1
THE MARTIAN HYDROLOGIC SYSTEM: MULTIPLE RECHARGE CENTERS AT LARGE
VOLCANIC PROVINCES AND THE CONTRIBUTION OF SNOWMELT
TO OUTFLOW
CHANNEL ACTIVITY
Patrick S. Russell1, 2
and
James W. Head III1
1
2
Department of Geological Sciences, Brown University, Providence RI 02912 USA
Now at: Division of Space and Planetary Sciences, Physikalisches Institut, University of
Berne, Sidlerstrasse 5, 3012 Berne, Switzerland
Submitted to Planetary and Space Science
November 22, 2005
Revised March 29, 2006
Accepted April, 2006
In Press
2
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 coldbased 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 1.5x105 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 ~ 0.3 - 2 My through the snowpack melting mechanism at Tharsis and in ~5 x 104 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.
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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., 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 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
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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 (Figure
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., 2004;
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 ≥ 108 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 (Figure
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 (Figure 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
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subsurface flow is able to account for estimated discharge rates of 105 - 107 m3 s-1 at the surfacesubsurface interface at outflow channel sources, given permeabilities of 10-9 m2. Such a
permeability is significantly higher than the range proposed for nominal megaregolith models of
the martian subsurface which are 10-12 m2 at 1 km depth to 10-15 - 10-16 m2 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 permeability structure. Their favored Tharsis recharge model
supplies a minimum volume of water to carve the outflow channels (3x106 km3) within 250 My,
whereas the south polar recharge model requires 810 My. The maximum average subsurface
permeability considered was 2x10-13 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 m3 s-1 at
250 My after the start of significant discharge (translating to 100 m3 s-1 in their maximum
permeability model), to ~0.6 m3 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,
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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 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
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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 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; Figure
8
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 post-outflow subsidence. We also consider the
maximum surface elevation at which the water flowed to also have important implications for
subsurface hydraulic conditions (Figure 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 downcutting, 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
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 (Figure 2, 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 pre-outflow 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 hydraulic gradient, but in some instances may also
be required.
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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
sub-cryosphere groundwater system. In summary, while the transient climate models may allow
10
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 (Figure 2, 3) consists of a broad, ~ 700 km-wide dome that reaches an
elevation of ~1400 m, 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
11
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
(Figure 5). 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 Figure 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
(Figure 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
12
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 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 (~108
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
13
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:
log(K) = 0.4 + 2.9log( ρ ) ,
€
(1)
where K is thermal conductivity of the snowpack (in W m-1 K-1) and ρ is snowpack density (in g
cm-3 - note that the text and Figure 1 of Carr and Head (2003) imply ρ is in kg m-3, however their
Eqn 1 is written for ρ in g cm-3). The relationship in Eqn 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, Figure 1, 2003). The relationship between density and depth is:
ρ = ρ i − ( ρ i − ρ s )exp[−C (gMars gEarth )z],
€
(2)
where ρi is the density of ice (917 kg m-3), ρs 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 K (current) or 230 K (early
greenhouse) and a geothermal heat flux of 0.1 to 0.15 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 0.05 to 0.08 W m-2 by the Middle Noachian, from which point it decreased
gradually to current values of 0.02 to 0.03 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
14
to >0.06 W m-2 in the Noachian and ≤0.03 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 Figure
6. As is the case beneath the south pole, a groundwater mound would build beneath this region of
recharge (Figure 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
groundwater mound would be capable of providing a greater hydraulic head relative to the
outflow location (Figure 6b) than would a sub-south polar groundwater mound (Figure 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 (Figure 6b). Figure 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) (Eqns 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, Tsurf, of 210 K and a
basal temperature, Tmelt, of 273 K, the most conservative condition for melting. At the interface of
15
the ground surface and the snowpack base, the amount of heat flow out of the ground (Qo) is
equal to the amount of heat flow into the base of the snowpack (Q1) plus the amount of latent
heat consumed while melting a thin layer of snow/ice at the interface:
Qo = Q1 + 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 (3.3x105 J kg-1). Q1 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
recharge is then:

R = Qo −


€
z
∫ K(T
0
− Tsurf )  1
 ,
L
z

melt
(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 Figure 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. 1 kg
yr-1 m-2 (equivalent to a 1 mm column under 1 m2 of surface in 1 year) of water is produced under
1100 m of snowpack, and ~8 kg yr-1 m-2 is produced under 2000 m. If the geothermal heat flow
(Qo) is as high as 0.15 W m-2, melting first occurs under a snowpack ~550 m thick and reaches
rates of ~9 kg yr-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, Q1, making more energy available at the snowpack-ground interface (Eqns 3 and
4). The increase in thermal conductivity with density (Eqn 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 Q1 in thicker snowpacks, suppressing
melting. Because conductivity exponentially approaches a constant maximum value, however,
16
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
hr-1 (Fetter, 2001). In comparison, the rates of melt production obtained above (Figure 7) are on
the order of 10-3 to 10-4 mm hr-1. Modeling of the water table within terrestrial volcanic edifices
finds that a decay of permeability with depth from 10-13 m2 to 10-17 m2 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 (as per Eqn 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 self-compaction depth (where porosity is <1% and therefore negligible) of ~8.5 km
below the surface (following Clifford, 1993). The resulting time to recharge this column
equivalent is less than 1.5x105 years for snowpacks over 1100 m thick (Figure 8).
Harrison and Grim (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 threedimensional 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
17
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
plausible mechanism for providing a source of water for recharging the regional groundwater
supply and creating an elevated mound of groundwater.
7. Evidence for Recharge at Volcanic Rises
Is there geologic evidence and support in modeling for the former presence of liquid or
solid surface water at the major volcanic provinces, Elysium and Tharsis? It is both in these
regions that basal melting as described above would have been most likely to occur, and around
these rises that a significant fraction of martian outflow channels have their source. The Tharsis
rise contains four large volcanic edifices over 14,000 m (Olympus, Arsia, Pavonis, Ascreaus
Montes) and is central to major instances of outflow activity (Figure 2, 3). The Chryse outflows,
to the east of Tharsis, are sourced in chaotic terrain and depressions near the eastern end of
Valles Marineris (Carr, 1996). Mangala Vallis, to the west of Tharsis, emanates from a graben
extending from the southeast of Tharsis (Tanaka and Chapman, 1990; Ghatan et al., 2005).
The gently sloping volcanoes Hecates Tholus, on the northern periphery of the Elysium
rise, and Ceraunius Tholus, on the northern side of the Tharsis rise, are covered with a radial
system of valley networks documented in detail by Gulick and Baker (1990). The pattern and
morphology of the valley systems strongly suggest that the upper, central reaches of the
volcanoes were a source of surface and/or near-surface water. The valleys are interpreted to have
been formed by both overland flow and seepage in a highly porous medium, resulting from
hydrothermal convection (Gulick and Baker, 1990) or melting beneath a snowpack (Fassett and
Head, 2004, 2006). The hydrothermal interpretation is dependent on the presence of groundwater
in the upper elevations of the volcanoes and is consistent with the existence of a regional
groundwater system. In the basal melting scenario a portion of meltwater would have infiltrated
the subsurface, providing water to the groundwater system. The latter process is not only a
plausible mechanism for charging the regional groundwater system, but is also arguably the most
18
direct method for providing the water at high elevations necessary to a hydrothermal system. The
formation of valleys by overland flow of meltwater requires production of basal meltwater in
excess of the infiltration rate. Given surface infiltration rates and near-surface permeability as
discussed above (Hurwitz et al., 2003), excess meltwater production would require substantially
higher heat flow, consistent with the effect hydrothermal advection of heat would have on the
base of a snowpack. In summary, the development of a snowpack and the onset of basal melting
by elevated geothermal heat flow is consistent with the subsequent initiation of hydrothermal
activity. In turn, hydrothermal activity could have contributed to valley formation either directly
(Gulick and Baker, 1990) or by increasing meltwater production rates above the infiltration rate
at the base of the overlying snowpack (e.g., Fassett and Head, 2004, 2006).
Elysium Mons and the four large Tharsis volcanoes do not display extensive valley
systems. The surfaces of Hecates and Ceraunius Tholi are dated to the Late and Early Hesperian,
respectively (Gulick and Baker, 1990). Effusive lava flow activity continued to cover the flanks
of the much larger Elysium Mons until at least the Early Amazonian (Tanaka et al., 1992). At
Tharsis, the majority of surfaces of Olympus Mons are Late Amazonian (Hartmann and
Neukum, 2001), most of Arsia Mons is Mid Amazonian (Hartmann and Neukum, 2001), and the
northeast and southwest flank eruptions of the Tharsis Montes, as well as the vast surrounding
lava plains, are Mid to Late Amazonian (Scott and Tanaka, 1986). If valley networks formed
from excess meltwater at the base of a snowpack or from hydrothermal circulation within highelevation groundwater on the Tharsis or Elysium Montes, they would have since been buried by
these later lava flows.
The Tholi valley networks provide good evidence for snowpack accumulation,
subsequent hydrothermal activity, and groundwater charging. In a self-consistent model of
groundwater charging, outflow events must occur during or after the period of charging activity.
The outflows from the northwest Elysium Fossae are Early Amazonian, the Chryse outflows
around Valles Marineris off the east edge of Tharsis are Late Hesperian, and Mangala Vallis off
to the southwest of Tharsis is Late Hesperian (Scott and Tanaka, 1986; Greeley and Guest,
1987). Thus, the ages of postulated snowpack activity on the Tholi (Hesperian) and active
outflow (Late Hesperian, Early Amazonian) are broadly consistent. Outflow at Cerberus is much
younger as previously discussed (Burr et al., 2002; Berman and Hartmann, 2002).
19
Lack of evidence for liquid water run-off on the large Montes of the Elysium and Tharsis
rises does not mean that meltwater production, such as that observed at the neighboring Tholi,
has not occurred. Aqueous channels and valleys could have been obliterated by continued
eruptive activity of the larger Montes later into martian history or meltwater production rates
may simply not have been great enough to exceed infiltration rates and produce valley-forming
run-off. However, evidence for the past presence of liquid or solid water on the large volcanic
provinces beyond that at the relatively small Tholi would be necessary to establish the case that
the Elysium and Tharsis rises were significant centers of snow or water accumulation, with the
resulting potential of vertically recharging the groundwater system. Here we synthesize evidence
from the literature that significant snow or ice deposits have accumulated at all four of the major
Tharsis volcanoes in the past, making them plausible candidates for groundwater recharge
centers through the process of basal melting and infiltration.
A series of geomorphic units on the northwest flanks of the four large Tharsis Montes
have been interpreted as deposits left by cold-based glaciers based on morphology and
comparison to terrestrial wet-based glaciers (Lucchitta, 1981) and based on high-resolution
MOLA topography and comparison to Antarctic cold-based ice sheets (Head and Marchant,
2003; Shean et al., 2005; Milkovich and Head, 2003; Milkovich et al, 2006; Parsons and Head,
2004). A typical sequence of facies, from distal to proximal deposits, as summarized from this
latter group of workers is as follows: a ridged facies consisting of hundreds of concentric thin
ridges, interpreted as drop moraines by a retreating cold based glacier; a knobby facies consisting
of small, sometimes elongated knobs, interpreted to be sublimation till resulting from downwasting of an ice deposit with a significant sediment component; and a smooth facies of gently
sloping terrain and lobate features that are interpreted as debris-covered alpine and piedmont
glaciers. These deposits on all of the three major Tharsis Montes are similarly interpreted as
resulting from the activity of cold-based, debris-covered piedmont glaciers, as are the piedmont
lobes seen along the base of the northwestern Olympus Mons scarp (e.g., Head et al., 2005;
Milkovich et al., 2006).
Cold-based glaciers are frozen to their bed, meaning that underlying topography may
remain relatively unaltered in their presence, and that flow occurs within the overlying ice (Benn
and Evans, 1988). By definition, there is no basal melting associated with such features.
However, the demonstration that significant ice deposits collected around the Tharsis Montes in
20
the Late Amazonian lends support to the plausibility that they could also have formed here
earlier and that they may have previously undergone basal melting to supply water to the
subsurface. In at least one instance, evidence that a lava flow banked up against a formerly
existing ice deposit, indicates that ice deposits were not restricted solely to times after lava flow
activity, and potentially heightened heat flow, had ceased (Shean et al., 2005). The latest
manifestations of these ice deposits were significant in volume, covering areas of ~180,000 km2
and 75,000 km2 at Arsia Mons and Pavonis Mons, respectively, and an elevation range of ~5 km
on the slopes of both volcanoes (Head and Marchant, 2003; Shean et al., 2005). Evidence from
the lava flows interpreted to have been banked up against ice deposits yields a minimum ice
sheet thickness of ~250 m (Shean et al., 2005). Given the area and elevation range covered by
these ice sheets, profile modeling of the ice sheet suggests that thicknesses on the order of 2 km
are reasonable (Shean et al., 2004, 2005; Fastook et al., 2005).
The morphological and topographic evidence for significant ice deposits on the Tharsis
rise is supported by modeling studies which suggest that if massive ice is going to form at
martian low latitudes, it would likely form on the Tharsis and Elysium rises (Mischna et al.,
2003, 2004; Haberle et al., 2004; Forget et al., 2006). Supporting previous predictions (Jakosky
and Carr, 1985), the latest global circulation models predict that at obliquities higher than ~ 45°,
which is estimated to be the most probable obliquity of Mars throughout its history (Laskar et al.,
2004), ice should accumulate at high elevations, especially at the Tharsis Montes, and persist
year-round (Mischna et al., 2003, 2004; Haberle et al., 2004; Forget et al., 2006). In addition,
planetary scale images of Mars from MOC and laser reflections from MOLA show that areas of
regional cloud cover in today's climate include the Tharsis rise (Neumann, et al., 2003). Three
strong lines of geologic, climate modeling, and atmospheric evidence, as well as radial valley
networks on Tholi, provide a plausible case that snow and/or ice is likely to have been present on
large volcanoes at times during the martian past.
Ice deposits on the flanks of Tharsis Montes appear geologically recent, having been
emplaced on top of and below Mid to Late Amazonian lava flows clearly visible in topography
data (e.g., Head and Marchant, 2003; Shean et al., 2006). Modeling suggests that deposition
occurred at periods of higher obliquity than that of today (Mischna et al., 2003, 2004; Haberle et
al., 2004; Forget et al., 2006). The variation of orbital parameters necessary to shift the climate
from that of today to one in which low latitude snow or ice deposits would be common are likely
21
to be typical of much of the history of Mars (Laskar et al., 2004). Between 10 and 5 Mya,
obliquity ranged from 25° to 45°, significantly higher than the range of 15° to 35° since 5 Mya
(Laskar et al., 2004). It is in periods of high obliquity that the Tharsis ice deposits are likely to
have formed. Prior to 20 Mya, calculations of the nature of Mars' orbit become chaotic and thus
impossible to reasonably predict (Laskar et al., 2004). However, statistical analyses predict that
at any point in the last 4 Gy of martian history, the most probable obliquity at that time was
~41°, with a ~90% probability that obliquities over 60° were reached (Laskar et al., 2004). Thus,
the orbital conditions resulting in the climate that produced the observed latest Amazonian
evidence for surface ice on Tharsis were likely to be the orbital conditions most prevalent over
extended periods of the martian past. Orbital influences on climate would be superposed on
monotonic and/or endogenic influences on climate such as atmospheric loss and decline in
volcanic activity. Background, or non-orbital, climate conditions similar to those of today are
considered to have been achieved by the end of the Noachian (Clifford and Parker, 2001).
Orbitally forced climate conditions that we know from geologic analysis to have resulted in ice
accumulation on Tharsis thus had the potential to result in similar ice accumulations throughout
the Hesperian and Amazonian. This firm extrapolation allows the assertion that basal melting of
significant snow or ice deposits on the Tharsis rise is a viable mechanism to have operated in the
past to charge the regional groundwater system. This is also a plausible process at Elysium where
significant outflow occurred, recharge of the groundwater system is necessary to explain the
outflows, and modeling suggests snow accumulation would also occur.
What additional evidence exists for the presence of surface snow and ice on Tharsis and
Elysium that might have been sources for melting and recharge? At Elysium, in addition to the
evidence for recent discharge of water (Cerberus) and water-rich flows (Utopia lahars), there is
evidence for past snow and ice melting at the summit of Hecates Tholus, producing radial valley
networks (e.g., Fassett and Head, 2006) and evidence for recent glaciers at the base of the
volcano (e.g., Hauber et al., 2005). Furthermore, there is evidence elsewhere at this latitude
range for deposition of snow and ice and glacial flow (lineated valley fill and lobate debris
aprons; Head et al., 2006a, b) earlier in the history of Mars (see map in Head and Marchant,
2006) related in part to upwelling, adiabatic cooling and snow and ice deposition at volcanoes,
massifs and lowland-highland margins. Although most of Elysium is covered with later-stage
22
lavas, these data strongly suggest that snow and ice accumulation sufficient to promote melting
and recharge occurred at earlier times.
On Tharsis, most of the shields and the surface of Tharsis itself are covered with Late
Hesperian and Amazonian lavas flows. The presence of extensive cold-based glacial deposits
(e.g., Head and Marchant, 2003), and the likelihood that circulation patterns in the past would
continue to supply snow and ice to Tharsis (e.g., Forget et al., 2006), strengthens the probability
that there was sufficient precipitation to cause melting and recharge during earlier periods of
higher heat flux. Further recent evidence for such deposits lying under the veneer of Amazonian
lavas come from the discovery of dendritic valleys on the flanks of Tharsis in the Valles
Marineris region (Mangold et al., 2004). Here, Late Hesperian fluvial landforms interpreted to
be related to atmospheric precipitation and subsequent melting are found on the older preAmazonian deposits underlying the Amazonian lava cap (Mangold et al., 2004). Furthermore, a
system of very large valleys northwest of Tharsis has been interpreted to represent the results of
melting and runoff from Tharsis during a Late Noachian-Early Hesperian climatic event (Dohm
et al., 2000). These observations and interpretations, together with models for episodic
hydrological cycles (e.g., Baker et al., 1991, 2000), support the likelihood of significant sources
of snow and ice, its melting, influx into the groundwater system, and runoff.
To assess the capacity of Tharsis and Elysium to supply recharge we apply the melting
rates obtained in the previous section to a snowpack only 1100 m thick. If basal melting occurred
over all of Tharsis, supplying a column equivalent of 200 m of water over ~ 8 x 106 km2 every
1.5 x 105 years, the minimum outflow channel volume estimate of 3 x 106 km3 can be provided in
0.3 My. Smaller recharge areas, which are more likely given the extent of evidence for ice
accumulation discussed above, would increase this time. For example, a Tharsis recharge area of
~1 x 106 km2, a few times larger than the most recent largest individual ice sheet, would provide
the minimum outflow volume channel estimate in ~2.3 My. These times required for entry of
water into the groundwater system are of the same order as the ~ 1 My needed to provide the
same volume to outflow channel sources through subsurface flow in the maximum permeability
case of Harrison and Grimm (2005). The approximate equivalence of recharge and subsurface
flow rates suggests that subsurface flow from the recharge area to the outflow source is not
limited by the amount of water that can be supplied by the present mechanism of basal melting.
The problem still exists, however, that both basal melting and subsurface flow rates provide
23
sufficient water over a time scale of 105 - 106 years, whereas outflow channel morphology
suggests channel formation on time scales that are orders of magnitudes less (e.g., Carr, 1979,
1996).
At the smaller Elysium rise, ~ 5 x 105 km2, approximately 3 x 104 km3 of water is
required to account for the observed channels and lahar deposits (Russell and Head, 2003). Using
the same conditions as above for Tharsis, basal melting can provide this volume in ~5 x 104
years. Thus, the present mechanism of basal snowpack melting is a very plausible mechanism for
charging the groundwater system near large volcanic rises with enough water to account for
observed outflow volumes in relatively short periods of geologic time.
8. Importance of Dikes to Permeability and Groundwater Flow
The present model of snow and/or ice accumulation and melting accounts for the location
of outflow channel sources in providing mechanisms for gathering water, melting it, and
transporting it from the surface into the groundwater system, all of which are correlated with
characteristics of martian volcanic rises. Dikes are an additional feature of volcanic provinces
that are key to subsurface hydrologic processes and may be another factor in the spatial and
causal relationship between major volcanoes and outflow channels. Propagation of dikes
associated with magmatic intrusion has been shown to be very effective at forming fractures on
the Moon (Head and Wilson, 1993), Venus (Ernst et al., 1995), and Mars (Mège and Masson,
1996; Wilson and Head, 2002a, b). On Mars, dike intrusion produces the additional result of
disrupting the cryosphere and enabling the escape to the surface of groundwater under hydraulic
pressure, as evidenced at Elysium Fossae, Cerberus Fossae, Mangala Vallis, and potentially east
of Tharsis in the region of Valles Marineris and related parallel chasms (Russell and Head, 2001,
2003; Head and Wilson, 2002; Wilson and Head 2002a, b; Head et al. 2003; Ghatan et al., 2004;
Mège and Masson, 1996).
The fracturing associated with planar dike intrusion would preferentially increase the
permeability of the subsurface in the direction parallel to their propagation. A slowly cooled dike
itself would represent a relatively low permeability barrier, creating a directional anisotropy in
hydraulic conductivity (Fetter, 2001). The creation of a highly anisotropic subsurface
permeability structure with higher permeability parallel to dike propagation directions is
illustrated in Figure 9. Dikes inhibit the passage of water across them and further enhance
24
subsurface flow along-strike to the point where the dikes intersect the surface, as proposed for
the northwest Elysium outflows (Chapman, 1994; Russell and Head, 2003). Just as dikes have
demonstrably been involved in initiating conditions necessary for the escape of groundwater in
many cases of major outflow, they also significantly increase effective permeability in the
direction from the groundwater storage region to the outflow source (e.g., Russell and Head,
2003).
In modeling outflow events in the Chryse region with a regional source of groundwater at
relatively high elevation within the Tharsis rise, Carr (1979) required permeabilities of 10-9 m2 to
achieve estimated outflow discharges. While the permeability of the average martian uppermost
crust (≤10-12 m2; Clifford and Parker, 2001; Hanna and Phillips, 2005a) is consistent with that of
unfractured basalts on Earth (10-15 - 10-11 m2; Heath, 1987; Welhan and Wylie, 1997), fractured
basalts and surface lava flows on Earth have permeabilities in the range of 10-10 - 10-6 m2 (Heath,
1987; Welhan and Wylie, 1997). The major volcanic provinces and the montes themselves are
largely constructional in nature, built of successive layers of overlapping lava flows and different
generations of lava flow units (Scott and Tanaka, 1986; Tanaka et al., 1992). Construction by
lava flows and lateral dike emplacement are the primary contributors to the subsurface structure
of Hawaiian shields used in representations of volcanic groundwater systems on Earth (Figure 9).
Permeability in thick sequences of lava flows further fractured by dike propagation in these
provinces may thus be expected to be higher than the permeability of the nominal martian upper
crust considered by Clifford and Parker (2001) and Harrison and Grimm (2004). A maximum
surface permeability of 10-6 m2 (Heath, 1987; Welhan and Wylie, 1997) would result in an
average effective permeability of the martian subsurface between the base of the nominal
equatorial cryosphere (2.5 km depth) and the depth of self-compaction (8.5 km) of 9x10-9 m2,
following the subsurface conventions of Clifford and Parker (2001). This permeability is within
the range required by Carr (1979) in modeling high outflow channel discharge rates (see also
Wilson et al., 2004). The presence of groundwater under hydrostatic pressure would tend to
widen fractures and increase effective permeability above the dry permeability values given
above (Hanna and Phillips, 2005a). The interaction of water with fractures represents an
additional mechanism for potentially increasing flow in confined, highly pressurized regions of
an aquifer, although rapid expansion of fractures occurs only when hydrostatic pressure
approaches lithostatic pressure. Channeling of flow in the processed zone around individual
25
dikes will also increase permeability. Release of hydrostatic pressure during an outflow event
would eventually lead to a reduction in fracture width and a decrease of effective permeability
(Hanna and Phillips, 2005a).
An alternative method of increasing discharge rate is to increase the gradient driving
groundwater flow. Hanna and Phillips (2005b) have shown that the displacement accompanying
dike intrusion can lead to pressurization of the aquifer above the ambient hydrostatic pressure.
Thus, both intrusion of the dike and subsequent aquifer pressurization may contribute to
rupturing the cryosphere and providing a path for water to the surface. Pressurization also
increases the efficiency of discharge to the surface (Hanna and Phillips, 2005b) in addition to the
effects of increased permeability and focusing of flow discussed above.
We have shown that the environment at the large martian volcanic rises has the potential
to collect snow, melt it, and create an elevated reservoir of groundwater. Likewise, the high
permeability medium, and possibly the increased driving pressure (Hanna and Phillips, 2005b),
on which interpreted outflow rates appear to depend (Carr, 1979; Harrison and Grimm, 2004)
may also be a consequence of the volcanic province environment.
9. Conclusions
Several lines of geologic evidence and modeling indicate that significant snow and ice
deposits have accumulated at high elevations on Mars in the past. The only major differences
from the climate of today required for such deposits are those predicted to result from orbital
variations typical for Mars. Elevated heat flow at these very locations due to magmatic activity
provides a mechanism for raising subsurface temperatures above freezing and melting this snow,
thus introducing meltwater into the subsurface. The structural nature of the subsurface in these
volcanic areas provides a medium for groundwater flow with effective permeabilities in excess
of those expected in the nominal martian subsurface.
This model of regional recharge centers is proposed to have operated concurrently with
and superposed on the global cryosphere-hydrosphere system of subsurface flow and south polar
recharge of Clifford (1993) (Figure 1). However, we stress that the regional recharge model
(Figure 6) has advantages in accounting for major outflow events. Accumulation and infiltration
of water at high planetary elevations on the flanks of volcanic rises relatively close to outflow
breakout locations increases the potential groundwater hydraulic gradient over values provided
26
by the south polar cap. The effect of a steeper hydraulic gradient is quantified by the
groundwater flow modeling results of Harrison and Grimm (2004), which show that a Tharsis
source of groundwater can provide the volume of water estimated to have flowed through the
outflow channels more rapidly than a south pole source. The south polar recharge model requires
saturation of the entire, global subsurface to elevations above the pre-outflow surface elevations
at outflow source regions. The regional model requires only that the subsurface be saturated in
the area incorporating the recharge zones and outflow zones. This significantly reduces the
amount of water implied to be in the global inventory to account for outflow elevations within
the context of a martian hydrosphere-cryosphere system. Finally, the spatial distribution of many
of the major outflow channel sources around the periphery of volcanic rises suggests this may be
a causal relationship: The outflows occur where there is a large accumulation of groundwater
with steep hydraulic gradients. Adjacent to volcanic rises, this groundwater may be confined
under hydrostatic pressure by the cryosphere. The central areas and flanks of high volcanoes are
the areas most likely to collect snow and to have the elevated heat flow necessary to melt the
snow and provide a non-frozen infiltration path to the subsurface. Thus, these volcanoes
represent both an entry point for water into the groundwater system and an elevated reservoir for
the outflows.
Calculations show that melting at the base of a snowpack several hundred meters thick
can occur under conditions of high geothermal heat flow (≥ 0.1 W m-2), current average surface
temperatures, and modeled snowpack thermal conductivity (Carr and Head, 2003). On postNoachian Mars, the possibility of basal melting would be restricted to regions of anomalously
high heat flow, such as volcanic rises experiencing magmatic intrusion (e.g., Gulick, 1998;
Fassett and Head, 2004, 2006). Basal melting could be initiated under a ~1 km-thick snowpack
and would yield ~1 kg m2 yr-1 of water under 1.1 km of snow (Figure 7), which is far less than
expected infiltration capacities. At this rate, a volume of water equivalent to the pore space in a
column of the subsurface would be introduced into the groundwater system in <1.5 x 105 years
(Figure 8).
Morphologic and topographic study of deposits around the Tharsis Montes and Olympus
Mons are consistent with the former presence of extensive cold-based ice sheets (Head and
Marchant, 2003; Shean et al., 2005; Milkovich and Head, 2003; Milkovich et al, 2006; Parsons
and Head, 2004), demonstrating the geologic viability of the occurrence of significant amounts
27
of snow and ice on volcanic rises. The latest global circulation models also predict volcanic rises
to be preferential locations of permanent snow or ice cover during times when obliquity is high,
≥45° (Mischna et al., 2003, 2004; Haberle et al., 2004; Forget et al., 2006), yet well within the
range of likely obliquities for most of martian history (Laskar et al., 2004). Given the melting
rates above, the area of Tharsis and Elysium are capable of introducing minimum outflow
channel volume estimates of water into the subsurface in 0.3-2 x 106 and 5 x 104 years,
respectively.
The permeability of the nominal martian upper crust (Clifford and Parker, 2001) and
unfractured basalts on Earth (Heath, 1987; Welhan and Wylie, 1997) is below that required to
account for estimated outflow discharge rates (Carr, 1979). The prevalence of dike intrusion at
Tharsis and Elysium (Russell and Head, 2001, 2003; Head and Wilson, 2002; Wilson and Head
2002a; Head et al. 2003; Ghatan et al., 2004; Mège and Masson, 1996) should lead to a highly
anisotropic subsurface permeability structure (Figure 9) with increased permeability parallel to
dike propagation directions (e.g., Russell and Head, 2003). Dikes are thus another characteristic
of volcanic provinces that may have a genetic influence on outflow, both by significantly
increasing the ability of the subsurface to transport water from high accumulation zones to
outflow sources relative to what is possible elsewhere on Mars and by providing a mechanism
for cryosphere disruption and outflow initiation (Russell and Head, 2003; Head et al., 2003).
The combination of geomorphologic evidence, climate modeling, snowpack and upper
crustal thermal considerations, and subsurface geologic characteristics of volcanoes strongly
supports a mechanism of regional vertical recharge of groundwater at major volcanic rises
(Figure 6). These factors may also explain the preferred occurrence of outflow source regions
around volcanic provinces in the context of a post-Noachian Mars characterized by an
impermeable subsurface cryosphere. While recharge at the south pole and global horizontal
transport, as introduced by Clifford (1993), may still be operative processes, the regional vertical
recharge of groundwater at Tharsis and Elysium is a more efficient and perhaps dominant
process in supplying water to surrounding outflow channel sources.
Acknowledgments: We gratefully acknowledge NASA Grants (to JWH) from the Planetary
Geology and Geophysics Program and the Mars Data Analysis Program. Thanks are extended to
28
Nancy Christy, Anne Côté, James Dickson and Peter Neivert for assistance in manuscript
preparation.
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Figure Captions
Figure 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 Figure 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 ~200x. 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 subcryosphere region. (b) 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) 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 ho), the water can flow to the surface.
Figure 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 Figures 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.
37
Figure 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).
Figure 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).
Figure 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 sub-cryosphere 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).
38
Figure 6. Regional recharge of the groundwater system at the Tharsis and Elysium rises rather
than at the south pole (compare to Figure 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) providing a closer reservoir
and higher hydraulic pressure head (ho) for flow to resupply water at the outflow breakout
location. (c) same model represented in (b) for the Elysium region.
Figure 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 porositydependent 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 xintercept of each curve.
Figure 8. Using the melting rates in Figure 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 Figure 7) the time to fill any
amount of pore space is infinite, and not shown on this graph.
Figure 9. Cross-sectional sketch of important geologic and hydrologic conditions at a Hawaiian
volcanic island. Left and right sides of the diagrams represent the same volcanic dome at
different times as indicated. Features pertinent to the discussion of groundwater in martian
volcanic rises are in large type. The movement of groundwater that has infiltrated from above is
severely controlled by the presence and orientation of dikes. In this instance, lateral groundwater
39
flow is inhibited to the left and right and is concentrated in directions into and out of the page.
Figure from Takasaki (1978), as adapted from Cox (1954).
40
Table 1. Outflow source locations, grouped by region. 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]. C=Chaos, F=Fossae, V=Vallis/es.
Outflow feature
(Source - Channel)
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
Approx.
Lat
Approx.
Lon
Base/floor
elevation (km)
Surrounding/rim
elevation (km)
2.0
2.6
3.1
2.8
4.5
-5.0
3.0
0.5
-1.2
-8.2
-13.8
-12.1
-0.6
-1.4
341.7
335.4
323.2
327.3
330.9
323.0
341.2
315.4
317.4
316.7
318.7
316.9
278.9
298
-3.3
-2.8
-4.1
-4.1
-4.0
-4.2
-3.4
-3.5
-2.2
-3.6
-3.8
-0.2
-1.0
-1.7
-1.7
-1.3
-1.7
-1.6
-1.8
-0.2
-1.0
0.6
0.1
1.0
1.3
1.3
1.6
1.6
-18.3
210.3
-0.3
0.5
38.9
33.5
26.4
30.2
138.8
141.9
135.9
138.1
-4.3
-3.9
-3.8
-3.5
-4.2
-3.7
-3.6
-3.4
10.3
156.7
-2.7
-2.4