The Martian hydrologic system: Multiple recharge centers at large volcanic provinces and the contribution of snowmelt to outflow channel activity

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1 ARTICLE IN PRESS Planetary and Space Science 55 (2007) The Martian hydrologic system: Multiple recharge centers at large volcanic provinces and the contribution of snowmelt to outflow channel activity Patrick S. Russell a,b,, James W. Head III a a Department of Geological Sciences, Brown University, Providence, RI 02912, USA b Division of Space and Planetary Sciences, Physikalisches Institut, University of Berne, Sidlerstrasse 5, 3012 Berne, Switzerland Accepted 30 March 2006 Available online 7 September 2006 Abstract Global recharge of the martian hydrologic system has traditionally been viewed as occurring through basal melting of the south polar cap. We conclude that regional recharge of a groundwater system at the large volcanic provinces, Elysium and Tharsis, is also very plausible and has several advantages over a south polar recharge source in providing a more direct, efficient supply of water to the outflow channel source regions surrounding these areas. This recharge scenario is proposed to have operated concurrently with and within the context of a global cryosphere hydrosphere system of the subsurface characteristic of post-noachian periods. To complement existing groundwater flow modeling studies, we examine geologic evidence and possible mechanisms for accumulation of water at high elevations on the volcanic rises, such as melting snow, infiltration, and increased effective permeability of the subsurface between the recharge zone and outflow source. Evidence for the presence of large Amazonian-aged cold-based piedmont glaciers on the Tharsis Montes has been well documented. Climate modeling predicts snow accumulation on high volcanic rises at obliquities thought to be typical over much of martian history. Thermal gradients causing basal melting of snowpack over 1 km thick could provide several kg m 2 yr 1 of water, charging a volume equivalent to the pore space in a square meter column of subsurface in less than yr. In order to account for estimated outflow channel volumes, the subsurface volume above the elevation of the outflow channels must be charged several times over the area of Tharsis. Complete aquifer recharge can be accomplished in My through the snowpack melting mechanism at Tharsis and in years for channel requirements at Elysium. Abundant radial dikes emanating from large martian volcanic rises can crack and/or melt the cryosphere, initiating water outflow and creating anisotropies that can channel subsurface water from a high-elevation groundwater reservoir to outflow sources. In this model, snow accumulation, infiltration of meltwater, and increased effective permeabilities are a consequence of the geologic, thermal, and climatic environment at Elysium and Tharsis, and may have had a genetic influence on the preferential distribution of outflow channels around volcanic rises on Mars. r 2006 Published by Elsevier Ltd. Keywords: Outflow channel; Mars; Hydrological system; Groundwater recharge; Snowmelt 1. Introduction The presence of outflow channels and evidence for their formation through catastrophic release of large volumes of groundwater has intrigued planetary scientists since their initial documentation (see review in Carr, 1996). Models of groundwater systems and cryospheric structures designed to account for the observed outflow channels (e.g., Corresponding author. address: patrick.russell@space.unibe.ch (P.S. Russell). Clifford, 1993) called on a global hydrologic system recharged through basal melting below the south polar cap. Uncertain, however, were the permeabilities required for global recharge, whether the system was indeed globally interconnected, and how recharge and subsurface flow could take place rapidly enough in order to sustain the implied flow rates. In this work we focus specifically on the formation of catastrophic outflow channels sourced in chaos regions or fossae. The conditions that allow for collection, storage, outbreak, subsurface flow, and possibly recharge of source water for these outflow channels are still /$ - see front matter r 2006 Published by Elsevier Ltd. doi: /j.pss

2 316 ARTICLE IN PRESS P.S. Russell, J.W. Head III / Planetary and Space Science 55 (2007) puzzling. A major step forward was the Clifford (1993) end-to-end model of the martian hydrological cycle, focusing on subsurface processes. This model allows for conditions under which large volumes of groundwater confined beneath a cryosphere would be available to form the observed outflow channels upon its release. This model provides a context in which to test theory with geologic observations. In previous studies we have used geologic observations to constrain and modify the global model on a local and regional basis (e.g., Russell and Head, 2002a, 2003). Investigations of outflow initiation mechanisms, elevation distributions, and relationships with geologic units and structures around the Elysium and Tharsis rises at Elysium Fossae (Russell and Head, 2001, 2003), Cerberus Fossae (Head et al., 2003), and Mangala Valles (Head and Wilson, 2002; Ghatan et al., 2005) have demonstrated that observed outflow activity is consistent with and facilitated by a two-layer hydrosphere cryosphere system as described by Clifford (1993). However, in these analyses a definitive distinction could not be made between groundwater that ultimately came from south polar cap recharge, as in the global model, and groundwater that was regionally charged from above, such as at the Elysium rise (Russell and Head, 2003). Several issues concerning the global extent and interconnectedness of a martian groundwater system have also been raised (Russell and Head, 2002a, b). In this paper we address the hypothesis that regional recharge of the groundwater system at the Tharsis and Elysium rises, rather than south polar recharge of a global groundwater system, is the dominant source of water for outflow channels in these areas. The hypothesis of groundwater recharge acting at the large martian volcanic provinces calls on data concerning climatic, hydrologic, and geologic conditions that can reasonably be expected to have prevailed on Mars over the majority of its history (Carr, 1979; Clifford, 1993; Clifford and Parker, 2001). The hypothesized regional recharge model is developed within the context of the global cryosphere hydrosphere system of Clifford (1993). The mechanism by which water enters the groundwater system in Clifford s (1993) model is by melting at the base of the south polar cap (e.g., Head and Pratt, 2001), from where it is able to reach all other parts of the planet in a highly hydraulically interconnected subsurface (Fig. 1). Water is prevented from entering the subsurface water system elsewhere by the impermeable frozen zone of the uppermost crust, or cryosphere. This cryosphere is a predictable consequence of estimates of post-noachian surface temperature distribution, geothermal heat flow, and subsurface thermal conductivity (Clifford, 1993). The cryosphere is also able to confine groundwater beneath it as long as the hydrostatic pressure of the groundwater does not exceed the lithostatic pressure of the overlying frozen crust. Comprehensive modeling of the hydrologic characteristics of the martian megaregolith suggests that it is unlikely that hydrostatic pressure ever exceeded lithostatic pressure to potentially cause outflow (Hanna and Phillips, 2005a). The process of dike intrusion however, is recognized for its ability to disrupt the confining seal of the cryosphere and is considered an important factor in initiating groundwater outflow on Mars (Head and Wilson, 2002; Head et al., 2003; Russell and Head, 2001, 2003; Ghatan et al., 2005; Hanna and Phillips, 2005b; Wilson and Head, 2002a, b). According to Clifford (1993), the hydraulic head imparted to an interconnected groundwater system due to groundwater mounding beneath the south polar cap is sufficient to provide water to an aquifer in the vicinity of the outflow channels and planet-wide on a time scale of X10 8 years. Clifford and Parker (2001) envision a scenario in which the hydraulic potentiometric surface resulting from sub-polar groundwater mounding is located above the elevation of the sources of the outflow channels, thus providing a source of water and elevational head to supply flow to the outflow event (Fig. 1d). In this polar recharge model, the distance from the recharge zone to the major outflow centers at Elysium and Tharsis/Chryse is approximately one-quarter of the planetary circumference, or over 5000 km (Fig. 2). Carr (1979) modeled outflow of groundwater from beneath a cryosphere in the Chryse region, considering flow rates across the subsurface/surface boundary at an outflow breakout location and comparing them to estimated outflow channel discharge rates. He found that subsurface flow is able to account for estimated discharge rates of m 3 s 1 at the surface-subsurface interface at outflow channel sources, given permeabilities of 10 9 m 2. Such a permeability is significantly higher than the range proposed for nominal megaregolith models of the martian subsurface which are m 2 at 1 km depth to m 2 at 10 km depth (Clifford and Parker, 2001; Hanna and Phillips, 2005a). Carr (1979) proposed that groundwater under Tharsis was lifted to higher elevations as the rise formed, thus providing an elevated and proximal groundwater source. This model is consistent with the hydrosphere cryosphere partitioning of the subsurface in the model of Clifford (1993). However, the Tharsis rise is believed to have been present since early in the Noachian, very early in Mars history (Phillips et al., 2001), and groundwater elevated during formation by uplift at these early times would have had time to drain away before the onset of outflow channel formation in the Hesperian (Scott and Tanaka, 1986). Here we develop the hypothesis that vertical recharge of the martian groundwater system at large volcanic provinces, on a more regional scale and closer to outflow sources (e.g., Elysium; Russell and Head, 2003), is a more viable and more efficient means of supplying the large amounts of water required for outflow events than south polar recharge alone (see also Harrison and Grimm, 2004, 2005). Recently, Harrison and Grimm (2004, 2005) compared rates of subsurface flow from the south pole and from Tharsis that would be required to supply water to the Chryse outflow channel events using a conservative

3 ARTICLE IN PRESS P.S. Russell, J.W. Head III / Planetary and Space Science 55 (2007) permeability structure. Their favored Tharsis recharge model supplies a minimum volume of water to carve the outflow channels ( km 3 ) within 250 My, whereas the south polar recharge model requires 810 My. The maximum average subsurface permeability considered was m 2, in which case discharge of a given volume occurs 250 times faster for both the Tharsis and south pole models. Harrison and Grimm (2004, 2005) show how the proximity and elevation difference of Tharsis and Chryse translate into hydrologic conditions that are better able to provide water for the observed outflows, as we explore in more detail below. In their Tharsis model, instantaneous rates of discharge range from 0.4 m 3 s 1 at 250 My after the start of significant discharge (translating to 100 m 3 s 1 in their maximum permeability model), to 0.6 m 3 s 1 at 1000 My (Harrison and Grimm, 2005). These values are significantly less than discharge rates estimated for the outflow channels (Carr, 1979). In our analysis, we focus on mechanisms and geologic evidence for a regional source of water and for mechanisms for its entry into the groundwater system. We also identify geologic conditions, such as dike-intruded volcanic constructs, that might facilitate the transport of this water to outflow locations. The latter topic is important in assessing Carr s (1979) requirement of high permeabilities to sustain high discharge rates, and in providing the potential for higher discharge rates within the groundwater transport system of Harrison and Grimm (2004). Our investigation of how and where water is likely to enter the subsurface in Fig. 1. The hydrologic model of Clifford (1993). Each part (a d) is an identical composite altimetric profile from the south pole to the north pole across Tharsis and Chryse (see locations on Fig. 2), illustrating the elevational and horizontal relationships of the martian topographic features important in this analysis. Vertical axis is kilometers above and below mean planetary radius; horizontal axis is in latitude, with north positive. Key features are labeled in part (a). Vertical exaggeration 200. Dark (grey) shading is the basement, within which porosity is predicted to be negligible. Light blue shading is the cryosphere, the porous zone of the upper crust in which the temperature is below the freezing point of water. Tan is the porous zone in which the temperature is above freezing, but contains no water in its pore space. These three zones represent the structural and thermal conditions of the upper crust as envisioned by Clifford (1993) and Clifford and Parker (2001). In (a) there no groundwater represented in the sub-cryosphere region. (b) It illustrates how water (dark blue shading) introduced to the groundwater system would become distributed over geologic time, conforming to an equipotential surface. Deposition of water on the south polar cap cold trap leads to melting at the base of the underlying cryosphere. This meltwater forms a groundwater mound beneath the cap, flowing from there northward to charge the subsurface in other parts of the planet. (c) It illustrates the same configuration as above but with a greater total volume of water in the global subsurface. Note that the confining ability of the cryosphere can support groundwater under hydraulic pressure beneath the surface at greater elevation than the adjacent, exterior surface (arrow). Diagrams based on those of Clifford (1993) and Clifford and Parker (2001). (d) Same subsurface configuration as (c), illustrating that if the cryosphere is disrupted in a region where groundwater was previously confined under the hydraulic pressure head resulting from south polar basal melting (represented by h o ), the water can flow to the surface.

4 318 ARTICLE IN PRESS P.S. Russell, J.W. Head III / Planetary and Space Science 55 (2007) Fig. 2. Global topography of Mars from MOLA, labeled with features and areas discussed in the text. White lines labeled 1 and 2 are locations of profiles across Tharsis/Chryse (composite profile) and Elysium, respectively, that are depicted in Figs. 1 and 6. Large white circle at the Tharsis rise is Olympus Mons. The three smaller white circles are the Tharsis Montes, from southwest to northesast, Arsia, Pavonis, and Ascraeus Montes. the context of a global cryosphere (Clifford, 1993), and how subsurface transport rates may be increased, complements the hydraulic modeling of Harrison and Grimm (2004). 2. The general conceptual model Consider a groundwater recharge system analogous to that proposed for the south pole (Clifford, 1993), but located at higher elevations and closer to the outflow channel breakout locations. The groundwater mound in this regional case would: (1) represent a large reservoir of water concentrated closer to the breakout location; and (2) provide a greater initial hydraulic head compared to that at the outflow source region, thus steepening the subsurface hydraulic gradient between the reservoir and breakout location. As the subsurface aquifer immediately surrounding an outflow breakout location transfers water to the outflow, the aquifer drains and the hydraulic gradient driving flow across the subsurface/surface interface at the breakout location is reduced (Carr, 1979). This situation is analogous to draw-down of the water level and reduction of the hydraulic gradient resulting from pumping water in a drill hole in a confined aquifer (Fetter, 2001). In order for outflow to continue at high discharge rates, water must be resupplied to the outflow source through the subsurface from more distant regions. A sufficient rate of resupply depends on: (1) the availability, or presence, of a subsurface reservoir of water; (2) the hydraulic gradient driving water flow from this reservoir to the breakout interface; and (3) the hydraulic conductivity, governed largely by the permeability, of the subsurface host rock. Regional recharge at high elevations closer to outflow source areas would be more effective in providing subsurface reservoirs and hydraulic gradients than would polar recharge, as documented by Harrison and Grimm (2004). Here we incorporate the model of regional recharge concept into our current understanding of the hydrologic cycle on Mars based on observational evidence and theoretical modeling. This model also addresses the issue of hydraulic conductivity by demonstrating that relatively high subsurface permeabilities are to be expected in the same geologic environments that favor conditions for recharge and where outflow channels have occurred. In summary, the basic points of the regional recharge model are as follows: (1) The groundwater system is charged regionally at several areas, rather than solely at the south pole, providing reservoirs of subsurface water closer to outflow sources; (2) groundwater at these regional recharge areas is elevated above the level possible due solely to polar basal melting; this provides steeper hydraulic gradients near outflow channel locations to drive subsurface resupply flow more efficiently; (3) water enters the groundwater system from above as a result of melting at the base of a snowpack, similar in concept to sub-polar basal melting; (4) there is a coincidence in the expected locations of preferential snow deposition, elevated heat flow, subsurface areas of high effective permeability, and locations of observed major outflow channels. Groundwater flow modeling results (Harrison and Grimm, 2004) provide the basis for us to incorporate new developments into the regional recharge model. These include: (1) new evidence of low-latitude ice deposits; (2) model results predicting low-latitude precipitation at high obliquity; (3) thermal calculations of surface heat flow from subsurface magma reservoirs; (4) thermal conditions necessary to cause melting at the base of a snowpack; (5) models of how propagating dikes disrupt a confining cryosphere and increase effective permeability near volcanic rises; and (6) spatial and vertical distribution of outflow channels. We use these new data and the global model of Clifford (1993) and Clifford and Parker (2001) to establish a more specific

5 ARTICLE IN PRESS P.S. Russell, J.W. Head III / Planetary and Space Science 55 (2007) model of regional supply and recharge centers for outflow channel events. 3. Elevations of outflow channel sources As a fundamental check of the south polar basal melting recharge model (Clifford, 1993; Clifford and Parker, 2001), Carr (2002) reviewed the elevations of post-noachian water-worn features on Mars. If these features are below the maximum possible recharge elevation, i.e. the elevation of the surface beneath the polar cap (1500 m), they are consistent with the model to a first order. According to Carr (2002), the floors of all major outflow sources fit this criterion. The main exceptions within the entire class of water-worn features include valleys on some volcanoes and fill deposits within Valles Marineris and other chasmata. The elevation of several channels and valleys approach the maximum possible recharge elevation at the base of the south polar cap. We retabulate and plot the elevations of the major outflow channels (Table 1; Fig. 3) in the following manner. The base elevation of outflow sources considered by Carr (2002) represents the minimum elevation to which groundwater was supported by subsurface hydraulic head during the outflow event, barring any postoutflow subsidence. We also consider the maximum surface elevation at which the water flowed to also have important implications for subsurface hydraulic conditions (Fig. 4). If the subsurface hydraulic head allows outflowing water to rise only to the currently observable floor elevation of source regions, this implies that the channels themselves were carved by water that never rose above this elevation. In this scenario, the channel formation would have to have started underground, near the floor elevation of the source region. Continued erosion presumably would have collapsed the overlying ground, which then would have had to have been removed to form today s topography. We favor the more widely accepted mechanism of channel formation by down-cutting over time, based on the morphology of the channels, as summarized in Carr (1996). If the channel is formed by down-cutting, water must once have been present and flowing at the local pre-outflow surface in the earliest stages of the outflow event, which in turn requires the potentiometric surface of the source groundwater to have been at (or above) this surface elevation. Even the water level in a crater or chasm lake fed by subsurface fractures is controlled by the hydrostatic head of the Table 1 Outflow source locations, grouped by region Outflow feature (Source Channel) Approx. lat Approx. lon Base/floor elevation (km) Surrounding/rim elevation (km) East of Tharsis Iam C Ares V Hydaspis C outlet to Ares V Hydroates C Simud V Hydroates C Tiu V Hydaspis C Tiu V Aurorae C North outlet Aram C Ares V Shalbatana V Ravi V Ganges Chasma East outlet Eos Chasma NE outlet Capri Chasma East outlet Echus Chasma Kasei V Juventae Chasma Maja V West of Tharsis fracture Mangala V Northwest of Elysium Galaxis F NW outlet Galaxis F Hrad V Elysium F Tinjar/Granicus V Elysium F Apsus V Southeast of Elysium Cerberus F Athabasca V Note: The last column lists the minimum elevation of the potentiometric surface of groundwater at the outflow source at the time of outflow initiation. This measurement is taken from the channel rim at the upstream-most reach of the channel. If the channel has its source in a wider enclosed depression, the measurement is taken where the depression narrows and transitions into the channel. Latitude and longitude coordinates apply to these measurements. The second to last column lists the lowest elevation of the floor of the channel in the area immediately adjacent to the rim measurement, e.g. in the mouth of the depression. Several of the surrounding rim elevations cluster around the maximum level attributable to a groundwater level developed solely due to basal melting solely at the south pole, or 1.5 km, according to Carr (2002). C¼ Chaos, F ¼ Fossae, V ¼ Vallis/es.

6 320 ARTICLE IN PRESS P.S. Russell, J.W. Head III / Planetary and Space Science 55 (2007) Fig. 3. Outflow source locations, grouped by region: (a) specific locations (see Table 1) of the channels on a MOLA digital altimetry map. The 1.5 km elevation contour is shown; (b) longitudes, and surface and floor elevations of the outflow the channel source regions (Table 1). The lower value is the minimum elevation of the potentiometric surface of groundwater at the outflow source at the time of outflow initiation. Several of these elevations are above the maximum level attributable to a groundwater level developed due to basal melting solely at the south pole, or 1.5 km (horizontal line), according to Carr (2002). groundwater source. In summary, water must have existed at the level of the contemporary surface terrain in order to have carved the observed channels adjacent to the outflow channel source, thus requiring the minimum total subsurface hydraulic head during outflow initiation to be at that elevation (column 5 in Table 1). Major outflow occurrences considered here are concentrated to the east and west of Tharsis and to the northwest and southeast of Elysium (Figs. 2 and 3; Table 1). In most of these cases, the elevation of the potentiometric surface of the local groundwater implied by outflow channel initiation as described above is below the elevation of the base of the south polar cap (1.5 km). However, several preoutflow surfaces to the east and west of Tharsis are within one kilometer below and even above the polar cap base elevation. As Carr (2002) points out, the global recharge model of Clifford and Parker (2001) does not explain groundwater at these elevations. Thus high elevation regional scale recharge not only appears more effective in supplying water for outflow down a steeper

7 ARTICLE IN PRESS P.S. Russell, J.W. Head III / Planetary and Space Science 55 (2007) surrounding, pre-existing terrain surface source depression minimum total hydraulic head implied by incipient outflow (a) surrounding, pre-existing terrain surface source depression minimum total hydraulic head implied by final outflow topography (b) incipient outflow channel late stage outflow channel Fig. 4. The effect of outflow elevation measurements on implied subsurface hydraulic head. Assuming outflow channels are incised from the top down, the original surface at the time of incipient outflow must have been similar to that of the current surrounding terrain. To get water to effuse from the ground to this elevation, the hydraulic potentiometric surface, or minimum hydraulic head, must have been at least at this elevation, shown as a dashed line in part (a). Only in late stages of outflow, when the channel and source region were excavated, would the minimum hydraulic head have been at the base elevations of the channels or source regions, shown as a dashed line in part (b). Taking the former case (a) into consideration increases estimates of the minimum hydraulic head necessary at the time of outflow, by an amount approximately equivalent to the depth of the channel source area below the surrounding surface, shown by the double-headed arrow in part (b). hydraulic gradient, but in some instances may also be required. 4. Sources of groundwater at high elevations Juvenile water from magmatic intrusions has been cited as one possibility for a water source responsible for forming valley networks on volcanoes (Gulick and Baker, 1990), and thus could also be a source of groundwater accumulation at high elevations. The other general method for charging a groundwater system is by addition of surface water from above. Early and/or episodic occurrence of rain or snow deposition of water on the surface have been proposed (1) during a warm early climate with a thick atmosphere (e.g., Craddock and Howard, 2002), (2) due to major volcanic eruptions of juvenile water and subsequent snow precipitation (e.g., Zent, 1999), (3) resulting from transient clement climates induced by magmatic disruption of a groundwater-cryosphere system (e.g., Baker et al., 1991, 2000) or by large impact events (e.g., Segura et al., 2002; Colaprete et al., 2003); and (4) due to the coincidence of climate-change induced snow accumulation and enhanced heat flow caused by magmatic intrusions (e.g., Fassett and Head, 2006). In the absence of an impermeable global subsurface cryosphere due to relatively high early geothermal heat flux (Clifford and Parker, 2001), a warmer, thicker atmosphere and/or relatively high rates of volcanism may have led to the potential for addition of water to the subsurface during the Noachian (e.g., Craddock and Howard, 2002; Zent, 1999; Jakosky and Phillips, 2001). Without the confining cryosphere, however, water could more easily flow on a global scale to areas of low geopotential and would be less likely to accumulate in large, regionally elevated zones of groundwater (Clifford and Parker, 2001). These conditions and the dominance of valley networks as opposed to outflow channels in the Noachian geologic record suggest an Noachian hydrologic system greatly different from that of the Hesperian or Amazonian (Carr, 1996). By the Hesperian, a substantial cryosphere on the order of a kilometer thick is expected to have formed due to the decline in geothermal heat flow (Clifford, 1993; Clifford and Parker, 2001), prohibiting water due to precipitation from accessing the subsurface as is thought to have occurred in the Noachian. Average surface temperatures above the melting point of water would be required for millions of years to significantly decrease the thickness of the cryosphere. Thus, any precipitation due to shorter transient magma- or impact-induced climate events (e.g., Baker et al., 1991, 2000; Segura et al., 2002; Colaprete et al., 2003) would be blocked from entering the subcryosphere groundwater system. In summary, while the transient climate models may allow for precipitation in the Hesperian, and while high elevations may be preferential sites of precipitation as on Earth, mechanisms are still required for maintaining subsurface temperatures above freezing to allow rainwater access to the subsurface and for melting of any precipitation that accumulates in the form of snow. Recently, Carr and Head (2003) presented an updated model of snowpack basal melting that provides a potential basis for charging and recharging a regional groundwater system from above. This mechanism of melting is optimized at times of, or in regions of, high heat flow. Volcanic centers represent not only areas of anomalously high heat flow, but also relatively high elevations, favoring orographic precipitation and the development of regionally elevated groundwater tables. What is required are climatic conditions producing significant depths of snowpack and geothermal conditions capable of melting the base of the snowpack. We now examine two centers of outflow channel activity, Elysium and Tharsis, to test for the plausibility of the regional vertical recharge mechanism. 5. Hydrologic considerations at the elysium rise The Elysium rise (Figs. 2 and 3) consists of a broad, 700 km-wide dome that reaches an elevation of 1400 m,

8 322 ARTICLE IN PRESS P.S. Russell, J.W. Head III / Planetary and Space Science 55 (2007) from which a steeper volcanic cone, Elysium Mons, extends up to 14,000 m. On the margins of this rise are two volcanic edifices, Hecates Tholus to the north (4000 m elevation) and Albor Tholus to the south (also 4000 m elevation). The Elysium rise itself is extensively flooded with Hesperian and Amazonian lavas (Tanaka et al., 1992). Early Amazonian fluvial channels, lava flows, and lahars emanate from fossae on the northwest flank of Elysium and flow into Utopia basin (Greeley and Guest, 1987). Using MOLA topography and MOC images we have resolved three main types of units (lavas, sediment-rich debris flows or lahars, and high water-content, late stage flows) and traced them back to their source fossae (Russell and Head, 2003). Those fossae that are the sources of lavas and water-related flows are clustered at elevations below 3100 m, with most below 3500 m. Fossae occurring at elevations higher than 3100 m on the Elysium rise are the sources only of lavas, not groundwater. This elevational dependence of water sources suggests that their subsurface source is also elevationally controlled, as would be the case for a zone of water-saturated subsurface. We interpret the radial Elysium Fossae to have been initiated by lateral propagation of dikes that intersected, or nearly intersected, the surface (Russell and Head, 2003). This conclusion is based on fossae morphology, associated features (Chapman, 1994), orientation (Mouginis-Mark, 1985), understanding of the behavior of magmas reaching neutral buoyancy in a volcanic rise (Wilson and Head, 1994), and the ability of near surface dikes to form graben (Wilson and Head, 2002a; Rubin, 1992). In our model of emplacement history dikes thermally and physically disrupted the cryosphere allowing water below the level of the saturated subsurface to escape to the surface (Fig. 5). Fig. 5. Perspective and cross-section sketch of the subsurface magmatic and hydrologic conditions leading to the effusion of water, lava, and lahars from fossae on the flanks of Elysium Mons. Dotted region is subcryosphere porous zone that is not saturated with groundwater. X marks the regional water table, or boundary between saturated subsurface (below) and relatively dry subsurface (above). Dikes propagating laterally through this subsurface disrupt the cryosphere, allowing water from an elevated level of saturated ground to flow to the surface. Dikes intersecting the surface above the zone of subsurface saturation produce only lava flows. Figure from Russell and Head (2003). Vertically oriented, planar dikes may have assisted in focusing subsurface flow to these surface fossae (Chapman, 1994; Russell and Head, 2003). Above the level of watersaturated ground (marked by the X on Fig. 5), lava flows, and not groundwater, erupted to the surface as a result of dike intrusion. This elevational configuration suggests a subsurface that is hydrologically stratified. This subsurface configuration is also locally consistent with the hydrosphere cryosphere model of Clifford (1993), but does not necessarily affirm the south polar source of groundwater in a global system. Another area of evidence supporting a local hydrosphere cryosphere configuration consistent with Clifford s (1993) model is found on the opposite side of Elysium Mons. On the southeast flanks of the broad volcanic rise, Cerberus Fossae, the sources of Athabasca Valles (Fig. 2), follow the same general northwest southeast orientation as the fossae in northwest Elysium, and thus may have formed under similar conditions of regional stress. These fossae are also hypothesized to have formed as a result of dikes propagating laterally from Elysium Mons and intersecting the (near) surface on the southeast flanks of the volcanic rise (Head et al., 2003). Along with lavas, massive outpourings of water flowed downslope from the fossae to interfinger with lava plains (Burr et al., 2002; Head et al., 2003). Calculations by Head et al. (2003) of how much water is expected to have been released as a result of cryosphere disrupting dike intrusion and tapping of a pressurized groundwater system are in general agreement with flow rates calculated by Burr et al. (2002) based on channel volumes. Comparison of the elevation and age of the Elysium and Cerberus Fossae outflows yields insight into whether recharge of the groundwater system within the Elysium rise is required to produce the observed relationships. The elevation of Cerberus Fossae ( 2500 m) is higher than that of the highest elevation of water-related outflow in the northwest Elysium ( 3500 m). In other words, hydrologic activity occurred at Cerberus Fossae in the elevation zone where only lavas were effused from Elysium Fossae to the northwest. In addition, the lava and water flows from Cerberus Fossae are believed to be very young, dating to the latest Amazonian (Berman and Hartmann, 2002). This age is several hundred million to more than a billion years younger than that of the water-related flows in northwest Elysium (Tanaka et al., 1992). This inverse relationship between elevation and age suggests that recharge of the groundwater system within the Elysium rise occurred between the two outflow events. Release of groundwater from the Elysium Fossae only occurred up to 3500 m in elevation, implying that there was not enough water in the Early Amazonian to release water at 2500 m. After outflow of water from Elysium Fossae, the total volume of water stored within Elysium Mons rise would clearly have been less than before outflow. Given several hundred million to over a billion years without outflow and without recharge between the outflow periods, water within the

9 ARTICLE IN PRESS P.S. Russell, J.W. Head III / Planetary and Space Science 55 (2007) Elysium rise should have easily reached elevational equilibrium, as the expected equilibration time on a planetary scale is only 100 My (Clifford, 1993). The resulting equilibrium level within the Elysium rise, given no recharge, must therefore be below the equilibrium level of the earlier outflows (from Elysium Fossae in the Early Amazonian). In this scenario of no recharge, disruption of the cryosphere several hundred million to over a billion years later is expected to release water only below the maximum level of previous outflow, i.e. below 3500 m. However, outflow at Cerberus occurs at 2500 m. Thus, considering the subsurface to be hydraulically interconnected on even conservatively long time scales (10 8 years), outflow at the relatively high elevation of 2500 m is not expected without recharge. For sufficient water to be present in the Elysium rise to account for the Cerberus outflow, recharge to the groundwater system subsequent to the Early Amazonian outflow at the Elysium Fossae is required. 6. A mechanism of regional recharge in a global cryosphere system Is recharge of a regional groundwater system from above a plausible process within a global cryosphere system? The main problem with charging a sub-cryosphere hydrosphere from above is that near-surface temperatures are below freezing, preventing infiltration and transport of water through the cryosphere. In this manner the cryosphere effectively acts as a seal inhibiting the passage of water from above as well as from below (Clifford, 1993). Carr and Head (2003) have shown that accumulation of snow or ice deposits could result in production of liquid water by melting at their base without the requirement of rainfall or a warmer climate, which are problematic from many theoretical standpoints (see summary by Haberle, 1998). The main factors leading to such basal melting are the insulating effect of the snowpack from the cold atmosphere and the geothermal heat flow from below. Carr and Head (2003) consider the effect of porosity on snowpack thermal conductivity and conclude that the increase of conductivity with depth within the snowpack may be expressed as logðkþ ¼0:4 þ 2:9 logðrþ, (1) where K is thermal conductivity of the snowpack (in Wm 1 K 1 ) and r is snowpack density (in g cm 3 note that the text and Fig. 1 of Carr and Head (2003) imply r is in kg m 3, however their Eq. (1) is written for r in g cm 3 ). The relationship in Eq. (1) is considered a good approximation for Mars as it runs along the lower bounds of terrestrial estimates of the conductivity density relationships which are those terrestrial estimates done at the coldest temperatures (Carr and Head, Fig. 1, 2003). The relationship between density and depth is r ¼ r i ðr i r s Þ exp½ Cðg Mars =g Earth ÞzŠ, (2) where r i is the density of ice (917 kg m 3 ), r s is the density of the snowpack surface (300 kg m 3 ), C is a constant (normally between 0.02 and 0.03 on Earth; we use 0.025), g is gravity, and z is depth within the snowpack (Carr and Head, 2003). Given planetary average surface temperatures of 210 (current) or 230 K (early greenhouse) and a geothermal heat flux of W m 2, melting could occur at the base of a snowpack on Mars fifty to a few hundred meters thick (Carr and Head, 2003). For purposes of a conservative argument, we assume that planetary surface temperatures attained values similar to those of today soon after the end of the heavy bombardment. Pre-MGS models predict a heat flow in the Late Noachian of 0.15 W m 2, a value which declines through the Hesperian but remains around 0.1 W m 2 for this latter period (see review in Schubert et al., 1992). More current work, however, suggests that heat flow decreased much more quickly and was significantly less over the planet s history. Recent modeling indicates heat flow had already decreased to W m 2 by the Middle Noachian, from which point it decreased gradually to current values of W m 2 (see review in Spohn et al., 2001). This later study is supported by analysis of gravity and topography data suggesting that heat flow was 0.04 to W m 2 in the Noachian and p0.03 W m 2 in the Hesperian and later (Zuber et al., 2000; McGovern et al., 2002). Thus, nominal heat flow for the Late Noachian and later was likely well below the values used in the Carr and Head (2003) modeling study (Solomon et al., 2005). However, heat flow could be higher in local settings such as volcanoes. Fassett and Head (2004, 2006) model the diffusion of heat from a magma chamber beneath the center of a volcano under current martian conditions and demonstrate that local heat flow to the surface, especially towards the center of the volcano, can be increased significantly to at least 0.1 W m 2 (in the case that they modeled) by this mechanism. Thus, conduction from a local heat source as modeled by Fassett and Head (2004, 2006) could initiate basal melting beneath a few hundred meters of snow or ice in post-heavy bombardment conditions as calculated by Carr and Head (2003). The presence of a snowpack in a region of elevated heat flow as presented above essentially raises the melting isotherm up to the surface, to the base of the snowpack. In this configuration, if basal melting of the snowpack is occurring as predicted, the underlying upper crust will no longer be below freezing. In a basal melting situation, any liquid water produced is free to infiltrate the ground and percolate down to the local groundwater table. This hypothesized scenario is depicted at a volcanic rise representing Tharsis and the superposed montes in Fig. 6. As is the case beneath the south pole, a groundwater mound would build beneath this region of recharge (Fig. 6a), the height of which would depend on the rate of recharge and the permeability of the surrounding rock. Because of the high elevation of the recharge zone, this

10 324 ARTICLE IN PRESS P.S. Russell, J.W. Head III / Planetary and Space Science 55 (2007) (a) 10 Tharsis (b) h o (c) 10 Elysium (Fig. 1d). In addition, a nearby vertical recharge zone such as this could provide a large reservoir of groundwater at a given elevation in the region of outflow sources without necessitating the filling of all the planet s pore space to that elevation, as required by a system charged solely by basal melting at the south pole, a quarter-planet s distance away (Fig. 6b). Fig. 6c depicts an analogous scenario of regional vertical recharge and outflow at Elysium. We calculate a range of possible recharge-rate estimates based on thermal conductivities suggested by Carr and Head (2003) (Eqs. (1) and (2)) and a geothermal heat flow typical of a volcanic setting as suggested by Fassett and Head (2004, 2006) (0.1 W m 2 ). We consider a steady-state snowpack of thickness z with an average surface temperature, T surf, of 210 K and a basal temperature, T melt, of 273 K, the most conservative condition for melting. At the interface of the ground surface and the snowpack base, the amount of heat flow out of the ground (Q o ) is equal to the amount of heat flow into the base of the snowpack (Q 1 ) plus the amount of latent heat consumed while melting a thin layer of snow/ice at the interface: Q 0 ¼ Q 1 þ RL, (3) where R is the rate at which water is produced by melting at the base of a snowpack and L is the latent heat of fusion of ice ( Jkg 1 ). Q 1 can be found for a given snowpack thickness from the solution of the one-dimensional heat flow equation using the parameters above. The rate at which water is melted at the base of a snowpack is potentially made available for groundwater Fig. 6. Regional recharge of the groundwater system at the Tharsis and Elysium rises rather than at the south pole (compare to Fig. 1). Vertical axis is kilometers above and below mean planetary radius; horizontal axis is in latitude, with north positive. The source of water is snowpack that preferentially accumulates at high martian volcanic rises. Elevated heat flow due to magmatic activity elevates the melting isotherm, causing the base of the cryosphere to rise to the ground surface. This both eliminates the impermeable obstacle the cryosphere represents to infiltration and causes melting at the base of the snowpack. (a) as a result of basal melting and infiltration, a groundwater mound now builds Beneath Tharsis. (b) Provides a closer reservoir and higher hydraulic pressure head (h o ) for flow to resupply water at the outflow breakout location. (c) Same model represented in (b) for the Elysium region. groundwater mound would be capable of providing a greater hydraulic head relative to the outflow location (Fig. 6b) than would a sub-south polar groundwater mound Rate of Melting (kg -1 / yr / m -2 ) Q o = 0.15 Q o = Equilibrium Snowpack/Ice Thickness (m) Fig. 7. The relationship between the equilibrium thickness of an overlying snowpack and the rate of melting at its base, at two different surface heat flows (0.15 and 0.1 W m 2 ). A surface heat flow of 0.1 W m 2 is easily achieved in post-noachian times above a cooling magma chamber (e.g., Fassett and Head, 2004). Melting rate is calculated using an integrated porosity-dependent thermal conductivity within the snowpack and the assumption that the snowpack remains at a relatively constant thickness for the majority of its duration (Carr and Head, 2003). No melting occurs at the base of the snowpack for thicknesses below those indicated by the x-intercept of each curve.

11 ARTICLE IN PRESS P.S. Russell, J.W. Head III / Planetary and Space Science 55 (2007) recharge is then: R z 0 R ¼ Q o KðT melt T surf Þ 1 z L, (4) where the net effective thermal conductivity for a vertical column of the snowpack is obtained through integration of conductivity from the surface to the base of the snowpack. Rates of potential recharge to the subsurface are shown as a function of equilibrium snowpack thickness in Fig. 7. Under conditions outlined above including a local heat flow of 0.1 W m 2, the equilibrium snowpack thickness must be at least 1000 m for melting to occur beneath it. 1kgyr 1 m 2 (equivalent to a 1 mm column under 1 m 2 of surface in 1 year) of water is produced under 1100 m of snowpack, and 8kgyr 1 m 2 is produced under 2000 m. If the geothermal heat flow (Q o ) is as high as 0.15 W m 2, melting first occurs under a snowpack 550 m thick and reaches rates of 9kgyr 1 m 2 at 1 km thick and 17 kg yr 1 m 2 at 2 km thick. Melting rates are higher beneath thicker snowpacks because increasing the thickness of the snowpack decreases the heat flow across it, Q 1, making more energy available at the snowpack-ground interface (Eqs. (3) and (4)). The increase in thermal conductivity with density (Eq. (1)) causes the effective thermal conductivity of a column of snowpack to approach the relatively high value of ice with increasing depth. This conductivity trend tends to increase Q 1 in thicker snowpacks, suppressing melting. Because conductivity exponentially approaches a constant maximum value, however, suppression of melting in thicker snowpacks is overwhelmed by the increased melting that occurs with depth. The rate of infiltration and downwards movement of water depends on the properties of the surface and subsurface and will determine the portion of melted water that will reach the groundwater system. Typical infiltration rates of soils on Earth are measured on the order of mm h 1 (Fetter, 2001). In comparison, the rates of melt production obtained above (Fig. 7) are on the order of mm h 1. Modeling of the water table within terrestrial volcanic edifices finds that a decay of permeability with depth from to m 2 can accommodate infiltration rates of 1 m yr 1, or 0.1 mm hr 1, within the upper few kilometers of the subsurface of the Cascade range in Oregon, USA (Hurwitz et al., 2003). Thus, meltwater produced by basal melting on a martian volcano would be easily accommodated by the surface infiltration capacity and the permeability of the near surface. The effective rate of recharge to the subsurface is therefore considered equivalent to the melting rate. If the subsurface pore space were to fill completely, infiltration would then be limited by the flow of groundwater through and out of the system (see, for example, Harrison and Grimm, 2004). The time scale required to recharge the local groundwater system for estimation purposes is the time it takes for melting at the base of an equilibrium-thickness snowpack Time to change pore space volume of column of subsurface (kyr) Q o = 0.15 Q o = Equilibrium Snowpack Thickness (m) Fig. 8. Using the melting rates in Fig. 7, equilibrium snowpack thickness is plotted against the time it would take to fill a column of subsurface equivalent to the integrated pore space below the recharge area between the base of the cryosphere and the top of the basement, or 200 m. At snowpack thicknesses at which melting is initiated (see Fig. 7) the time to fill any amount of pore space is infinite, and not shown on this graph. (as per Eq. (4)) to fill the pore space of a column of the subsurface between the top of the basement and the base of the cryosphere. This estimate will be a minimum time because it assumes continuous melting and because water may flow through the subsurface away from the region of recharge. The available pore space is equivalent to a 200 m column of water, assuming a 2.5 km-thick cryosphere at low latitudes, a surface porosity of 20%, an exponential decay in porosity with depth, and a selfcompaction depth (where porosity is o1% and therefore negligible) of 8.5 km below the surface (following Clifford, 1993). The resulting time to recharge this column equivalent is less than years for snowpacks over 1100 m thick (Fig. 8). Harrison and Grimm (2004) and Fassett and Head (2004, 2006) also suggest that advection of heat by any groundwater present would increase heat flow to the surface (including slightly increased heat flow further from the summit). Gulick (1998) demonstrates that water may circulate to the surface of a volcano in a purely hydrothermal-convection model. Recent three-dimensional hydrological modeling of water convection in the subsurface below a cryosphere suggests that the cryosphere may be thinned from its nominal thickness of several kilometers to a minimum of 300 m depending on model input parameters (Travis et al., 2003). Any such increase in heat flow (Fassett and Head, 2004, 2006), hydrothermal activity at the surface (Gulick, 1998), or thinning of the cryosphere (Travis et al., 2003) would only facilitate basal melting beyond the predictions of the nominal model we have presented based on the work of Carr and Head (2003) and Fassett and Head (2004, 2006). In conclusion, basal melting of an accumulated snowpack on regions with anomalously high heat flux, such as volcanoes, is a