Aqueous flows carved the outflow channels on Mars

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. E5, 5039, doi: /2002je001940, 2003 Aqueous flows carved the outflow channels on Mars Neil M. Coleman U.S. Nuclear Regulatory Commission, Washington, D. C., USA Received 19 May 2002; revised 6 October 2002; accepted 3 January 2003; published 15 May [1] The role of water in carving the Martian outflow channels has recently been challenged by the hypothesis known as White Mars. This hypothesis claims that the channels were cut by CO 2 gas-supported debris flows that also resurfaced the northern plains. However, proposed analogs of cryoclastic flows are either inappropriate (i.e., submarine density flows) or are primarily depositional rather than erosional (i.e., pyroclastic flows). Subaerial mass movements on Earth do not carve long deep channels like those on Mars. I review runout efficiencies for mass movements on Earth and Mars. The efficiencies required for cryoclastic flows to resurface the northern plains of Mars are so large that they appear unattainable. White Mars seeks to resolve carbonate and floodwater paradoxes that probably do not exist. It is also doubtful whether reservoirs of CO 2 could persist over geologic time in the crust. Overall, the CO 2 hypothesis fails key tests and should be abandoned as a means to carve outflow channels. I present a new interpretation of the fluid source that created Aromatum Chaos and Ravi Vallis. Water, not liquid or gaseous CO 2, was the causative fluid, and the source was an ice-covered impoundment in ancestral Ganges Chasma. At that time the canyon had no eastern outlet, and groundwater flowed northward to discharge at Aromatum Chaos and Shalbatana Vallis. The presence of ice-covered water bodies can help to calibrate models of volcanichydrologic climaxes during Hesperian time. The outflow channels, like the spectacular landforms of the Channeled Scabland, are monuments to the erosive power of catastrophic aqueous floods. INDEX TERMS: 6225 Planetology: Solar System Objects: Mars; 5415 Planetology: Solid Surface Planets: Erosion and weathering; 5470 Planetology: Solid Surface Planets: Surface materials and properties; KEYWORDS: White Mars, outflow channels, Ravi Vallis, Ganges Chasma, groundwater, carbon dioxide Citation: Coleman, N. M., Aqueous flows carved the outflow channels on Mars, J. Geophys. Res., 108(E5), 5039, doi: / 2002JE001940, Introduction [2] The enormous outflow channels of Chryse Planitia and Amazonis Planitia provide the best evidence that large quantities of water were released onto the surface early in Martian history. For example, Kasei Valles is a system of anastomosing channels 2000 km long that begins in Echus Chasma and ends in Chryse Planitia (Figure 1). Carr [1996] provides a detailed summary of the distribution, characteristics, and possible origins of the outflow channels. Catastrophic flooding remains the most accepted origin because features of the channels resemble those found in the Channeled Scabland of eastern Washington [Baker, 1978, 1981, 2001; Carr, 1996]. The ages of Martian outflow channels are estimated to range from Noachian to Amazonian, but most of the outflows to Chryse Planitia occurred in the Hesperian [Ivanov and Head, 2001]. Dohm et al. [2001] propose the existence of huge, structurally controlled valleys located southwest of Olympus Mons that transported catastrophic floods from western Tharsis into Amazonis This paper is not subject to U.S. copyright. Published in 2003 by the American Geophysical Union. Planitia. Zuber et al. [2000] provide geophysical evidence of buried channels in Chryse Planitia that possibly represent earlier episodes of flooding. [3] The role of water in carving the outflow channels has been challenged by a White Mars model in which huge landslides occurred on steep slopes, and CO 2 in various phases erupted to form chaotic terrain [Hoffman, 2000; O Hanlon, 2001]. Resulting gas-supported cryoclastic debris flows allegedly carved the outflow channels and resurfaced the northern plains. The model is described as resolving several paradoxes, such as the presumed inability of an aquifer to yield enough water to produce the floods, the seeming lack of carbonates at the surface, and the fate of floodwaters. If this model were correct it would mean that little liquid water existed near the surface of Mars for the last 3 billion years, reducing the probability that life could have evolved there. However, the CO 2 model is inconsistent with studies of terrestrial and Martian mass movements. The model is also inconsistent with a regional view of paleohydrology in which many outflow channels emanate from source areas that could have contained surface impoundments [Carr, 1996]. It is no coincidence that most outflow channels on Mars are closely associated with volcanic 5-1

2 5-2 COLEMAN: WATER CARVED THE OUTFLOW CHANNELS ON MARS Figure 1. Kasei Valles is a complex system of channels 2000 km long that begins near the equator and terminates in western Chryse Planitia. These channels display a wide array of flood-carved features, including teardrop-shaped islands, scoured bedrock surfaces, deep inner channels, and smooth depositional plains at their termini. The north channel includes an inner channel 2 km deep (MOLA pass 233) within a wide swath of flood-scoured terrain. The south channel is also deeply incised, reaching a depth of 3 km at the crossing of MOLA pass 252. The image is 1400 km across. Image resolution is 1/64th degree per pixel, with dark areas being lower in elevation than light areas (Image credit: MOLA Experiment Gridded Data Record, available from the Planetary Data System Geosciences Node at provinces. As discussed below, White Mars is not a viable alternative to an aqueous flood origin for the outflow channels. 2. Discussion 2.1. Terrestrial Analogs [4] Hoffman [2000] identifies pyroclastic flows and submarine density flows as terrestrial analogs of the Martian events that created chaotic terrain and carved the outflow channels. Specifically, Hoffman [2001a] claims that a cryoclastic flow formed Aromatum Chaos, the source of Ravi Vallis (Figure 2). Hoffman [2000] makes the highly dubious claim that liquid water has not been present on the surface for eons, and then cites aqueous submarine density flows as erosive analogs of gas-supported flows. However, once released, gas-supported flows (e.g., pyroclastic flows) rapidly lose their overpressure to the surrounding atmosphere, and this effect is most pronounced on a planet like Mars with a thin atmosphere and reduced gravity. Swift pressure loss causes rapid fallout of the debris load, such that gas-supported flows are almost continuously depositional from near their source to the distal zones. The water column that supports submarine density flows cannot

3 COLEMAN: WATER CARVED THE OUTFLOW CHANNELS ON MARS 5-3 Figure 2. Aromatum Chaos, located at 1 S, 43 W (Map source: USGS, 1981). Topography along transect A - A 0 shows that the chaos formed on virtually level terrain that slopes less than one degree to the east-northeast. Line A - A 0 is 170 km long. Ravi Vallis was born out of the chaos and flowed eastward into Simud-Tiu Valles. A broad area west and southwest of the chaos also collapsed, but did not produce typical chaotic terrain. Several pronounced pits are labeled. Pit X is 1800 m deep, Y is 1400 m deep, and Z collapsed 1100 m (MOLA data at 1/32 degree resolution are available from the Mars MER 2003 and Global Data Visualization website at At the equator on Mars one degree of longitude is 59 km. decompress in this way and therefore can foster efficient, long-distance transport of debris. Clearly, submarine density flows should not be considered as valid analogs of gassupported flows on Earth or Mars. Hoffman [2000] proposes an ice-grinding mechanism to try to sustain pressures in cryoclastic flows, but Stewart and Nimmo [2002] show that the mechanism is not likely to produce significant amounts of CO 2 vapor. Put simply, rocks can float on air, but not for long. [5] Hoffman [2000] cites gas-supported mass flows like pyroclastic flows as another analog of cryoclastic flows. But pyroclastic flows are primarily depositional, cause little erosion beyond eruption zones, and do not carve long, deep channels like those on Mars. Deposition of debris from a rapidly moving pyroclastic flow may be nearly continuous from near the eruptive source to the terminus of the flow [Fisher, 1966]. Pyroclastic and other debris flows typically form lobate or sheet-like deposits of positive relief. Low velocity flows tend to be thick and to pond in depressions and valleys, smoothing the topography. Higher velocity pyroclastic flows can surmount topographic barriers and create thin deposits that drape over the terrain. Large caldera systems have produced extensive ignimbrite sheets with volumes that can exceed 2000 km 3 [Sparks et al., 1997]. Rather than cutting their own channels, debris flows typically follow and partially fill preexisting fluvial channels, a tendency clearly seen in the 18 May 1980 eruption at Mount St. Helens [Lipman and Mullineaux, 1981]. This tendency to follow fluvial channels could wrongly lead to the idea that gas-supported flows are strongly erosive and can cut long, deep channels. Even when pyroclastic flows cause local erosion, this is usually manifested either as unconformities within volcanic deposits or as surface scour. For example, the initial blast surge at Mount St. Helens probably reached supersonic velocities in the proximal zone. This produced furrows in the surface and scoured vegetation and soils, intermixing them with pyroclastic material to deposit a ground layer up to several meters thick [Fisher, 1990]. Avalanche-style erosion can also occur in eruption zones on the steep flanks of volcanoes where erosion can keep pace with or outstrip deposition. For example, an erosional feature named the stairsteps was eroded by pyroclastic flows on the north slope of Mount St. Helens [Rowley et al., 1981, Figure 287]. This and other erosional channels largely coincide with stream channels that existed before the 18 May eruption. The pyroclastic flows widened and deepened the preexisting fluvial channels. I emphasize that it would be inappropriate to extrapolate this spatially limited process (i.e., avalanche-style erosion) to carve deep, low-gradient channels on Mars that are thousands of kilometers long. In other words, outflow channels should not be interpreted as low-gradient landslide scars of great length. [6] Although pyroclastic flows cause great destruction to vertical obstacles in their path, they typically produce minimal basal erosion. In 79 A.D. (C.E.) Vesuvius erupted, engulfing the seaside town of Herculaneum in a series of

4 5-4 COLEMAN: WATER CARVED THE OUTFLOW CHANNELS ON MARS pyroclastic surges and flows [Sigurdsson et al., 1985]. The town was destroyed with great loss of life, but the entombment and substantial preservation of streets, walls, fountains, artifacts, and the skeletal remains of inhabitants attests to the net accumulation of deposits and minimal basal erosion. Items excavated at Herculaneum include wall paintings, mosaic pavements, statuary, charred papyrus scrolls, coins, mounds of grain, and even eggs [Parslow, 1995]. Much thicker pyroclastic flow deposits exist in the southwestern Nevada volcanic field. For example, at Yucca Mountain, Miocene-age tuffs and lavas exceed a thickness of 1200 m [Carr et al., 1986] and may reach a thickness of 3000 m. A thin, incipient soil developing on volcanic ash [unit bt4, Moyer et al., 1996] was overrun by the 150-m thick Tiva Canyon Tuff of the Paintbrush Group [Buesch et al., 1996]. Although absent in some places, the reddish paleosol is so widely preserved that it serves as a marker bed in outcrops, tunnels, and drill cores. The evidence that analogs of cryoclastic flows (i.e., pyroclastic flows) are primarily depositional is an acute problem for the White Mars hypothesis of channel formation. As discussed below, other lines of evidence reveal further difficulties and lead us back to a fluvial origin for the outflow channels White Mars Rocket Chamber Model [7] Hoffman [2001a] considers a relatively low-pressure rocket combustion chamber to be an analog for the system that created Aromatum Chaos (Figure 2), with escape of the exhaust through the venturi-like constriction [i.e., Ravi Vallis] to the east. However, the movements of expanding gas-charged debris flows are not sensitive to small topographic gradients, especially on a planet like Mars with reduced surface gravity and a thin atmosphere. Mars Orbiting Laser Altimeter (MOLA) data from Mars Global Surveyor show that Aromatum Chaos formed on terrain that slopes less than one degree to the east-northeast (Figure 2). Therefore, if this chaos had been created by catastrophic release of CO 2, the result should have been a radial eruption and debris field, especially given that no channel was initially present to contain and direct the flows. In this nearly level terrain it is difficult to see how crustal blocks could have become unstable and slid downslope, as described by the White Mars hypothesis [Hoffman, 2000]. The origin of Ravi Vallis, which is born out of Aromatum Chaos (Figure 2), is consistent with the disruption of a cryospheric confining layer and flow from a confined aquifer, as proposed by Carr [1979]. It is an important clue that Ravi Vallis directly followed a gentle topographic slope leading away from Aromatum Chaos. The erosional agent was likely an incompressible fluid like water that must closely follow topographic lows, carving well-defined channels. MOLA data show that Aromatum Chaos is 1500 m deeper than the channel that flows away from it. A broad area west of the chaos also subsided without producing chaotic terrain (Figure 2). The flows clearly originated at considerable depth, and the initial potentiometric surface for the confined aquifer was higher than the local terrain elevation. Hoffman [2000] questions a fluvial origin because shorelines are not found in Aromatum Chaos. However, it would be very unusual for such features to be preserved on the margins of collapse basins because the continued lowering of regional water levels would progressively induce mass wasting at the chaos margins, removing evidence of shorelines. By similar reasoning, evidence of shorelines from former ice-covered lakes would be poorly preserved on the steep, unstable margins of the Valles Marineris canyons. On the other hand, shorelines might be preserved in more stable environments like the northern plains, providing possible evidence of former lakes or oceans [Parker et al., 1989, 1993; Clifford and Parker, 2001]. However, over hundreds of millions of years, surface processes like aeolian erosion and deposition of dust and volcanic ash would tend to degrade relatively delicate features like shorelines. The true nature of the hypothesized shorelines in the northern plains remains controversial Debris Transport [8] Hoffman [1999] claims that Martian outburst features were not produced by aqueous floods, but instead were formed by a new class of cold, gas-supported density flows (i.e., cryoclastic flows). These hypothetical flows would have been much larger than the largest mass movements previously documented for Earth or Mars. Because of their huge volumes, high speeds, and entrained debris, cryoclastic flows are supposed to have been immensely erosive in their upper and mid portions [Hoffman, 2000] and capable of eroding channels. It is therefore useful to examine the characteristics of large mass movements on Mars to gain insights about their morphology, emplacement processes, and scaling phenomena. Lucchitta [1978] described an enormous landslide in Ganges Chasma and compared it with large terrestrial landslides. To explain the relatively high runout efficiency of the Martian slide she considered that the debris may have contained water derived from ground ice in the canyon wall. She also suggested that the slide may have been emplaced on a cushion of gas, either from a denser atmosphere, or evolved from the substrate or the debris itself. Using new MOLA data (pass 13088), Coleman and Casteel [2002] found that the landslide in Ganges Chasma had a lower runout efficiency than estimated by Lucchitta [1978]. Lucchitta [1979] examined 35 large Valles Marineris landslides that were well resolved in Viking images. These images reveal slump blocks and large fan-shaped deposits of positive relief that were emplaced on the canyon floors. Lucchitta [1979] reported that the Martian slides follow the terrestrial trend of increasing runout efficiency with increasing slide volume. She hypothesized that the canyon walls may have consisted of saturated, poorly consolidated materials behind an icecemented free face. Lucchitta [1979] concluded that the unstable canyon walls may have collapsed under the influence of Marsquakes, and that material in the lower wall sections underwent liquefaction and rushed out as enormous mudflows. McEwen [1989] analyzed data for 29 landslides in western Valles Marineris, many of which were previously studied by Lucchitta [1979]. McEwen [1989] plotted runout efficiency versus landslide volume for the Martian slides and for terrestrial dry-rock avalanches, and fitted a linear least squares trend line to each data set. He found that the slopes of the terrestrial and Martian trends are similar but offset. He reports that a typical Martian landslide must be 2 orders of magnitude more voluminous than a terrestrial landslide to achieve similar runout efficiency. McEwen [1989] estimated yield strengths for 12 Martian slides.

5 COLEMAN: WATER CARVED THE OUTFLOW CHANNELS ON MARS 5-5 Table 1. Runout Efficiencies for Various Mass Movements Types Runout Efficiencies a Source 37 Terrestrial Debris Flows 2 12 Lucchitta [1978] 12 Terrestrial Debris Flows 2 25 Iverson [1997] 5 Pyroclastic Flows (1980, 5 9 Coleman b Mount St. Helens, WA) 41 Debris Flows 3 95 Fisher and Schmincke [1984] 29 Landslides in Valles 2 23 McEwen [1989] Marineris, Mars Landslide, Ganges Chasma, <30 Lucchitta [1978] Mars (lat 9 S, long 44.5 W) <13 Coleman and Casteel [2002] Hypothetical Martian mass <80 Coleman c movement (volume 10 6 km 3 ) White Mars cryoclastic flows Coleman d (hypothetical efficiencies c needed to reach 30 N in Chryse Planitia) Aureum Chaos >450 Echus Chasma >300 Ganges Chasma >400 Hydraotes Chaos >400 Hydraspis Chaos >450 Iani Chaos >500 Juventae Chasma >350 a Efficiency = runout distance of debris flow divided by descent height [Iverson, 1997]. b Estimated using Figure 295 of Rowley et al. [1981]. c Estimated by extending Malin s [1992, Figure 10] trend line for Martian landslides. d Descent height estimated using approximate canyon and chaos rim heights (MOLA data at 1/32 degree resolution are available from the Mars MER 2003 and Global Data Visualization website at dataviz/) and an average elevation of minus 4000 m in central Chryse Planitia at 30 N. Using these results and the estimates of runout efficiency, he suggested that the Martian landslides were not water saturated. The recently available high-resolution topographic data (MOLA) will permit more refined studies of Martian landslides. Such studies will improve our understanding of physical processes in mass movements on both Earth and Mars. [9] Hoffman [2000] claims that gas-supported cryoclastic flows not only carved the outflow channels but also transported debris long distances, thereby resurfacing and smoothing the northern plains. We can evaluate this claim by comparing runout efficiencies of Earth-based and Martian debris flows, which range from 2 to 95 (most are less than 30), with hypothetical cryoclastic flows on Mars (Table 1). Extreme (100) runout efficiencies would be needed for cryoclastic flows to travel from large canyon and chaos source areas via outflow channels to the northern plains. Such extreme efficiencies have not been demonstrated for subaerial mass movements on Earth or Mars (Table 1) and appear unattainable. I suggest that high runout efficiencies for gas-supported flows might be possible on Venus where the atmosphere is extremely dense (92 bars), more closely mimicking a fluid state, and might therefore support efficient mass movements. Malin [1992] estimated runout efficiencies for 15 landslides on Venus and found that all were less than 20. Bulmer [1997] plots fall heights versus runout lengths for various aprons on Venus, indicating that some had runout efficiencies >60. [10] Malin [1992, Figure 10] plotted volumes of terrestrial and Martian landslides versus their runout efficiencies. He fitted trend lines to the data, illustrating the offset in trends described by McEwen [1989]. We can extend Malin s [1992] trend line for Martian landslides to estimate efficiencies for enormous mass movements. For example, a hypothetical landslide volume of 10 6 km 3 would correspond to a runout efficiency of approximately <80. This value is far below that required for cryoclastic flows to reach 30 Nin Chryse Planitia (Table 1). Although we can imagine landslides on the order of 10 6 km 3, it seems unlikely they could actually occur on Mars. Volumes of mass movements are intrinsically limited by the degree of topographic relief and the corresponding volume of unstable, elevated terrain. The high relief associated with the Valles Marineris helps explain the voluminous landslide deposits seen in the canyons. Hoffman s [2000] proposed genesis of cryoclastic flows invokes trains of sliding crustal blocks. If this process actually occurred it would create a series of debris flows of moderate to large scale rather than wholesale liquefaction of the upper crust. However, it is questionable whether liquid CO 2, the alleged energy source for cryoclastic flows, can be preserved in crustal reservoirs over geologic time [Stewart and Nimmo, 2002], especially in source regions for outflow channels. [11] The hypothetical efficiencies shown in Table 1 for cryoclastic flows only consider transport to 30 N in central Chryse Planitia. Much higher efficiencies would be needed to resurface the northern plains at higher latitudes. Extreme runout efficiencies are characteristic of liquids, like water, which readily transport debris and cut channels. The runout efficiency of an aqueous flood is limited only by the distance from source to ponding areas, and rates of infiltration, evaporation, or freezing. As pointed out by Hoffman [2000], large efficiencies are also seen in submarine density flows, but these are not appropriate analogs for Mars because they are aqueous, not gas-supported, flows. On the other hand, density flows could have happened on Mars if catastrophic floods penetrated ice-covered water bodies in the northern plains. Analysis of channel termini in Chryse Planitia by Ivanov and Head [2001] shows that a northern ocean could possibly have existed when the Hesperian out-

6 5-6 COLEMAN: WATER CARVED THE OUTFLOW CHANNELS ON MARS flows occurred. However, the hypothetical existence of seas or oceans in the large basins on Mars remains controversial. [12] The energy to sustain long runout distances of cryoclastic flows supposedly comes from the CO 2 phase transition from liquid to vapor, aided by mechanical grinding and vaporization of CO 2 solids entrained in the debris flows [Hoffman, 2000]. However, Stewart and Nimmo [2002] estimate that if all of the potential energy of a debris flow were used to vaporize CO 2 solids, the amount of vapor liberated would not be enough to maintain the flow. They conclude that the vapor needed to support the debris load must be generated from the initial decompression of liquid CO 2. Therefore, Hoffman s [2000] CO 2 -ice-grinding mechanism for inducing extreme runout efficiencies appears not to be viable. Terrestrial pyroclastic flows can also be rich in volatiles and often overrun streams and lakes because of their tendency to follow alluvial valleys. As seen at Mount St. Helens, the emplacement of hot lateral blast and pyroclastic deposits produced large phreatic explosion pits [see Rowley et al., 1981, Figure 288]. Many ephemeral fumaroles and thermal ponds were created west and south of Spirit Lake [Dethier et al., 1981]. Despite abundant sources of volatiles for many pyroclastic flows, they do not display the extreme runout efficiencies required under White Mars. The very low gradients of channel floors pose another problem for gas-supported debris transport under White Mars, but these low gradients are fully consistent with a fluvial origin. Williams and Phillips [1999] have shown that the average gradients along outflow channel floors have absolute values of <0.003 (m/m). Several channels have reverse slopes, requiring either deep aqueous flows or secondary tectonic and depositional changes in channel bed elevations. [13] Smaller scale mass movements have been seen on Mars. Malin and Edgett [2000] describe geologically recent gullies and debris aprons that have formed on steep slopes at mid to high latitudes, mainly in the southern hemisphere. Various authors have proposed an aqueous origin for these features [Cabrol et al., 2001; Costard et al., 2001; Gaidos, 2001; Knauth et al., 2001; Paige, 2002; Andersen et al., 2002]. Musselwhite et al. [2001] and Hoffman [2001b] suggest that breakout of liquid CO 2 generated gas-supported density flows that carved the gullies. In a test of the CO 2 hypothesis, Stewart and Nimmo [2002] estimate the mass of vapor needed to support the mass of small debris aprons found at the base of recently-formed gullies. They find that frictional heating of CO 2 solids is unlikely to produce significant CO 2 vapor, and therefore most of the vapor needed to support the debris flows would have to be generated by the initial decompression process. Because of the difficulty of preserving CO 2 reservoirs in the crust, Stewart and Nimmo [2002] conclude that CO 2 gas-supported flows could not have formed the young gullies on Mars. Hoffman [2002] notes that arguments against crustal storage of liquid CO 2 may have some validity, and suggests a revised model for the formation of polar gullies. The revised model invokes gas-lubricated flow based on avalanching of seasonal CO 2 snowpack and clastic debris. Some gullies may be carved by infrequent flows of meltwater produced by solar heating. Paige [2002] describes conditions under which liquid water runoff is physically plausible during periods of high obliquity, even at mid to high latitudes. He considers this may be the dominant explanation for the gullies. [14] One additional point should be made about mass movements at various scales, based on the general commentary of Iverson [1997]. The available energy for these movements consists of gravitational potential energy which is converted to vertical and horizontal translation of debris. The ability of a mass movement to travel long distances is constrained by energy losses due to internal and external grain contact friction, inelastic collisions, and the dissipation of energy by viscous fluids. Debris flows lose mass due to deposition and can acquire mass by eroding the surface they are moving across. Mass acquired on steep slopes has its own gravitational potential energy that can contribute to the flow. However, mass accumulated by erosion in low gradient areas would contribute very little energy and instead would decelerate the mass movement. Long runouts of mass movements are only possible when internal friction and interaction with the substrate are minimized. The notion that gas-supported flows could transport debris hundreds or thousands of kilometers along gentle slopes and also excavate channels that are kilometers deep is entirely inconsistent with the limited energy available to do this work. This is a fundamental weakness of the White Mars hypothesis Source of Groundwater for Ravi Vallis and Shalbatana Vallis [15] Hoffman [2001a] argues that groundwater could not have produced the outflows from Aromatum Chaos (Figure 2) because water would have to be locally recharged in many episodes to provide enough discharge to excavate the chaos and carve Ravi Vallis. Hoffman [2000] refers to a volumetric misfit between outburst channels and the chaos zones from which they emerge. He explains that chaos collapse involves regolith alone which generates its own fluids from liquid CO 2 and CO 2 -bearing ices within its own volume. But these assertions appear to be incorrect because the source of the fluid was a distant surface impoundment, not local recharge. Carr [1996] describes a 400-km-long zone of subsidence that extends northward from Ganges Chasma to the outflow source of Shalbatana Vallis. MOLA data (Figure 3) confirm this subsidence, and also reveal that it extends eastward to Aromatum Chaos, the Figure 3. (opposite) MOLA shaded topography for the region centered on 6 S, 46 W. Features include Ganges Chasma (G) and its eastern outlet (O), and layered deposits (LD) within the canyon that may be of volcanic origin. A prominent subsidence zone (SZ) extends northward from Ganges Chasma to the source area for Shalbatana Vallis (S). The subsidence zone (SZ) also extends eastward to Aromatum Chaos (A), the source of Ravi Vallis (R). The area immediately west of Aromatum Chaos is marked by a broad downwarping and isolated depressions (see Figure 2). The Simud-Tiu channel system is located east of this view, and the main canyons of the Valles Marineris lie to the south and west. The frame is 900 km across. Resolution is 1/64th degree per pixel, with dark areas being lower in elevation than light areas (Image credit: MOLA Experiment Gridded Data Record, available from the Planetary Data System Geosciences Node at wufs.wustl.edu/missions/mgs/mola/egdr.html).

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8 5-8 COLEMAN: WATER CARVED THE OUTFLOW CHANNELS ON MARS source of Ravi Vallis. The field relations show that a liquidfilled impoundment in Ganges Chasma drained northward via subterranean flowpaths to maintain surface flows in Shalbatana and Ravi Valles. The evidence that the flows were connected to a surface impoundment virtually eliminates liquid CO 2 as the flowing agent. Liquid CO 2 would not be stable at the surface, unless the atmospheric pressure exceeded 5 atm, and could not have persisted in the crust or beneath an ice cover over geologic time [Stewart and Nimmo, 2002]. However, liquid water is relatively close to its stability field even on present-day Mars [Lobitz et al., 2001; Haberle et al., 2001] and could have been stored in a Hesperian-age impoundment given a thick ice cover veneered with ash or dust. A small increase in atmospheric pressure (5 10 mbar) would have further enhanced the ability of water to be stored near the surface. The presence of young channels in Amazonis [Carr, 1996] and Elysium (Cerberus) [Burr et al., 2002] shows that outflow channels can form even under the present atmosphere and climate. Outflows from these youngest channels might have created shallow (<1 m) ice deposits that could possibly be detected with neutron instruments onboard the Mars Odyssey orbiter. [16] Today Ganges Chasma has an eastern outlet (Figure 3) with a floor elevation of 3720 m [MOLA 1/32 degree grid at S, W] that would prevent the formation of large lakes in the canyon. At the time of flows in Shalbatana and Ravi Valles, Ganges Chasma would have been smaller and the eastern outlet would not have yet formed. It was probably an isolated ancestral basin of the type described by Lucchitta et al. [1994]. Hebes Chasma, a large fully-enclosed canyon with thick layered deposits, shows us how the ancestral canyons may have looked before most of them merged to form Valles Marineris. MOLA topography shows that gradients would have favored groundwater flow northward from Ganges Chasma, but only if ice and water deeply filled the ancestral canyon. The land surface at the outflow zone of eastern Aromatum Chaos has an elevation near the Martian datum. The elevations of highland terrain adjacent to western Ganges Chasma exceed 1100 m. Given that the regional slopes have not significantly changed since the Hesperian, a small topographic gradient of (1.1 km/500 km) would have induced northerly subsurface flow from an ice-covered lake in ancestral Ganges Chasma toward the outflow zones at Shalbatana Vallis and Ravi Vallis. Rising groundwater and lake levels would have raised pressures in confined aquifers to the north where the potentiometric surface could have eventually risen above the land surface, creating conditions for groundwater release and artesian flow. Meteorite impact or seismic activity may have triggered the groundwater release. Shalbatana Vallis is significantly deeper (elev m at N, W) than Ravi Vallis (elev m at S, W) [MOLA 1/32 degree grid]. Therefore, after the discharges had commenced, flow could have continued in Shalbatana Vallis even if the potentiometric surface dropped far enough for Ravi Vallis to dry up. Flow in both channels would have ceased after an eastern outlet was carved from Ganges Chasma. This outlet would have catastrophically drained the canyon and helped to form Simud-Tiu Valles. The subsidence zone north of Ganges Chasma shows how the eastern outlet may have formed. Perhaps the outlet began as a similar zone of subsidence that ultimately collapsed far enough that the flow of water transitioned from underground to aboveground, draining the canyon and permanently lowering regional groundwater levels. The eastern outlet parallels the dominant east-west structural trend in the Valles Marineris. Structures along this trend may have favored creation of the eastern outlet by enhancing groundwater flow, leading to undermining, collapse, and catastrophic surface flow and erosion. There is evidence elsewhere on Mars that groundwater erupted from massive fissure zones or graben at rates sufficient to carve fluvial channels. Burr et al. [2002] describe geomorphic evidence for aqueous flooding that erupted from the Cerberus Fossae near 10 N, 203 W, forming the channel system of Athabasca Valles. Tanaka and Chapman [1990] relate the formation of Mangala Valles to catastrophic discharges from Memnonia Fossae at 18 S, 149 W Origin of the Largest Channels [17] The largest outflow channels on Mars were produced quite differently from Ravi Vallis. Kasei Valles [Williams et al., 2000], Ares Vallis, and Simud-Tiu Valles likely formed through the cyclic release of flood waters from impoundments dammed by ice and debris, analogous to the scabland flooding of eastern Washington [Baker, 1978]. The main Valles Marineris canyons, along with Ganges Chasma, are likely sources for the floods that formed Simud-Tiu. Layered deposits within the Valles Marineris may be volcanic in origin, possibly generated by subice eruptions [Chapman and Tanaka, 2001]. Ivanov and Head [2001] concluded that the Simud-Tiu system seems to crosscut all major channels that emptied into Chryse Planitia. Simud-Tiu Valles may therefore represent the last large flow events. This is logical in the context of regional hydrology because the draining of ice-covered lakes in the main canyons of Valles Marineris would have rapidly and permanently lowered the groundwater elevations in adjacent regions, thus preventing the recurrence of large outflows in other channels. Detailed analysis of the ancestral canyons of Valles Marineris and the possible locations, volumes, timing, and longevity of canyon lakes is beyond the scope of this paper. However, a wealth of newly acquired data (i.e., topography, imagery, and surface composition data) is available from Mars Global Surveyor [Albee et al., 2001] and Mars Odyssey that can be used to interpret the history of the canyons and the channels that issued from them. [18] Ares Vallis, which is 1.6 km deep (MOLA pass 21 at 10 N) [Smith et al., 1998], is the final drainage channel for what may be the longest known surface water pathway in the solar system [Parker et al., 2000; Clifford and Parker, 2001]. This system began as channels that drained the south polar region, then flowed into the Argyre basin, and finally overflowed Argyre at a low point near 35 S, 36 W. Although the course is imprinted by some craters, MOLA data suggest that hydraulic continuity could have existed northward from Argyre via Ares Vallis to Chryse Planitia. Ares Vallis also received fluid inputs from chaotic terrain such as Aram Chaos, Iani Chaos, and Margaritifer Chaos. But the possibility that a sea in Argyre may have overflowed helps identify the agent that carved Ares Vallis. Only a liquid, like water, could have migrated to Argyre, slowly accumulated under an ice cover, and ultimately filled the

9 COLEMAN: WATER CARVED THE OUTFLOW CHANNELS ON MARS 5-9 basin to overflow capacity. Hypothetical CO 2 gas-supported flows from the south polar region would have rapidly dissipated and dropped their sediment loads in the large basin. [19] The channels of Kasei Valles were likely formed by flood waters released from impoundments in Echus Chasma, including the southernmost extremity that may earlier have been fully enclosed like Hebes Chasma. The south channel of Kasei Valles reaches depths of 3km (MOLA pass 252 at 23 N) (see Figure 1). Downcutting of 1 3 km in various outflow channels requires intense hydrodynamic erosion, especially if the upper crust consists of layered volcanic rocks. Cavitation, the most powerful erosive process in floods, can cause local pressures that are strong enough to shatter indurated rocks and would have been greatly enhanced in Martian catastrophic floods [Baker, 1979]. [20] Hoffman [2000, Figure 12] shows erosional grooves on ridges located on the floor of Kasei Valles. He presents these crossover valleys as evidence that erosive, gassupported density flows overtopped the ridge. He also asserts that if the crossover valleys were water carved, the floodwaters would need to have reached the top of the ridge. We can now reexamine this part of Kasei Valles using MOLA data (Figure 4). In comparing Figure 4 to Hoffman s Figure 12, please note that his Figure 12 is reversed from left to right. The scoured channels, streamlined islands, and erosional grooves shown in Figure 4 provide strong evidence that deep floodwaters swept through this region. Multiple episodes of flooding are probably required to produce the observed features and channel depths. The power and depths of the flows were so great that some crater ramparts were obliterated, leading to erosion of the teardrop-shaped islands in the lee of the craters. MOLA data (orbits 1290 and 9815) show that the large channel at E in Figure 4 is more than 600 m deeper than the adjacent ridges. The deepest parts of this channel may have been cut by later floods that did not reach the tops of the islands. MOLA data (Figure 4) reveal deep pits at the sites of some former craters. Debris flows would have filled in these depressions, but catastrophic floods would more likely have further eroded the pits. A deep cataract can be seen at C in Figure 4. The MOLA image at 1/64th degree resolution does not resolve all of the erosional grooves shown in the upper frame of Figure 4. However, the grooves shown at D in the center of a ridge were incised to about the same depth as the channel floor south of the ridge. Therefore, these grooves represent incision through most of the height of the ridge and not just ridge-top erosion. Overall, I conclude that the erosional features in Kasei Valles are entirely consistent with a catastrophic flood origin and do not resemble the landforms created by gas-supported mass movements like pyroclastic flows. Gas-supported flows would have filled in and smoothed the preexisting topography, which is inconsistent with the sculpted and eroded landforms in Kasei Valles Composition of the Upper Crust of Mars [21] Hoffman [2001c] considers the upper crust of Mars to be a thick layered regolith, as much as 10 km thick, produced mainly by impact events. A thick icy regolith would theoretically contain a sizable fraction of volatiles, would more easily be disrupted to form cryoclastic flows, and could allegedly be eroded by such flows to carve channels. However, early views that Mars has a thick regolith are being revised because of the depth and pervasiveness of the layering. McEwen et al. [1999] report extensive horizontal layering to depths of at least 8 km in the Valles Marineris canyons. The detailed layering is inconsistent with a deep regolith, or megabreccia, which should only be coarsely layered. Based on morphological and compositional data, McEwen et al. [1999] suggest that these layers were mostly formed by volcanic flood lavas, and that Mars was probably very volcanically active until after the period of heaviest impact bombardment. The production rate of lava may have been high enough to dominate the surface processes and prevent the formation of a pervasive thick regolith. Hoffman [2001c] doubts a volcanic origin for the crustal layers because of a lack of volcanic vents and fissures, and because of the obvious low strength of the layered materials. However, Caruso and Schultz [2001] perform mechanical modeling of slope stability in Valles Marineris and find the wallrock is consistent with igneous rock such as layered basalt with columnar jointing. Surface rocks at the Viking and Pathfinder landing sites resemble vesicular basalts, and thermal emission data suggest that basalts and andesites dominate the low-albedo surface units on Mars [Christensen et al., 2001]. On Earth, andesites are produced at crustal plate margins where subduction occurs, such as in the Cascade Range of North America. The presence of abundant andesite would therefore be hard to explain on a planet like Mars where little or no evidence of subduction has been seen. The signature of andesites is concentrated in the northern plains of Mars, where oceans or seas may have existed [Clifford and Parker, 2001]. Wyatt and McSween [2002] reexamine TES (thermal emission spectrometer) data and conclude that the northern plains can be interpreted as weathered basalt instead of andesite. They suggest that these lowland plains may consist of basalts weathered under submarine conditions, or that sediments derived from weathered basalts were transported into the lowlands. Of course, Martian meteorites provide actual samples of the upper crust. They are igneous rocks from parent magmas that were emplaced at shallow depths in the crust or extruded as lavas [Carr, 1996] The Carbonate Paradox [22] Hoffman [2000] rejects the abundant evidence of fluvial erosion on Mars and cites the apparent lack of carbonates at the surface as a paradox that would be resolved if liquid water were absent. Large carbonate deposits are not currently exposed at the surface, but could escape detection at surface abundances of <10% [Christensen et al., 2001]. The so-called carbonate paradox may not exist because carbonates are found in some Martian meteorites, although their origin is still being debated. The ancient meteorite ALH84001 contains carbonates at levels of several percent [Jakosky and Phillips, 2001]. Carbonates could be very difficult to detect using remote sensing [Fonti et al., 2001], and they are also susceptible to destruction at the surface by sulfur-containing gases from volcanism [Blaney, 1999; Clark, 1999]. This is supported by the fact that Martian soils are enriched in sulfur, approximately 100 times higher than the average in the Earth s crust [Clark et

10 5-10 COLEMAN: WATER CARVED THE OUTFLOW CHANNELS ON MARS al., 1977]. Newsom et al. [1999] conclude that hydrothermal processes could also have helped enrich the S, Cl, Na, and K content of soils. It is possible that a Hesperian hydrologic climax did not last long enough for thick sequences of carbonates to accumulate in surface impoundments, especially if thick ice covers on lakes reduced the degree of interaction between the water bodies and the atmosphere. In summary, the nondetection of carbonates by remote sensing has reasonable explanations, and is not an adequate basis to conclude that liquid water has been absent for eons The Floodwater Paradox [23] An additional paradox described under White Mars concerns the destination of waters released in the catastrophic floods. Hoffman [2000] observes that the Martian polar caps seem to consist mainly of water ice, but are much too small to account for the water needed to fill a former ocean in the northern plains. Indeed, Smith et al. [2001b] estimate that the polar layered terrains contain a small global equivalent layer of about m. Hoffman [2000] notes that it is unclear whether the water from that ocean was lost to space or buried as ice in the crust. Neutron data from Mars Odyssey provide strong evidence that a large reservoir of water is preserved as ground ice in the uppermost crust at high latitudes. The method can assess the hydrogen content, as a surrogate for water, in the uppermost meter of soil or rock. Large quantities of hydrogen have been unambiguously identified poleward of 60 S. Boynton et al. [2002] suggest that the hydrogen exists in a subsurface layer as ice, which constitutes 35 ± 15% of the layer by weight, or 40 to 73% ice by volume. The presence of water ice is probably required because the sheer abundance of hydrogen exceeds what would be found in weathering products such as clays. Also, the areas with enriched hydrogen generally coincide with regions where ground ice is predicted to be stable under the present climate [Mellon and Jakosky, 1995]. A hydrogen-rich layer may also exist in the northern hemisphere, but the north polar region was largely obscured by a seasonal CO 2 cover at the time of initial data collection by Mars Odyssey [Boynton et al., 2002]. Although the neutron data represent only the uppermost meter of soil or rock, it is probable that rich ice deposits extend to greater depths at mid to high latitudes. The total inventory of water that is sequestered as ground ice will depend on crustal thermal properties and porosities. This inventory will add substantially to that estimated for the polar layered terrains. A large volume of water has probably also been lost to space over geologic time [Jakosky and Jones, 1997; Jakosky and Phillips, 2001; Hodges, 2002]. The enrichment of D/H in the Martian atmosphere requires loss of substantial quantities of hydrogen to space, with water providing the source. Jakosky and Leshin [2001] estimate that approximately 2/3 of the exchangeable water must have been lost. Therefore, considering the evidence that water ice is still abundant on Mars today, a paradox may not exist regarding the fate of floodwaters that carved the outflow channels Crustal Recharge of Volatiles [24] The apparent accumulation of ice and water in ancestral Ganges Chasma, as discussed in section 2.4, would require significant sources of liquid water at that time. A detailed analysis of recharge on Mars is beyond the scope of this paper but is the subject of ongoing study. The following sections discuss previous work and ideas for future research on processes that could produce liquid water on Mars. The potential for recharge of liquid CO 2 is also discussed Polar Basal Melting [25] Several authors have explored the potential for recharge of volatiles at the base of the Martian polar caps. Clifford [1987, 1993] proposes that polar basal melting of water ice occurred in the geologic past and caused a global water table to rise during the Hesperian, leading to conditions favoring outburst flooding from confined aquifers. Clifford s analysis implicitly assumes that an interconnected global aquifer existed on Mars and that most groundwater recharge occurred in the polar regions. Clifford and Parker [2001] conclude that the present-day polar caps may be too thin to support basal melting of water ice under the current climate and inferred geothermal conditions. [26] Ross and Kargel [1998] conclude that the north polar cap cannot be made of CO 2 ice because its surface is too warm. Kieffer et al. [2000] find no evidence of water in the south polar seasonal cap and little evidence of water in the atmosphere above the receding cap. However, they expect that the southern perennial CO 2 cap overlies water ice because temperatures are persistently low enough to coldtrap water. Durham et al. [2000] find that the rheological weakness of pure CO 2 ice is so pronounced that it cannot be Figure 4. (opposite) Scoured channels and teardrop-shaped islands formed by catastrophic floods in Kasei Valles (Map source for upper frame: USGS, 1987). Flow direction was from lower left to upper right. Streamlined islands develop in areas protected from the strongest currents, such as in the lee of crater ramparts. Note locations of impact craters at the island tips labeled A, B, and C. The floodwaters were sufficiently deep and powerful that the crater ramparts were undermined and mostly obliterated by the flows. Deep pits at B and C, shown in the MOLA frame, are remnants of the former craters. These pits would have been filled in if large-scale debris flows had swept through this area. Instead, they were preserved and may have been widened and deepened by floodwater hydrodynamic erosion and plucking of the country rock that had been weakened by the former impacts. Loss of the protective crater ramparts led to increased erosion of the islands themselves. MOLA data suggest that the island at B was formerly much longer (east to west) and more than twice as wide. The eastern end of the island was deeply incised by the largest flooding events. Erosion at D cut through the center of an island. Longitudinal grooves (E) were probably caused by differential erosion of bedrock on channel floors. The image frames are centered on 27 N, 61.9 W. The Viking imagery (upper frame) is outlined in the lower frame which shows MOLA elevations at a resolution of 1/64th degree per pixel. Dark areas in the MOLA image are lower in elevation than light areas. (Image credit for lower frame: MOLA Experiment Gridded Data Record, available from the Planetary Data System Geosciences Node at

11 COLEMAN: WATER CARVED THE OUTFLOW CHANNELS ON MARS 5-11 a major component of either polar cap. Hoffman et al. [2002] agree that CO 2 ice is probably too soft to sustain the topography of the polar caps. They consider a clathratebased model to be the only viable one for basal melting. [27] Direct precipitation of CO 2 clathrate at the poles is severely limited by the availability of water vapor in the atmosphere. Clathrates could form if CO 2 and water ice became buried together in a polar cap to depths greater than 10 m [Carr, 1996]. Mellon [1996] investigated the phase stability of CO 2 ice and CO 2 clathrate hydrate and found that their low thermal conductivity restricts the amounts of CO 2 that can be sequestered. Under realistic conditions, CO 2 will be limited to no more than a few 10s of millibars in the north polar deposit. Mellon [1996] suggested that the south polar deposit may have a similar composition, but he assumed the deposits in the south were thinner, which made it difficult to place limits on the CO 2 content. Mellon [1996] had assumed that the north polar cap was 4 6 km thick, and the southern cap was up to 2 km thick. MOLA data [Smith et al., 2001b] now reveal that the thickest part of the southern cap is similar to the northern cap, 3 km thick. These thicknesses differ from those used by Mellon [1996], but probably do not alter his main conclusion that large quantities of CO 2 are not found in the polar deposits. Kolb