melting temperatures quite efficiently
811
(Huppert et al., 1984; Huppert and Sparks. 1985). Locally, komatiite flows have thermally eroded felsic ground rocks to depths in excess of 1OOm (Barnes et al.. 1988a) in the Yilgarn Block, a 1000 x 700 km quasicircular Archean shield consisting of granitic intrusive rocks and greenstone belts.
Moon The sinuous rilles (Fig. 7a) are one of the most interesting volcanic landforms on the Moon. A fluvial origin was suggested by Lingenfelter et al. (1968). However. the common association of sinuous rilles with volcanic plains, and their morphological similarities to terrestrial lava channels leads most investigators to interpret them as lava drainage channels (e.g. Strom, 1966). The lunar sinuous rilles have their source areas either on the perimeter of the mare surface or close by in the highlands, and the emanating lavas flowed consistently toward the inner parts of maria (Guest and Murray. 1976). Sinuous rilles narrow and shallow downstream, originate from collapsed pits. These are characteristics often observed for terrestrial lava channels. Nevertheless, there are important differences between sinuous rilles and typical terrestrial-type lava channels (Figs 6 and 7a and b). In contrast to typical terrestrialtype lava channels, many sinuous rilles do not have clear association with flow units. In general, sinuous rilles have much larger depth-to-width ratios. The meanders of sinuous rilles have high width-to-wavelength ratios compared with typical terrestrial-type lava channels (Komatsu and Baker, 1994a). Oberbeck et al. (1969) and Greeley ( 1971) thought sinuous rilles may have originated as lava tubes based on the observation that there are chains of pits which may evolve to sinuous rilles. Hulme (1973, 1982) and Carr (1974) proposed thermal erosion as an important formation mechanism for the sinuous rilles. Many lunar basalt rocks are enriched in Ti and Fe, and this makes the basalt a high temperature, low viscosity lava. These are factors favorable for thermal erosion. In addition, the size dimensions of the sinuous rilles are very large compared with those of terrestrial lava channels. This may mean that the eruption rates were very high, which would also contribute to high thermal erosion rates.
Venus The first Venusian channel was discovered on the radar images of the Venera 15/ 16 mission. However, this channel was mapped as a tectonic feature (Kotelnikov et al., 1989) or as a narrow depression (Sukhanov et ul., 1989). and it was named “Hildr Fossa”. Among the major discoveries made by the Magellan radar mapping mission. the channels and valleys are one of the most unexpected. The very first channel identified in Magellan imagery. named later a canali-type channel, was sinuous, had a constant width, and no tributaries. It had no clear association with flow units, and was very long ( > 500 km) (Head et NI., 1991). This unique combination of characteristics has never been seen in terrestrial or Martian fluvial chan-
812
nels, nor in lava channels on the terrestrial planets. These unique morphologies attracted the attention of researchers, leading to hypotheses concerning the channel’s origins, considered as a type of lava channel (Komatsu et al., 1992; Baker et al., 1992; Gregg and Greeley, 1993 ; Komatsu et al., 1993; Kargel et al., 1994). Later in the mission, researchers discovered a wide range of channel morphologies (Komatsu et al., 1993). Lava channels on Venus have the widest range of morphologies in the Solar System. These channels probably represent a large spectrum of lava emplacement processes. Eruption factors, such as chemical composition, eruption temperature, eruption rate and duration, in addition to environmental conditions such as ambient temperature, atmospheric pressure, and topography, must have varied to form the remarkable range of morphologies. There are a number of Venusian channels similar to typical terrestrial-type lava channels (Fig. 8). They have clear association with flow units (levees/lateral flow deposits) and tend to narrow and shallow downstream. Although their scales are often an order or two of magnitude larger than those of terrestrial counterparts, these Venusian lava channels are considered to have a similar formation mechanism to terrestrial ones. Sinuous rilles (Fig. 9) were so named because of their close resemblance to lunar sinuous rilles. They narrow and shallow downstream, they originate from collapsed pits, have deep sections (up to several hundred meters), and normally do not have a clear association with flow units. Meander characteristics separate many of the Venusian and lunar sinuous rilles from Venusian canali and from lava channels on Earth (Komatsu and Baker, 1994a; Kargel et al., 1994). This was attributed to the possible involvement of erosional processes, particularly thermal erosion, in sinuous rille formation. Their close association with coronae, which are possible mantle upwelling or mantle diapir related features (Stofan et al., 1992) indicates that the channel-forming lavas may have come form mantle. Kallistos Vallis (Fig. lo), originally named “the Outflow Channel”, deserves special attention (Baker et al., 1992 ; Parker et al., 1996). This channel is wide, originates from a collapsed pit, and has braided reaches. These characteristics are typical of Martian outflow channels, and this implies the channel’s origin by high discharge flows. Kallistos Vallis forms radar bright levees/lateral flow deposits at its terminus, which indicates that the channel was formed by lava. Canali-type channels (Fig. 11) remain a mystery. It has been found that mafic lava, such as tholeiitic lava would not retain turbulence, which is probably required for a channel’s erosional origin, over the observed channel length (Komatsu et al., 1992). Ultramafic lavas have the same problem. However, mafic and ultramafic lava can flow the required distance with a thickened crust (Komatsu et al., 1992). Increased capacity of Venusian atmosphere to remove heat helps thickening of a crust which may promote longer flow length (Gregg and Greeley, 1993). Calculations suggest that canali-type channels may have formed by constructional processes, and some of the canali may have roofed (Komatsu et al., 1992 ; Gregg and Greeley, 1993). However, a lack of clear association with flow units (absence of levees/lateral flow deposits) has to
G. Komatsu and V. R. Baker: Channels in the solar system
be explained (Komatsu et al., 1992, 1993). An alternative explanation is that canali were formed by lavas that are exotic, by terrestrial standards (Komatsu et al., 1992; Baker et al., 1992 ; Kargel et al., 1994 ; Treiman, 1994 ; Gregg and Greeley, 1994). Sulfur would retain its fluidity and carbonatite may also stay fluid for a long time under Venusian conditions. Both have very low viscosities at their melting temperatures. Some canali have deformed longitudinal profiles (Baker et al., 1992 ; Komatsu and Baker, 1994b). These profiles are evidence of the regional tectonic movement and provide clues to understand the tectonic history of the region after the channel formation.
Mars
Martian lava channels are observed on the flanks of shield volcanoes such as Alba Patera (e.g. Cattermole, 1987). They are simple to complex, and mostly associated with clearly observable flow margins. They are thought to have formed by tube-fed flows (Cattermole, 1987). There are anastomosing channel networks on some shield volcanoes, such as Alba Patera (Mouginis-Mark et al., 1988 ; Gulick and Baker, 1990) and Hecates Tholus (Fig. 12) (Mouginis-Mark et al., 1982). These may originate as lava channels but Mouginis-Mark and co-workers considered fluvial origin instead based on their morphology and the fact that they are incised into the relatively unconsolidated materials compared with lava rocks. Gulick and Baker (1990) discussed fluvial modification of lava channels on the flank of Alba Patera by citing similarities between these channels and valleys on Hawaiian volcanoes.
Discussion
Other types of channels formed by water include meltwater channels on glaciers (Ewing, 1972) and channels formed by the solution of limestone by carbon dioxide rich water (Sweeting, 1972). These are orders of magnitude smaller than other fluvial channels, and not particularly significant in planetary landscapes. Based on preliminary mapping of Venus, Earth, Mars, and the Moon, we know that lava channels are a common phenomenon on the terrestrial planets. However, their morphologies vary widely. Venus has the widest range of lava channel morphologies in the Solar System (Tables 2 and 3), almost comparable to that for terrestrial fluvial systems (Baker et al., 1992; Komatsu et al., 1993). Atmospheric conditions (environmental factors) for Venus, particularly its high ambient temperatures, are important factors. As Head and Wilson (1986) estimated, the average flow length on Venus would be about 10% longer than its counterpart on Earth, if other conditions are held constant. Venus’ atmosphere actually increases the cooling rate because of efficient heat removal from flow surface in the dense atmosphere, resulting in enhanced crustal growth (Head and Wilson, 1986). Diverse eruption conditions, driven by the catastrophic resurfacing (Schaber et
G. Komatsu
and V. R. Baker: Channels in the solar system
813
Table 2. Lava channel morphology
Meandering Braided Oxbows Streamlined hills Distributary Tributary 0. observed
Venus
Moon
Mars
Earth (modern)
Earth (komatiite) ~~_~~ NK NK NK NK
Earth (fuvial)
0 0 0 0 0
0 0 NK 0 0
0 NK NK NK NK
0 0 NK NK NK
NK
NK
NK
NK
NK NK
0 0
Earth (fluvial)
0 0 0 0
; NK. not known.
Table 3. Maximum
dimensions
Length
Width Depth
of lava channels
Venus
Moon
Mars
Earth (modern)
Earth (komatiite)
-6800km
-3OOkm
NK -5OOm
-5km -3Om
NK -5OOm
- 6700 km -- 20 km” - 150kmh
NK
NK
- 1OOm
-_ 60 m” - 100mh
-3Okm -3OOm
-1Okm -3OOm
NK. not known. i’ River. ‘Catastrophic floods. al., 1992) and subsequent mantle plume volcanism, are probably the most important factors responsible for the diversity of channel morphologies on Venus. Are there channels on other terrestrial planets? Mercury has extensive evidence of volcanism (Strom, 1984), but there are no lava channels observed on the half of Mercury observed by Mariner 10. This is important because the maximum resolution in Mariner 10 images should allow recognition of lava channels at a scale equivalent to some of the largest lunar sinuous rilles (R. Storm, personal communication). This may be due to the possible difference in the volcanic style. In particular, the Mercurian surface is considered to be low in Ti and Fe compared with the Moon (Hapke et al., 1975). Other potential objects which may have channel structures in the Solar System are outer Solar System satellites such as IO and Triton, and asteroids such as Vesta. These objects are known to have experienced volcanism on their surfaces. The volcanism on outer Solar System satellites is quite exotic. IO is erupting sulfur-rich lava, and lavas on icy satellites are mixtures of a variety of ice compositions. Whether these objects have channel structures or not has to wait for future planetary missions.
Conclusions
Channels are common features in the Solar System. Fluvial channels are found on Earth and Mars. Recent studies have focused on the paleochannels. In the late Pleistocene. catastrophic outburst of water from the glacier-dammed lakes scoured bedrock and formed channels on the North American and Eurasian continents. Use of spaceborne radar in the Sahara desert revealed a number of paleochannels under the sand. Submarine channels are considered to be part of turbidite systems, but their formation mechanism is not well understood.
Lava channels are known to exist on the Moon and all terrestrial planets except for Mercury. In particular, lava channels on Venus are comparable to the fluvial channels on Earth in their morphology and scale. This suggests that volcanism can produce the range of channels formed by water. Acknowle~yements. This project was supported by National Aeronautics and Space Administration through the Venus Data Analysis Project, grant NAGW-3515, and the Planetary Geology and Geophysics Program. grant NAGW-285. Goro Komatsu is a Research Fellow of Japan Society for the Promotion of Science for Young Scientists, Ministry of Education.
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