- © 2015 by the Mineralogical Society of America
The surface of Mars has been sculpted by flowing water and shaped by wind. During the first two years of its exploration of Gale Crater, the Mars Science Laboratory mission's Curiosity rover has recorded abundant geologic evidence that water once existed on Mars both within the subsurface and, as least episodically, flowed on the land surface. And now, as Curiosity presses onward toward Mount Sharp, the complexity of the Martian surface is becoming increasingly apparent. In this paper, we review the nature of the surface materials and their stories, as seen through the eyes of Curiosity.
Potential preservation of a diverse geologic past led to the selection of Gale Crater as the landing site for the Mars Science Laboratory (MSL) mission (Grotzinger et al. 2012). Orbital imagery provided geomorphic evidence (Malin and Edgett 2000; Anderson and Bell 2010; Siebach and Grotzinger 2014) for a protracted history of aqueous activity in the crater, and orbitally derived geochemical data indicated a diversity of mineral components (Milliken et al. 2010; Fraeman et al. 2013) that have the potential to provide and preserve key nutrient and energy sources necessary for habitability.
Notable surficial deposits include those related to a well-defined alluvial fan, the Peace Vallis fan, that extends into the crater interior from its little-denuded rim (Palucis et al. 2014), a central mound that exposes a nearly 5 km thick section of layered rock (Malin and Edgett 2000), and a complex series of layered strata that is exposed within the topographically low region between the Peace Vallis fan and Mount Sharp (Fig. 1) (Grotzinger et al. 2014; 2015 this issue). Gale Crater also records snapshots of the broader Martian environment, such as exotic lithologies transported into the crater via impact processes and the near-ubiquitous, wind-transported fines that form a veneer on the Martian surface.
In an effort to evaluate the habitability potential of environments in Gale Crater, Curiosity's cameras have scanned a wide range of surficial deposits to seek evidence of water, the medium for all known biochemical reactions. The presence of past water is often apparent in the depositional fabrics of surficial materials. Clues gained from the texture of surface materials can also be used to predict how persistently environments may have interacted with water, either within the shallow subsurface or at the ground surface. Curiosity's extensive suite of analytical instruments also allow for the search for both the nutrients necessary for life (e.g. C, H, N, S, etc.) (Mahaffy et al. 2015 this issue) and for possible energy sources (e.g. disequilibrium redox pairings in minerals) that could fuel microbial metabolisms.
THE EYES OF CURIOSITY
The eyes of Curiosity include six engineering cameras designed to support terrain assessment, hazard avoidance, and traverse planning (Maki et al. 2012), as well as four dedicated science cameras: the Mast Cameras (Mastcam) (Bell et al. 2013), the Mars Hand Lens Imager (MAHLI) (Edgett et al. 2012), and the Remote Microscopic Imager (RMI) associated with the ChemCam instrument suite (Maurice et al. 2012). Together, these cameras provide high-resolution, black and white and RGB color images, from distances of 2.1 cm to infinity. These images aid in the interpretation of rock fabrics and, with the use of a suite of multispectral filters, permit limited detection of ferric, ferrous, and hydrated minerals.
SURFACE MATERIALS IN GALE CRATER
One of the most striking features of the Mars Science Laboratory landing site is the Peace Vallis fan (Palucis et al. 2014). This landform is interpreted to be the surficial expression of a long-lived alluvial-to-fluvial system deriving from the Gale Crater rim. Curiosity's landing site, at the distal end of this fan, allows assessment of the extent to which the region experienced inundation by fluids emanating from the fan. Some of the earliest Mastcam images from Curiosity revealed glimpses of such fluvial activity in lithified, pebble-rich conglomerates that had been exhumed from beneath the surface of Bradbury Landing by the jets of Curiosity's landing system. Then, within the first 100 meters of Curiosity's exploration of Gale Crater, a series of isolated outcrops revealed well-lithified deposits of rounded sand grains, granules, and pebbles with textures typical of fluvial conglomerates (Williams et al. 2013). Since these first glimpses of fluvial processes, Curiosity has investigated a number of key outcrops that confirm the widespread nature of fluvial activity in Gale Crater. The Shaler outcrop, for example, consists of inter-stratified pebbly sandstone and finer-grained intervals (Fig. 2a) and contains well-developed, large-scale trough cross-stratification indicative of channelized flow.
In addition to fluvial sandstones, Curiosity has encountered a broad variety of sheet sandstones (Fig. 2b and c). These sheet-like deposits occur at nearly every waypoint imaged by Curiosity (e.g. Gillespie Lake member in Yellowknife Bay, Renssalaer at the Cooperstown waypoint, and Square Top at the Kimberley waypoint), and are often interbedded with conglomeratic deposits (Fig. 2d). These beds are generally massive, although poorly defined planar lamination and cross-lamination can also be present. Most of the observed sheet sands are medium grained and contain a variety of larger pebbles. Pebbles are rounded to angular and may represent, in part, material entrained from underlying strata during periods of enhanced surface flow. Combined, the clast-supported nature of these varied facies, the presence of coarse sand grains to fine pebbles, and the occurrence of partially rounded clasts suggest that these deposits represent sediment transport by flowing water. The absence of a well-sorted fabric in the coarser-grained deposits, however, suggests that at least some of these facies may have been rapidly deposited during flood events, rather than being the result of sustained fluvial activity.
The observation of widespread fluvial material in bedrock exposures along Bradbury Rise suggests that the Peace Vallis fan is the surficial expression of a long-lived alluvial-to-fluvial system deriving from the Gale Crater rim. Detailed mapping of the toe of the Peace Vallis fan further indicates that fan deposits interfinger with a distinctly different succession of strata located downslope from and adjacent to the fan. That these light-toned, layered rocks might represent distal alluvial or even lacustrine environments was critical in influencing the MSL team's initial decision to detour north toward Yellowknife Bay, rather than south toward Mount Sharp.
The detour into Yellowknife Bay proved to be a superb decision, especially with regards to the investigation of the Yellowknife Bay formation (Grotzinger et al. 2014), the base of which is composed of fine-grained strata of the Sheepbed member and sheet-sands of the overlying Gillespie Lake member. Following brushing of the Sheepbed outcrop, high-resolution MAHLI images (Fig. 3) revealed an absence of grains larger than approximately 50 μm and the presence of grooves where the Dirt Removal Tool (DRT) scraped the surface. Combined, these observations provided the first evidence of a mudstone on Mars. The tailings from the first drill hole (Fig. 3) also provided a glimpse of something rarely seen on Mars: elements in their reduced state, as suggested by the grey color. Once analyzed by Curiosity's CheMin X-ray diffraction instrument (Bish et al. 2013; Vaniman et al. 2014), these sediments were found to contain a nonequilibrium mineral assemblage that includes primary igneous phases (primarily basaltic minerals, such as plagioclase, pyroxene, and forsteritic olivine), a substantial (>30%) X-ray-amorphous component, and a variety of secondary phases. The secondary phases included a high proportion (>20%) of trioctahedral smectite clay (including saponitic smectite and a potentially Mg-bearing smectite), calcium sulfate minerals (anhydrite and bassanite), and iron oxides (including the iron oxide–hydroxide/chloride mineral akaganeite) (Downs and the MSL Science Team 2015 this issue). The analysis of chemical trends (McLennan et al. 2014) further suggests that diagenesis of the Sheepbed mudstone took place at near-neutral pH and at water:rock ratios low enough to reflect essentially isochemical alteration conditions.
Additional information regarding the deposition and diagenesis of the Sheepbed mudstone has been obtained from a variety of preserved diagenetic features (Fig. 4). These features include spheroidal nodules (Stack et al. 2014), a dense network of mineralized fractures (Siebach et al. 2014), and calcium sulfate–filled veins (Nachon et al. 2014). Taken together, these features indicate at least two distinct diagenetic episodes.
Nodules in the Sheepbed mudstone consist of millimeter-sized spheroids that are more resistant to erosion than the host mudstone. Some nodules are solid while others have a hollow interior. This range of textures suggests a common origin, possibly as concretions or via preferential cementation around a mineral phase that has since dissolved. Alternatively, the hollow nodules may have originated via compaction and incipient lithification of the host mud at the perimeter of gas bubbles, similar to the origin of terrestrial “molar-tooth” structures (Pollock et al. 2006). Preservation of such gas-expansion features in the Sheepbed mudstone is consistent with terrestrial data that suggest retention of exsolved gas within sediment that contain sufficient (>15%) clay content to enhance cohesion. Although most gas production in terrestrial pore fluids occurs during the decomposition of organic matter, temperature- or pressure-driven exsolution of dissolved atmospheric gases could also result in such features.
Early diagenetic nodules occur in the Sheepbed mudstone along with locally dense networks of mineralized fractures, although these two features show an antithetical spatial relationships. Fracture networks consist of short (<50 cm), curvilinear to planar mineralized fractures with a range of orientations, from vertical to subhorizontal. Fractures are filled by multiphase cement consisting of two isopachous (i.e. equal thickness), erosionally resistant outer layers and a central less-resistant fill. Physical relationships suggest that the fractures may have formed both as interconnected voids and as discrete crosscutting features. The occurrence of early diagenetic concretions and fracture networks suggests a possible common origin via subaqueous exsolution of gases from the sediment–water interface (Stack et al. 2014; Siebach et al. 2014). In this scenario, gas release within weakly cohesive subsurface sediments would have driven expulsion of shallow pore waters, resulting in an increase in the cohesive strength of the sediment. Local differences in sediment strength and rate of gas production could have then resulted in the formation of either discrete spheroidal voids or fracture networks.
In addition to containing early diagenetic nodules and fracture networks, Yellowknife Bay strata also preserve evidence of later fluid flow. Planar veins containing a single phase of calcium sulfate mineral fill occur throughout the Yellowknife Bay formation and crosscut early diagenetic features. These late-stage mineralized veins penetrate up to tens of centimeters of vertical section and are characterized by a low spatial density and orientations that are predominantly vertical or horizontal (the latter following bedding planes). The light-toned veins are reminiscent of fracture networks generated when subsurface fluid pressures exceed the yield strength of the overlying rock, resulting in hydraulic fracturing. In this case, sulfur-rich fluids may have originated from burial dehydration of gypsum that was deposited prior to the Sheepbed mudstones, or from unrelated units near the base of Mount Sharp.
Eolian processes have played a dramatic role in transforming the materials seen in Gale Crater. Persistent wind is indicated by the polishing of depositional materials and exotic boulders, and by the ever-present veneer of dust that tends to obscure the rover's view of the underlying geologic materials. The winds, however, were also the primary agent responsible for exposure of Gale Crater strata, providing the team with a critical cross section of depositional processes. In addition, K–Ar exposure ages (Farley et al. 2014) have focused the team's strategy on identifying solid samples that have not experienced prolonged exposure to radiation, which could be detrimental to the preservation of organic molecules.
Early in the mission, Curiosity examined eolian materials of the Rocknest sand shadow (Fig. 5), first visually (Minitti et al. 2013) and then geochemically (Bish et al. 2013; Leshin et al. 2013). The depression left by Curiosity's scoop shows that the bedform surface is composed of a thin veneer of 1–2 mm grains (Fig. 5a). A combination of fractures within this veneer and of displacement of rafts of this veneer during scooping indicates incipient cementation and stabilization of the bedform surface. Stabilization may also be reflected in the presence of distinct banding 12–21 mm beneath the exposed surface of the bedform. Although the origin of the banding is uncertain, the orientation of the banding parallel to the present-day surface of the bedform suggests that it may represent either a change in the relative percentage of dust deposition during bedform accumulation or chemical changes within the sediment driven by diffusion of atmospheric volatile components.
The interior of the sand shadow is composed primarily of a mixture of dark grey, red, and orange-brown lithic fragments <150 μm in diameter, with only the larger size fractions resolved in the highest-resolution images. Mineralogical analysis (Bish et al. 2013) revealed the soil to be predominantly basaltic in composition (plagioclase, forsteritic olivine, augite, and pigeonite, with minor K-feldspar, magnetite, quartz, and ilmenite), but with a relatively large percentage of iron-rich, X-ray-amorphous material. In a sieved portion delivered to Curiosity's Observation Tray, most of the material is below the 3 pixel minimum necessary for resolving individual grains. Resolved grains, however, include three distinct morphologies (Minitti et al. 2013). The first of these consists of well-rounded and nearly equant, translucent, yellow grains with no indication of external crystal faces or internal cleavage planes. These grains are tentatively identified as quartz or olivine (Fig. 5b). A second type consists of small, opaque, angular to subangular, white grains that have distinctly flat (or faceted) sides, suggesting a mineral phase with two distinct cleavage planes, such as feldspar (Fig. 5c). Finally, near the limit of the image resolution are a small number of spheroidal grains that are distinctly symmetrical, round, and highly reflective; these may be impact spherules (Fig. 5d).
It would not be a surprise to find materials generated by hypervelocity impacts in the surface materials of Gale Crater. Evidence for impact-derived materials dates to Curiosity's earliest days on Mars, when both Mastcam and RMI imaged a variety of exotic clasts of probable igneous origin (Fig. 6). These included a large number of grey-colored, faceted boulders with uniform, aphanitic textures and compositions consistent with alkali basalt (Wiens et al. 2015 this issue) (Fig. 6a and b). More rarely, Mastcam and the ChemCam RMI imaged boulders that revealed hints of a porphyritic texture on dust-free surfaces (Fig. 6c and d). The most conspicuous phenocrysts are centimeter-scale and lighter in color than the surrounding groundmass. These exotic components are distinct from other surface components in that they are irregular in their distribution, are larger and more angular than locally derived clasts, and show less evidence of abrasion, suggesting an origin potentially beyond the walls of Gale Crater, and emplacement as impact ejecta (Yingst et al. 2013).
In addition to individual exotic components, Curiosity has imaged a variety of possible impactite deposits (Newsom et al. 2015), including upturned bedrock clasts, fractured bedrock, and probable impact breccias (Fig. 7a). Impact breccias display a wide range of clast types, clast lithologies, and angularity of individual clasts. Although these characteristics alone do not uniquely define an emplacement by impact, they are broadly inconsistent with the inferred hydrological emplacement of the vast majority of coarse-grained units within Gale Crater.
The critical role of impacts in providing material to Gale Crater is also evidenced by the occurrence of impact spherules and the occasional meteorite sighting. Impact spherules result from the atmospheric condensation and solidification of impact vapor, forming small (<1 mm diameter), generally spheroidal, glassy particles (Fig. 7b). These are identifiable by their smooth and highly reflective external surface (from which dust readily falls during physical disruption). Impact spherules are observed most commonly in disturbed soils, although they also have been found in lithified materials. By contrast, potential iron meteorites (Fig. 7c) occur as conspicuous exotic boulders on the landscape, much like those documented by the Mars Exploration Rovers (Schroder et al. 2008).
Curiosity has provided us with tantalizing glimpses of a world that we are only beginning to understand. Evidence of once-abundant surface water contrasts with evidence of only minor geochemical alteration of the materials that interacted with these fluids. Geochemical evidence also reveals distinct periods of moderate salinity and near-neutral pH, as well as periods of high-salinity, with sulfate-dominated fluids and periods of migrating iron-rich groundwaters. Collectively, these observations suggest that Gale Crater was rich in potentially habitable environments.
The ultimate challenge, however, still lies ahead. Can we define the extent of these previously habitable environments, can we determine the persistence of these environments through time, and can we know whether these environments were simply habitable or were actually inhabited? The answers to these questions may lie within the flanks of Mount Sharp, and the MSL team will continue to look for clues as Curiosity traverses olivine-bearing sands and a stratigraphic succession containing Fe–Mg-smectite clay, hematite, and hydrated sulfate. There are many ideas about what we might find, but we must remember the words of Winston Churchill, who said, “It is always wise to look ahead, but difficult to look farther than you can see.”
Thanks to M. Malin and K. Edgett for introducing me to Mars, and to J. Grotzinger and the Mars Science Laboratory mission for the support of an extraordinary team of scientists and engineers who work and learn together. Thoughtful reviews from team members, external reviewers A. Knoll and E. Simpson, and principal editor G. Brown helped improved this manuscript.