JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, E02007, doi:10.1029/2010JE003709, 2011
Early Mars hydrology:
2. Hydrological evolution in the Noachian and Hesperian epochs
Jeffrey C. Andrews‐Hanna1 and Kevin W. Lewis2
Received 4 August 2010; revised 14 November 2010; accepted 24 November 2010; published 24 February 2011.
[1] Mars was warmer and wetter during the early to middle Noachian, before a hydrologic
and climatic transition in the late Noachian led to a decrease in erosion rates, a change in
valley network morphology, and a geochemical shift from phyllosilicate to sulfate formation
that culminated in the formation of widespread sulfate‐rich sedimentary deposits in
Meridiani Planum and the surrounding Arabia Terra region. This secular evolution
was overprinted by episodic and periodic variability, as recorded in the fluvial record,
sedimentary layering, and erosional discontinuities. We investigate the temporal evolution
of Martian groundwater hydrology during the Noachian and early Hesperian epochs using
global‐scale hydrological models. The results suggest that the more active hydrological
cycle in the Noachian was a result of a greater total water inventory, causing a saturated
near‐surface and high precipitation rates. The late Noachian hydrologic, climatic, and
geochemical transition can be explained by a fundamental shift in the hydrological regime
driven by a net loss of water due to impact and solar wind erosion of the atmosphere.
Following this transition, the water table retreated deep beneath the surface, except in
isolated regions of focused groundwater upwelling and evaporation, producing the playa
evaporites in Meridiani Planum and Arabia Terra. This long‐term evolution was modulated
by shorter‐term climate forcing in the form of periodic and chaotic variations in the orbital
parameters of Mars, resulting in changes in the volume of water sequestered in the polar
caps and cryosphere. This shorter‐term forcing can explain the observed periodic and
bundled sedimentary layering, erosional unconformities, and evidence for a fluctuating
water table at Meridiani Planum.
Citation: Andrews‐Hanna, J. C., and K. W. Lewis (2011), Early Mars hydrology: 2. Hydrological evolution in the Noachian
and Hesperian epochs, J. Geophys. Res., 116, E02007, doi:10.1029/2010JE003709.
1. Introduction
[2] The ancient surfaces of Mars preserve a record of
warmer and wetter conditions in which liquid water played
an important role in shaping the surface. Multiple lines of
evidence are converging toward a consensus picture of the
phenomenological history of water on early Mars. In the early
Noachian, conditions were generally warmer and wetter than
today, though likely arid by terrestrial standards [Howard
et al., 2005; Stepinski and Stepinski, 2005; Barnhart et al.,
2009]. These conditions led to the formation of dendritic
valley networks [Carr and Malin, 2000; Craddock and
Howard, 2002; Hynek and Phillips, 2003] and resulted in
the alteration of some of the near‐surface materials to phyllosilicates [Poulet et al., 2005; Bibring et al., 2006]. A significant shift in the ambient environment in the late Noachian
1
Department of Geophysics and Center for Space Resources, Colorado
School of Mines, Golden, Colorado, USA.
2
Department of Geosciences, Princeton University, Princeton, New
Jersey, USA.
Copyright 2011 by the American Geophysical Union.
0148‐0227/11/2010JE003709
led to drier conditions, as recorded in a transition from dense
runoff valleys to more sparsely dissected networks [Baker
and Partridge, 1986; Harrison and Grimm, 2005], the cessation of valley network formation [Fassett and Head, 2008],
a decrease in erosion rates [Golombek et al., 2006], and a
change from widespread phyllosilicate alteration [Poulet
et al., 2005] to the formation of localized evaporitic sulfate
deposits [Bibring et al., 2006; Squyres et al., 2006]. The late
Noachian to early Hesperian epochs were characterized by
the formation of widespread playa evaporites in Meridiani
Planum and related sedimentary deposits throughout the
surrounding Arabia Terra region [Andrews‐Hanna et al.,
2010]. This secular evolution was overprinted by temporal
variability on shorter time scales, expressed in the geologic
record in evidence for a pulse of enhanced valley network
formation in the Late Noachian [Irwin et al., 2005], periodic
layering and bundling of layers within sedimentary deposits
[Lewis et al., 2008], and erosional unconformities within the
Meridiani sulfate deposits [Wiseman et al., 2010].
[3] This evidence for hydrologic and climatic variability
over multiple time scales motivates a more detailed treatment
of the time evolution of the hydrology of early Mars. We here
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climatic evolution of Mars. Early in Mars history, a large total
water inventory fueled high precipitation rates and a saturated
near‐surface zone, consistent with the widespread formation
of phyllosilicates and valley networks in the early to middle
Noachian. A net loss of water then drove a hydrologic and
climatic transition in the late Noachian from wet conditions
with a high level of surface and near‐surface hydrologic
activity, to arid conditions with a more sluggish hydrologic
cycle and a surface that is desiccated across much of the
planet. Following this transition, predicted regions of groundwater upwelling and evaporation match the observed distribution of sulfate‐rich sedimentary deposits. Overprinting this
long‐term evolution, short‐term changes in the available
water inventory in response to periodic and chaotic changes
in obliquity resulted in significant short‐term variations in the
precipitation and groundwater upwelling rates and the depth
to the water table. These variations are shown to be consistent
with the evidence for layer periodicity and bundling, and
erosional discontinuities in the sedimentary record.
2. Observational Constraints on the Temporal
Evolution of Martian Hydrology
Figure 1. Evidence for the global hydrological evolution of
Mars from the distribution of (a) valley networks [adapted
from Hynek et al., 2010]; (b) hydrated minerals, with phyllosilicates in red, sulfates in blue, and other hydrated minerals
in yellow [adapted from Bibring et al., 2006]; and (c) layered
sedimentary deposits [adapted from Malin and Edgett, 2000].
Figures 1b and 1c are reprinted with permission from
American Association for the Advancement of Science,
http://www.sciencemag.org.
use models of the global‐scale groundwater hydrology to
investigate the distribution and behavior of water on Mars
during the Noachian and early Hesperian epochs, and the
temporal variability of the hydrological cycle. In a companion
paper, we focused on the hydrology in the late Noachian to
early Hesperian, at the time that the Meridiani Planum playa
deposits and other sulfate deposits were forming in response
to sustained groundwater upwelling [Andrews‐Hanna et al.,
2010]. That study investigated the hydrology under stable
surface environmental conditions without any external forcing. In this study, we focus on the temporal variability of the
hydrologic cycle through the Noachian and Hesperian
epochs. The current work incorporates the effects of secular
changes in the hydrologically available water inventory
driven by solar wind stripping and impact erosion as well as
short‐term variability induced by ice sequestration in the
polar caps and high‐latitude permafrost in response to periodic and chaotic changes in obliquity. This study considers
only the effects of the changing water inventory on the
hydrology, and does not consider the effects of likely contemporaneous changes in temperature.
[4] The model results are shown to be consistent with the
evidence for both the long‐ and short‐term hydrologic and
2.1. Secular Evolution and the Late Noachian
Transition
[5] The widespread dendritic valley networks found
throughout the low‐latitude Noachian‐aged terrains provide
compelling evidence for more Earth‐like conditions on early
Mars [Carr and Clow, 1981; Baker, 1982; Carr and Chuang,
1997]. On the basis of the low drainage densities [Carr and
Chuang, 1997] and morphologies similar to terrestrial
groundwater sapping valleys on the Earth, it has been suggested that liquid precipitation and surface runoff may not
have been required [Goldspiel and Squyres, 2000]. However, groundwater sapping mechanisms still require aquifer
recharge, which must be brought about through either liquid
precipitation or the accumulation and melt of snow, both of
which would require warmer and wetter conditions than the
current hyperarid cold desert. More recent high‐resolution
imaging and topographic data (Figure 1a) revealed that the
drainage densities across much of Mars were significantly
higher than previously thought [Irwin and Howard, 2002;
Hynek and Phillips, 2003; Hynek et al., 2010], strengthening
the argument for liquid precipitation.
[6] Quantitative geomorphic analyses of the valley networks and comparisons to terrestrial fluvial systems suggest
that conditions on early Mars were comparable to those on the
Earth in arid environments today [Stepinski and Stepinski,
2005; Barnhart et al., 2009], consistent with the low erosion rates calculated for early Mars [Golombek et al., 2006].
A progression of valley network morphology is generally
observed, from early formation of densely dissected, V‐shaped,
degraded valley networks suggestive of precipitation‐induced
runoff, to later formation of more sparsely dissected, U‐shaped,
pristine valley networks [Baker and Partridge, 1986; Williams
and Phillips, 2001; Harrison and Grimm, 2005]. This transition
has been interpreted as a result of either an increased contribution from groundwater sapping [Baker and Partridge,
1986; Williams and Phillips, 2001; Harrison and Grimm,
2005] or occasional flash flooding under arid conditions
[Lamb et al., 2008]. Either interpretation of the transition in
valley network morphologies suggests a drying of the climate
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with time, with a decrease in the precipitation and erosion
rates and an increase in the depth to the water table below the
surface [Harrison and Grimm, 2005]. This transition in the
fluvial geomorphology was approximately contemporaneous
with a decrease in the average surface erosion rates by several
orders of magnitude in the late Noachian to early Hesperian
[Golombek et al., 2006]. Similarly, landscape evolution
models argue for a transition from both diffusive and
advective erosion by rain splash and runoff, to primarily
diffusive erosion by rain splash and mass wasting, arguing for
increased infiltration and decreased runoff on an increasingly
arid Mars [Craddock and Howard, 2002]. The record of both
fluvial and nonfluvial erosion thus suggests that conditions in
the Early Noachian were at least transiently similar to arid or
semiarid environments on the Earth, but the surface became
increasingly desiccated with time.
[7] These geomorphic interpretations of a climatic and
hydrologic transition are corroborated by spectral observations of the Martian surface indicating a concomitant mineralogical transition. Spectral signatures indicative of the
presence of phyllosilicates, including smectite, montmorillonite, and kaolinite, have been detected within ancient
Noachian terrains across much of Mars (Figure 1b) [Poulet
et al., 2005; Bibring et al., 2006; Mustard et al., 2008b].
Stratigraphic relationships suggest that phyllosilicate alteration preceded the formation of the fluvial channels in at least
some portions of the planet [Mustard et al., 2008a]. This is
supported by improved estimates of valley network ages that
place the termination of valley network formation at the
Noachian‐Hesperian boundary [Fassett and Head, 2008],
after the termination of phyllosilicate formation prior to the
end of the late Noachian [Bibring et al., 2006]. The early era
of phyllosilicate formation was followed by an era of sulfate
deposition in the late Noachian to early Hesperian [Bibring
et al., 2006]. Unlike the widespread phyllosilicate deposits
interspersed within the crust, the sulfates are generally stratigraphically above the ancient Noachian surfaces [e.g.,
Wiseman et al., 2008], and are localized primarily within the
Meridiani Planum region in western Arabia Terra, as well as
within the Valles Marineris canyons and chaos regions at the
heads of the outflow channels [Gendrin et al., 2005; Murchie
et al., 2009; Roach et al., 2010b]. The hydrated sulfate deposits are commonly associated with layered outcrops interpreted as sedimentary rocks (Figure 1c) [Malin and Edgett,
2000].
[8] In situ observations by the Mars Exploration Rover
Opportunity at Meridiani Planum led to the interpretation of
these deposits as evaporitic sulfate salts that have been
extensively reworked by fluvial and aeolian processes in a
playa‐like environment, followed by multiple episodes of
groundwater‐mediated diagenesis [Grotzinger et al., 2005;
McLennan et al., 2005; Squyres and Knoll, 2005; Arvidson
et al., 2006; Squyres et al., 2006]. A similar origin has been
suggested for the deposits in the canyons and chaos regions
to the west, based on the stratigraphic similarities and the
co‐occurrence of sulfates and hematite observed in both
locales [Glotch and Rogers, 2007; Murchie et al., 2009;
Roach et al., 2010a], as well as similarities in the hydrological
environment [Murchie et al., 2009]. Sulfate deposits outside
of Arabia Terra, Valles Marineris, and the chaos regions
appear to be restricted to isolated crater lakes such as
Columbus crater to the west of Tharsis [Wray et al., 2011].
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The transition from global phyllosilicate alteration to local
and regional sulfate deposition has been suggested to signal a
distinct change in the surface geochemical environment to
more acidic and sulfate‐rich conditions [Bibring et al., 2006],
the cause of which is investigated in this work.
[9] The phyllosilicate‐sulfate mineralogical and geochemical transition approximately coincides with the transitions in erosion rates and valley network morphology,
suggesting a significant global hydrological, geochemical,
and climatic change in the Late Noachian. This transition
resulted in an increasingly arid surface, an increase in the
depth to the water table, a decrease in the precipitation rates,
and a transition from phyllosilicate to sulfate deposition.
Arid, yet not fully desiccated, conditions then persisted in the
Late Noachian and Early Hesperian, allowing the growth of
local evaporite deposits while other evidence for water on the
surface waned. Eventually, Mars transitioned into a colder
climate, and further hydrological activity was limited to
occasional outburst floods escaping from pressurized aquifers
beneath a thick cryosphere to carve the outflow channels
[Hanna and Phillips, 2006; Andrews‐Hanna and Phillips,
2007].
2.2. Short‐Term Periodic and Episodic Evolution
[10] The long‐term secular drying of the surface and
near‐surface environment during the Noachian and Early
Hesperian was overprinted by shorter‐term temporal variability. Layering at the millimeter to centimeter scale observed
by the Mars Exploration Rover Opportunity [Grotzinger et al.,
2005] records the shortest time scale of variability. This
finest‐scale layering may correspond to annual cycles, individual sand avalanches down the slip faces of dunes, or other
short time scale phenomena, but likely tells us little about
the long‐term climate evolution. A regular periodicity in the
thickness of meter‐scale layers deduced from HiRISE images
and stereotopography for deposits across much of Mars
argues for a periodic forcing mechanism (Figure 2a), likely
supplied by orbital‐driven periodicity in the climate evolution
[Lewis et al., 2010] (also K. W. Lewis, Geologic setting of
rhythmic sedimentary rocks on Mars, manuscript in preparation, 2011). Orbital control of the sedimentation is most
clearly manifest in the cyclical bundling of layers into packets
of 10 at Becquerel crater (Figure 2b), which correlates with
the 1.2 Myr cycle that modulates the maximum obliquity
achieved during the shorter 120 kyr obliquity cycles [Laskar
et al., 2004; Lewis et al., 2008].
[11] Variability of a more episodic, rather than periodic,
nature is also observed in the geomorphic and sedimentary
record. Erosional unconformities are observed in outcrops
observed by Opportunity (such as the Wellington contact;
Figure 2c) [Grotzinger et al., 2005] as well as in sedimentary
layering in HiRISE images (Figures 2d–2e), suggesting
temporary hiatuses in deposition accompanied by erosion of
the deposits that could result from a change in the hydrologic
or climatic environment. While angular unconformities such
as these are observed relatively rarely from remotely sensed
data, other depositional hiatuses may not have led to actual
erosion of underlying strata before deposition resumed. The
resulting parallel stratification would make detection of such
an unconformity nearly impossible from orbit. Smaller‐scale
truncation surfaces in homogeneous lithologies such as the
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Figure 2. Evidence in the sedimentary record for temporal variability in the hydrological cycle, including
rhythmic strata and stratigraphic unconformities. (a) Uniformly spaced rhythmic bedding exposed on the
floor of Danielson crater (8°N, 353°E; HiRISE image PSP_002733_1880). (b) Multiple scales of cyclicity
preserved within a sedimentary mound in Becquerel crater (22°N, 352°E; HiRISE image PSP_001546_
2015). Rhythmic beds several meters in thickness are bundled into larger stratigraphic units at the scale
of tens of meters. Roughly 10 beds comprise each of the larger bundles, which may be linked to a similar
ratio of frequencies in the Martian obliquity signal. (c) Truncation surface within cross‐bedded eolianites at
Meridiani Planum, as seen from the Opportunity Rover (sol 278 Pancam image mosaic from http://pancam.
astro.cornell.edu/). In this example, it is difficult to determine whether the exposed truncation surface is evidence of a widespread deflationary episode in the Meridiani stratigraphy, or simply a local scour surface.
The exposed outcrop is roughly 7 m thick in the vertical direction. (d) Prominent angular unconformity
within cyclically bedded sediments in a crater on the north rim of the Schiaparelli basin (HiRISE image
PSP_002508_1800). (e) Angular unconformity in Gale crater (HiRISE image PSP_009149_1750). The relatively large impact crater preserved at this stratigraphic horizon is good evidence of a long depositional
hiatus before emplacement of the overlying strata.
example from Meridiani Planum may be common, but would
be indiscernible to orbital instruments. On a larger scale,
geomorphic and mineralogical mapping of the Meridiani
deposits revealed a lateral facies change caused by erosion
into preexisting layered deposits followed by deposition of
relatively late stage sulfates, also suggestive of a hiatus in
deposition leading to net erosion of the deposits, followed by
continued sulfate deposition [Wiseman et al., 2010]. A similar erosional unconformity has been documented within the
interior layered deposits in western Candor Chasma [Fueten
et al., 2008].
[12] Evidence also suggests temporal variability in the rate
of valley network formation. Globally, valley network
formation appears to have peaked toward the end of the
Noachian [Howard et al., 2005; Irwin et al., 2005; Fassett
and Head, 2008], reflecting a late increase in fluvial activity. Smaller‐scale spatiotemporal variability in valley network activity is observed on the scale of individual drainage
basins [Hoke and Hynek, 2009]. Combined with the evidence
for a drying of the climate and a mineralogical transition in
the late Noachian to early Hesperian, this late stage pulse of
fluvial activity suggests that the transition from wet to dry
conditions may have been both rapid and complex, possibly
involving some overlap or interfingering of the periods of
fluvial erosion and sulfate deposition.
[13] Taken together, these various lines of evidence point
toward short‐ and long‐term variability in the hydrologic and
climatic cycle of early Mars. This variability includes both
periodic variations on time scales of 120 kyr to 1.2 Myr
expressed in sedimentary layering, as well as episodic shifts
in the hydrologic‐climatic conditions expressed in erosional
unconformities and changes in the rate and style of fluvial
modification of the surface. While the concept of a transition
from warm and wet to cold and dry conditions still holds
to first order, this secular evolution was overprinted by the
more complex behavior expected of a real climate system.
While previous models of the hydrologic evolution of Mars
have been successful in reproducing the observed equatorial
sulfate deposits in Meridiani Planum and Arabia Terra
[Andrews‐Hanna et al., 2007, 2010], it is necessary to consider a more realistic representation of the temporal evolution
of the hydrologic‐climatic system. In the following sections,
we investigate the temporal evolution of the hydrological
cycle on early Mars from a groundwater perspective, focusing
on the distributions of valley networks and sedimentary
deposits, the underlying mechanisms controlling the Late
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Noachian hydrologic, climatic and geochemical transition,
and the short‐term variability in the hydrological cycle.
3. Groundwater Model Description
[14] We model the groundwater hydrology of early Mars
using a fully explicit finite difference code representing
unconfined groundwater flow in a spherical shell grid
[Andrews‐Hanna et al., 2007, 2010]. The models were run at
a spatial resolution of 2.5 degrees per pixel. The aquifer
properties are taken from the megaregolith model of Hanna
and Phillips [2005]. Groundwater flow is driven by gradients in the hydraulic head, which are established as a result
of precipitation‐induced recharge in the low latitudes and
evaporative loss of groundwater in regions where the water
table reaches the surface. Full details and benchmarking of
the hydrological model can be found in the companion paper
[Andrews‐Hanna et al., 2010].
[15] Conditions on early Mars are assumed to be arid by
terrestrial standards, even during the period of valley network
formation, as suggested by the low drainage densities and
immature state of the fluvial landscape [Howard et al., 2005;
Stepinski and Stepinski, 2005; Barnhart et al., 2009].
Groundwater is assumed to evaporate upon reaching the
surface, thereby limiting the water table to remain at or
beneath the surface. The presence of small‐scale lakes and
seas would not greatly affect the large‐scale patterns of
groundwater flow, since they would exist below the model
resolution and would only change the local hydraulic head by
a small amount. Large oceans in the northern lowlands [e.g.,
Di Achille and Hynek, 2010] or the Hellas and Argyre impact
basins [Moore and Wilhelms, 2001] would have a more significant effect on the patterns of groundwater flow, though the
existence of such water bodies is controversial. The model
does not include such possible ancient oceans or seas, though
their possible effect is discussed in section 4.
[16] The globally integrated evaporative loss of water
in each time step is redistributed over the surface in a low‐
latitude precipitation belt, following a half‐cosine distribution
between ±45° latitude to match the first‐order distribution of
valley networks. This implicitly assumes that the conditions
that prevented high‐latitude valley network formation would
have inhibited aquifer recharge as well. The lack of high‐
latitude valley networks may be a result of either a lack of
precipitation, or persistently cold temperatures that would
have prevented both runoff and infiltration of precipitation.
[17] The companion paper [Andrews‐Hanna et al., 2010]
demonstrated the importance of the dynamic interaction
between the hydrology and the surface topography.
Groundwater‐mediated sedimentation alters the surface
topography, which in turn alters the distribution and flow
paths of groundwater. The formation of evaporites and
evaporite cemented sedimentary deposits is represented in the
models by assuming a groundwater salinity upon reaching the
surface equal to 80% that of seawater [e.g., Handford, 1991],
and that the deposits incorporate 50% by volume of clastic
sediments based on Mini‐TES observations at Meridiani
[Glotch et al., 2006] and similar to other dirty evaporites in
playa settings, such that evaporation of 100 m of groundwater
produces 2 m of sediments. Assuming either different fluid
salinities or different aquifer permeabilities results in a simple
scaling of the rate of fluid flow and deposition. While there
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are great uncertainties in both of these parameters, the previous study found that the predicted rates of sedimentary
deposition in Becquerel crater closely match the rates inferred
from a correlation of the layer periodicity with the time scale
of obliquity variations [Lewis et al., 2008; Andrews‐Hanna
et al., 2010].
4. Long‐Term Hydrological Evolution During
the Noachian and Hesperian
4.1. Hydrological Controls on Sulfate Deposition
[18] We first briefly review the results of the earlier work on
the groundwater‐mediated formation of evaporitic sulfates in
the Late Noachian to Early Hesperian [Andrews‐Hanna et al.,
2007, 2010]. Previous groundwater hydrology models found
that Meridiani Planum and the surrounding Arabia Terra
region were areas of focused groundwater upwelling to the
surface, driven by the regional topography of Arabia Terra
[Andrews‐Hanna et al., 2007]. In short, the breaks in slope
on the northern and southern edges of Arabia Terra separating
it from the lowlands and highlands, together with its location
in the low latitudes where precipitation‐induced recharge of
aquifers was likely, led to a net flux of groundwater to the
surface resulting in the formation of evaporites and evaporite
cemented sediments.
[19] Subsequent high‐resolution global and regional
groundwater models that explicitly included the growth of
sedimentary deposits through evaporite deposition and
cementation of aeolian debris strengthened this prediction
[Andrews‐Hanna et al., 2010]. These models successfully
predicted the approximate thickness and spatial extent of the
Meridiani deposits, but also predicted that they should be part
of a more extensive set of deposits covering much of the
Arabia Terra region. These predictions were borne out by
observations of layered deposits, large intracrater deposits,
and pedestal craters that preserve the remnant of this once
more extensive set of deposits [Fassett and Head, 2007;
Fergason and Christensen, 2008; Andrews‐Hanna et al.,
2010; Zabrusky and Andrews‐Hanna, 2010]. The model
results were found to successfully explain the measured dip
[Hynek and Phillips, 2008] and inferred deposition rate
[Lewis et al., 2008] of the deposits. Deposits are also predicted within the Valles Marineris canyons, where large
sulfate‐rich interior layered deposits are observed [Murchie
et al., 2009]. Predicted deposits in topographic lows in the
northern lowlands and large impact basins may be obscured
beneath younger volcanic, sedimentary, and aeolian cover.
Some support for ancient hydrological activity in the lowlands comes from recent observations of hydrated phyllosilicate outcrops exposed by craters in the northern lowlands
[Carter et al., 2010], and evidence for mud volcanism possibly arising from a buried volatile‐rich layer in Acidalia
Planitia [Oehler and Allen, 2010].
[20] While the previous groundwater models considered
the time evolution of the hydrological cycle for a fixed set
of environmental conditions, they did not address the
hydrological evolution in the face of an evolving climate on
early Mars. Building on the success of the earlier work in
explaining the salient features of the late Noachian to early
Hesperian sedimentary record, we now investigate the temporal evolution of the hydrological cycle throughout early
Mars history.
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Figure 3. Hydrological model outputs for different initial mean water table depths (wt), representing different values of the hydrologically available water inventory. (left) Depth to the water table, (middle) predicted sedimentary deposit thickness after 50 Myr, and (right) distribution and rate of groundwater
upwelling and evaporation after 50 Myr, for initial mean water table depths ranging from (top to bottom)
200 m to 1000 m.
4.2. Effect of the Total Water Inventory: Wet and Arid
Hydrological Regimes
[21] From a groundwater hydrology perspective, the
hydrological state of the planet is determined primarily by the
total water inventory available to the hydrologic system relative to the storage capacity of the global aquifer system,
which is established in the models by the assumed initial
mean water table depth below the surface (initialized at a
constant depth prior to hydrological equilibration). This
hydrologically available water inventory represents only that
portion of the total water inventory that directly takes part in
the hydrological cycle at any one time. The previous study
[Andrews‐Hanna et al., 2010] assumed a single constant
value for the water inventory, for which the model predictions
were found to be in agreement with the observed distribution
of sulfate deposits. However, the hydrologically available
water inventory of Mars is poorly constrained, and likely
experienced significant periodic and secular changes over
Mars history. A suite of global hydrological models were
run with different assumed initial mean water table depths,
ranging from 200 m to 1000 m. The models were run for
100 Myr to achieve hydrological equilibrium without any
sedimentary deposition, and then run for an additional 50 Myr
while tracking the evolving groundwater flux to the surface
and sedimentary deposit thickness (Figure 3). This set of
models assumed arid conditions leading to evaporite formation over the full range of initial mean water table depths
and precipitation rates, an assumption we will break with in
section 5.
[22] Models initialized with different initial mean water
table depths produce substantially different outcomes. The
strongest constraint on the nature of the hydrological cycle
comes from the observed distribution and thickness of sulfate
deposits formed in the Late Noachian to Early Hesperian.
These deposits are localized primarily within Meridiani
Planum and the surrounding Arabia Terra region, the Valles
Marineris canyons, and the chaos regions at the sources of the
outflow channels. The predicted extent of the deposits agrees
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Figure 4. Predicted equatorial precipitation rate in balance
with the rate of deep groundwater flow prior to sedimentation
for initial mean water table depths ranging from 200 m to
1000 m. The shaded bar at top schematically represents the
transition from wet to arid conditions. The arrow represents
the initial mean water table depth for which the model results
most closely match the observed distribution of deposits in
Meridiani and Arabia Terra.
best with the observed distribution for models with an initial
mean water table depth of ∼600 m (Figures 3g–3i).
[23] For an initial mean water table depth of 200 m,
corresponding to a greater hydrologically available water
inventory, the water table intersects the surface throughout
the low‐latitude precipitation belt as well as within the
northern lowlands and large impact basins (Figures 3a–3c). If
all groundwater is assumed to evaporate immediately upon
reaching the surface, this leads to the prediction of widespread evaporite‐cemented deposits covering much of the
surface, in conflict with their observed extent. The shallow
water table and short path lengths of groundwater flow result
in rapid cycling of water between the subsurface and the
atmosphere, driving high precipitation rates. The predicted
equatorial precipitation rate in hydrologic equilibrium prior to
the initiation of sedimentation is 12.1 mm/year, compared to
0.13 mm/year for an initial mean water table depth of 600 m
(Figure 4). In contrast, models with a lower hydrologically
available water inventory (e.g., initial mean water table depth
of 1000 m) predict the water table to lie deep beneath the
surface throughout both the southern highlands and most of
Arabia Terra, resulting in minimal sedimentary deposition in
Meridiani Planum and its surroundings (Figures 3m–3o).
Groundwater reaches the surface and evaporates primarily
within the large impact basins and in the northern lowlands,
and the precipitation rate is only 0.021 mm/year (Figure 4).
[24] These models reveal two end‐member regimes of
global‐scale water cycling. For lower total water inventories,
Mars is in the “arid hydrological regime,” in which the water
table lies deep beneath the surface over most of the planet and
the precipitation rate is limited by the rate at which deep
groundwater flow supplies water to the surface. As the total
water inventory is increased, the system enters the “wet
hydrological regime” in which the water table reaches the
surface throughout most of the low‐latitude precipitation
belt, and the precipitation rate increases asymptotically.
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In the wet regime models, the equilibrium storage capacity
of the low‐latitude aquifers is exceeded, so that the precipitation falls primarily on a saturated surface and the excess
water continues to circulate through the climate system.
While there is a dramatic difference between the hydrological behavior of Mars under wet and arid conditions, the
transition between the two regimes is gradational rather than
discrete (Figure 4).
[25] The high precipitation rates and the saturation of
the surface in the wet hydrological regime violate the arid
assumption in the models, and the precipitation would lead
to runoff and ponding. With abundant water available at the
surface, the rates of evaporation and precipitation would be
dictated by the climate system [e.g., Richardson et al., 2007;
Soto et al., 2010] rather than the groundwater system. The
water table would lie at or near the surface over most of the
planet, causing the groundwater hydrology to be dominated
by short wavelength flow in local topographic basins [Grimm
and Harrison, 2003]. The widespread evaporites predicted
by the models as a consequence of the arid assumption
(Figure 3b) would be replaced by fluvial erosion and ponding,
dictated by the combination of the climatically controlled
distribution of precipitation and the groundwater flow
patterns.
[26] As the total water inventory is increased further, the
wet hydrological regime would approach the present‐day
terrestrial hydrological cycle. The total water inventory on the
Earth greatly exceeds the storage capacity of the crustal
aquifers. The largest reservoir of terrestrial water is in the
oceans, with less than 1% contained within aquifers
[Deming, 2002]. As a result, there is at all times a large
supply of water in direct communication with the Earth’s
atmosphere, and the rates of precipitation and evaporation
are dictated by the climate system rather than being limited
by the rate of groundwater flow. Whether early Mars ever
possessed oceans or a truly Earth‐like hydrological system
remains an open question.
[27] The model‐predicted precipitation rates are likely
lower bounds. These models assume that all of the precipitation infiltrates the surface (where it is not saturated) and
contributes to recharging the deep aquifers. Thus, the precipitation rate in the models reflects only that part of the
hydrological cycle involving exchange with the deep subsurface, with no explicit representation of runoff and ponding
followed by evaporation prior to infiltration. Inclusion of a
surface hydrological cycle would increase the actual precipitation rates significantly. In particular, the high precipitation
rates in the wet regime would lead to runoff and ponding of a
significant fraction of the precipitation. This surface reservoir
of water would drive higher rates of precipitation [Soto et al.,
2010] and a more vigorous hydrological cycle than can be
represented in these groundwater hydrology models without
an explicit representation of the atmospheric component of
the hydrological cycle. The actual precipitation rates in the
wet regime would be determined by the climate system rather
than the groundwater hydrology. Similarly, during the arid
regime a significant fraction of the precipitation may evaporate prior to infiltration, increasing the actual precipitation
rate relative to the model value. For example, if only 20% of
the precipitation infiltrated the surface to reach the deep
aquifers, the actual precipitation rates would be a factor of
5 higher than predicted by the models.
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4.3. Hydrological Controls on the Distribution
of Valley Networks
[28] Just as the predicted regions of groundwater upwelling
in the arid regime match the observed distribution of sulfates,
the predicted distribution of near‐surface groundwater during
the wet regime with a greater available water inventory shows
some correlation with the observed distribution of valley
networks. The models predict a shallow water table in the
low‐latitude precipitation belt, with a much deeper water
table in high southern latitudes resulting from the lack of
precipitation‐induced recharge at high latitudes and the
presence of the Hellas basin acting as a hydrological sink.
While this contrast between low and high latitudes is imposed
in the models by the assumed precipitation distribution, the
models also predict significant longitudinal differences in
the midlatitudes, breaking with the longitudinal symmetry of
the imposed zonal precipitation belt.
[29] In the southern midlatitudes, the water table lies much
deeper beneath the surface surrounding the Hellas impact
basin, while lying closer to the surface in the vicinity of
Tharsis (e.g., Figure 3d). As a result of its great depth, Hellas
acts as a large hydrological sink, drawing down the water
table in the surroundings. The imposed low‐latitude recharge
belt over the northern half of the basin results in an asymmetric drawdown cone toward the south. In contrast, the high
topography of Tharsis drives groundwater flow southward
into the surrounding highlands, pushing the water table closer
to the surface south of Tharsis. This large‐scale pattern in the
distribution of shallow groundwater is in first‐order agreement with the observed distribution of valley networks
(Figure 1a) [Carr, 1995; Hynek and Phillips, 2003; Stepinski
and Collier, 2004; Hynek et al., 2010]. While the northern
edge of the valley network distribution is imposed by the
dichotomy boundary and the resurfacing of the northern
lowlands, the southern edge follows an approximate great
circle path dipping southward near Tharsis and northward
near Hellas [Mutch et al., 1976]. The southern edge of the
valley network distribution approximately coincides with the
transition from a shallow water table in the low latitudes to a
deep water table in the middle to high latitudes.
[30] This similarity suggests that the distribution of
groundwater may exert some control over the formation of
the valley networks, as might be expected given the importance of base flow from groundwater to rivers and streams on
the Earth [Deming, 2002]. However, the specific fit of the
valley networks from recent high‐resolution mapping [Hynek
et al., 2010] to the predicted regions of shallow water table is
only approximate, suggesting that other processes likely also
exerted a significant control over valley network formation,
including spatial variations in the rate of precipitation on the
surface [Soto et al., 2010].
4.4. Implications for the Long‐Term Hydrologic
and Climatic Evolution of Mars
[31] We suggest that the wet and arid hydrological regimes
map approximately onto the Noachian and late Noachian–
early Hesperian periods, respectively. Throughout much of
the Noachian, the widespread valley networks, high erosion
rates, and evidence for phyllosilicate formation suggests
abundant near‐surface water and elevated precipitation rates
throughout the low latitudes. The decrease in erosion rates
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[Golombek et al., 2006], transition in valley network morphology [Baker and Partridge, 1986; Harrison and Grimm,
2005], and ultimate cessation of widespread valley network
formation [Fassett and Head, 2008] in the Late Noachian is
consistent with the dramatic decrease in precipitation rates
and the drop in the water table predicted for a wet to arid
hydrological transition (Figures 3 and 4).
[32] Similarly, the geochemical change from widespread
phyllosilicate formation to localized sulfate deposition
[Bibring et al., 2006] would have been a natural outcome of
the wet to arid hydrological transition. During much of the
Noachian, the abundance of low salinity, near‐neutral pH,
meteoric rainwater and shallow groundwater would encourage widespread alteration of the primary igneous mineralogy
to phyllosilicates. After the hydrological transition, the precipitation would rapidly infiltrate the arid surface over most
of the planet, recharging aquifers deep beneath the surface
without prolonged interaction with the near‐surface crust.
However, in a few regions, that coincide with the locations of
observed sulfate deposits, this deep groundwater would reach
the surface after flowing long distances in the subsurface.
This upwelling groundwater would have had ample time to
equilibrate with the aquifer host rocks and acquire a high
solute concentration. Upon reaching the photochemically
oxidized shallow subsurface, the fluids would develop a low
pH and high concentration of sulfates [Burns, 1993; Burns
and Fisher, 1993; Hurowitz et al., 2010]. Oxidation of the
near‐surface materials and fluids would have been encouraged by physical mixing of the playa deposits within dunes
which would have enabled atmospheric and photolytic oxidation to penetrate to greater depths, and fluctuations in
the height of the water table which would have allowed
alternating oxidation of the near‐surface sediments and
fluid‐sediment interaction. Laboratory experiments and
geochemical models in which basaltic weathering fluids are
evaporated under oxidizing conditions successfully predict
the observed mineralogy at Meridiani, and further show a
decreasing pH with evaporitic concentration [Tosca et al.,
2005]. The surface geochemistry in regions of groundwater
upwelling would be dominated by the chemistry of the
upwelling brines, with evaporation leading to localized precipitation of sulfate minerals, including jarosite [Burns,
1993].
[33] Previous studies suggested that the geochemical transition from phyllosilicate to sulfate deposition may have been
triggered by an increase in Tharsis outgassing in the Late
Noachian [Bibring et al., 2006]. However, Tharsis outgassing
alone fails to explain the change from broadly distributed
phyllosilicates to locally concentrated sulfates within a few
isolated regions [Bibring et al., 2006; Mustard et al., 2008b].
Layered deposits that appear to be genetically related to the
Meridiani sulfate deposits extend to the eastern edge of
Arabia Terra [Andrews‐Hanna et al., 2010; Zabrusky and
Andrews‐Hanna, 2010], a distance of more than 5000 km
from the edge of Tharsis. More than half of the planet lies
within this distance from Tharsis, and yet sulfate deposits
outside of Tharsis are largely isolated to locations in Meridiani Planum and Arabia Terra.
[34] Tharsis outgassing also fails to explain the timing of
the sulfate formation. Tharsis‐induced tectonism peaked during the Noachian [Anderson et al., 2001]. Tharsis outgassing
would thus have been active throughout the Noachian, much
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of which was dominated by phyllosilicate rather than sulfate
formation. Volcanic outgassing would have also been a significant source of CO2 and H2O to the early Martian atmosphere [Phillips et al., 2001], and the release of sulfur and
other greenhouse gasses has been invoked to explain the
warm and wet conditions on early Mars during the period of
phyllosilicate formation [Halevy et al., 2007; Johnson et al.,
2008]. Tharsis outgassing cannot simultaneously explain
both the warm and wet conditions during the era of phyllosilicate alteration and valley network formation, and the later
transition to arid conditions and sulfate deposition. While
Tharsis outgassing was likely an important contributor of
sulfur to the atmosphere and hydrosphere of early Mars, we
suggest that the transition between phyllosilicate alteration
and sulfate deposition in the Late Noachian and the specific
distribution of sulfates are the results of a hydrologic and
climatic transition driven by a decrease in the hydrologically
available global water inventory relative to the storage
capacity of the crustal aquifers.
4.5. Water Loss Mechanisms and the Hydrological
Transition
[35] The wet to arid hydrological transition corresponds to
a change in the initial mean water table depth from approximately 200 to 600 m, resulting in a transition from a saturated
near‐surface throughout the low latitudes to a focusing of
hydrological activity at Meridiani Planum and Arabia Terra.
This corresponds to a net loss of a global equivalent layer
(GEL) of ∼60 m of water, assuming a mean near‐surface
porosity of 0.15 [Hanna and Phillips, 2005]. Such a change
could be brought about by either decreasing the total water
inventory, or by increasing the storage capacity of the crustal
aquifers or other storage reservoirs.
[36] Evidence for a decrease in the total water inventory of
Mars is found in the elevated D/H ratios in the Martian
atmosphere, interpreted as resulting from the erosion of
atmospheric water by solar wind sputtering [Yung et al.,
1988; Donahue, 1995; Jakosky and Jones, 1997]. Impact
erosion of the atmosphere would have also contributed significantly to the loss of water [Melosh and Vickery, 1989].
The combined effects of solar wind sputtering and impact
erosion may have eroded 95–99% of the early Martian
atmosphere since the onset of the geologic record, with the
vast majority occurring during the early Noachian [Brain and
Jakosky, 1998]. Lesser amounts of atmospheric water loss are
possible if the elevated deuterium concentration in Martian
water were due in part to a greater cometary contribution to
the primordial water inventory of Mars relative to the Earth
[Lunine et al., 2003]. In section 5.1, we show that the loss
rates due to solar wind stripping and impact erosion combined
with estimates of the present‐day water inventory predict the
loss of a GEL of ∼80 m of water during the late Noachian and
early Hesperian epochs, which is sufficient to trigger a wet
to arid hydrological transition.
[37] Alternatively, a number of mechanisms exist for
increasing the total storage capacity of Martian water
reservoirs, which would be accompanied by a drop in the
water table and could trigger a wet‐arid hydrological transition. The storage capacity of the aquifers could be increased
by an increase in porosity due to continued impact‐induced
brecciation of the surface. An increase in the porosity of the
top 5 km by only 0.01 could trigger a wet to arid hydrological
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transition. Water can also be removed from the active
hydrological system by sequestration within the polar caps
and the high‐latitude ground ice as the climate cooled. It is
not known whether the polar caps on early Mars under different pressure, temperature, and humidity conditions would
have been larger or smaller than their present size, but they
nevertheless constitute a possible reservoir for increased
storage of water as ice. The present‐day south polar cap
and layered deposits are volumetrically dominated by water
ice [Zuber et al., 2007] and are estimated to contain a GEL
of 11 m of water [Plaut et al., 2007]. Assuming flat basal
topography beneath the north polar layered deposits [Phillips
et al., 2008], this repository contains a GEL of ∼5 m of
water. The near‐surface high‐latitude regions today contain
also a significant fraction of ground ice [Feldman et al.,
2004]. The top 1 km of the crust poleward of 60°S latitude
could contain a GEL of ∼6.5 m of water, assuming a mean
fractional ice content of 10%. Freezing or melting of north
polar permafrost will not significantly change the total water
storage, since its location at low elevation would result in this
region being water saturated under warm conditions, and the
replacement of water with ice does not significantly change
the total storage. These cryosphere reservoirs add up to the
potential to change the total storage of water as ice by a 22 m
GEL of water, which falls short of the volume required to
induce a wet to arid transition but may have played a prominent role in short‐term forcing of the hydrological cycle as
discussed in section 5.
[38] While it is difficult to place firm bounds on the loss
of water or increase in storage capacity that can be attributed
to any one mechanism, it is clear that the sum total of all
mechanisms is more than sufficient to account for the 60 m
GEL loss or storage of water necessary to induce the wet‐arid
hydrological transition in the late Noachian. The net loss of
water from Mars over time is supported by both geomorphic
evidence for a wetter surface on early Mars and isotopic
evidence for water loss from the atmosphere. The model
results and interpretations above suggest that the observed
geomorphic and geochemical evidence for the hydrologic and
climatic evolution of Mars can be understood in terms of a
wet to arid hydrological transition in the late Noachian driven
by this net loss of water. While the specific loss history is
poorly constrained, one scenario for the evolving water
inventory and its hydrological ramifications is explored in
detail in section 5.
5. Secular, Episodic, and Periodic Variability
of the Hydrologic Cycle
5.1. Short‐ and Long‐Term Evolution of the Available
Water Inventory
[39] Based on the evidence for both the secular evolution
and shorter‐term perturbations to the hydrologic and climatic
cycle described in section 2, and the sensitivity of the
hydrological cycle to variations in the available water
inventory demonstrated in section 4, we now investigate in
more detail a specific scenario for the temporal evolution of
the hydrology of early Mars. We first generate a simplified
scenario for the evolution of the hydrologically available
water inventory that reproduces qualitatively and at least
semiquantitatively the magnitudes and time scales of the key
processes at work. At present, there are no firm quantitative
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Figure 5. Model scenario for the evolution of the change in
total water inventory (Dw) in contact with the atmosphere
due to solar wind stripping and impact erosion over 4.5 Ga,
calculated by extrapolating the present water inventory backward with time.
constraints on the temporal evolution of the Martian water
inventory, though numerous studies have investigated the
various processes thought to have played a role. We synthesize the results of these studies into a quantitative representation of the evolving water inventory, including secular loss
to solar wind stripping and impact erosion, and the changing
storage of water in the cryosphere in response to orbital
forcing. This scenario is not meant to be a unique predictor of
the changes in water inventory, but rather to serve as a
physically plausible proxy in order to enable an investigation
of the hydrological ramifications.
[40] We first consider the secular evolution of the total
water inventory, driven by loss over time to solar wind
stripping and impact erosion of the atmosphere. These processes both act in a fractional sense on the amount of water in
direct contact with the atmosphere at any one time. In order to
construct a time series of the change in water inventory, we
take the present‐day inventory of an estimated 29 m GEL of
water within the polar caps and near‐surface ground ice from
the section 4 (including the ground ice in the northern high
latitudes) as representative of the volume in contact with the
atmosphere in the present epoch. The present‐day deep water
inventory is unknown, but may be small due to the gradual
desiccation of the subsurface [Grimm and Painter, 2009] and
is not currently in direct communication with the atmosphere.
We then reverse the loss rate to calculate the change in water
inventory with time in the past, assuming that any increase in
the water inventory in the past relative to the present remained
in direct communication with the atmosphere. While this
approach is an oversimplification, it allows the generation
of an evolutionary scenario for the total water inventory that
is both quantitative and consistent with hypothesized loss
mechanisms.
[41] Water is lost to solar wind stripping at a constant
fractional rate of 90% [Brain and Jakosky, 1998]. This
neglects the possible shielding effect of an early actively
generated magnetic field. Demagnetization of the giant
impact basins suggests that the dynamo ceased by the early to
middle Noachian [Mohit and Arkani‐Hamed, 2004; Lillis
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et al., 2008], prior to the formation of much of the observed
population of valley networks. Water is also lost to impact
erosion of the atmosphere at a rate that varies with time in
proportion to the impact flux [Melosh and Vickery, 1989].
Combining the impact and solar wind erosion of the atmosphere, the resulting evolution in the total water inventory
yields a loss of 109.1 m GEL between 4.0 and 3.0 Ga,
representing the middle Noachian through late Hesperian
(Figure 5). During this time, an 80.4 m GEL is lost between
4.0 and 3.7 Ga, representing the middle and late Noachian and
bracketing the time of the hydrologic and climatic transition.
This loss history is consistent with the loss of a ∼60 m GEL
required between the onset of the geologic record and the
late Noachian in order to induce the hydrologic and climatic
transition from wet to arid conditions.
[42] Over shorter time scales, the evolution of the hydrologically available water inventory was likely driven primarily by changes in the storage of water in the polar caps and
high‐latitude permafrost induced by the orbital evolution of
Mars. Short‐term climate change is driven primarily by the
120 kyr obliquity cycle, with the maximum obliquity within
this cycle modulated on a longer 1.2 Myr time scale [Laskar
et al., 2004]. This regular behavior is overprinted by significant shifts in mean obliquity, commonly reaching mean
values as high as 50° or as low as 10°. The evolution of the
orbital parameters of Mars are reasonably well constrained
for the past ∼20 Myr, but over longer time scales the behavior
is chaotic and unpredictable [Laskar et al., 2004]. It is
impossible to uniquely determine the evolution of Mars’
obliquity and other orbital parameters over the past 4.5 Ga.
However, the fundamental nature of the periodic and chaotic
variations in the orbital parameters is similar in multiple
simulations of the past 250 Ma. The 120 kyr obliquity cycle
and its 1.2 Myr modulation persist through all simulations,
while chaotic shifts in mean obliquity lead to periods of
sustained high and low obliquity of variable duration. There
is no indication that this general behavior would have changed
appreciably since the orbits of the giant planets were stabilized after the Late Heavy Bombardment at 3.9 Ga [Gomes
et al., 2005]. We adopt a single 250 Myr orbital simulation
from Laskar et al. [2004] (solution 301003BIN_A.P001,
available on the Web at http://www.imcce.fr/Equipes/ASD/
insola/mars/mars.html), as representative of the periodic and
chaotic changes in orbital parameters. This simulation
includes long periods of stability at both high and low
obliquity, as well significant shorter obliquity excursions.
[43] We next relate this orbital evolution to the changing
storage of water as ice in the cryosphere. Conditions on early
Mars may have been similar to those on the Earth today,
in which the total volume of water stored as ice in the polar
caps, glaciers, and permafrost of the Earth is highly sensitive
to changes in climate driven by orbital Milankovitch cycles
[Crowley and Kim, 1994; Huybers and Wunsch, 2005]. The
specific climatic conditions that prevailed on Mars in the
Noachian and Hesperian are unknown, as are the conditions
that governed the stability of the polar caps and cryosphere at
that time. Nevertheless, the stability of the present‐day polar
caps and cryosphere have been shown to be highly sensitive
to changes in the latitudinal distribution of insolation driven
by the orbital evolution [Mellon and Jakosky, 1995; Levrard
et al., 2007]. The stability of the north polar layered deposits
has been parameterized as a function of the orbital parameters
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Figure 6. (a) Obliquity, (b) insolation at southern summer
solstice (iss), and (c) change in volume of water stored in the
polar caps and subsurface cryosphere (Dw) over 250 Myr
based on the orbital evolution model P001 of Laskar et al.
[2004] and the parameterization of the stability of the north
polar layered deposits of Levrard et al. [2007].
for present‐day Mars [Levrard et al., 2007]. The polar ice loss
is primarily a function of the insolation at the summer solstice, while the polar ice gain is primarily a function of
obliquity [Levrard et al., 2007]. The south polar layered
deposits appear to be more stable than the north, though the
specific dependence of the cap stability on the orbital evolution has not been investigated. As a first‐order approximation, we use the stability conditions for the present‐day
north polar layered deposits as a function of obliquity and
summer solstice insolation as a proxy for the stability of both
polar caps and the permafrost on early Mars. While this
approach is not strictly valid for the higher atmospheric
pressure, temperature, and humidity likely on early Mars, it
provides us with a simple means of representing the sensitivity of the storage of ice within the cryosphere on the
orbital evolution and must suffice until future climate models
provide more firm constraints on the early cryosphere and
hydrosphere evolution.
[44] This approach neglects the possibility of the
redeposition of ice in the low latitudes. Under the present cold
climatic conditions, ground ice becomes more stable in the
low to middle latitudes during periods of high obliquity while
liquid water is everywhere unstable, and obliquity cycles
primarily redistribute ice between the polar caps and ground
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ice [Mellon and Jakosky, 1995]. Under warmer climate
conditions, we expect a significant fraction of the ice released
from the poles at high obliquity to enter into the hydrological
cycle, as occurs on Earth. Periods of high obliquity decrease
the temperature contrast between the equator and poles. If the
global mean temperature was at or above the freezing point,
then polar warming at high obliquity would likely lead to
release of water to the hydrosphere rather than ice deposition
in the low to middle latitudes, analogous to the sea level rise
during interglacial periods triggered in part by high obliquity
on the Earth [Crowley and Kim, 1994]. The polar cap stability
models implemented here [Levrard et al., 2007] are specifically for the case of exchange with the atmosphere, and do not
include the effects of polar basal melting [Clifford, 1993;
Clifford and Parker, 2001]. Polar basal melting associated
with changes in obliquity would lead to short‐lived groundwater mounds beneath the polar caps following excursions to
high obliquity and would diminish the hydrological response
to short‐term forcing, but would not alter the long‐term
behavior of the system.
[45] The parameterization of polar cap stability from
Levrard et al. [2007] is applied to the orbital evolution scenario from Laskar et al. [2004] to construct a representative
250 Myr time series of the obliquity, insolation at summer
solstice, and the resulting change in water storage in the polar
caps and cryosphere (Dw; Figure 6). The maximum polar cap
volumes were set equal to those of the present‐day caps,
though larger polar caps may have been possible. The
imposition of this upper bound is necessary to keep the polar
caps from reaching arbitrarily large sizes at low obliquity,
which would likely be unstable to outward viscous flow. This
250 Myr time series is then reflected and repeated to generate
a 1 Gyr continuous time series. This short‐term periodic and
episodic forcing is superimposed over the secular loss of
water due to solar wind stripping and impact erosion of the
atmosphere for the period of interest between 4.0 and 3.0 Ga
(model P001a; Figure 7a), spanning the middle Noachian
through the late Hesperian epochs [Hartmann and Neukum,
2001]. A second representation of the variation in the available water inventory was generated by shifting the time series
representing the orbital forcing of the cryosphere storage by
250 Myr relative to the secular loss time series (model P001b;
Figure 8a), such that the shifts in mean obliquity and resulting
changes in cryosphere storage occur at different times relative
to the evolution of the total water inventory.
[46] The resulting representations of the change in hydrologically available water in Figures 7a and 8a provide reasonable representations of the possible effects of atmospheric
loss and orbital forcing on early Mars. We emphasize that
these are not intended to be unique solutions of the evolution
of the water inventory, as such is beyond the reach of our
current understanding of early Mars. Rather, these are
intended only to be plausible and self‐consistent scenarios to
enable a hydrological study of the implications of the time
evolution of the water inventory.
5.2. Modeling the Noachian‐Hesperian Hydrological
Evolution
[47] The secular evolution and short‐term perturbations to
the hydrologically available water inventory generated above
were input into the hydrological model as forcing functions.
As water is cycled between the subsurface and atmosphere in
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Figure 7. Results from a 1 Gyr simulation representing the effects of orbital forcing and secular loss of
water between 4.0 and 3.0 Ga for model P001a. (a) Change in hydrologically available total water inventory
(Dw), (b) the evolving precipitation flux in comparison with the mean groundwater upwelling rate within
Meridiani Planum and all of Arabia Terra, and (c) the mean depth to the water table within Meridiani Planum
and Arabia Terra.
the models through groundwater evaporation and precipitation, some fraction is added or removed in each time step to
match the requirements of the applied changes in total water
inventory due to the combined effects of atmospheric loss and
either cold trapping of water in the cryosphere or release of
water to the atmosphere and hydrosphere from melting or
sublimation of the cryosphere ice.
[48] In addition to the relative change in total water
inventory, it is necessary to establish a reference point at
which the total water inventory can be constrained. The initial
mean water table depth at the beginning of the simulation
(representing Mars at 4.0 Ga) is set such that Meridiani Planum and Arabia Terra are regions of localized groundwater
upwelling at the Noachian‐Hesperian boundary at ∼3.7 Ga.
This is achieved for an initial mean water table depth of
approximately 200 m. Substantially greater or lesser values
for the initial mean water table depth would result in models
in which groundwater does not ever reach the surface in
Meridiani, or the low‐latitude surfaces are saturated throughout the simulation, respectively. The hydrological model is
run for 100 Myr until equilibrium is achieved, and then the
1 Gyr time series representing the hydrological forcing function is applied to perturb the hydrologically available water
inventory beginning at a model time of 0 Myr.
[49] The models include a simple representation of the
different behavior of the system in the wet and dry hydrological regimes. In the wet regime, runoff and ponding of
water is likely, and substantial evaporite deposits are not
expected to form in the comparatively wet climate. In the arid
regime, low precipitation rates and groundwater fluxes are
expected to lead to evaporation of the groundwater upon
reaching the surface, leading to the formation of evaporites
and evaporite‐cemented sediments. While the results of
section 4.2 reveal the transition between the wet and arid
hydrological regimes to be gradational rather than discrete, in
order to simply represent these two behaviors in this model
we specify a threshold precipitation rate to define a discrete
boundary between the two regimes. Based on the results of
the equilibrium models (Figures 3 and 4), this threshold
precipitation flux is set to 2 × 10−4 m/year, which corresponds
to a focusing of hydrological activity at locations of observed
sulfate deposits. Higher precipitation rates are assumed to
result in fluvial activity and alteration of the surface to
phyllosilicates, and lower precipitation rates are assumed
to result in arid conditions with evaporite formation at loci
of groundwater upwelling. While this threshold precipitation
rate is lower than would typically be expected to result in
significant runoff and fluvial modification of the surface, the
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Figure 8. Results from a 1 Gyr simulation representing the effects of orbital forcing and secular loss of
water between 4.0 and 3.0 Ga for model P001b, with the orbital component of the hydrological forcing
shifted by 500 Myr relative to model P001a in Figure 6. (a) Change in hydrologically available total
water inventory (Dw), (b) the evolving precipitation flux in comparison with the mean groundwater
upwelling rate within Meridiani Planum and all of Arabia Terra, and (c) the mean depth to the water table
within Meridiani Planum and Arabia Terra. The vertical dashed lines mark the times at which the results are
presented in map view in Figure 9.
model precipitation rates are lower bounds and only reflect
that portion of the precipitation that leads to direct infiltration,
as discussed in section 4.2.
[50] The hydrological model was run for 1 Gyr using the
two scenarios for the changing water inventory, representing
the time period between 4.0 and 3.0 Ga (Figures 7a and 8a).
Evaporite‐mediated deposition was allowed only at precipitation rates below the specified threshold. The time evolution
of the precipitation rate and growth of sedimentary deposits
were tracked (Figures 7b and 8b). The mean rate of groundwater upwelling to the surface and the mean depth to the
water table as a function of time were calculated both over
Meridiani Planum and the associated etched terrain [Hynek,
2004], as well as over the entire Arabia Terra region, defined
here as the region bounded by the 0 and −2500 m elevation
contours centered on 0°N 0°E (Figures 7b, 7c, 8b, and 8c).
Global maps of the depth to the water table, deposit thickness, and groundwater upwelling rate are presented at key
times for model P001b (Figure 9).
5.3. Model Results: Long‐Term Evolution
[51] Mars starts out in the wet hydrological regime in the
beginning of the simulations, with a saturated near surface
and maximum precipitation rates of 60 cm/year (model
P001a; Figure 7b) and 1 cm/yr (model P001b; Figure 8b). The
difference in early precipitation rates is due to the fact that
model P001a transitions into a high obliquity phase with the
resulting release of water from the cryosphere into the
hydrosphere early in the simulation. The water table is close
to the surface throughout the low to middle latitudes early in
these simulations (e.g., model P001b at 50 Myr; Figure 9a),
but evaporite formation and induration of sedimentary
deposits is inhibited by the wet climate conditions. In both
models, fluvial erosion and phyllosilicate formation would be
expected to dominate throughout the first 100 to 200 Myr of
model time. As water is lost due to solar wind stripping and
impact erosion of the atmosphere, the water table drops
deeper beneath the surface in many regions of the planet, and
the precipitation rate drops (Figures 7b, 7c, 8b, and 8c). The
secular loss of water alone would drive a permanent transition from wet to arid conditions. However, the superimposed effects of orbital forcing result in a more complicated
transition.
[52] A shift from either wet to arid or arid to wet conditions
can be brought about by a perturbation to the hydrologically
available water inventory in response to a shift in mean
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Figure 9. (left) Predicted depth to the water table, (middle) deposit thickness, and (right) groundwater
upwelling rate during simulation P001b at 50, 240, 350, 390, and 600 Myr (see Figure 8).
obliquity. Mars is most susceptible to these chaotic obliquity
shifts when the total water inventory and precipitation rate are
close to the threshold level. In model P001a (Figure 7), after
∼110 Myr the total water inventory has dropped to the point at
which changes in the mean obliquity can drive temporary
shifts from the wet to arid hydrological regime. In this simulation, three short excursions from high to low obliquity
result in wet‐arid transitions, each lasting 2–4 Myr. These
brief excursions in mean obliquity would have caused alternating periods of valley network erosion and sulfate deposition, possibly leading to fluvial erosion of the sulfate deposits.
At 137 Myr elapsed model time, model P001a transitions into
a state of low mean obliquity that persists for 226 Myr,
forcing an abrupt and stable hydrological transition from wet
to arid conditions. At this point the increased aridity would
allow significant evaporite deposits to form in loci of
groundwater upwelling. The subsequent transition to high
obliquity at 363 Myr in this model is insufficient to cause
a shift back to the wet regime as a result of the cumulative
effect of the continued secular loss of water to impact and
solar wind erosion of the atmosphere.
[53] In model P001b (Figure 8) a shift from low to high
obliquity at 113 Myr reinforces the prevailing wet hydrological regime, causing an increase in the precipitation rate
and a rise of the water table. The hydrological response to this
release of water into the hydrosphere in this scenario provides
a possible explanation for the evidence that valley network
formation may have peaked in the Late Noachian [Howard
et al., 2005; Irwin et al., 2005; Fassett and Head, 2008].
Later in this simulation, short excursions to low obliquity
are sufficient to drive brief episodes of arid conditions
beginning at 140 Myr model time. These arid regime episodes
become more frequent as the total water inventory declines,
until the model transitions permanently to the arid regime at
212 Myr elapsed model time.
[54] Sedimentary deposition commences when the models transition into the arid regime, leading to the growth
of evaporite‐cemented sedimentary deposits in Meridiani
Planum and much of Arabia Terra (e.g., model P001b at
240 Myr; Figures 9d–9f). The mean groundwater upwelling
rates in Meridiani Planum are higher than in the broader
Arabia Terra region, since much of the surface of Arabia
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Terra is unsaturated. Similarly, the mean depth to the water
table over Meridiani is less than that over all of Arabia Terra.
Shortly after the transition to arid regime conditions, the water
table gradually rises to shallower depths beneath the surface
in Meridiani and Arabia Terra despite the continued secular
loss of water. This is observed in model P001a at 137–
175 Myr (Figure 7c), and in model P001b at 212–248 Myr
(Figure 8c). This rise of the water table is a result of the
gradual sedimentary infilling of the deepest craters and other
depressions within Meridiani and Arabia Terra, which allows
the rise of the water table throughout the region [Andrews‐
Hanna et al., 2010]. However, the water table later resumes
its drop to deeper levels below the surface as a result of the
declining total water inventory.
[55] The sedimentary deposition and diagenetic alteration
of the deposits in Meridiani Planum and Arabia Terra are
related to the rate of groundwater upwelling and the depth to
the water table. Active upwelling of deep fluids to the surface
under arid regime conditions will result in the formation of
evaporites and evaporite cemented sedimentary deposits.
Fluctuations in the depth to the water table may result in
diagenesis of the deposits at depth, with periods of stability at
a particular water table depth leading to formation of a discrete diagenetic front (e.g., the Whatanga contact observed by
Opportunity [Grotzinger et al., 2005]). A significant drop of
the water table would leave the deposits exposed to aeolian
erosion and deflation, possibly explaining the observed
unconformities in HiRISE images (Figure 2) as well as in
the large valley within the Meridiani deposits [Wiseman
et al., 2010]. Alternatively, a significant increase in the
precipitation rate during the period of sulfate deposition may
result in fluvial erosion of the deposits.
[56] Transitions to high mean obliquity and the associated
release of water from the cryosphere result in an abrupt
shallowing of the water table and increase in the precipitation
rate during arid regime conditions (e.g., at ∼370 Myr in model
P001a; Figure 7c). Similarly, transitions to low mean obliquity result in an abrupt deepening of the water table. Short
excursions in mean obliquity drive oscillations in the water
table height with amplitudes of 100–200 m. These chaotic
changes in mean obliquity generate disequilibrium hydrologic responses that can be observed in the mean depth to the
water table if the obliquity stabilizes at its new value after the
transition. Each decrease in mean obliquity results in an
abrupt increase in the depth to the water table as more water is
placed into storage in the cryosphere. This is followed by a
gradual shallowing of the water table to an intermediate depth
over the next ∼50 Myr (e.g., from 630 to 680 Myr in model
P001a; Figure 7c) as the planet comes into hydrological
equilibrium with the lower available water inventory. This
provides a measure of the mean hydrological response time of
the planet of ∼50 Myr. While the hydrological cycle responds
to forcing on much shorter time scales than this, as will be
seen in section 5.4, it only comes into equilibrium with an
applied forcing on this time scale.
[57] The loss of water and gradual drop in the water table
eventually brings to an end the groundwater upwelling and
sedimentary deposition in Meridiani and Arabia Terra after
190 Myr and 200 Myr, respectively, in model P001a.
In model P001b, hydrological activity in Meridiani and
Arabia Terra ceases after 180 and 400 Myr, respectively,
following a general pattern of a northward shift in the loci of
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hydrological activity in this region (Figures 9d–9f and
9g–9i). In both models, Arabia Terra experiences renewed
activity during periods of high obliquity, as deposition in the
bordering northern lowlands allows the water table to rise
further toward the surface (e.g., after ∼500 Myr in model
P001a, and 750 Myr in model P001b). However, the water
table remains below the surface throughout Meridiani
Planum in the latter parts of both simulations.
[58] Even during periods of low obliquity late in the simulation, hydrological activity in the Valles Marineris canyons
persists, due to their great depth relative to the surroundings
(e.g., model P001b at 600 Myr; Figures 9m–9o). This is
consistent with the generally younger age of the interior
layered deposits within the canyons and chaos regions
[Glotch and Christensen, 2005; Flahaut et al., 2010] and their
greater thickness in comparison with the Meridiani deposits.
Sedimentary deposition in the models is allowed to continue throughout the simulations wherever there is a flux of
groundwater to the surface. In actuality, a transition to cold
climate conditions sometime during the Hesperian likely
brought an end to both the precipitation‐induced aquifer
recharge and the groundwater upwelling to the surface, setting the stage for outflow channel formation during the
Hesperian. However, the presence of sulfate deposits that lie
stratigraphically above chaos regions at the sources of outflow channels [Glotch and Christensen, 2005] suggests that
this transition to cold temperatures was not a simple abrupt
and irreversible change.
5.4. Model Results: Short‐Term Periodicity
[59] In both simulations, the model behavior over 10–
100 Myr time scales is characterized by abrupt changes in the
depth to the water table and the rate of groundwater upwelling, driven by perturbations to the hydrologically available
total water inventory in response to chaotic changes in the
mean obliquity. Over shorter time scales, the hydrology is
dominated by the effects of the 120 kyr obliquity cycle and
the modulation of the maximum obliquity with a 1.2 Myr
period. This short‐term periodicity is observed in both the
precipitation rate and the groundwater flux at Meridiani
(Figure 10). This hydrological periodicity could be expressed
in the sedimentary record through modulation of the layer
thickness or strength due to either variations in the precipitation rate, which would affect the supply of clastic material
incorporated into the deposits, or variations in the groundwater flux, which would affect the supply of solutes and the
degree of induration of the deposits.
[60] The response of the aquifers to this short‐term periodicity is limited by the hydrologic diffusion time scale for
subsurface flow. The time scale for regional hydrological
equilibration observed in the response to changes in mean
obliquity in section 5.3 was ∼50 Myr. On a regional scale,
much of the groundwater flows at depths of up to a km or
more for distances of 100s to 1000s of km before reaching
the surface in Meridiani Planum and Arabia Terra. For the
aquifer model of Hanna and Phillips [2005], the hydraulic
diffusion time scale for subsurface flow with a water table at
1 km depth over horizontal length scales of 100–1000 km is
∼0.3–30 Myr. For a water table located 100 m below the
surface over the same range of length scales, the hydraulic
diffusion time scale ranges from 0.04 to 4 Myr. This simple
scaling analysis would suggest that the large‐scale hydrology
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Figure 10. Plots of the hydrological model results over 10 Myr intervals showing (left) regular periodic
behavior due to the obliquity cycle and (middle and right) a bundled periodicity due to the modulation of
the obliquity cycle on a 1.2 Myr time scale. (a–c) The obliquity models assumed to be driving the short‐term
climatic forcing and (d–f) result in a perturbation to the hydrologically available water inventory. (g–i) The
perturbation to the water inventory results in temporal variations in the equatorial precipitation rates, and the
mean rates of groundwater upwelling and evaporation in Meridiani Planum and Arabia Terra.
in regions with a particularly deep water table should be
unperturbed by the 120 kyr obliquity cycle, while regions
with a shallow water table will be more strongly influenced.
However, while the water table remains at depth below
the surface for great distances, the aquifers experience
precipitation‐induced recharge throughout the low latitudes,
including Arabia Terra. As a result, the groundwater
upwelling at Meridiani is sourced from precipitation‐induced
recharge from all points between Meridiani and the hydraulic
divide between Arabia Terra and the Hellas impact basin.
Due to the shallow aquifers in the vicinity of the deposits and
the range of flow path lengths that contribute to the groundwater upwelling, even the short 120 kyr obliquity cycle is
strongly expressed in the rate of groundwater upwelling.
[61] During some portions of the orbitally driven hydrological evolution, a relatively constant 120 kyr periodicity
is observed in both the precipitation and groundwater
upwelling rates at Meridiani (Figure 10g). During other periods, the 1.2 Myr modulation of the obliquity cycle is strongly
expressed in the hydrologic response (Figures 10h–10i).
These two hydrological behaviors provide a possible explanation for the observation that some sedimentary deposits
show a uniform layering periodicity at HiRISE scale while
others show layers bundled into packets of 10 [Lewis et al.,
2008, 2010]. The bundled layer packets have been corre-
lated to the 120 kyr obliquity cycle and its 1.2 Myr modulation [Lewis et al., 2008], suggesting that the individual
layers in similar deposits with uniform periodicity may reflect
forcing by the 120 kyr obliquity cycle without substantial
1.2 Myr modulation.
[62] Within the framework of this model, the pronounced
1.2 Myr modulation of the hydrologic response arises primarily during periods of stable low (Figure 10b) or high
(Figure 10c) mean obliquity. This appears to be in part an
effect of the imposition of a maximum polar cap size at
low obliquity, and a minimum (zero) polar cap size at high
obliquity. In times of sustained low mean obliquity at values
of ∼20° (Figure 10b), the polar caps reach their maximum
size, and only experience significant loss during the periods
of highest obliquity over the 1.2 Myr modulation of the
obliquity cycle. Similarly, in times of sustained high mean
obliquity at values of ∼35° (Figure 10c), the polar caps
disappear entirely and only experience regrowth at periods
of lowest obliquity during the 1.2 Myr modulation of the
obliquity cycle. The more pronounced 1.2 Myr modulation of
the precipitation and evaporation rates in Figure 10i is also
attributable to the more pronounced 1.2 Myr modulation of
the obliquity cycle at this time (Figure 10c).
[63] In actuality, the hydrologic and climatic cycle was
likely much more complicated than the simplified represen-
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tation in this model, leaving open the possibility of other
mechanisms for modulating the layer periodicity. Other factors related to climatic variability, including variations in
the supply of aeolian dust and sand [Lewis et al., 2010], may
have contributed to the observed layering periodicity as well.
Nevertheless, these model results provide one possible
mechanism by which perturbations to the hydrologically
available water inventory due to orbital forcing can explain
the observed periodicity and bundling of layers in sedimentary deposits on Mars. Significantly, this work demonstrates
that hydrologic and climatic forcing over 120 kyr time scales
would be expected to significantly affect the groundwater
hydrology at the locations of sedimentary deposition in
Meridiani Planum and Arabia Terra as suggested by the
sedimentary record.
6. Conclusions and Discussion
[64] There is abundant evidence for the presence of water
on the surface of Mars early in the planet’s history, and for
the importance of groundwater in the hydrological cycle.
We have used groundwater hydrology models to provide a
theoretical framework in which to understand and interpret
the observed record of water‐related activity throughout the
Noachian and early Hesperian epochs, and to integrate the
different lines of evidence into a coherent picture of the global
hydrologic and climatic evolution. The model results provide
a mechanism for causing the inferred geomorphic and geochemical transition in the late Noachian to early Hesperian,
and shed light on the factors controlling the distribution
and timing of the formation of sulfates, phyllosilicates, and
valley networks. While there is significant uncertainty in
many of the model inputs, the resulting hydrological behavior
is driven primarily by the large‐scale topography of Mars and
the effects of short‐ and long‐term changes in the hydrologically available water inventory. The model results are not
intended to provide a specific prediction of the actual time
evolution of the hydrologic cycle, but rather to provide a
qualitative and semiquantitative representation of a possible
hydrologic and climatic history that is built on our current
understanding of the key processes involved. In the context of
this work, we suggest the following scenario for the hydrologic and climatic evolution of Mars during the Noachian
and early Hesperian.
[65] Early Mars possessed an active hydrological cycle,
with deep groundwater flow driven by a combination of low‐
latitude precipitation, groundwater evaporation, and the long
wavelength topography. The nature of the hydrological cycle
at any given time was determined primarily by the hydrologically available total water inventory, which underwent
long‐ and short‐term variations driven by the secular loss
of water to impact and solar wind stripping, and periodic
and episodic changes in the volume of water stored as ice in
the polar caps and subsurface cryosphere induced by orbital
forcing.
[66] During the Noachian, a greater total water inventory
fueled a more vigorous hydrological cycle in what we term
the wet hydrological regime. Mean precipitation rates were
controlled by the climatic redistribution of water, with
significant surface and shallow subsurface reservoirs of water
in direct communication with the atmosphere. The high
precipitation rates and shallow water table throughout the low
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latitudes resulted in phyllosilicate weathering reactions in the
near surface and erosion rates comparable to those in arid and
semiarid regions of the Earth today. Wet conditions favored
surface runoff and ponding over evaporite deposition, though
these were not explicitly included in the models. The first‐
order correlation of the distribution of valley networks with
regions predicted to be underlain by a shallow water table
suggests that groundwater may have played a role in controlling valley network formation.
[67] The steady loss of water to impact and solar wind
erosion triggered a shift in the hydrological regime to arid
conditions in the late Noachian. This transition required a
loss of a GEL of only ∼60 m of water, which can be easily
accounted for by estimated atmospheric loss rates on early
Mars. In the arid hydrological regime, the water table
remained deep beneath the surface throughout much of the
low latitudes, with groundwater only intersecting the surface
in a few isolated locations, including the large impact basins
and the northern lowlands. In this arid hydrological regime,
the precipitation rate was dictated by the rate of deep
groundwater flow, leading to a dramatic drop in precipitation and erosion rates.
[68] At some point intermediate between the extreme wet
and arid hydrological regimes, Meridiani Planum and the
surrounding Arabia Terra region emerged as one of the few
regions of presently exposed Noachian‐aged crust in which
the water table reached the surface. The long path lengths of
groundwater flow allowed ample time for equilibration of the
fluids with the basaltic aquifers, resulting in high salinity,
sulfate‐rich fluids that became acidic upon interacting with
the oxidized surface environment [Hurowitz et al., 2010]. The
high ratio of deeply sourced groundwater to meteoric water in
regions of groundwater upwelling within a few key regions
resulted in evaporite deposition and cementation of aeolian
sediments. A steady flux of groundwater to the surface in
Arabia Terra sustained regional playa development in the
absence of a topographic basin, driven by the combination of
the general low topography of Arabia Terra relative to the
highlands and the distinct breaks in slope separating Arabia
Terra from the southern highlands and northern lowlands
[Andrews‐Hanna et al., 2007]. As Mars transitioned into
this arid hydrological regime in the Late Noachian, higher‐
resolution hydrological models predict a sequence in which
large craters within Arabia Terra first filled with evaporites
and evaporite‐cemented sediments, which allowed the regional
water table to rise toward the surface resulting in broad regions
of groundwater upwelling to fuel the playa formation at
Meridiani [Andrews‐Hanna et al., 2010]. Continued loss
of water eventually terminated hydrological activity within
Arabia Terra. Hydrological activity and groundwater‐
mediated cementation of sedimentary deposits would have
continued in Valles Marineris long after the water table had
dropped below the surface in Arabia Terra.
[69] Superimposed over this secular evolution, episodic
perturbations to the hydrologically available total water
inventory would have resulted from the change in storage of
water as ice in the polar caps and high‐latitude ground ice in
response to chaotic changes in mean obliquity [Laskar et al.,
2004; Levrard et al., 2007]. This change in available water
volume would have resulted in episodic changes in the precipitation rates, water table depths, and flux of groundwater to
the surface. These episodic changes may explain the terminal
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epoch of enhanced fluvial activity in the Late Noachian
[Irwin et al., 2005] and the rapid cessation of valley network
activity at the end of the Noachian [Fassett and Head, 2008].
In this scenario, valley network activity in the Middle Noachian was waning due to the secular loss of water with time,
under low obliquity conditions with increased storage of
water as ice in the cryosphere. A shift to high obliquity in the
Late Noachian then triggered melting and sublimation of
the high‐latitude ice deposits, driving a sudden increase in
the hydrologically available water inventory and a shift to
renewed wet conditions. A shift back to low obliquity could
then have driven an equally abrupt shift to arid conditions and
sulfate formation.
[70] Similar changes in mean obliquity in the Late
Noachian to Early Hesperian would have caused abrupt
changes in the rate of groundwater upwelling and the depth to
the water table in Meridiani Planum and Arabia Terra, leading
to sequences of groundwater upwelling and sedimentation,
water table retreat and erosion, and water table rise causing
diagenetic modification and renewed deposition. Alternately,
an increase in precipitation during the period of sulfate
deposition may have led to fluvial erosion of the forming
sedimentary deposits. Such episodic changes may explain
erosional unconformities expressed in HiRISE images of
layered sedimentary deposits, a large‐scale erosional unconformity in northern Sinus Meridiani [Wiseman et al., 2010],
and evidence for a fluctuating water table and diagenetic
episodes expressed in the Meridiani deposits [Grotzinger
et al., 2005; McLennan et al., 2005].
[71] On shorter time scales, perturbations to the hydrological cycle driven by the 120 kyr obliquity cycle and the
1.2 Myr modulation of that cycle resulted in similar periodicities in the precipitation and groundwater upwelling rates
in Meridiani and Arabia Terra, equivalent to Milankovitch
forcing of the Earth’s climate. Model results at different times
show both a regular 120 kyr periodicity, as well as a 120 kyr
periodicity that is modulated strongly by the 1.2 Myr cycle.
These model behaviors correlate with the observation of both
sedimentary deposits with a uniform periodicity at HiRISE
scale [Lewis et al., 2010], as well as deposits in which the
layer periodicity is bundled into packets of 10 [Lewis et al.,
2008].
[72] Previous work has focused largely on the role of
temperature changes in the climate evolution of Mars. The
scenario developed here highlights instead the importance of
the changing availability of water in both the hydrological
and climatic evolution. The decline in valley network formation, the decrease in erosion rates, and the shift from
phyllosilicate to sulfate deposition can all be explained by a
change in the available water inventory under warm climate
conditions. Changes in the mean surface temperature would
have also exerted a significant control on the rate of water
cycling and the nature of hydrological activity on the surface.
Ultimately, the onset of cold climate conditions in the Late
Hesperian trapped the aquifers beneath a thick cryosphere,
essentially halting the hydrologic cycle. Rather than a simple
transition from warm and wet to cold and dry conditions, we
suggest that Mars underwent a transition from warm and arid,
to warm and hyperarid conditions in the Late Noachian,
followed by a transition to cold and hyperarid conditions in
the Hesperian. Both the geologic record and these model
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results show that these transitions would have been gradual
and complex rather than discrete and simple, but the decoupling of the temperature and moisture conditions is an essential
aspect of the hydrologic and climatic history of Mars.
[73] Acknowledgments. We are grateful to Sandra Wiseman for comments on an early draft of the manuscript and to Tim Glotch for a thorough
and thoughtful review. This work was supported by a grant from the NASA
Mars Data Analysis Program to J.C.A.‐H.
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