White Mars: A New Model for Mars' Surface and Atmosphere Based on CO2

Icarus 146, 326–342 (2000) doi:10.1006/icar.2000.6398, available online at http://www.idealibrary.com on

White Mars: A New Model for Mars’ Surface and Atmosphere Based on CO2 Nick Hoffman Department of Earth Sciences, La Trobe University, Bundoora, Victoria 3083, Australia E-mail: [email protected] Received March 29, 1999; revised March 2, 2000

A new model is presented for the Amazonian outburst floods on Mars. Rather than the working fluid being water, with the associated difficulties in achieving warm and wet conditions on Mars and on collecting and removing the water before and after the floods, instead this model suggests that CO2 is the active agent in the “floods.” The flow is not a conventional liquid flood but is instead a gas-supported density flow akin to terrestrial volcanic pyroclastic flows and surges and at cryogenic temperatures with support from degassing of CO2 -bearing ices. The flows are not sourced from volcanic vents, but from the collapse of thick layered regolith containing liquid CO2 to form zones of chaotic terrain, as shown by R. St. J. Lambert and V. E. Chamberlain (1978, Icarus 34, 568–580; 1992, Workshop on the Evolution of the Martian Atmosphere). Submarine turbidites are also analagous in the flow mechanism, but the martian cryogenic flows were both dry and subaerial, so there is no need for a warm and wet epoch nor an ocean on Mars. Armed with this new model for the floods we review the activity of volatiles on the surface of Mars in the context of a cold ice world— “White Mars.” We find that many of the recognized paradoxes about Mars’ surface and atmosphere are resolved. In particular, the lack of carbonates on Mars is due to the lack of liquid water. The CO2 of the primordial atmosphere and the H2 O inventory remain largely sequestered in subsurface ices. The distribution of water ice on modern Mars is also reevaluated, with important potential consequences for future Mars exploration. The model for collapse of terrain due to ices that show decompression melting, and the generation of nonaqueous flows in these circumstances may also be applicable to outer Solar System bodies, where CO2 , SO2 , N2 , and other ices are stable. °c 2000 Academic Press Key Words: ices; Mars, surface; Mars, atmosphere; geological processes.

INTRODUCTION

Modern Mars is a distinctly cold, dry, dusty planet. However, the surface shows signs of apparent flow of water, most notably during the late Hesperian and Amazonian when “outburst floods” carved major channel networks leading to the northern lowland plains. These channels are the single largest line of evidence for water on Mars’ surface. If alternative explanations for these channels can be generated, then the paradoxical models

of a “warm and wet” early Mars can be discarded in favor of a more consistent cold ice world model. In this paper we develop an explanation for the “flood” channels as cold and dry surface density flows, supported by CO2 vapor. The source of the vapor is liquid CO2 and solid CO2 -bearing ices from the regolith. SURFACE OBSERVATIONS

Mars’ surface displays a complex and extended geologic history with at least two major episodes of fluid erosion and/or sedimentary transport: first in the Noachian when dendritic valley networks were incised, overlapping in time with the closing phase of planetary bombardment and the second in the Hesperian/Amazonian where outburst floods have carved major channel systems. The surface evidence and history is well summarized elsewhere (e.g., Cattermole 1992, Baker et al. 1991, Kieffer et al. 1992), but the following outline history and cogent details should be noted. Early Mars, like the other terrestrial planets, shows clear evidence of major cosmic bombardment during the final stages of accretion. And large parts of the present surface—the southern highlands—appear to be substantially unaltered since this time, on Viking era imagery. However, many Noachian craters do show subdued relief, and smaller craters of this age are missing, implying some local resurfacing or erosion process. MGS highresolution images show a smoothed and rounded topography with patchy development of thermokarst and ablation features, indicative of a long slow erosion history under a thin, cold atmosphere. Local dendritic valley networks in these highland terrains are indicative of fluid flow and erosion during the Noachian period (>3.8 Ga) with a significant component of groundwater sapping. Subsequently surface activity became more dominated by volcanic processes, with little sign of fluvial erosion or sedimentation. Mars’ surface appears to have progressively become dry and inactive after this time, although “splash” craters suggest the presence of a fluid stored at depth in the crust, below a dry surface layer about 500 m thick (Barlow et al. 1999). Widespread ground ice is predicted at depth over most of Mars (Rossbacher and Judson 1981). The layered composition of the upper crust (McEwen et al. 1999) may suggest extensive earlier

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sedimentary and/or igneous layering although in this paper an alternative mechanism will be investigated. In the more recent Late Hesperian to Amazonian episode (∼1.8 to 1 Ga), resurfacing rates increased significantly in local areas with catastrophic outburst floods originating at the sapped and collapsed edges of cliffs and craters and in linear tension features such as Chasmae. These “floods” are strongly erosive in their upper and mid portions and extend into very long-distance, low-gradient, transport/depositional systems with distinctive channel morphologies. The floods show evidence of breaching of topographic barriers, overtopping of obstacles, thick flow depths and generally catastrophic proportions. Repeated flow episodes have occurred in each channel and the deposits from different channel systems overlap, implying an extended chronology of fluid activity. The main problem with outbursts of underground water is the storage of such large volumes in the available pore space and the requirement for a water recycling system despite the absence of evidence for precipitation. Alternative explanations involve periglacial solifluction and rock or ice glaciers for the transport of these flood deposits (Rossbacher and Judson 1981, Kargel and Strom 1992, Squyres et al. 1992). These explanations cannot adequately explain the long transport distances and low gradients compared to Earth analogs. Characteristic of the outburst source regions are the zones of “chaos”—extensive areas of blockily fragmented ground and tension gashes, sometimes with no obvious outlet but with considerable loss of volume in highly disrupted areas. Carr (1979) was one of the main proponents of a liquid water-escape mechanism for generation of the chaos zones and the ensuing water floods. Milton (1974) suggested that the water was released by decomposition of CO2 –water clathrate, while Baker and Milton (1974) drew attention to the parallel with glacial lake bursts in the channeled scablands of Washington State (USA). Many chaos zones are broadly circular and may represent the surface expression of collapse due to subsurface melting from a geothermal heat source. Lambert and Chamberlain (1978, 1992) developed a very useful model for the generation of fluids in the subsurface from CO2 clathrate which elegantly explains the areas of chaos. Unfortunately, they focused on liquid CO2 as the active erosive agent in Amazonian times and therefore missed some key aspects of the model. Those defects will be addressed in this paper. MARS PARADOXES

A number of significant paradoxes crop up in trying to model the evolution of Mars. Principal among these is the Faint Young Sun Paradox. Our understanding of stellar evolution requires that the early Sun radiated some 30% less energy than it does now, with gradual warming as its hydrogen supply is steadily fused to helium and its core expands. However, if Mars in the past was cooler than it is now, and if it is too cold now for liquid water to flow on its surface, then how can we explain the channel

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features on its surface? Present day Mars has a mean surface temperature of 223 K (of which a mere 5 K is greenhouse effect in the thin atmosphere). Near Mars’ equator on the warmest of days, in the weak glow of the Sun the temperature can just rise high enough to melt water ice. Early Mars would have had a “bare rock” equilibrium temperature of a mere 196 K and even equatorial temperatures would have been glacial. Some progress can be made by invoking spin-axis instability of Mars, but these models generally only warm one part of Mars at the expense of another and the atmospheric cold-trap remains, preventing a true global warming. Standard methods of avoiding this paradox call for a dense early CO2 atmosphere on Mars with a strong greenhouse effect. However, this approach runs into other difficulties. First, if we compare Mars and Earth and assume that they had similar original volatile inventories, then they would also have had similar greenhouse atmospheres. For Mars to have had enough CO2 to raise its mean temperature to that of liquid water then Earth, being closer to the Sun, would have been even hotter. Earth’s “bare rock” radiative equilibrium temperature is about 40 K warmer than Mars, so if the greenhouse effect of early atmospheres was this strong, then Earth would probably have experienced a runaway greenhouse effect like Venus, becoming hot enough to boil its oceans into a steam atmosphere and sterilize any attempts at early life. The fact that Earth is not like Venus would require either that it had a lesser volatile inventory and/or much slower outgassing than did Mars. Everything that we know about Earth seems to work the other way. Earth is larger, would have accumulated more energy of accretion from incoming planetismals, has a more active tectonic system than does Mars, and seems to be richer in volatiles. Alternatively, we begin to doubt strong early greenhouse atmospheres on Earth and Mars. Second, even if Mars did have that greenhouse atmosphere, it would not be capable of sustaining surface temperatures high enough to avoid atmospheric collapse. Kasting (1991) pointed out that the 10 bars of atmosphere required would be unstable and would condense at altitude, leading to atmospheric cooling and eventual freeze out at the poles or globally, resulting in an ice world scenario with a thin atmosphere of a few bars at most of CO2 . Although this collapse can be circumvented by adding a number of species to the greenhouse inventory (e.g., Pollack et al. 1987) and by favorable radiative behavior of clouds (Forget and Pierrehumbert 1997), the resulting cocktail becomes somewhat contrived and still does not explain why Earth avoided a runaway venusian outcome. Note that the greenhouse effect is driven only by gaseous phases. The large amounts of water in Earth’s oceans do not contribute to the greenhouse effect while the small amounts of water vapor in the atmosphere are very important. Similarly, solid ices do not have a greenhouse effect. In fact, due to their high albedo there may actually be a negative greenhouse effect from such ices. Therefore the presence of large amounts of “greenhouse” phases on a cold planet do not necessarily lead to it warming up—witness modern Mars, which has abundant CO2 frost at

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its winter pole. For example, P. F. Hoffman (1998) showed that an episode of global glaciation occurred on Earth during the Neoproterozoic, due to runaway cooling and albedo feedback. The combination of a condensable atmosphere on Mars and the albedo-cooling feedback mechanism predisposes early Mars to a global deep freeze. Further paradoxes in the conventional water-based story of Mars surface are the volumetric misfit between the source areas of the catastrophic outburst channels and the implied flow depths and rates. In order for water to be the active fluid carving these channels and transporting the huge amounts of debris, then floods on Mars must have been larger by an order of magnitude, at least, than anything observed on Earth. On Earth, large volumes of water can be stored on the surface at ambient temperature and pressure behind rock, earth, or ice dams, which may fail suddenly leading to huge floods. On Mars, the surface conditions are too cold in the Amazonian epoch for storage of liquid water, so it must be stored underground in aquifers with limited porosity, and a more complex breakout mechanism is required. The chaos zones are spatially small compared to the channels that emerge from them and mass balance calculations require very large volumes of water in each of many flood episodes in each channel. For this water-based model to be true, the water needs to be stored around the chaos zone and recharged repeatedly. The ratio of water to debris required to form a dense mudflow or lahar on Earth is about 1 : 1. For the more liquid catastrophic flood outbursts implied by the morphologies of the channels, the ratio needs to be at least 2 : 1. However, the effective limit of porosity in any coherent rock or loose regolith is about 40%. If excess water is added through pressurization of the aquifer, the host rock will liquefy and itself flow as a slurry. In order to achieve the minimum 2 : 1 ratio for an outburst without exceeding the 40% maximum porosity, then a volume of saturated aquifer about 5 times the debris volume is required to be completely drained at a very rapid discharge rate. There are a number of difficulties with accumulating this volume of subsurface water on a planet where surface water cycling is very limited. In addition, the discharge rates required are excessive for flow out of a porous medium. More exotic models of underground caverns and cavities with subterranean lakes and rivers have been proposed, but these become progressively less credible. Additional paradoxes concern the destination of the water involved in the floods and of the CO2 from the primordial greenhouse atmosphere. The polar caps of Mars appear to be composed largely of water ice (Smith et al. 1999), but are too small by over an order of magnitude to account for the volume of water required to explain possible shorelines of a Boreal ocean (Baker et al. 1991, Parker et al. 1989). Hydrogen isotope data suggest significant loss to space over Mars’ history which can potentially explain this, but it is still not clear whether the water from that ocean is now buried as ice, or lost to space. As for the CO2 , there is a carbonate paradox on Mars. On Earth, the ∼90-bar primordial CO2 inventory is now locked into carbonate minerals but almost no carbonate has been detected on Mars

despite an extensive search program. If significant volumes of liquid water had been present on Mars, carbonate should have been profusely generated in the shallow lake and ocean basins and should now be exposed on the dry basin floors or in younger impact craters in these areas. Yet there is no such evidence. Interestingly, Bodnar (1999) has recently found liquid CO2 droplets, not H2 O, as a pervasive fluid phase from analysis of martian SNC meteorites. The significance of this discovery should be emphasized. On Earth, subsurface rocks are saturated with water, the dominant volatile in the hydrosphere. CO2 is almost never encountered in the subsurface as a free phase. For two independent samples from Mars to both have CO2 as the free volatile phase suggests that conditions on Mars are far more likely to be dominated by CO2 . In order to resolve these paradoxes, we need a fundamentally different model of Mars’ atmosphere and surface evolution, involving a more rigorous understanding of the pressure/ temperature behavior of the mixed volatile system CO2 + H2 O. Let us begin with a review of the quantity of volatiles expected on Mars. VOLATILE INVENTORY OF MARS

Venus has ∼90 bars CO2 atmosphere at present. Earth has a similar CO2 inventory locked up in carbonate rocks. If those 90 bars of CO2 were somehow frozen out on Earth or Venus, it would form a layer about 600 m deep over the entire planet (recall that Torricelli found that Earth’s 1-bar atmosphere can be balanced by a ∼10-m water column. Solid CO2 has a relative density of 1.5, so needs a shorter column). Mars is about half the diameter of its sister planets and if it had a similar volume fraction of CO2 , spread over its smaller surface area, the column density of CO2 would be about half that of Earth or Venus (volatile thickness scales as planetary radius, R, given equal volume fraction and outgassing efficiency). In fact, a global layer some 325 m thick of solid CO2 ice would equate to the primordial CO2 inventory of Mars. In Mars’ lesser gravity, this would exert a surface pressure of 18 bars. There is some uncertainty in these figures so a range of 10–30 bars is probably reasonable. In practice, a cold early Mars could have had an “ocean” or a polar ice sheet 1 km deep over about 1/4 of the planet, under a ∼5-bar CO2 atmosphere. The pressure obviously requires to exceed 5.11 bars before liquid CO2 is stable on the surface. Isotope models of Mars’ atmospheric evolution require a minimum initial inventory of a few bars of CO2 , but do not constrain the upper limit (e.g., Fox 1993, Jakosky et al. 1994). Over the lifetime of Mars, oxygen has been more strongly buffered than hydrogen, suggesting that an extensive CO2 reservoir in the regolith has interacted more strongly with the atmosphere than the equivalent H2 O reservoir. Estimates of the water inventory of Mars vary widely. In part, this is due to different reservoirs being measured. Recent Mars Obiter Laser Altimeter (MOLA) data show that the pole caps contain a volume equivalent to a global layer some 22 to 33 m

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deep (Smith et al. 1999). This water is believed to actively participate in cycling processes over a geological time scale, due to orbital variations. Carr (1979) calculated that the sum of all Amazonian outburst floods represents a volume equivalent to 40–80 m water globally. The SNC meteorites appear to be in equilibrium with a mantle with around 36 ppm water, yielding about a 130-m global layer assuming a degree of differentiation similar to that of Earth (Driebus and Wanke 1987). On the other hand, Earth’s wetter mantle may result from water recycling by plate tectonics so the dry mantle of Mars may not preclude a crustal water reservoir. Head et al. (1998) have modeled possible Boreal ocean shorelines and also find about 130 m global equivalent. Finally, scaling Earth’s oceanic water inventory (a 2to 3-km global ocean) would suggest that 1000 to 1500 m of water might be present in the surface layers of Mars—although this latter volume would be largely inactive ice which has not communicated with the atmosphere, otherwise the hydrogen isotope fractionation data cannot be satisfied. THE WHITE MARS MODEL

We now introduce our new model for the evolution of Mars as an ice world dominated by the behavior of CO2 . This model has the following important features, which are discussed in turn in the remaining sections of this paper: • The CO2 phase diagram has characteristics that can readily explain the chaotic outburst floods as density flows lubricated and supported by CO2 liquid and vapor at cryogenic temperatures. In contrast, the H2 O phase diagram has characteristics that make outbursts of water difficult to achieve on a cold planet. • The most common fluid in the subsurface of Mars is likely to be liquid CO2 , not water. • Clathrates can sequester H2 O, further limiting its availability in the subsurface of Mars. These clathrates provide an

additional source for the CO2 by disequilibrium melting in the “floods” while the water remains as solid ice. • The density flows have analogs on Earth in submarine turbidity currents and volcanic pyroclastic flows and surges. • Localized warming and instability of the layered terrain produces the floods by collapse of the terrain and its transition through an avalanche to a debris flow and then to a gas-supported density flow. • The White Mars model provides simple and complete explanations for the Carbonate Paradox and the Faint Young Sun Paradox and is compatible with the atmosphere and history of both Earth and Venus. PHASE DIAGRAMS

We now turn to a discussion of the important fundamental differences between the properties of H2 O and CO2 that lead to our White Mars model. Figure 1 shows the phase diagrams of water and CO2 side by side. Each of these volatiles is capable of existing as liquid, solid, or vapor at pressures and temperatures commonly encountered on the surface or subsurface of the terrestrial planets. For each volatile there exists a unique pressure and temperature at which all three phases can coexist—the triple point. Water’s triple point (0.0098◦ C, 0.006 bar pH2 O) is of vital influence for life on Earth. The phase changes between ice, water, and vapor drive Earth’s weather and climate systems, form the cycles of erosion, transport, and deposition of sediments, and ultimately permit life to flourish. CO2 is more volatile than H2 O and its triple point is both colder and at higher pressure (−56.4◦ C and 5.11 bar). Water is unique in having a negative slope to its solid–liquid phase boundary. This is due to the lower density of ice compared to water, unlike most solids. The consequences of this permit us to skate on ice and build snowballs, but mean that subsurface water on Mars will at least partially freeze when it decompresses

FIG. 1. The phase diagrams of H2 O and CO2 . Solid lines are phase boundaries where two phases can coexist. These meet at the triple point, where liquid, solid, and gas can all coexist. The gas/vapor phase boundary terminates at the critical point, above which temperature both gas and liquid merge into a single supercritical fluid phase. Note that due to the negative slope of the liquid/solid boundary for water, H2 O shows pressure melting and decompression solidification. CO2 , on the other hand, solidifies under pressure but melts on decompression. With extreme decompression, both volatiles flash partially to gas from either solid or liquid phase, but H2 O generates far lower pressures and sublimes at a much slower rate due to its higher latent heat capacity and lower vapor pressure.

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in an outburst, unless it is distinctly hot. Solid CO2 , on the other hand, is prone to partial melting on adiabatic decompression from high pressures. The arrows on the diagram illustrate the phase behavior on decompression from high pressures, such as would be expected from the lithostatic or hydrostatic load at several kilometers’ depth. In reality, the path would not cross the phase boundary, but would track it. The ratio of solid and liquid at any point can be calculated from the physical properties of the volatile and the energy of the system. CLATHRATES AND SUBSURFACE FLUIDS

Carbon dioxide, like many other small molecules, forms a joint gas hydrate or clathrate with water. A complex cage of 46 H2 O molecules link into a framework with cavities for up to 8 guest molecules at full occupancy. Thus, a relatively small amount of CO2 can lock up large amounts of water. Clathrates have slightly larger cell volume than an equivalent quantity of normal water ice, but slightly less than the sum of the ice and the guest molecules as separate phases; therefore, they are the preferred phase under pressure and are likely to occur in the subsurface. Miller (1974) describes in detail the CO2 /H2 O system at both “normal” and cryogenic temperatures. Figure 2 shows the combined phase diagram for the mixed volatile system. The phases that coexist at any point on the diagram are determined not only by the pressure and temperature, but also by the relative abundance of CO2 and H2 O. If one component dominates, then phases involving that component will be favored. A large relative abundance of one or another volatile species will mop up the other species and lock it into clathrates, in the appropriate temperature and pressure range. Given the relative

FIG. 2. Mars pressure and temperature conditions compared to CO2 and H2 O phase diagrams. Planetary mean temperatures are plotted. As with modern Mars, temperature variations exist between the icy polar night and the warm equatorial day. These variations will be explored in later sections of this paper. Early Mars requires a substantial heat source to prevent atmospheric collapse. If sufficiently intense, the main phase of planetary bombardment can provide this heat temporarily, but as soon as the impactor flux dies down, the atmosphere collapses to a polar ice sheet with a few millibars of atmosphere. Subsequent warming of the Sun has warmed Mars’ surface by a few Kelvins.

molecular weights of H2 O and CO2 , a fully occupied clathrate lattice represents a ratio of CO2 to H2 O of 1 : 2.35 (w/w). On Earth, CO2 (as carbonate) and water coexist in the ratio 2 : 3 by weight. In the colder conditions existing on Mars, a similar ratio of volatiles would result in effectively all of the water being locked into solid clathrates in the subsurface, with significant excess volumes of CO2 available as polar and subsurface ice in high latitudes, as atmospheric gas, and most importantly, in low latitudes as liquid CO2 . If an equivalent inventory of volatiles exists on Mars as on Earth, then we would expect the regolith (∼10 km thick) to contain ∼15% clathrate and ∼5% CO2 by volume. Clathrates melt in interesting ways. At fixed pressure, if the clathrate is warmed, then equilibrium melting occurs with clathrate transforming to liquid water and CO2 vapor. CO2 is highly soluble in cold water, especially at elevated pressures, but excess CO2 still remains as a free vapor phase. If, instead, temperature is fixed and the pressure is reduced, then clathrates display disequilibrium melting. Solid clathrate transforms to solid water ice and CO2 vapor. In this case, the solubility of CO2 in ice is negligible and large amounts of free CO2 are formed. The process of disequilibrium melting is kinetically unfavorable and may take some time to complete (many hours to days), with continued decrepitation of the unstable ice mixture as gas pockets burst out to the ambient atmosphere. Near-surface conditions on modern Mars favor disequilibrium melting, unless a strong geothermal heat source is available. If we consider the behavior of cold mixed subsurface ice (i.e. dry ice plus clathrate) on Mars which is being warmed by an igneous intrusion, we observe an interesting sequence of behavior. First, at around 216 K, the dry ice transforms to liquid CO2 . This is the easiest formed liquid in the martian subsurface. Once it has formed, a “hydrothermal” system will initiate, based on liquid CO2 , not water. Depending on the details of porosity and heat input, the CO2 may either be expelled from its source region or may participate in a closed heat transport cycle. Liquid CO2 can react directly with minerals without liquid water. For example, hydroxides are readily converted to carbonates in situ. If the heat source is in the very near subsurface (<100 m), the pressure of CO2 gas may be sufficient to drive a weak phreatic volcanic eruption, or a CO2 geyser. Recent Mars Global Surveyor images (e.g., MOC 24906) show features resembling phreatic cones in the northern plains. These are far more likely to be due to CO2 activity than H2 O, due to CO2 ’s higher volatility and the cryogenic prevailing conditions. Further warming leads to the next phase transition at ∼283 K. Here, solid clathrate breaks down to water and additional CO2 . Most of the CO2 thus liberated is soluble in the water, resulting in highly acidic carbonated water, but coexisting with excess liquid CO2 , which forms a separate, immiscible liquid phase. A mixed hydrothermal system with both liquid CO2 and liquid water will result. At ∼300 K, liquid CO2 passes its critical point and becomes an indeterminate phase with neither true vapor nor true liquid properties. This highly compressible and mobile phase has interesting solvent properties, with solubility

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depending inversely on density. On Earth, supercritical CO2 can be used for dry cleaning and for the removal of organic contamination from delicate mechanisms, since it is an excellent solvent for light organic molecules. At higher temperatures, CO2 is less soluble in water, so the CO2 phase will persist. At the hottest temperatures likely in the regolith of Mars—in excess of 400 K—water transforms to vapor and a mixed steam/supercritical CO2 hydrothermal system will result. THE DISTRIBUTION OF SUBSURFACE ICES AND FLUIDS ON MODERN AND ANCIENT MARS

We can also view the above progression of ices and fluids in terms of the subsurface temperature of Mars, which depends on the geothermal gradient and the surface temperature distribution (Fig. 3). For modern Mars, we have adequate data on the surface temperature but must assume a “reasonable” geothermal heat gradient. Without measurements of planetary heat flow and thermal conductivity or direct borehole temperature measurements we must make some assumptions. On Earth, a typical geothermal gradient in sedimentary basins with several kilometers of sediment is about 30 K/km. Mars is a smaller world, so is likely to have lost heat better than Earth. It also has a thicker mechanical lithosphere, confirming lower temperature gradients through the crust. We assume here that modern Mars has a geothermal gradient ∼10 K/km. Varying this number will merely change the depth range over which solid and liquid phases are encountered. Figure 3 shows the comparison between a conventional waterdominated Cryosphere on modern Mars (right-hand side) and the equivalent White Mars CO2 -dominated cryosphere (left-

FIG. 3. Subsurface distribution of ices and fluids on Modern Mars. A comparison of Mars’ cryosphere in conventional water-dominated volatile models (right) and CO2 -dominated models (left). In both cases, clathrate occurs in the subsurface due to the high pressures, but other phases are different. The waterdominated model has several kilometers of cryosphere even in equatorial regions, while the CO2 -dominated model has liquid CO2 “aquifers” at modest depth up to 35◦ latitude. The model assumes present-day temperatures and a geothermal gradient of 10 K/km.

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hand side). Note that in both cases a dessicated zone extends down to ∼500 m thickness in equatorial latitudes. For the waterdominated case, the cryosphere extends from pole to equator, but is much thinner near the equator. The cryosphere is composed of water ice and clathrate (relative proportions depend on the CO2 : H2 O ratio). Note that the surface distribution of ices in the polear regions is not addressed here. The poles appear to consist of nonequilibrium assemblages of water ice and dry ice (which should not coexist, were it not for the slow kinetics of clathrate formation in the solid state). The presence of large amounts of CO2 extends the depth to the top of the aquifer by about 1 km, since CO2 clathrate at pressure melts at about 283 K. In this model, extensive geothermal heating or crustalscale breakthrough of aquifers is required to get liquids near to Mars’ surface. For the CO2 -dominated case, a rather more interesting distribution results (left-hand side). The cryosphere sensu strictu is confined to latitudes above ∼35◦ and consists of dry ice and clathrate. Below this, and subcropping the dessicated zone near the equator, is a partial “aquifer” where the dry ice has melted to liquid CO2 , but the clathrate remains solid. As discussed above, the volume fraction of CO2 in Mars’ regolith is expected to be ∼5% by volume. The liquid CO2 will tend to migrate upward as the regolith compacts, supplying CO2 fluids to shallow reservoirs and to potential CO2 geysers at surface. An additional “dessicated” zone has been added where the volatile liquid CO2 can escape to surface, leaving a residuum of clathrate which seals off the deeper fluid system. It is interesting that the top of the CO2 fluid zone corresponds approximately with the onset depth for “splosh” craters, suggesting that the fluid responsible for the splosh geometries may be liquid CO2 , rather than water. At depth in the regolith, the clathrate also melts to give an interesting mixed aquifer of coexisting liquid CO2 and liquid water. In this zone, direct reactions with rock should be promoted and the CO2 will be converted to carbonate. However, this occurs at a depth of several kilometers and these carbonates will not be exposed at surface by other than massive impact excavations. For early Mars, the uncertainties are even larger. In Fig. 4 I show the results if we assume Mars’ present-day atmosphere, but 70% solar energy input. This results in surface temperatures ∼22 K cooler. The geothermal gradient would have been larger in the past. Here, 20 K/km has been used but again, varying this figure simply varies the depths at which the various fluid regimes are encountered. Near the equator, the trade-off of colder surface temperatures and higher geotherm yields similar cryosphere thickness, but in the polar regions, the geothermal gradient dominates to give a thinner palaeo-cryosphere. Each zone is thinner, due to the higher geothermal gradient. The zone of stable near-surface liquid CO2 extends only to ∼15◦ . By comparison of these two figures, it is apparent that in the CO2 -dominated White Mars model, there is a zone between 15◦ and 30◦ where the cryosphere warms up through geologic time and previously stable solid CO2 transforms to liquid in the subsurface—a circumstance likely to lead to unstable terrain. In

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FIG. 4. Subsurface distribution of ices and fluids on early Mars. The fluid zones are thinner due to the higher geothermal gradient in the past. The waterdominated model has several kilometers of cryosphere even in equatorial regions, while the CO2 -dominated model has liquid CO2 “aquifers” at modest depth up to 15◦ latitude. The model assumes temperatures about 22 K below present-day and a geothermal gradient of 20 K/km.

the next section we address the conditions of terrain collapse and generation of flows. AMAZONIAN MARS: DENSITY FLOWS

In this model, Mars is covered by a thick layered regolith (e.g., McEwen et al. 1999), and the layering will contain a significant

fraction (10–30%) of ground ice. In low latitudes this will be exclusively clathrate with the excess CO2 as a pore fluid, in higher latitudes the CO2 will also be solid—dry ice. In contrast to the standard model of relatively warm, porous regolith, saturated with liquid water, our model contains cold (−20 to −40◦ C) layered regolith and ices many kilometers thick. The ices act as a rigid cement, binding the loose material. However, near to the steep slopes into the northern plains of Mars (the structural dichotomy), or at the rim of large impact craters, considerable lateral stress will develop. Oversteepening of the slope may also have occurred due to tectonic uplift and magma inflation. The stress will concentrate in ice-rich layers which can undergo solid-state creep under the high strains. Warming by regional heating or local geothermal activity will also lead to a degree of melting, weakening the matrix at shallower depths. Local warming is not a necessary component of the instability, but it does aid or provide a focus for collapse. Under these circumstances, the regolith will be unstable and begin to slide away sideways, lubricated by a partial melt of liquid CO2 , the most volatile component of the ice. Ultimately, a large and deep-seated landslide will occur (Fig. 5), as in equivalent oversteepened slopes on Earth. Developing vertical and lateral fractures will expose the ices and CO2 to lower confining pressures by breaching impermeable layers. Lambert and Chamberlain (1978, 1992) have already discussed just such a process for the breakdown of terrains containing subsurface clathrates. Their goal was to show that liquid CO2 could be a viable erosive agent at or near the surface in Amazonian times, but the atmosphere was probably too thin by this time for any liquids

FIG. 5. Landslides of CO2 -saturated regolith on Mars. The “White Mars” model provides an explanation for the instability of the thick (5–10 km) layered regolith. Initial slip planes develop near oversteepened slopes and sole out onto volatile- rich mobile layers. Progressive slope failure occurs with the slip planes working deeper and further back. Lighter layers are more volatile rich with deposits of CO2 clathrates and dry ice.

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at the surface. However, the instability of the icy regolith would extend to a depth of many kilometers and encourage large-scale collapse on decollement surfaces, generating extensive debris flows. In the uppermost 50 m of any debris sheet so produced, the confining lithostatic pressure will be less than the stability limit of liquid CO2 . Therefore, in the upper parts of the sheet, liquid CO2 will flash to CO2 vapor and residual dry ice, blasting at least the finer material into a turbulent cloud. As the upper layers are blown off, the decompression front will work down through the debris sheet, generating a larger and thicker cloud of dust, sand, rocks, ice, and gas. The process is analagous to the violent degassing of a can of soft drink or beer that has been shaken up, or to the discharge of a pressurized CO2 fire extinguisher. Depending on the original temperature of the regolith (which buffers the liquid CO2 temperature), the equilibrium vapor pressure of the CO2 may vary from as little as 5 bars at the critical point to ∼35 bars at 0◦ C, and to even higher pressures if the temperature were above zero. As the landslide develops, more and more vapor forms as the CO2 is exposed to low atmospheric pressures (Fig. 6). This process is remarkably similar to the early stages of the collapse of lava domes such as at the Soufriere hills volcano in Martinique or the Mount St. Helens flank collapse and lateral

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blast. Although we are dealing with a cold system, the presence of abundant CO2 ices gives a plentiful fluid and vapor source. The potential overpressure is only of the order of 5–50 bars, rather than the hundreds of bars possible in terrestrial volcanism when wet magma at up to 1200◦ C is exposed to air, but even 5 bars is sufficient to drive a powerful expansion process. As the cold debris sheet slides away downslope, it begins to disintegrate and grind block against block, exposing and depressurising more sediment, and leading to greater fluidity. Meanwhile, the headwall from which this slide collapsed is now laterally unsupported, internally pressured, and ready to fail in turn. Thus, a series of self-lubricated blocks detaches from the headwall and slides downslope, fragmenting into gas-supported debris flows analogous to Terrestrial Nuee Ardentes. On Earth, the energy to sustain the gas pressure that supports these flows comes from the hot magma, which warms entrained air. Here on Mars, the energy comes from the CO2 phase transition to vapor, aided by mechanical grinding in the flow which generates a little friction and helps expose and warm overpressured CO2 ice, leading to rapid decompression. These flows are best described as cryoclastic, rather than pyroclastic. The flows will initially be far less energetic and explosive than terrestrial volcanism, so we do not see constructive volcanic edifices or phreatic cones in the source areas.

FIG. 6. A model for cryoclastic outbursts on Mars. (Top) Blocks initially slide gently on mobile layers but this is prone to runaway acceleration. The blocks crumble and fragment, releasing gas. A thick slurry initially develops, lubricated by liquid and vapor CO2 . (Middle) If the debris continues to flow downslope and fragment further, it transforms into a fully gas-supported cryoclastic density flow analagous to terrestrial pyroclastic surges or submarine turbidite flows. If the flow is arrested, a ponded slurry settles out to a smooth flat plain. (Bottom) The cryoclastic flow proceeds rapidly downslope. Progressive comminution of the icy load generates self-sustaining buoyancy and low-friction gas support. The turbulent head of the cloud and the basal load are strongly erosive. The main body of the flow is in the laminar flow regime and has very low internal friction, allowing long transport distances and rapid flow velocities. The flow is able to surmount and erode “minor” topographic obstacles (a few hundred meters to 1 km in height) and continue on the far side. In the waning stages of the flow, collapse occurs and the coarse load is dumped in an unsorted layer, leaving “tide” marks at its margins.

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Because the material shed into the debris flow contains unfragmented blocks of regolith, there remains a volatile source within the flow. In addition, the energy budget of CO2 phase transitions does not permit all of the CO2 to instantaneously transform to the vapor phase. As an example, on Earth, the industrial process for producing dry ice does not use a cryogenic deep freezer to chill liquid CO2 to below the solidus. Instead, pressurized liquid CO2 is chilled to around −40◦ C, and then the liquid is released through an expansion valve. A portion of the CO2 flashes to vapor which expands rapidly. This adiabatic decompression cools the residual liquid to below the solidus, including the latent heat of solidification. A buildup of solid CO2 is produced, which is harvested as dry ice (the CO2 vapor is typically recompressed, cooled, and recycled). On Mars, therefore, although a large proportion of the CO2 is initially transformed to vapor, a significant proportion remains as dry ice, chilled to the solidus temperature. As the flow transports the mixture of material, heat energy is transferred from the regolith (at −20 to −40◦ C) to the overcooled solid CO2 , generating more CO2 gas and sustaining the internal pressure of the flow. Any clathrates in the debris are also transported, largely untransformed. The kinetics of disequilibrium melting are too slow for the clathrates to participate in the initial explosive transfor-

mation. As a consequence, the initial density flow contains large quantities of volatiles which will continue to outgas over a period of minutes, hours, or days (further experimental data are required to establish the exact kinetics of clathrate breakdown in these circumstances). The principal determinant for the rate of outgassing of the residual CO2 is the exchange of thermal energy between the rocky particles and the icy particles. This is determined by the grain size, thermal conductivity, original temperature structure of the regolith, and the turbulence of the flow. No attempt is made in this paper to quantify the rates. Figure 7 illustrates the typical morphology of a martian outburst flood channel. The channel originates from a collapsed chaotic terrain, but rather than large volumes of water having burst out of a subterranean aquifer and carried blocks of regolith away, instead we see that the collapse involves regolith alone which generates its own fluids from liquid CO2 and CO2 bearing ices within its own volume. In this way, the volumetric misfit between the channels and the chaos zones is explained. Dense CO2 phases within the regolith expand to large volumes of CO2 gas, which support the flow, and the flow is sustained for the long distances observed on Mars, because a continuing source of volatiles is carried along with the flow in the form of untransformed solid CO2 and clathrate.

FIG. 7. Schematic morphology of a martian outburst flood channel. Flow originates from a collapsed zone of kilometer-scale slumped, rotated, and slipped blocks at an area of significant topographic relief (1–5 km over short lateral distances). Rather than generating a conventional avalanche or debris flow deposit, the pressurized volatiles within the kilometer-deep layered regolith outgas abundant volumes of CO2 , fragmenting the host rock and converting it into a dense cloud of debris lubricated by gas. The flow proceeds downslope, rapidly amalgamating from a series of V-shaped ravines or chutes and scouring a broad shallow channel and possessing enough momentum to spill over “minor” topographic obstacles (up to several hundred meters in elevation). Characteristic features include teardrop-shaped islands and parallel lineations in the channel floor (caused by second-order linear rolls within the flow). Adjacent terrain may be coated by airfall deposits and overspill flow lobes. In the terminus of the flow—the northern lowlands—the flow spreads out into a low-relief depositional sheet. Since density flows are bottom-seeking, any topographic lows are preferentially filled in. The net result is an exceedingly flat and smooth depositional surface, with a gentle gradient northward—an equilibrium profile for these low-friction flows. Scale: A–B, 5 to 50 km long, 1 to 50 km wide; B–D, 100 to 2000 km length, 10 to 250 km width; D–E, up to another 1000 km long and wide.

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FIG. 8. Schematic morphology of a volcanic pyroclastic flow. Pyroclastic flows are recognized on both Earth and Mars. Flow originates from collapse of a recently extruded hot dome of pasty magma perched over a cliff or ravine on the flanks of an acid stratovolcano. Rather than generating a conventional avalanche or debris flow deposit, the pressurized volatiles within the magma outgas abundant volumes of gas, fragmenting the host rock and converting it into a dense cloud of debris lubricated by gas and entrained air which expands as it is heated by the hot rock fragments. The flow proceeds downslope, rapidly amalgamating from a series of V-shaped ravines or chutes and scouring a broad shallow channel and possessing enough momentum to spill over “minor” topographic obstacles (1000-m ridges have been overtopped on Earth). Characteristic features include teardrop-shaped islands and parallel lineations in the channel floor (caused by second-order linear rolls within the flow). Adjacent terrain may be coated by airfall deposits and in the terminus of the flow—an ignimbrite plain—the flow spreads out into a low-relief depositional sheet. Since density flows are bottom-seeking, any topographic lows are preferentially filled in. The net result is an exceedingly flat and smooth depositional surface, with a gentle gradient away from the volcano—an equilibrium profile for these low-friction flows. Scale: A–B, up to 1 km long and 500 m wide; B–D, up to 10 km length, 500 m width; D–E, up to 50 km long and wide. Larger-scale pyroclastic flows also result from collapse of a convective eruption column. In these cases the entire flow system can extend for up to 250 km from the source vent.

Terrestrial analogs for the transport of these density flows are best found in volcanic pyroclastic flows (Fig. 8), in submarine turbidity currents (Fig. 9), and in long runout landslides and snow avalanches. All of these are density flows at the base of the atmosphere or ocean, with support and lubrication provided by entrained fluid, and by particle interactions. The “particles” in these flows can include multi-meter-scale boulders, even in the case of pyroclastic flows which are supported entirely by hot air. Figure 10 shows boulders deposited at the surface by a pyroclastic flow on the Island of Martinique in 1997. Compare this to the Pathfinder landing site imagery (Fig. 11) with an essentially identical distribution of boulders. Although these have been transported longer distances, the mechanism is identical— support by gas. On Earth, pyroclastic flows typically travel a few tens of kilometers before the initial heat and volatile supply is depleted. In the case of the very large-scale pyroclastic surges generated by collapse of a buoyant eruptive column, transport for many tens or a few hundred kilometers has been observed. The outburst floods on Mars are of even a larger scale than the largest pyroclastic surges observed in Earth’s geologic record. In addition,

they carry a source of new volatiles with them, while pyroclastic flows on Earth are initially more explosive, but shorter lived, and hence have shorter transport distances. The cryoclastic flows on Mars are more akin to submarine turbidite flows in this regard. The deep oceans of Earth bear witness to the passage of density flows that extend for thousands of kilometers. The flows persist for this distance because flow energy is maintained by gravitational potential energy gain through steady downslope movement on a long, gentle slope of gradient ∼1 : 1000. For submarine turbidites, continual input of water at the head of the flow replaces fluid lost out of the top and sides, so the flow persists. On Mars, the major source of volatiles is internal, from continued degassing of cold CO2 bearing ices rather than external, but the flow support is conceptually similar. Table I compares the processes of flow origin, support, and exhaustion between martian cryoclastic flows and terrestrial pyroclastic and turbidite flows. The geometries and dimensions of Figs. 7, 8, and 9 are consequences of these underlying processes. Although all three are density flows, they vary in detail. Pyroclastic flows are very explosive near to source due to the

FIG. 9. Schematic morphology of a submarine turbidite channel on Earth. Flow originates from a collapsed zone of kilometer-scale slumped, rotated, and slipped blocks at an area of topographic relief. Rather than generating a conventional avalanche or debris flow deposit, the high water content within the layered sediments and entrained seawater disrupt the sediment and convert it into a dense cloud of debris lubricated by liquid. The flow proceeds downslope, rapidly amalgamating from a series of V-shaped ravines or chutes and scouring a broad shallow channel and possessing enough momentum to spill over “minor” topographic obstacles (a few hundred meters is about the limit). Characteristic features include teardrop-shaped islands and parallel lineations in the channel floor (caused by second-order linear rolls within the flow). Adjacent terrain may be coated by rainout deposits or overspill flow lobes. In the terminus of the flow—the abyssal plains—the flow spreads out into a low-relief depositional sheet. Since density flows are bottom-seeking, any topographic lows are preferentially filled in. The net result is an exceedingly flat and smooth depositional surface, with a gentle gradient away from the continents—an equilibrium profile for these low-friction flows. Scale: A–B, 10 to 50 km long, 1 to 50 km wide; B–D, 100 to 1000 km length, 1 to 20 km width; D–E, up to another 500 km long and wide.

FIG. 10. Aftermath of a pyroclastic flow on Earth. Meter-scale boulders are scattered liberally across the ground surface after a blast of hot air and debris rushed down a valley on the flanks of the Soufriere Hills volcano, Martinique. These rocks were transported in a density flow, lubricated and supported by hot gas. 336

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FIG. 11. Aftermath of a cryoclastic flow on Mars? Meter-scale boulders are scattered liberally across the martian surface around the Pathfinder landing site. Contrary to conventional thought, these are not diagnostic of a powerful flood of water, but may well have been deposited from a cryoclastic density flow, lubricated and supported by CO2 gas. Reprinted, with permission, from the Jet Propulsion Laboratory archive.

high magmatic temperatures involved, but they are typically low volume and relatively short flow distance due to the rapid loss of heat energy and volatiles. Turbidite flows originate by nonexplosive collapse of medium to large scale and have very long transport distance due to the maintenance of both the driving force (gravitational energy down long gradients) and the fluid source from entrained ambient seawater. Cryoclastic flows on Mars are large to catastrophic in scale and, again, have long flow distances due to the downslope gradient and the maintenance of volatile support by continued slow degassing of cold ices exposed to low pressures. It is important to note that cryoclastic flows are coldtemperature phenomena and are never anywhere near to magmatic temperatures. Mars exhibits plenty of examples of conventional pyroclastic flows around volcanic constructs, but these are essentially identical to pyroclastic flows on Earth. They are highly explosive, originate from vents, and have relatively limited transport distances.

Characteristic features of density flows include transport in wide, flat-bottomed “channels,” with flow-parallel linear striations and scour marks. Individual turbidity currents can travel for thousands of kilometers down very gentle gradients, since the main body of the current is in the low-friction laminar flow regime, and can transport coarse debris including boulders for many hundreds of kilometers. The flows are often strongly erosive until they flatten out into broad depositional areas with little structure. Flows may individually and collectively build leveed margins. The flow thickness is significant (tens to hundreds of metres), with scour marks and debris extending up side valleys and confining hills, and a “tide” mark is often left to mark the level of the flow. The flows are able to travel upslope for short distances to breach topographic barriers and overtop obstacles, which are then incised, eroded, and eventually removed if the flows persist for long enough. On Earth, subsea continental margins worldwide contain areas of large-scale slumping and turbidite generation very similar in morphology to the chaotic

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TABLE I Density Flows on Earth and Mars: Mechanisms and Processes Pyroclastic flows

Turbidite flows

Cryoclastic flows

Location

Volcanic vent or edifice

Continental Shelf edge or slope

Chaos zone at crustal dichotomy, crater rim, fissure, or canyon

Source

Collapse of eruption column or lava dome avalanche

Collapse of sediments following an earthquake or by local slumping

Collapse of regolith following marsquake, or by local slumping

Nature of debris

Hot ash and pasty lava

Sand, mud, rocks, and boulders

Sand, mud, rocks, ice and boulders

Nature of initial expansion

Expansion of volatiles from magma and heating of entrained air

Entrainment of ambient water and turbulent mixing

Expansion of volatile liquid CO2 from regolith

Nature of continuing flow support

Heating of entrained air

Entrainment of ambient water and turbulent mixing

Outgassing of new CO2 from dry ice grains and from disequilibrium melting of clathrate

Energy inputs

Gravitational potential energy

Gravitational potential energy

Gravitational potential energy

Energy losses

Cooling of hot debris, escape of gas, friction

Friction

Friction, escape of volatile gas

Fluid losses

Gas and hot air escapes upward and sideways

Water escapes upward and sideways

CO2 escapes upward and sideways

Solid losses

Coarse sedimentation from base of flow, ashfall from upper portions

Coarse sedimentation from base of flow—Bouma sequence. Hemipelagic rainout from upper portions

Coarse sedimentation from base of flow, dustfall from upper portions.

Duration

Minutes to ∼1 h

Many hours to days

Many hours to days

Distance

Tens to hundreds of km

Hundreds to thousands of km

Hundreds to thousands of km

Speed

100–300+ kph Sometimes supersonic

50–100 kph

100–500 kph?

terrains on Mars. Methane hydrates are implicated as a factor in the cementation and then abrupt collapse of the shelf margin following clathrate decomposition in response to pressure changes driven by sea level falls. This CO2 -based collapse model explains all of the observed features of the Amazonian outburst floods and the chaotic terrains. The source areas are categorized by their origin at or near steep topography. Most are near Mars’ equator in regions where surface temperatures would be higher and liquid CO2 stable in the relatively shallow subsurface. Many are associated with contemporaneous or prior vulcanism, which provides a local heat source for additional melting. They are often associated with linear tension faulting and locally grouped into dominostyle collapse terraces and range through undeformed blocks that have slid a few hundred or thousand meters to more fragmented debris fields with occasional huge floating blocks. In the main flood channels, catastrophic gas-supported flows would have rushed downhill on shallow gradients due to low internal friction. The flows would have been immensely erosive in their upper and mid portions due to the huge volumes, high speeds, and entrained debris. Run-out distances would be significant and thick debris flows would have scoured high up valley walls and jumped “small” obstacles like crater rims a few hundred meters high. Figure 12 is a classic Viking photomosaic of part of the wide floor of Kasei Valles. Prominent flow-parallel ridges a few hundred meters high have been overtopped by flows that have crossed diagonally from one side to the other. If this were a water

flood, then the water depth would need to be the full 300 m across the full width of Kasei Valles—here over 250 km wide. Cryoclastic flows, on the other hand, do not need to span the entire valley width at any one time and are quite capable of overtopping ridges of this magnitude through momentum-driven runup. Once the main flow has passed by, the debris load will subside in a cloud of dust and dump boulders along hillsides, as seen at the Pathfinder landing site (note “tide” marks of huge boulders on “Twin Peaks”—Pathfinder image 81977). The existence of low-density expanded gas as the transport medium for the flows avoids the volumetric problem of supplying enough water for frequent deep flows. The original icy component of the regolith can supply the lubrication for each flow from within its own volume. Some linear ridges on Mars that are currently interpreted as wind-eroded yardangs might in fact be primary depositional and erosive features of energetic, debris-laden, gas-supported cryoclastic flows that have been accentuated or exhumed by later wind erosion. The lowland surface of Mars can now be interpreted as a vast “volcanic” wasteland of cryoclastic flows, similar to terrestrial ignimbrite plains, although the scale of some martian debris fields would make the Valley of Ten Thousand Smokes look insignificant. At the surface, large blocks are scattered in loose linear trains across the proximal, strongly channeled areas, as seen in Viking, Pathfinder lander, and MGS imagery. In the flat northern plains and near flood channels, the dust clouds raised by the major density flows will have rained out like

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FIG. 12. Large-scale scour marks in Kassei Vallis. The groove and scour marks of the flat-bottomed outburst flood channels are diagnostic of density flows. Note that a major ridge is preserved in the lee of an eroded crater and that some flows have overtopped this ridge to cross diagonally from one side to the other. If these crossover valleys were carved by a water flood, the water depth would need to be the full height of the ridge. Cryoclastic density flows can travel uphill for several kilometers laterally and several hundred meters vertically, driven by momentum. These crossover valleys are further evidence for density flows as the erosive and transport mechanism for the “flood” channels. Reprinted, with permission, from the Jet Propulsion Laboratory archive.

volcanic ash and buried small-scale topography on surrounding “ash plains.” Lucchitta (1987) discussed an interesting flow feature within Coprates Chasma. This relatively young flow is transitional between a dry avalanche and an outburst flood and shows only partial transition to a flow, which travels only a few hundred kilometers. Although he discussed this flow in the context of water as the fluid agent, much of his discussion can be transferred to a CO2 -based cryoclastic flow model. His interpretation was that the flow represented the collapse of material at a later epoch than the outburst floods and which contained barely sufficient fluid content to initiate a flow. In the White Mars context, we interpret the fluid as CO2 , but otherwise agree with Lucchitta that this flow is a significant step between fully developed outburst floods (i.e., cryoclastic flows) and smaller and younger dry avalanches. The more recent looking collapse events seem weaker and slower, as the subsurface reservoirs dried up to some extent, or

the region of vulnerable ground (in terms of original porosity and volatile content and ground slope) was consumed. The final collapse events, which are continuing to the present day, are “dry” slumps and avalanches, without the spectacular lubrication effects of the CO2 ices. The model for the origin of the cryoclastic flows also allows us to explain some oddities relating to chaotic terrain. In many instances, a chaos zone is clearly associated with a flood channel, yet the floor of the chaos is demonstrably lower then the adjacent channel, with an obvious spillway crossing cliffs up to 1 km high. Water floods would be expected to flow downhill from the source area and this upward step is hard to explain. Conventional models sometimes invoke a lake in the enclosed basin, feeding the channel, but the geometry is not like lake basins on Earth. In particular, no shoreline is observed around the basin. Komatsu et al. (2000) suggest that total melting of clathrate after magmatic or geothermal heating leads to the generation of both liquid water and overpressured CO2 gas, the former flowing in a conventional

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FIG. 13. Water flood vs CO2 models for origin of depressed chaos zones. (a) In the conventional water flood model, liquid water emerges under pressure into a circular depression, causing collapse of the regolith. The basin fills with water and overflows. Later, the water freezes and slowly sublimes away leaving a depression. CO2 may be associated with the water and aid the collapse process. One would expect to see one or more shorelines associated with the base level of the overspill, and the entire active lip of the overspill should reflect the same water level. (b) In the CO2 density flow model, an eruption-like column of CO2 and debris boils out of collapsing regolith. This cloud is forced over the lip by internal pressure, spilling at a variety of levels. Downslope, the overflow reforms into a dense flow down the “flood” channel. No watermark is eroded in the basin, and the active lip of the overspill can have significant topographic relief.

catastrophic flood and the latter assisting with the collapse of chaotic terrain. In the White Mars model, an energetic vapor/density flow is generated in the chaos zone, following geothermal heating and collapse. If the temperature of the cloud is 0◦ C, then the CO2 exerts ∼35 bars pressure, sufficient to lift a cloud of density 0.2 g/cc over a 1-km topographic step (Fig. 13). Once expelled from the source “crater” the flow pours away downslope and gradually condenses into a thicker cryoclastic flow with more erosive power. In these circumstances, little debris would be expelled and a relatively fines-dominated flow would result. One chief diagnostic feature between water floods and cryoclastic flows is the elevation of the spill point(s) across the lip. Typically, a wide expanse of the lip is simultaneously active, allowing a high volume flow to emerge across a wide spillway. For a water flood, this wide lip will be at essentially identical altitude, and there will be a matching shoreline around the lake margin. For a cryoclastic flow, the vapor column has no fixed top and can be simultaneously boiling over different parts of the rim, at different elevations. Equally, with no fixed top to the density cloud, it erodes no sharp shoreline around the basin. In effect, these areas of chaos act as wide and gentle eruptive craters, with distributed activity across their entire floor area, rather than focused volcanic vents. In other examples, totally enclosed areas of chaos and deep pits and fissures occur, with no visible outlet yet significant vol-

ume loss. Each is an area where this collapse process has begun within an internal basin such as a crater or along a fissure or fracture. The collapse has consumed the edges of the feature, working outward into flatter terrain. The collapse process would have blasted a lot of the fine debris into a cryoclastic debris/vapor cloud, which would overspill the edges of the closed basin. Some of this material might rain out locally as a surface deposit, but much of it would either flow radially away from the source as cryoclastic flows or be dispersed by prevailing winds. Later, surface winds would pick up much of the fine, uncemented material and blow it away. In this way, material is removed from an enclosed area with little sign of its deposition around the source zone. Volume loss in the source is also enhanced by later sublimation of the significant volumetric component of CO2 and H2 O ices and to the deflation of dust-size debris released from the cementing effect of the ice. In this way, a layered icy regolith some 6 to 8 km thick could lose 35 to 50% of its volume to the atmosphere, and thus the resulting surface would be as much as 2–3 km below the surrounding plains. Deposition of broadly smooth-surfaced deposits with detailed small-scale hummocks or striations and residual blocks and boulders infills the resulting basin, with peripheral areas of chaos representing an arrested state of partial collapse. These enclosed chaos zones can be interpreted as an extreme form of thermokarst or as a novel type of CO2 -driven volcano.

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DISTRIBUTION OF VOLATILES ON MARS TODAY

The Mars we see today is characteristically dusty. Under the White Mars paradigm we expect the regolith to be composed of fine-grained material (“dust”) bonded largely by ice. The dearth of liquid water and fluvial action implies a general lack of cements (on Earth, carbonate is the commonest cementing material in rocks). Instead, the regolith of Mars has undergone a dry-cleaning process where CO2 solvents have washed through it and have now evaporated away, leaving dry crumbly dust behind. It may be more helpful to think of Mars as a planet with a crust of dirty ice (or icy dirt) and a dry and dusty surface layer, rather than simply as a rocky planet with icecaps and subsurface ice. Significant volumes of CO2 -bearing ices are inferred to still exist at depth in many parts of the martian crust. Likely search areas would be thick layered terrains in mid-latitudes where annual mean surface temperatures remain below 215 K. In equatorial latitudes, clathrates remain stable to the present day. Also of interest are the thick sedimentary accumulations of the northern plains where unsublimed CO2 ices may exist in the distal outwash flows, and earlier sedimentary ices may be buried under later floods. These destination zones would be relatively depleted in CO2 , though, due to its consumption as a lubricant in the flow process when it is lost to the atmosphere. Rather, these are likely to be zones where ordinary water ice has been winnowed and concentrated by the transport processes and CO2 outgassing of the cryoclastic flows. The presence of extensive water ice deposits in equatorial regions is an encouraging factor for future exploration of the surface of Mars. Water on Mars has probably seldom been liquid, except in the subsurface, near significant hot spots like buried impact melt bodies or major igneous features. In the new model of Mars proposed here, the water ice produced by disequilibrium melting of clathrates will remain as solid ice at temperatures far below 0◦ C and will be transported by the flows as part of their debris load, to be deposited as sediment in the lowland plains. Water, in fact, will have behaved largely as a solid mineral on Mars and will be present in sedimentary deposits as primary or accessory grains rather than as a pervasive matrix phase. Water ice is likely be found at shallower depths than the residual CO2 deposits due to its lower vapor pressure and higher melting point. The mobility of water ice due to pressure-induced creep will allow it to behave diapirically or intrusively, as does rocksalt on Earth. For example, Ori and Baliva (1999) describe domal uplifts in three martian craters which may represent the surface expression of diapiric ices. The restricted occurrence of liquid water in Mars’ past and the very different solvent properties of the dominant fluid phase, CO2 , suggests that conventional evaporite minerals and salts will not, in general, have been scavenged from the crust, as has occurred here on Earth. Therefore, salt deposits and crusts should be relatively rare on Mars, and associated with especially warm

conditions, or deep erosion to levels where water was once fluid due to subsurface heat sources. Conversely, more exotic mineral species related to CO2 liquid phase solution and transport are anticipated. CO2 is a very good solvent for small organic molecules. Some study into this aspect may reveal interesting predictions of martian “evaporite” mineralogy for comparison with orbital spectrography and ground analyses. Potentially the most significant implication is that there may well be much larger amounts of water ice in the regolith of Mars than are envisaged by conventional models. The simplistic view of Mars is that a fluid cycling and sediment processing system was intermittently active over its life, but has effectively run down at the present day and much of the fluid has been lost. In the conventional view, this fluid system is seen as water-based, and the implication is that Mars today is substantially depleted in water. This is strongly supported by the hydrogen isotope fractionation data which shows that 90% of the active water inventory has been lost to space. However, in the alternative CO2 based model presented here, it is the CO2 which has been active and has been transported to polar regions where it is now locked into extensive Polar Layered Terrains. In the White Mars model, it is likely that much of the primordial water ice inventory of Mars is still buried in the crust as a passive mineral phase which never participated in the active water cycle that the hydrogen isotope data are measuring. Thick seams and layers of water ice may still exist at low latitudes and relatively shallow burial depths (less than 1 km and perhaps as little as a few hundred meters locally). These deposits of ice may form a significant resource for the future evolution or exploitation of the planets hydrosphere. CONCLUSIONS

Our new model proposes that Mars’ surface has broadly warmed through most of geological time due to the increasing solar constant, and that the observed loss of volatiles is due to a slow bake-out of a cryogenic CO2 phase—rather than the classic scenario of a cooling Mars with a freeze-out of water. The outburst floods are mediated by liquid and vapor CO2 , not water. Previous models of a “warm, wet Mars” focused on the triple point of H2 O. In this new model a focus on the CO2 triple point brings improved understanding of the present surface and evolution of Mars, and opens up analysis of a whole new class of transport processes—CO2 -driven density flows. Other planets and moons may exhibit similar phenomena based on other volatiles, such as N2 , CO, SO2 etc. The White Mars model brings together a broad range of observations and knowledge of flow processes to explain Mars with a single, simple, obvious, and nonparadoxical model. The new model allows further development but the fundamental insight which exploits the physical characteristics of the CO2 phase diagram provides compelling and simple explanations of many hitherto paradoxes of Mars. The flow features on Mars’ surface are not like those on Earth. This has long been recognized, albeit incompletely. The new model explains why. The Amazonian

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NICK HOFFMAN

flood channels are seen to be inadequate evidence for liquid water now that an alternative CO2 -based explanation is available. The search for liquid water on Mars’ surface is likely to be a futile pursuit. The CO2 -based evolutionary model described here fits all the observed surface features of Mars better than a model involving liquid water and resolves the apparent paradox between the observed “fluvial” features and low present and palaeo temperatures—the Faint Young Sun Paradox. Simultaneously, the Carbonate Paradox is resolved, since liquid water and CO2 only ephemerally coexisted. The bulk of the CO2 and water inventory of primordial Mars is still sequestered as solid ices in the thick layered regolith. Water ice is likely to be common in Mars’ subsurface, with a distribution controlled by surface activity. We propose that future exploration of Mars needs to be retargeted to exploit the advantages this new understanding gives us. ACKNOWLEDGMENTS I am deeply indebted to NASA and the U.S. public for the missions to Mars, which collected the observational data, and for maintaining public archives of images, data, and planetary geology conferences. I also thank my previous employer, BHP Petroleum, for giving me time to think about the Red Planet when I should have been mapping the Blue one! I am grateful to Howard Houben and an anonymous reviewer for helpful comments and to David Jamieson for helping rub off some rough spots. Scott Sandford also planted an idea about snowballs on Mars.

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