Mars-Relevant Field Experiences in Morocco: The Importance of Spatial Scales and Subsurface Exploration

3.1. Comparison of remote sensing versus in situ observations

Various surface features characteristic of desert landscapes (Garvin et al., 1981; Christensen, 1986; Golombek and Rapp, 1997; Levy et al., 2009) were recorded on-site as panoramic photos and were then compared with satellite images. The observed surface features were grouped in such a way as to reveal similarities and differences between the two image types to improve interpretations when using satellite images (such information is exclusively used for landing site selection on Mars; Bridges et al., 2003; Arvidson et al., 2008). We also sought to estimate how far we could extrapolate by using remotely visible structures below the spatial resolution of remote data. For example, how would the presence of structures below the spatial resolution be detected by using the remote data available. The analyzed surface features at the Ibn Battuta field sites were classified into groups termed cliffs (<100 m diameter, isolated and elevated rock exposures), hills (>100 m diameter elevated structures), general rocky desert (reg) surfaces with scattered rock blocks and/or gravel, general sandy desert surfaces with sand dunes (ergs or sand seas), lakes, wadis, and transition zones between each characteristically different surface texture areas (Mabbutt 1977).

Characteristic image pairs of remote sensing-based and in situ recorded surface features are seen in Figure 2. By analyzing these images, the following features are visible (coordinates are given in brackets at the end of each subset caption below).

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FIG. 2.

Examples for the Google Earth-based remote (1 and 3 columns) and in situ recorded (2 and 4 columns) image pairs. Image insets are marked with letters and the image numbers of visited locations in bracket behind. Numbered arrows mark the same feature observable on image pairs. More details can be read in the 3.1 section.

In Figure 2a, cliffs (1 in Fig. 2a) could only occasionally be identified in remote sensing images due to illumination geometry that casts shadows and thereby hinders recognition. Shadows might infer the existence of natural outcrops and could be used together with regional geological context to identify proper locations in vertical strata for on-site analysis. The joint use of remote sensing images and on-site photos helps to link the collected samples to regional-scale geological units (31.430361 N, −4.303556 W; Google Maps coordinates).

In Figure 2b, illumination created brightness differences between dunes and their subunits (1 in Fig. 2b); dark surface gravel cover (2 in Fig. 2b) and bright underlying evaporites can be easily recognized in satellite images. Due to the dark gravel surface that overlies evaporites, however, the identification of stratigraphy required in situ field work. Spatial arrangement, including stripe-like features at the edges of the dark area, can roughly be identified in the satellite images (31.193278 N, −4.109361 W), but the origin may be diverse and unidentifiable in remote sensing images only.

In Figure 2c, large, tilted bedrock layers are easily recognizable in this image due to their different albedos (1 and 2 in Fig. 2c). Although a sampling may seem to be identifiable with remote sensing data, the proper sampling point to be selected requires further analyses at higher resolution (1–10 m scale) images (31.008889 N, −7.102444 W).

In Figure 2d, steep edges (1 and 2 in Fig. 2d) of ephemeral river (wadi) banks form outcrops and can be spotted on images taken only under proper illumination conditions (31.040806 N, −7.124944 W).

In Figure 2e, linear dunes, barchans (1 and 2 in Fig. 2e), and sharp contact lines between dune sand and bedrock material are clearly visible on remote sensing images. These image pairs demonstrate that substantial albedo differences help to identify ideal locations for in situ work (31.280222 N, −3.989361 W).

In Figure 2f, sharp contact lines (1 in Fig. 2f) between dune fields; dune-free, deflation surfaces; and large star dunes (2 in Fig. 2f) can be recognized in this image (31.211028 N, −3.999667 W).

In Figure 2g, strong albedo differences are visible in these image pairs between dark basaltic rocks (1 in Fig. 2g) and yellowish-reddish carbonates (2 in Fig. 2g), which are emphasized by mass wasting (3 in Fig. 2g) from local heights (31.450167 N, −5.665667 W) and transport of the darker material onto the brighter carbonates.

In Figure 2h, outcrops of horizontal layers characterized by different albedos (1 in Fig. 2h) form a horizontally stratified hill (2 in Fig. 2h) (31.167472 N, −7.472917 W).

In Figure 2i, hills of different heights and slope exposure (1–3 in Fig. 2i) are present as barely observable structures in remote sensing images, demonstrating the importance of geometric conditions for satellite image acquisition (31.547639 N, −4.881556 W).

In Figure 2j, rock layers with their vertical outcrop walls and sharp edges (1–3 in Fig. 2j) emerge in this photo and the identification is aided by shadow, useful for locating sites where the vertical strata could be analyzed (31.297944 N, −4.402556 W).

In Figure 2k, typical reg (specific desert type) surface with reddish clayey and iron oxide-coated material in between blocks (30.748389 N, −6.68625 W).

In Figure 2l, typical reg surface differing from Figure 2k in boulder and gravel sizes and color, where ephemeral fluvial channels are obvious on satellite images (30.780417 N, −6.661861 W).

The analyzed terrain types at Ibn Battuta analogue fields were often sedimentary in origin, including old and recent ones, occasionally with ongoing fluvial and/or aeolian formation processes. As sedimentary successions and often water-related terrains will be targeted in future Mars missions (Doran et al., 1998; Peeters et al., 2009; Coradini and Orosei, 2011), we emphasize on the analysis of features of various sediment deposition mechanisms. Changes in the environmental parameters during deposition could directly be observed in the strata (thickness, lithology, mineralogy), which are obviously rarely seen in satellite images due to viewing geometry and limited spatial resolution. However, several sedimentary patterns/structures could be identified on satellite images, too.

Debris flows (talus slopes) can be observed in many pictures, both on the satellite and on-site images. The shape and exact size can be estimated by using satellite pictures for structures >100 m in size, as the whole structure cannot be seen in the field. Their occurrence is indicative of erodible bedrock affected by mass wasting due to mainly mechanical weathering and erosion. In situ analysis could highlight the type of the mass wasted sediments, the thickness, grading, and sorting, and improve our understanding of the transport process.

Two basic types of deserts are present in Morocco among the Ibn Battuta sites: stone deserts and sandy deserts (regs and ergs). It is well known that the approximate size of large rock blocks can be discerned with the use of satellite images, while in situ images allow for determination of lithology sorting and dune size within sand features and varying sized gravel and pebbles. Based on a local in situ survey, the stone deserts show local diversity (rock type, pebble/gravel size, occurrence), and their satellite images reveal a diverse number of features. No connection, to date, has been made between the appearance of a given rock desert type in remote sensing data and appearance in in situ images, which might be an important topic for future research.

In some cases, dried lake beds can be identified in satellite images when aided by recognition of feeding channels and bright evaporites that were left behind after an active period. Nevertheless, sensing data acquired remotely have proven not to be sufficient for sure such identification. A smooth depositional area in remotely acquired images does not necessarily indicate a former lake bed. Images taken on-site, however, can provide further clues, in particular when the former lake bed is indiscernible due to cover such as (1) wind transported dust, (2) clay layers (without bright evaporites), or (3) rock fragments and gravels entrained by areal flash floods, leading to a dark surface appearance when viewed from above (see the example in Section 3.2.1 later). The sediment types (clay, evaporite, sand) and their relative location to each other in the Ibn Battuta region allowed for determination of the stratigraphy during in situ analysis and confirmation of the existence of former lakes. On Mars, former lake beds are often identified by their bank structures and feeding channels (Cabrol and Grin, 1999; Fassett and Head, 2008), but it should be noted that other lines of evidence will be needed for future missions to successfully identify lake bed features.

A comparison of in situ and satellite-based observations on different surface structures is summarized in Table 1, focusing on consequences for surface exploration.

Based on the general characteristics outlined in Table 1, satellite images support the identification of surface areas covered by the investigated structures, aid in route planning, support rough target identification and estimation of potential source regions of surface debris, and generally provide useful context for in situ work. By contrast, local images allow for constraining formation mechanisms, provide a record for temporal changes in the sedimentary environment, and aid in identification of compositional/lithological characteristics of source regions. These considerations highlight the importance of acquiring local images of outcrops and surface structures before sampling, which, together with satellite images, would provide the proper context for interpretation of sample analysis results.

3.2. The shallow subsurface zone

During the Mars analogue field work in Morocco, we tested how the shallow subsurface exploration could contribute to understanding the formation of the region: especially what type of Mars-relevant information could be gained with a 10–30-cm-deep pit excavated at the analogue field. The rationale behind such activity is to support the understanding of the formation of a given area and get field experience that supports the planning of forthcoming missions. Few surface missions have had or will have such shallow subsurface exploration capabilities, such as the neutron spectrometer onboard Curiosity (Grotzinger et al., 2012) or WISDOM (Dorizon et al., 2016) and the driller (Di Iorio et al., 2012; Bost et al., 2015) onboard ExoMars. Experiences related to this topic are fairly limited, and here we provide some general geological and methodological lessons learned at Mars analogue sites.

Natural outcrops reveal subsurface zones, but these are not necessarily abundant, especially at smooth terrains filled with sedimentary material. Numerous outcrops are expected at impact craters (Hynek, 2004; Clark et al., 2005); however, other types such as on fluvial terrains can also be important and were analyzed at analogue terrains. All the examples presented in this study are images of natural outcrops (Fig. 3 and Table 2) that were related to fluvial terrains (wadis). In such valleys, branches that formed recently incised older sediments and uncovered the top 0.5–1.0 m section of the subsurface zone.

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FIG. 3.

Images of various fluvial processes related and by fluvial incut excavated natural outcrops. Periodic changes in deposited grain sizes (a–d), cemented gravel bar shielding the underlying deposits (e), fine laminated sand and aleurite size particles (f, g), desiccation produced laminated clays (h) could be seen on them. Most of the images show such fluvial stratigraphy, exhumed by later fluvial activities with deeper incut into earlier accumulated deposits. For specific details see Table 2 in Supporting Online Material.

By pure surveying of channel walls (similar to those in Fig. 3), the recent geological evolution history of the given fluvial terrain can roughly be estimated. This might have consequences on the planning of Mars exploration too, as the existence of outcrops requires relatively large, regional-scale surface structures that cut and expose subsurface features. Such large structures could be fluvial channels or tectonic faults at plains; however, hills could also expose their layers along steep walls. All the images in Figure 3 were recorded along 100–1000-m-long outcrops producing topographic undulations in branches, which formed at a later period of the fluvial activity, and they cut into the former deposited fluvial sediment at the same site of the same river.

As natural outcrops are rarely present on smooth terrains, artificial outcrops were excavated by spade and hand forced drillings to expose the subsurface sedimentary layers (see Table 3 for locations). To identify the sites for drilling, local geological context and existing natural outcrops nearby (if any) were considered, targeting usually at the deepest locations of fluvial riverbed sample sites that experienced the longest and most recent periods of liquid water cover. For lacustrine environments, another approach was applied; locations close to the bank of ephemeral lakes were sampled to have a stratigraphic sequence that includes fresh evaporites at the top, other lake bed sediments, and the bedrock below to examine the whole stratigraphic sequence in its entirety thereby supporting comprehensive reconstructions of processes acting in this sedimentary environment.

The excavation sites displayed in Figure 4 targeted fluvial locations with fine sand (Fig. 4b); cobbles/gravels (Fig. 4b) where the particles' size, layering, and imbrication could be easily analyzed. Former lakes (Fig. 4f) with sequences of sand, aleurite, and clay deposits were also analyzed; just like various rock desert types (Fig. 4a, c, e, g). Further details on these locations are found in Section 3.2.1 below.

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FIG. 4.

Comparison of images of shallow subsurface exploration. The horizontal bars mark 10 cm length, relevant as scales around the middle of each image. More information on these locations can be found in Table 3 in the Image ID No. column.

3.2.1. Observing the shallow subsurface to identify the origin of rock debris

Based on some observations, even only a few cm deep subsurface excavations can provide such basic information that could not have been achieved otherwise by using only remote sensing-based observations. In the following, such examples are presented, being of high importance to Mars, and they also allow for improving research methods for coming missions.

The site No. 0203 (30.780417 N −6.661861 W, Ouarzazate area—Adad) was a typical stony desert (reg type) with gravels/cobbles/boulders having different sizes (mostly 2–30 cm) and shapes on the surface. (The terms “reg” or “serir” were originally used for stony desert surfaces covered by sheets of gravel) (Thomas, 2016.) The blocks were mostly angular/subangular, originated from the same type, and did not show any imbrication or other regularity in their spatial arrangement. Generally, it is hypothesized that these desert surfaces formed by a combination of aeolian activity (deflation vs. dust deposition) (Cooke et al., 1993; Wells et al., 1995) and in situ weathering from basement rock as the source material.

The 30-cm-deep excavation (Fig. 4a) revealed that there is no bedrock at this depth, which is what would have been expected for blocks produced by in situ weathering. Altogether, two trenches were deepened some meters away from each other into the subsurface (down to 30 cm depth and at least 40 cm width), but neither of these exposed the bedrock. The surface blocks varied in size between 1 and 25 cm; some of them were relatively large and covered the surface almost homogeneously. The spatially homogeneous arrangement of gravels/cobbles and the lack of bedrock in the shallow subsurface led us to conclude that wind deflation and dust accretion, surface runoff, and/or lateral transport by creep (Cooke et al., 1993; Wells et al., 1995) are among those mechanisms shaping this landscape, while in situ weathering may safely be excluded.

The usefulness of simple shallow subsurface exploration is further demonstrated by another example, the homogeneous small particle debris field that remained at an ephemeral lake site after the active period (site No. 0403, coordinates 31.193278 N, −4.109361 W, Erfoud area) (Fig. 5). Here the surface debris resembles many other rocky desert surfaces: the terrain was covered by 1–2 cm diameter homogeneously distributed rectangular gravels. Some observations suggest that these grains were transported by areal floods; at the end of some small elongated depressions, these gravels were accumulated in fan-like structures (Fig. 5b). Shallow subsurface excavation (Fig. 5c) demonstrated that the debris is present only on the surface, while below, at least down to 20 cm depth, only fine material composed of silt/clay exists. Thus, the rock debris was vertically unmixed with the aleurite–clay layer filling of the plain, but covered only the surface of the fine material, pointing to the sequence of accumulation events.

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FIG. 5.

Dry clay-covered lake bed (bright) (a) with subsequent flash flood-transported small dark rocky debris (b) at site No. 0403, with no rocks in the shallow subsurface (c).

3.3. Drilling and borehole-wall scanning

Besides the inspection of natural or artificial outcrops, the shallow subsurface zone was explored by simple hand-guided drilling (Table 4), accompanied with image acquisition of the borehole wall. More detailed analyses of the collected granular samples were planned to be performed in the laboratory of the home institute and this work is ongoing. The drilling activity was almost exclusively performed in wadis, where selection of a best site for sampling required a brief survey of exposed bank walls. Site selection always involved different spatial scales; it started with satellite image analyses at the landscape/landform scale (approximately 10²–10³ m), continued on-site at the local scale (10⁰–10¹ m), and finished at the scale of sedimentary features (approximately 10⁻¹–10⁻³ m). An example sequence of analyzed scales by using this approach is presented in Figure 6 (drilling site 0601).

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FIG. 6.

The pictures from (a–f) show the location of the fourth drilling site of the expedition (a, b) Google Maps and the others: local camera images; (g) the drilling process; (h) the outcrop of the wall of the wadi with different layers; (i) example images of the borehole wall at different depths.

In Figure 6, both the context (satellite images) and on-site images are presented for the fourth drilling site at 30.983 N, 7.151 W. The investigated structure is a 400–600-m-wide branching wadi that can be seen in Figure 6a and b. Figure 6c–e displays a closer view of branches with different surface debris cover, including exposed fluvial sediments with gravel, sand, and aleurite–clay covers. Fluvial erosion resulted in natural outcrops, as floodwater cut into earlier deposited fluvial sediments, occasionally down to around 1 m depth. Topographic lows in channels produced during the last active fluvial episode were often filled with desiccated clay–evaporite layers and/or wind-driven sand accumulations. The fluvial and wind-driven fine material can be differentiated from ca 10–20 m distance on-site; a similar situation may be expected on Mars in exposed surface fluvial channels, and thus, a closer on-site inspection could be useful there.

Natural outcrops with fluvial sedimentary sequences can be seen in Figure 6h. Differences in bedding and grain size along the strata could be observed by inspection (from m to dm distances), which, although a trivial matter on Earth, would require time and effort to conduct a survey along outcrops during Mars exploration. This has been done a number of times with recent Mars missions; however, no sophisticated methodology has been developed as to how to conduct these surveys and optimize the number of nearby outcrops to visit. This context generation task could also be greatly improved by the use of drones (Singer, 2012; Svendsen et al., 2013). Based on the field experiences, the layering structure, the existence of agglomerates, the occurrence of larger grains could be enhanced by wind-driven selective erosion, and thus, such small-scale features could be easy to identify from open-air outcrops.

Using borehole-wall scanning in the drilled hole (Fig. 6g), images were acquired (Fig. 6i1–i5). While the contour of each particle could occasionally be blurred, individual grains above 0.2 mm diameter could firmly be identified. It is clearly visible that the larger grains are not rounded. The grain surfaces look smooth, but this observation may be affected by the quality of pictures as these features are around the limit of spatial resolution. No structure or regularity can be seen in these borehole-wall images, despite the fact that layering was observed on the same sedimentary strata exposed at a nearby outcrop. However, it is worth mentioning here that particle size and the level of cementation were the main differences between the different layers, as opposed to albedo or color. This implies that layering might be difficult to observe inside drilled boreholes on Mars.

The samples seem to be relatively well sorted (no more than two dominant particle size ranges are present: fine sand and aleurite), except those in the Figure 6i 3 and i4. In these two cases, larger grains (>1 mm diameter) appear, and thus, these stratigraphic levels are less well sorted than the others, probably referring to higher energy transport conditions. Aggregates cannot be identified (they are present, however; see next paragraph), and the layering is almost invisible in the borehole-wall scanning compared with the outcrop wall.

The shape of the grains is similar in all samples acquired along the vertical strata; they are usually angular without sharp edges. In samples collected at 30 and 40 cm depths and close to the surface, the finest grains often stick to bigger grains to form aggregates. The aggregates from shallow (1 cm) depths are often cemented by evaporate minerals and clays that formed during the final stage of the wet period.

Higher quality laboratory images (Fig. 7) improved the information gained on particles. It is of note that images with similar resolution will be taken by the CLUPI camera onboard the EXM rover. Differences in image quality between the images taken on-site and those recorded under ideal lighting and geometric conditions in the laboratory are displayed in Figure 7, highlighting the importance of image acquisition conditions. This observation may have a methodological aspect for Mars exploration; quality of images to be recorded by CLUPI will likely be lower than determined during laboratory testing by MicrOmega (20 × 20 μm).

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FIG. 7.

Samples from 1, 4, 10, 30, and 40 cm depth (from the top row to the bottom, respectively). The last two rows show fragments (as agglomerates) of the evaporite and subsequently formed clay crust. The photos in the first column were made by A4-Tech PK-910H web camera. The pictures of second and third columns were made by NICON Eclipse E600 POL microscope in the laboratory. For the detailed explanation on the inset images see the description on section 3.3.

The better quality of laboratory images resulted in improved size and shape determinations in the smaller size range (especially <0.2 mm), and more details of the grain surface patterns became visible. Among the minerals present, quartz, carbonate, mica, Fe-minerals, and some salt could be identified. The grains are more visible in the laboratory pictures, not just because of the better viewing geometry but also because fewer aggregates were present as they fell apart during sampling and transport. Besides, aggregates were hardly visible while embedded in the sediment matrix at their original location (wall scanning), but could be easily identified once the samples were collected and weakly pulverized due to simple transport. These aggregates were loose and easily fragmented, and the cement proved to be a mixture of iron oxides and clays.

The difference in characteristic grain size between samples collected at various depths along the vertical sampling column could be easily seen both in the field and in the laboratory. Smaller particle size in the upper samples (Fig. 7A and B) (range: 100–200 μm) and larger for the underlying horizons (Fig. 7C and D layers) (range: 800–1500 μm) imply decreasing sediment carrying capacity of a gradually weakened flow. The layer characterized by larger grain size may allow for estimation as to which sediments were laid down during elevated discharge. However, this analysis includes the data obtained in vertical strata of only one drilling. Open air natural outcrops offer a better overview at more locations, however, and their analysis may allow a spatially better identification of the sediment layer associated with the peak discharge at a given area.

Evaporites and clays appear as cements between grains in Figure 7f and g insets. These samples provided many aggregates, and grains were attached stronger to each other than in other sampled layers. These cements may have formed after the termination of fluvial activity, during the desiccation phase when little liquid remained and finally evaporated.

4.1. Shallow subsurface exploration

Shallow subsurface exploration resulted in further information with regard to the geological history of the given location that could not have been identified from remote sensing images or data collected at the surface on-site. Analysis of the top 20–30 cm shallow subsurface zone provided insight into the origin of the surface debris, as shown by some examples discussed in the points below.

4.2. Results of sub-mm-scale analysis

The smallest scale particle and layer analysis (around the mm regime) were achieved by way of two methods: (1) an optical borehole-wall scanner and (2) later laboratory analysis at the home institute. A comparison of the two methods reveals the advantages and disadvantages of each, including the more difficult particle size estimation for the smaller grains and the lack of information on particle surface texture when using in situ (nonideal) observing conditions relative to those in the laboratory. (It should be noted that such differences strongly depend on particular detectors that were used, but tests with such instrument are crucial and point to possible future usage of high-resolution optical images of deposited particles on Mars.)

Based on the moderately wide particle size distribution at two layers (especially in Fig. 6i 4), the action of water could be identified as a more probable agent than wind transport (when more narrow grain size distribution and lack of larger grains are characteristic) (Nichols, 2009) and mass wasting (when wider grain size distribution with large and extremely irregular shaped grains is characteristic) (Blum et al., 2005). Other than size, the shape of grains can also be roughly estimated by in situ images that support the identification of the transport method; however, the quality might not be enough to separate theoretical end cases of wind-transported, well-rounded, and fluvial transported poorly rounded grains for the sand fraction.

Aggregates are important components of many samples, which can inform with regard to aggregation processes before or during deposition (with Mars-relevant processes such as wet surface adhesion or duricrust formation), but these could be identified only under certain conditions. Agglomerates could be identified in the acquired and weakly pulverized sedimentary samples (during the later laboratory analysis) and not in the borehole-wall or in the outcrop surfaces observed during in situ work. However, it should be noted that the effective mechanical pulverization by the crushing station of ExoMars rover could also destroy such aggregates. Their identification could ideally be determined after sample acquisition and before pulverization, as recorded by the CLUPI camera.

4.3. Connections between different spatial scales and the use of Earth analogues

Based on the past decades of Mars surface exploration, in situ work has been targeted primarily with the use of remote images and landing site selection processes. For local work, “real-time” selection should be applied to identify and select ideal locations at meter and centimeter scales. This trend (focusing on on-site recorded images in target selection) has become more important for future Mars sample return mission (O'Neil and Cazaux, 2000; Sephton et al., 2013). Analogue work provides example methods and experiences as to how to target ideal locations by using various scales of data collectively (Table 5). Here, Ibn Battuta Centre's field sites could provide a specific contribution with regard to spatial scales, timescales, and hydrological systems as described below.

During the Mars exploration in recent decades, target types shifted from larger toward smaller spatial scales, visible in the 1–4 columns of Table 5, given that targeting of missions has become more accurate with our improved ability to land at smaller ellipses. Thus, analogue work should follow this trend, as it provides a test bed with which to improve methods as to how geological information of different spatial scales can be used to fine-tune the location of sampling sites. Here, the joint usage of satellite and in situ images could be improved by analogue studies specifically at Ibn Battuta Centre's field sites, including the extrapolation to rocky desert types from remote images and identification of probable fluvial incut produced outcrops. Another important aspect of Earth-based desert analogue sites has to do with the low speed of surface modification and the long duration of evaluated periods. Environmental conditions for deserts on Earth generally take longer to change than approximately 10¹–10² years, and surface modifications are influenced substantially by climatic changes (order of 10⁵–10⁶ years). In this aspect, the region where Ibn Battuta Centre sites are located suffered important changes since the Miocene (see details in an accompanied article by Ori in this special issue).

The large number of different analogue desert sites at the Atlas region that are close to each other provide the opportunity for easy logistical access and the development of future Mars missions' methodology, that is, how to consider and use regional and local context to identify the most water-relevant samples for astrobiology. Based on the present scientific view, older locations have been of primary importance with regard to astrobiological studies on Mars; however, young and even recent features at Ibn Battuta sites could help to develop new methodologies for the analysis of very slow surface modification of many old martian surface structures that can be remarkably intact even at the present.

The small-scale in situ analyzed features and related shallow subsurface characteristics of regional hydrological systems have widespread Mars relevance, which include highly changeable discharge, gradually diminishing water at the lower reach, evaporite and clay-rich deposits, and ephemeral standing water bodies. The investigated desert areas are dominated by ephemeral streams that flow southward from the Atlas Mountains, which are active only during rainy periods in the mountains. The most important of these rivers is Oued Ziz, its source located in the High Atlas Mountains, which flows though the Tafilalt region where a great deal of field work has been carried out. In the Tafilalt plain, the Oued forms alluvial plains rich in clay and evaporite deposits. In this area, the river loses much of its water such that it ceases to exist at the border of Sahara. In the mountain reach, the discharge is close to 1000 m³/s in winter and does not exceed 10 m³/s in summer.

In the investigated area, the Oued Ziz is strongly ephemeral, and it forms a set of terminal sabkhas (salty flats formed by evaporation in arid regions) (Hugget, 2007) made up of extensive clay, fine-grained sand, and evaporitic strata at the alluvial plains that accumulate during flooding. A portion of this water evaporates, but some of it typically recharges the shallow water aquifer. The water table is at a depth that ranges from 10 to 30 m. The area of these terminal plains is about 20 km wide and 60 km long. However, the Oued Ziz represents a major river system in the Quaternary evolution of Sahara. During humid climatic periods, when Sahara was predominantly a savanna rich in lakes and rivers, Oued Ziz flowed from the High Atlas up to the Erg Inguid in Algeria, a distance of about 600 km. Therefore, this major river is now composed of several independent reaches that are active locally depending on local and rare episodes of precipitation.

In winter, the water table can rise up to the surface in a few locations in the area of Erfoud (Figure 8). This area corresponds to two different systems: the sebkhas (ephemeral standing bodies of water on the order of 1 km wide, linked to the alluvial plain) and the border of the Erg Chebbi. This latter lake is the product of an aquifer that lies between the base of the sand of the Erg Chebbi and the substratum that consists of Paleozoic rocks. This aquifer is likely charged by rain that falls over the erg and penetrates to highly porous eolian sand of the erg. The lake lasts 2–3 winter months and disappears in the spring due to the decrease of rainfall as is the case for the sebkhas-type lakes. However, apparently the deposits are not composed of evaporites but of fine-grained clastic sediments. The site No. 2038 (mentioned above as ephemeral lake in 4.1 section) comprises such clay-like deposits and plain-shaped sediment accumulations without inflow-related channels.

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FIG. 8.

The depositional areas of Oued Ziz (flow direction according to arrows) marked by the bright terrains in the middle (a). Wintertime ephemeral lake produced by rain provided discharge at the alluvial plain (b).

4.4. Applying the analogue results for Mars

Several recent observations suggest the range of spatial scales (from orbital image scale to cm–mm scale in certain outcrops) that should be used in forthcoming missions, including the planned Mars sample return, where highly targeted sampling is planned, partly from the subsurface. Researchers were able to effectively connect orbital data and in situ observations by the highly mobile Curiosity rover, in this case in situ results helping to extrapolate beyond the rover's traverse (Milliken, 2015). At the foundation of Mount Sharp in Gale crater, the border between thin and cross-bedding layers was observed by the Curiosity rover at a site within Gale crater, the location of which could not be determined from orbital data alone, but required in situ imaging from a meter-scale distance (Grotzinger et al., 2015). Curiosity also analyzed the Shaler outcrop (fluvial sand body) of only 0.7 m thickness with seven separate facies based on grains and bedding along the feature (Edgar et al., 2014). The Opportunity rover also demonstrated the importance of centimeter-scale analysis of outcrops in attempts to reconstruct past conditions by using scale cross-lamination observed from a decameter distance in Erebus crater, Olympia outcrop (Grotzinger et al., 2006).

However, there are few case study examples on the ideal number and spatial setting necessary to establish in situ observations supported by “high-resolution” geological context to facilitate centimeter-scale sample targeting. With the many lessons learned from analogue field tests kept in mind, future methodological needs for effective sample collection on Mars will need to be determined. A sequence of steps toward realization of these goals (observations at different levels) are summarized in Table 6.

The results gained by the analysis of the two main research questions noted here could support the targeting of forthcoming missions by the following topics (especially the knowledge gaps listed in the rightmost column in Table 6).

The acquisition of several orbital images of a same target area under different geometries should be tested, which would be quite difficult and expensive.

Early tests with drones would help to clarify the range of ideal and required observation types. It would also be useful to initiate a discussion with mission experts to discern how this suggestion would fit with the possibilities and capabilities of a regular orbiter mapping mission.

For the determination of the ideal numbers and spatial arrangement of outcrops required to be imaged nearby on the surface for firm geological context, establishment should be tested at analogue key sites.

The last level of targeting: the selection of a given layer or number of layers to be sampled should also be tested at analogue locations with sample acquisition from more locations than foreseen to select the most relevant ones consistent with the above acquired context. The possibility of more sampling and subsequent later analysis, including better laboratory facilities on Earth than are available onboard a rover, makes analogues ideal to develop and improve the method that should be followed during the Mars sample return.