While the presence of water on the surface of early Mars is now well known, the volume, distribution, duration, and timing of the liquid water have proven difficult to determine. This study makes use of a distinctive boxwork-rich sedimentary layer on Mount Sharp to map fluid-based cementation from orbital imagery and estimate the minimum volume of water present when this sedimentary interval was formed. The boxwork structures on Mount Sharp are decameter-scale light-toned polygonal ridges that are unique compared to previous observations of Martian fractured terrain because they are parallel-sided ridges with dark central linear depressions. This texture and the sedimentary setting strongly imply that the ridges are early diagenetic features formed in the subsurface phreatic groundwater zone. High-resolution orbital imagery was used to map the volume of light-toned cemented ridges. Based on the cemented volume, a minimum of 5.25 × 105 m3 of cement was deposited within the fractures. Using a brine composition based on observations of other Martian cements and modeling the degree of evaporation, each volume of cement requires 800–6700 pore volumes of water, so the mapped boxwork ridge cements require a minimum of 0.43 km3 of water. This is a significant amount of groundwater that must have been present at the −3620 m level, 1050 m above the current floor of Gale Crater, providing both a new constraint on the possible origins of Mount Sharp and a possible future science target for the Curiosity rover where large volumes of water were present, and early mineralization could have preserved a once-habitable environment.
The past decade of rover and orbiter missions make the influence of liquid water on the surface environment of ancient Mars very clear [Bibring et al., 2006; Malin and Edgett, 2003; McEwen et al., 2007; Murchie et al., 2009; Squyres et al., 2004]. However, the volume of water that was once available on the surface of Mars, and which likely was subsequently lost to space [e.g., Jakosky, 1991], sequestered as ice caps and frozen grounds [e.g., Plaut et al., 2007], incorporated in rocks and minerals [e.g., Mustard et al., 2012], etc., is much debated. An early estimate that attempted to approximate the surface water inventory based on the cumulative volume of sediment excavated by the numerous outflow channels and valley networks on Mars indicated that a minimum 500 m global equivalent layer (GEL) of water must have been present on early Mars [Carr, 1987]. Other estimates have considered the volume of water needed to fill the northern ocean (100 m GEL) [e.g., Head et al., 1999], the amount of water integrated into hydrated minerals (150–1800 m GEL) [Mustard et al., 2012], or the total amount of hydrogen lost to space (95–99% of initial inventory) [e.g., Jakosky, 1991]. These large-scale estimates serve as a starting point for more specific discussion of local phenomena that may have contributed to regional water budgets.
Recent approaches to quantifying past water abundances exploit the increased resolution of recent orbiter missions to focus, for example, on well-defined geomorphic features that permit better constraints and allow modeling of the minimum water volumes required to form those features, including channels [e.g., Baker and Milton, 1974; Burr et al., 2009], basins [e.g., Goldspiel and Squyres, 1991; Jerolmack and Mohrig, 2007], and alluvial fans or deltas [e.g., Di Achille and Hynek, 2010]. These smaller-scale, better constrained, conservative calculations are particularly useful when coupled with mineralogic evidence for water or when they provide context for larger sedimentary or geologic structures [e.g., DiBiase et al., 2013; Jerolmack et al., 2004]. Here we evaluate a site where groundwater flowed through fractured rock and precipitated cements that formed large-scale boxwork structures. These structures enable estimation of water volumes required to form a specific interval of the sedimentary layers that comprise Aeolis Mons (informally known as Mount Sharp) in Gale Crater. The calculation of water volume at this height, many hundreds of meters above the current crater floor, places important constraints on processes occurring during diagenetic modification of Aeolis Mons. Furthermore, this site is characterized by a once water-rich environment that underwent early mineralization, which is known on Earth to help facilitate preservation of once-habitable environments [Grotzinger et al., 2012]. This is therefore recommended as a priority target for the Curiosity rover that successfully landed at the base of Mount Sharp in August 2012.
2 Boxwork Structures: Mapping Observations and Formation Discussion
Mapping of the boxwork texture was accomplished using orbital imagery available through the NASA Planetary Data System. The gridded topographic data set (463 m/pixel) from the Mars Orbiter Laser Altimeter (MOLA) instrument onboard the Mars Global Surveyor [Smith et al., 2003] was used as a reference for correlating orbital data over the fractures. Thermal Emission Imaging System (THEMIS) global IR daytime imagery (232 m/pixel) was correlated to the MOLA reference, Context Camera (CTX) images (6 m/pixel) were correlated to the THEMIS reference, and finally High Resolution Imaging Science Experiment (HiRISE) imagery (25 cm/pixel) was correlated to the CTX images. The fracture networks were identified in the HiRISE images and exposures of the fractures were mapped over approximately 1 km2.
HiRISE digital terrain models (DTMs, 1 m/pixel) provided by the U.S. Geological Survey (USGS) [Mattson et al., 2011] were coregistered where available to determine fracture elevations and stratigraphic relationships. The USGS creates DTMs based on the method described in [Kirk et al., 2008]. The absolute elevations of these DTMs is determined by comparison to MOLA elevations and is on the order of a few tens of meters, but the expected vertical precision (EP) within the DTM can be calculated based on the viewing geometry and resolution of the HiRISE image [Kirk et al., 2008]. For the two DTMs used in this study, the EP, assuming 0.2 pixel matching error, is 7 cm for DTEEC_001488_1750_001752_1750_U02 and 15 cm for DTEEC_019698_1750_019988_1750_U01. Relative elevations within the fractured bed were determined using the HiRISE DTMs, and absolute elevations to compare with the landing site were determined by comparing averaged HiRISE DTM and MOLA gridded values over the mapped fracture networks to the MOLA-based landing site elevation. Fractures were mapped to the limit of resolution of HiRISE 25 cm pixels, so ridges were detectable if they were about 50 cm across, and their heights could be measured to within about 10 cm based on DTM resolution.
Resistant fracture networks (boxwork textures) on Mount Sharp were first identified during Mars Science Laboratory (MSL) landing site assessment [Thomson and Bridges, 2008; Thomson et al., 2011]. Anderson and Bell placed the fractures in the context of geomorphologic units based on HiRISE mapping, noting that the best developed cemented fractures are in a dark-toned layered unit [Anderson and Bell, 2010]. Stratigraphically, these cemented fracture exposures are found in the upper member of the lower formation of the mound strata (Figure 1), which exhibits a spectral signature dominated by sulfates [Milliken et al., 2010] and has a thermal inertia of 260 to 420 J m−2 K−1 s−1/2 [Fergason et al., 2012].
Mapping of the boxwork texture in this study revealed that it is exposed in a stratigraphic interval at an average elevation of −3620 ± 50 m with reference to the geoid. This interval is approximately 880 m above the Bradbury Rise landing site for MSL Curiosity [Parker et al., 2013], and 1050 m above the current base level in Gale Crater [Anderson and Bell, 2010]. The upper limit of boxwork-containing strata is sharply bounded and coincides with a bedding plane. The interval is abruptly overlain by light-toned strata that lack the boxwork texture. In contrast, the lower boundary of boxwork-containing strata is diffuse; boxwork textures gradually dissipate downward through ~40 m of strata as measured in two visible sections in HiRISE DTMs (e.g., Figure 2e). Although exposure is intermittent, the boxwork texture can be traced through more resistant intervals. The boxwork-bearing unit is primarily exposed along cliff faces and in open-ended topographic lows between resistant outcrops of the capping unit (Figure 3). The light-toned ridges that help define well-developed boxwork textures are fairly densely spaced; these ridges make up an average of 35% of the surface area in these sections (ranging from ~20 to 50%, Figures 2a–2d).
The geometry of the fracture network that defines the boxwork texture is delineated by the trends of light-toned ridges, expressed in raised relief. Fractures are mostly straight; however, the longest fractures often show slight curvature (e.g., Figures 2a–2d). The fractures tend to intersect at 90° angles and show a preferred orientation; secondary fractures often end when they intersect primary fractures, although secondary fractures may crosscut primary fractures. The fracture networks are less well organized at deeper stratigraphic levels, showing increasing fracture curvature and greater intersection angles (Figures 2e and 2f). Ridge widths vary between exposures, but they average about 5 m in width. Some light-toned ridges show a dark line running down the center of the ridge, which varies in width up to 1.5 m (Figure 4).
The hollows between elevated boxwork-defining fractures are filled with dark sediment that forms dunes in larger accumulations. These hollows tend to be quasi-circular in plan view and range in diameter from tens of meters to below 1 m, with an average of ~10 m. HiRISE DTMs were used to measure the elevation difference between ridge tops and the middle of the hollows. In many cases the hollows were quite shallow or filled with sand, and there was not a measureable difference in elevation. For 25 profiles where there was a measureable difference in elevation, the average elevation difference was 0.4 m, and the elevation differences typically ranged from 0.1 m to 1.0 m (see Figure 4). The maximum elevation difference found was 3.5 m between the ridge and hollow.
2.3 Boxwork Formation Discussion
The boxwork texture is defined by light-toned, decameter-scale, polygonal ridge networks. The distinctive attribute of these light-toned ridges as compared to other fracture networks observed from orbit on Mars is that they are parallel sided with dark center lines that demarcate either linear central depressions or later cements. This texture strongly implies that the ridges are postdepositional diagenetic features formed in the subsurface when mineral-saturated groundwater flowed through fractured, lithified rock, and cement precipitated within fractures and pores. The parallel walls of the filled fractures are comparable to “isopachous” void-filling cements described commonly in rocks on Earth, where minerals create linings, or coatings, along the margins of voids—be they fractures or intergranular pore spaces. Such isopachous cements form when the void is within the phreatic (water saturated) zone [Tucker, 2009]. This differential cementation makes the fractures more resistant to erosion than the less cemented host rock, and thus, the fracture fills stand as topographically higher rims around the eroded host rock (Figure 1).
Boxwork formation implies a series of postdepositional processes: sediment was lithified and fractured, then saturated fluids percolated through the fracture network and cemented fractures and residual pore spaces in the subsurface, finally the boxwork interval was exhumed, and erosion of the less-indurated rock formed the now exposed polygonal ridges. These observations help to clarify the series of events at this stratigraphic interval on Mount Sharp.
The presence of fractures in a distinct stractigraphic succession, with increasing organization toward the top of that succession, and the absence of similar fractures in overlying strata imply that this stratum was already lithified and then exposed at the surface when the fracturing occurred. Lithification on Mars is not well understood, but in general, compaction by burial decreases porosity of sediments and fluids cement and lithify sediments [Grotzinger and Milliken, 2012]. Conceivably, this boxwork-bearing interval could have been buried, infiltrated by cementing fluids, and converted from sediment to rock, prior to exhumation and fracturing.
The fracture geometry helps narrow down the cause of fracturing. In general, large nontectonic surface fractures originate from contraction, impact processes, loading and unloading of lithospheric stresses, jointing, or fluid pressure [Long et al., 1996]. Jointing and fluid pressure are ruled out by stratigraphic relationships—the fracturing occurred at the surface—and impact processes are ruled out by the systematic organization of fractures (Figure 2). Contraction processes are the most likely to have formed the fractures. The observed orthogonal fracture patterns indicate that the fractures did not form in extended freeze/thaw cycles, which create 120° joints over 106 year time scales [Sletten et al., 2003]. Rather, the fracture geometry is most similar to those formed in noncyclic isotropic or slightly nonisotropic contraction stress fields [Olson et al., 2009], which could be related to sediment desiccation or more intense short-term freezing of lithified sediment [Long et al., 1996].
After lithification and fracturing, the voids in the host rock and fractures are inferred to have been filled with a second generation of cement. The dark line in the center of the ridges helps delineate the parallel-sided or “isopachous” nature of precipitated fracture-filling minerals along the walls of the fractures (e.g., Figure 4). The isopachous morphology of the light-toned elevated ridges is consistent with cementation in the phreatic groundwater zone, where cement formation occurs evenly on all available surfaces.
Approximations of the water volume required to form the ridges depend on the relative proportion of primary host rock and secondary cements forming the raised ridges. Two scenarios are proposed for the relative proportions of primary rock and secondary cement in the light-toned ridges: (1) the ridges are primarily composed of extensive secondary cements filling pores and fractures or (2) the ridges are mostly composed of the host rock, hardened by an early cement within the pore spaces. For either case, there are then two options for the dark central lineations in the ridges; the dark lines could represent residual porosity in the fracture network that was back-filled with wind-blown dark sand or dark-toned cement that completely fills the fracture void. Since the dark central lines are a relatively minor component of the ridges, the primary distinction between these scenarios is whether the light-toned ridges are mostly cement or if they are host rock with pore-occluding cement, perhaps ~30% cement fill. Orbital data cannot discriminate between these two scenarios. However, in either case, the observed boxwork represents a significant volume of cement precipitated from groundwater.
Polygonal ridges, albeit at a much smaller scale, were originally analyzed and described in Wind Caves, South Dakota, where the term “boxwork” structures was coined [Bakalowicz et al., 1987]. Large-scale intersecting filled fracture networks are also present in sulfate-bearing units in Candor Chasma [Okubo and McEwen, 2007] and northeast Syrtis Major [Ehlmann and Mustard, 2012], but the structures within Mount Sharp are distinguished as a dense, parallel-sided network of filled fracture boxwork structures.
Alternative formation hypotheses for the boxwork were considered, but they do not explain the observed fracture characteristics and the dark line in the center of the ridges. Volcanic dikes, for example, could also leave raised ridges posterosion, but these often form in clusters or irregular parallel geometries [Hoek, 1991], and this formation mechanism does not explain the centered dark lines. Some freeze-thaw thermal contraction polygons have dark central lines that demarcate accumulations of loose sediments between ice polygons, and the edges of the ice polygons may accumulate raised shoulders, but the central linear depressions (sand wedges) are generally wider than the raised polygon shoulders [Sletten et al., 2003]; this does not match the measured elevated ridges described here. Finally, boxwork patterns could be similar to large-scale honeycomb or tafoni salt-weathering patterns [Rodriguez-Navarro, 1998], but this is unlikely because of the high ridge width relative to hollows and the albedo variation in the ridges. Based on the fracture geometry, presence of sulfates, ridge characteristics, and local geomorphic features consistent with water-induced bedrock erosion [Anderson and Bell, 2010], the cementation-based hypothesis for boxwork formation is accepted as the most likely interpretation.
3 Fluid Volume Calculation
Quantitative estimates of water flow on Mars are essential to understanding water-rock interactions that inform an understanding of diagenesis and alteration, as well as habitability. At Gale Crater, these estimates would help provide context for Curiosity's mission of exploration. The boxwork structures described here constitute an unusual opportunity where quantitative measurements of a volume of diagenetic cements can be acquired from orbital imagery. Cement volumes can be related to the volume of water required to deposit the cement if a few simple assumptions are made about the ion saturation of the groundwater and the degree of evaporation of the brine. An approach to fluid pore volume calculations based on terrestrial studies of carbonate cementation is employed here to determine the water volume required to deposit the mapped cements [Banner and Hanson, 1990; Bethke, 1985]. Although the cement composition of the ridges was not uniquely determined from orbit, previous observations of evaporite deposits on Mars by the Opportunity rover [Grotzinger et al., 2005; McLennan et al., 2005] have constrained models of Martian brines. Here we use an evaporation model derived from acid-sulfate weathering of synthetic Martian basalts and constrained by the Opportunity rover findings in Meridiani Planum [Tosca et al., 2004, 2008].
Calculation of the minimum volume of water required to form the boxwork layer is based on the following equation:
Where the volume of cement (Vcement) is derived from mapping of orbital imagery and the minerals precipitated are derived based on a reasonable model for evaporation of a Martian brine [Tosca et al., 2008]. The volume of water (Vwater) evaporated per volume of cement precipitated (Vcement), or unitless “pore volume,” is calculated for a given degree of brine evaporation based on the volume of precipitated minerals (mmineral/ρmineral) per volume of water in the initial brine (Vbrine). The results of this calculation are shown in Figure 5, where pore volumes of water (Vwater/Vcement) are plotted against percent of water evaporated (Vwater/Vbrine). The plot seems to approach an asymptote, indicating that more water evaporation does not immediately lead to more cement precipitation, at two times; first, after the precipitation of jarosite, gypsum, copiapite, and bilinite and later, after the precipitation of epsomite, melanterite, anhydrite, and halite [Tosca et al., 2008]. Points near the two asymptotes are selected to describe the range of pore volumes of water that would be required to deposit the cement volume. As indicated in Figure 5, if 99% of the starting brine evaporates, then 6700 pore volumes of water are required, or (based on the last point in the modeled evaporation sequence) if 99.97% of the brine evaporates, 800 pore volumes of water are required to form the cements. These values fit well in the range of pore volumes estimated for terrestrial porosity occlusion scenarios [Banner and Hanson, 1990; Bethke, 1985].
To obtain a minimum estimate for the water volume, the volume of cement is derived from the surface area of ridges within the mapped HiRISE unit, 0.35 km2, multiplied by the thickness of the resistant boxwork layers in the vertical stratigraphy, 5 m, giving a minimum cemented ridge volume of 1.75 × 106 m3. This is a conservative estimate as the boxwork can be traced vertically through up to 40 m of stratigraphy in some locations and may well extend laterally for some distance into the subsurface beneath Mount Sharp, based on continuity of stratigraphy around the entire mound [Milliken et al., 2010]. Two end-member hypotheses were used to calculate the cement volume based on the ridge volume: ridges are 100% cement (cements occur within fractures) or ridges are 30% cement by volume (cements occlude pores within primary rocks adjacent to fractures). The pore-occluding scenario is comparable to several analogous locations on Earth, where rocks in close proximity to a fracture or fault can become strongly cemented, forming diagenetic “halos” [Knipe, 1992; Nelson et al., 1999]. Assuming 99.97% of the brine evaporates, the fracture-filling scenario requires a minimum of 1.4 km3 of water, and the pore-occluding scenario requires 0.42 km3 of water to form the measured ridges (Figure 5).
4 Implications for Mount Sharp Formation
These results are surprising because they imply a significant amount of water once percolated through pores in rocks 1050 m above the current base level for Gale Crater. Several scenarios could deliver the required water, but this analysis must begin with several firm constraints: (1) the boxwork fabric is developed along a bedding plane that emerges from within the stack of layers that define Mt. Sharp, (2) the boxwork fabric terminates abruptly against the overlying stratum, but extends downward for several tens of meters, (3) the isopachous element of the boxwork fabric indicates mineral precipitation in the phreatic zone, below the local groundwater table. These constraints require the boxwork to be an ancient feature, dating back to the time of sediment accumulation; development of the fractures was terminated before the time of deposition of the overlying stratum; mineral precipitation occurred in the fractures in the phreatic zone, below the local groundwater table. This volume of groundwater-based cement indicates that the mound was formed not simply by aeolian sediments cemented in minor wetting events [e.g. Kite et al., 2013] but that instead there was extensive groundwater flow and aqueous processing of sediments involved in mound formation (see Figure 6).
Perhaps the simplest explanation is that the boxwork texture reflects groundwater supplied by atmospheric precipitation either as rain or snow; basin-filling strata accumulating within the lower elevations of Gale crater could have absorbed water in this fashion. The waxing and waning of water derived from seasonal or longer-term climatic cycles could have provided a mechanism for fracture formation [Lachenbruch, 1963]. However, this mechanism cannot account for the mineral abundance required to create the fracture-filling cements. Meteoric waters derived from rainfall or melting snow on the −3620 m bedding plane surface would have been strongly undersaturated and almost certainly would have resulted in dissolution and karst formation given the high solubility of sulfate salts. In contrast, the evidence presented in this paper strongly supports mineral precipitation at this stratigraphic level. There is no evidence for dissolution fabrics which have been detected elsewhere based on remotely sensed image data [Belderson et al., 1978; Manda and Gross, 2006].
A derivative scenario assumes that Mt. Sharp had topographic expression at the time the boxwork-containing strata were accumulating and that precipitation occurred in those highlands. The recharge area underwent leaching to provide the ions required to cause mineral precipitation at some down-gradient distant site—for example, where the boxwork fabric is observed (Figures 6a and 6b). Mt. Sharp has considerable surface area and, depending on the mechanism for its formation, it is conceivable that its summit varied in position; the current summit is about 45 km southeast from the exposed boxwork bedding plane. The stratigraphic interval represented by the fracture network would have been within an aquifer, transporting these fluids down gradient from the undersaturated recharge area. As fluids moved through this aquifer they would have become increasingly saturated due to dissolution of minerals along the way, and eventually reprecipitated these dissolved ions where physical conditions caused local oversaturation. (A variety of soluble minerals might have been involved in dissolution updip, to precipitation downdip, but such considerations go beyond the scope of the current paper.) This mechanism would have required Mt. Sharp to have a higher elevation than the −3620 m bedding plane to drive the flow. It also likely would have required a perched water table (unless the whole mountain was saturated), underlain by an aquiclude, to prevent infiltration and loss of the fluid at sites far removed from the recharge area. It is conceivable that in the Mt. Sharp stratigraphy there exist lithologies heterogeneous enough to provide contrasts in hydraulic conductivity. This scenario allows for the possibility, but does not require, that Gale Crater was mostly filled with sediment (Figure 6b); as long as the aquifer including the boxwork has sufficient topography and area to serve as a recharge area for the groundwater flow, the sediment need not have filled the entire crater (Figure 6a).
A less conservative but viable scenario would invoke filling of Gale Crater with sediments up to the level of the −3620 m bedding plane (Figure 6c) [Malin and Edgett, 2000]. These sediments could have lithified and fractured based on sediment water flux or thermal expansion-contraction. Subsequent burial due to continued sediment accumulation could have resulted in circulation of mineral-saturated groundwater in the fracture network, with later precipitation of minerals in the fractures. Eventually, these strata were exhumed, creating Mt. Sharp and exposing the bedding plane on which the boxwork fabric is developed.
Detailed mapping of the filled fracture network on Mount Sharp indicates that this sedimentary layer most likely represents large-scale boxwork fabrics. The original sedimentary rock was lithified, likely by early cementing fluids during shallow burial. Subsequently this layered unit was exposed at the surface and fractured, and then it was again buried. Circulation of mineral-saturated fluids in the phreatic zone further lithified sediments adjacent to the fractures and also at least partially filled the fractures. This sequence of events requires circulation of mineral-saturated groundwater, supporting mound-formation scenarios in which groundwater could migrate from an undersaturated recharge area to precipitate within the boxwork level at least a kilometer above the current crater floor.
Volumes of diagenetic cements can be measured from orbital imagery, and based on these measurements and an assumed chemistry, the minimum volume of water required to form the cements measured was calculated to be about 0.4 km3. These deposits provide evidence for extensive and relatively rapid cement formation, which could be beneficial to the preservation of organic compounds, 1050 m above the current floor of Gale Crater.
The Mars Science Laboratory Curiosity rover is capable of driving to the boxwork layer [Grotzinger et al., 2012] from its landing site on the floor of Gale Crater and investigating the chemical composition and textures of these deposits from the surface. This site is a possible target for investigation by Curiosity as a location where a series of postdepositional water-based processes are interpreted that left extensive exposed diagenetic cements, which are indicative of possibly favorable conditions for preservation of organic compounds.
This work was supported by the Caltech GPS fellowship fund and a NASA Astrobiology Institute grant NNA13AA90A to JPG. We thank K. Stack for help determining the DTM precision, N. Tosca for providing brine evaporation results, and Mars Science Laboratory science team colleagues for helpful discussions. Helpful reviews by M. Ramy El-Maarry and R. Irwin improved this manuscript.