FROM UTAH TO MARS: REGOLITH – LANDFORM MAPPING AND ITS APPLICATION Jonathan D. A Clarke * Colin F. Pain† Regolith-landform mapping is an approach to describing the earth’s surface in terms of both landforms and the surface materials, whether of transported material, weathered rocks, duricrust, or fresh bedrock. The techniques of regolith-landscape mapping have been developed in Australia in recent years, chiefly under the auspices of CRC LEME, and have proved and important part of any field study on earth of surface materials, landscape, biota, and hydrology. It is also an essential platform on which to base more specific applications, such as civil engineering, resource extraction, landscape ecology, or geophysical and geochemical research. Regolith-landform mapping can be readily applied to the site scale and can also incorporate data from the subsurface derived from geophysical surveys and drilling. This paper describes the successful application of regolith-landform mapping to a Mars analog landscape at the Mars Desert Research Station in Utah. Many maps of surface composition and materials, landforms, and stratigraphic units have been prepared during almost 40 years of spacecraft imaging of the surface. To our knowledge, none have provided the same integration of landform morphology and surface material composition that has proved so useful in understanding terrestrial environments. Given the utility of such an approach, it is our recommendation that regolith landform mapping and site description be an integral part of studying both terrestrial analogs and of Mars itself. For human activities associated with establishment of permanent bases and settlements, understanding the regolith will also be critical. Mapping and otherwise characterizing regolith materials, distribution and architecture will be essential in selecting the best site for facilities, civil engineering works, and extraction of resources. It is also vital for understanding real and potential environmental hazards posed by the Martian surface.
INTRODUCTION A four-week, international multi-disciplinary expedition was carried out at the Mars Desert Research Station (MDRS) near Hanksville, Utah in February-March 2003 (Figure 1). The expedition was coordinated by Mars Society Canada in conjunction with Mars Society Australia and various chapters of the Mars Society in the United Sates and Europe. Investigations included field science, engineering, and human factors 1. The lead author was Principle Investigator for geology and carried out regolith and landform studies of the field area. It is our contention the regolith studies are essential to geological and biological research on both earth and Mars, and to the application of the knowledge for sustainable resource management on both planets. We maintain that a crucial first step in understanding the regolith is regolith-landform mapping, an approach that has been developed largely in Australia
*
Department of Geology, Australian National University, ACT 0200, Australia Email:
[email protected]
†
Cooperative Research Centre for Landscape Environments and Mineral Exploration (CRCLEME), c/- Geoscience Australia, PO Box 378, Canberra ACT 2601, Australia
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over the past two decades. In this paper we will outline the nature of terrestrial regolith, how it can be mapped and described (using the MDRS field area as an example), and some of the various applications of regolith-landform data. We then illustrate the application of the same techniques to representative areas and sites on Mars and finally briefly discuss the importance of detailed understanding of the regolith to exploration and settlement of the Martian environment, as highlighted by Duke2.
Figure 1. MDRS field area with the tuna can shaped habitat in foreground.
WHAT IS REGOLITH? “Regolith” is a general term used to refer to all weathered earth surface materials extending from the interface with unweathered bedrock at depth to the surface3. It includes in situ weathered material derived directly from the underlying bedrock, and transported ma terial derived from both bedrock and other regolith materials, and in many ways is similar in its usage to “surficial material”. The study of regolith as a discipline is relatively new4 5 but regolith materials have been studied in a number of disciplines for some time. These disciplines include pedology, geomorphology and parts of sedimentology, but rarely traditional geology. Regolith studies as discussed in this paper are concerned with approaches to understanding the three-dimensional distribution of regolith materials, and its representation in map form rather than vertical profiles or cross sections. Regolith is very closely associated with landforms, which makes a landscape approach to regolith mapping most appropriate 6. Landforms and regolith are formed by essentially the same groups of processes, and once the inter-relationships between regolith and landforms are understood, landforms can be used to predict regolith patterns. Thus landforms are used as a proxy for the largely hidden regolith. This means understanding the dynamics of the present landscape. It also means understanding the dynamics under which relict landforms and regolith materials formed. Regolith resulting from intense chemical weathering is common on Earth, and consists largely of various kinds of clay minerals. Thick intensely weathered in situ regolith owes its origin to long periods of geomorphic stability, and to the presence of water. Biota may also play a role, although this has yet to be fully studied. On Earth we make a fundamental distinction between transported and in situ regolith. Transported regolith is common on Mars, and can be recognized by its similarities to Earth regolith. Water-borne material shows up as fans and alluvial features, as does colluvial material such as talus deposits. These materials are derived from easily recognized source areas. Blanket deposits such as wind-blown sand, volcanic ejecta, and ejecta from impact craters are also common, and appear to follow the same rules on Mars as they do on Earth.
TECHNIQUES Many techniques may be used in regolith/terrain mapping. Recently A recent collection of papers concerned with geophysical and remote sensing methods for regolith exploration is that of Papp 7. Only some of the techniques are discussed here. In some cases it seems that the more techniques the better, but some workers and organizations appear to be quite limited in their range. Extrapolation of results and the mapping of boundaries are done from aerial photographs and from other airborne and space borne imagery. Our experience is that aerial photographs are by far the best tool for this job, but
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other types of imagery are used where time is limited, and for understanding of the regional picture. Air Photo Interpretation Air photo interpretation or photogeology is extremely useful. Tone, texture, pattern, and association with landforms are usually very well revealed. Practicing regolith mappers get a great deal out of air photograph interpretation, and generally prefer them to more ‘advanced’ imagery.
Figure 2. DEM image of study area, 1km grid, north at top.
Multispectral Remote Sensing Remote sensing of the earth’s surface using multispectral data obtained by satellites has some obvious advantages over air photos. They can be produced in color, false color, enhanced by various computer-based techniques, and they usually cover a wide area. Some such as SPOT imagery can be seen in stereo vision. They may be used in two basic ways: 1. The image may be used as a map, like an air photo but richer in detail, and providing a sort of instant ‘terrain classification’ by dividing the region into obvious parts, which usually have vegetational truth, probable geomorphic and therefore regolith truth, and possibly geological truth. In this instance it is the spatial patterns, shapes and locations which are important. The cause of these spatial patterns is of secondary importance. Pain 8 combined landform mapping with the use of Landsat imagery. He devised a classification of landforms that uses lines, texture, and shape, all observable on Landsat images. The system was tested in the area of Hastings, New South Wales, Australia, where it worked successfully. The basic geomorphic divisions are Erosional Landforms and Depositional Landforms. The map produced is essentially geological, and could be used only as a basis for a regolith map - the regolith was not directly observed.
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Figure 3. Scanned MAS image of MDRS field area, reduced to grayscale North at top.
2. Images may be used to ‘target’ certain areas, or to obtain specific information, from the spectral characteristics of the land surface. A spectral signature is a single feature or pattern of features whose relative intensities in the wavelengths of the image are completely characteristic of a particular material. It has been claimed, for instance, that remote sensing can be used to recognize carbonate, clay and iron oxides. This is a much more dubious use because remote sensing only senses the top few microns of whatever is sensed. The material whose signature we wish to detect is not usually visible . In a typical real world situation the sensor would have to see through the tree canopy. If it manages that it would have to see through the undergrowth. The next obstacle is grass or leaf litter, and only after that can the soil be sensed. Even then the top micron might not reveal much about the underlying regolith or bedrock. Only techniques such as radar, radiometric imagery and airborne electromagnetics can overcome this problem. For this reason the standard success stories of the use of multispectral images come from places like Death Valley, with extreme aridity and devoid of vegetation. Of course even in deserts there are features such as salt crusts and desert varnish, which can confuse interpretation. Wilson9 introduced the term ‘geodermatology’ to stress the fact that in most remote sensing we get reflections from only the top few microns. It is as if we are trying to look through a wall with a hand-held torch. We might hope that by changing the battery or bulb, by using a different tint of lens, or analyzing the reflected light in a computer we might somehow see through the wall. In reality, not only can we not see through the wall, but we cannot even see the wall itself. We can only see the whitewash. The same is true on Mars. Young and White10 suggest that remote sensing has not yet had much impact on the study of landforms. They suggest that this is because it has been considered inappropriate for the detailed scales at which most modern geomorphology is carried out. As far as mapping goes, remote sensing images have been used with some success to map landscape units. They have met with much less success in determining the nature of materials below the earth’s surface, in most cases merely confirming what was already known from other kinds of survey, and especially fieldwork. Nevertheless spectral remote sensing is a valuable tool, though if there had to be a choice most seasoned regolith workers would opt for color air photos or their equivalent, and after that radiometric imagery. This has
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implications for regolith mapping on Mars, where, despite many systems, the mineralogical interpretation of the multi-spectral imagery is still contains many uncertainties. Gamma-ray Spectrometric (Radiometric) Surveys Airborne gamma -ray spectrometry, commonly called airborne radiometrics, is increasingly being used for regolith mapping in Australia 11. Aircraft-mounted instruments flown 60-100 metros above ground level measure the abundance of potassium (K), thorium (Th) and uranium (U) in rocks and weathered materials by detecting gamma -rays emitted during the natural isotopic radioactive decay of these elements. Ninety percent of gamma rays emanate from the top 30-45 cm of dry rock or soil. The intensity of gamma rays emitted from the land surface relates to the mineralogy and geochemistry of the bedrock, and the nature of weathering. Interpretation of this data allows separation between different bedrock types, and between different weathering patterns on the same bedrock type. Present day geomorphic activity must be considered, and allows actively eroding surfaces with shallow regolith to be separated from stable surfaces with deeper regolith and paleo-landforms. It also allows separation of depositional regolith materials derived from different sources, or of different ages. Where landform boundaries were indistinct, radiometrics allowed boundaries to be completed. Radiometrics also allow much more confident extrapolation of field results to areas not visited - an important consideration in all reconnaissance surveys. Radiometric response of regolith materials differs with bedrock type. Within areas of homogeneous bedrock, radiometric signatures can be used with confidence to map regolith types in some detail. Clear distinctions can be made between transported and in situ regolith and between rocks with varying degrees of weathering. Equally important, the radiometrics give a clear indication of the degree of erosional and depositional activity. High-resolution airborne radiometrics thus shows great potential for mapping surface materials. However, more work still must be done on the radiometric signatures of regolith materials. 3-D mapping Although beyond the scope of this paper we note in passing that 3-D mapping of the regolith is possible using such techniques as air and ground-based electromagentic, ground-penetrating radar, and shallow seismic magnetic surveys. Ideally, these need to be supplemented by shallow drilling to provide factual control on the regolith architecture12. REGOLITH LANDFORM MAPPING IN THE MDRS FIELD AREA Data sources Four main sources of spatial data were used in regolith-landform mapping in the MDRS field area. They were: 1. Quarter-million scale geology map of the Salina Quadrangle 13. 2. 1:24,000 scale topographic maps, 3. 1:100,000 scale DEM (Figure 2). 4. Multi-spectral and panchromatic imagery
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The imagery was acquired during an overflight by the NASA ER-2 (civil version of the U-2) on Thursday, December 12, 2002. Approximate flight altitude was 20 km. Although the flight took place within one hour of solar noon, the sun angle was approximately 28 degrees due to the time of year. Data were gathered with a multi-channel spectrometer known as MAS14 shown in Figure 3, a 15-cm black and white film camera, and a 30-cm color IR camera. The B&W camera had a swatch width of approximately 3-km with a resolution of 3-8 meters. The Color IR camera should have a swath width of approximately 1.5 km with a resolution of 1.5 to 4 meters. The mineralogy and water content of regolith materials collected in the field were determined back at the MDRS using a bench-top Portable Infrared Mineral Analyzer (PIMA) manufactured and loaned to the lead author for Expedition One by the manufacturer, Integrated Spectronics Ltd. The PIMA is a short wave infrared reflectance spectrometer which operates in the wavelength region 1300 to 2500nm. Minerals that contain hydroxyl (such as layer silicates, hydroxides, amphiboles, and some sulfates) and carbonate radicals absorb incident radiation at specific wavelengths and in relative amounts that produce diagnostic reflectance spectra (see Shen et al.15 for a typical application). PIMA is also able to measure soil moisture. Methodology For MDRS the regolith mapping system has been optimized at two scales: map (Table 1) and site (Table 2) specific scale. Boundaries at the map scale (Figure 4) were drawn using the airborne imagery as a base. Those at the site scale were drawn using hand held camera photos, and field sketches (Figure 5). Mineralogy of clays and sulfates was determined using the PIMA. At both scales each regolith landform unit is captured by three main descriptors, a landform descriptor in lower case letters, a regolith material descriptor in upper case letters, and a numerical induration modifier (surface crusts at the site scale). Each descriptor is unique. Mapping and site description is a three-step process: 1. Examine the feature of interest and decides what sort of landform it is (hill, rough plain etc.) and assign an appropriate lowercase two-character code. 2. Decide what material the feature is made of (quartz or clay rich rock, sand silt, gravel etc.) and assign the appropriate upper case character. 3. Determine whether there is any surficial induration or binding to form a duricrust (gypcrete, ferricrete, etc.) and assign the correct numeric code. For site scale description this includes surface efflorescence, organic binding, or varnish. Examples: Small sandstone hill Dissected clay slope dsC0 Calcreted gravel terrace atG1 A simplified version of the mapping legend is used in the summary regolith-landform map in Figure 4.
Figure 4. Simplified regolith-landform map of study area.
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REGOLITH-LANDFORM UNITS Map scale regolith landform systems in the MDRS field area The MDRS field area can be divided at the map scale into 18 regolith landform systems. A summary of the stratigraphy is contained in Table 316 17 18. Dissected plains and hills of cracking clays. This regolith landform system is formed on the Brushy Basin Member of the Morrison Formation and forms the North Pinto Hills. Cracking or swelling clays (smectites) are typically formed in arid to sub-arid environments, such as prevailed during the deposition of this unit. The PIMA indicates the presence of montmorillinite and minor nontronite (Figure 6a). Coarse clear gypsum fragments are minor. Clay plains often exhibit patchy efflorescence of sulfate and halite, especially when the surface dries out after rain or in areas of groundwater seepage or surface water ponding. Vegetation is absent, possibly because of the high salinity. Repeated wetting and drying cycles are probably responsible for the concentration of rock fragments on the surface.
Table 1 MAP SCALE REGOLITH-LANDFORM UNITS LANDFORMS Unit
REGOLITH MATERIALS
Code
Unit
SURFACE CRUSTS
Code
Unit
Code
Erosional rise
er
Clay-rich bedrock (undifferentiated)
C
None
0
Smooth slope
ss
Non-cracking clay-rich bedrock (kaolinite, illite)
K
Carbonate
1
Dissected slope
ds
Cracking clay-rich bedrock (nontronite, montmorillinite)
S
Sulfate
2
Wash
ew
Iron
3
Unconfined stream
ch
Quartz rich bedrock
Q
Halite
4
Smooth plain
ep
Silty sediment
L
Rough plain
dp
Sandy sediment
A
Erosional stream terrace
et
Gravel
G
Alluvial plain
ap
Boulders
B
Talus deposit
ta
Silt+sand
LA
Colluvial fan
fc
Silt+gravel
LG
Eolian plain
wp
Silt+boulders
LB
Dunes
wd
Sand+gravel
AG
Sand+bould ers
AB
Polymict sediment
PM
Prefix to indicate significant weathering
W
Relict sediment deposit
rl
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Smooth plains of non-cracking clays. Non swelling clays form in deeply weathered environments (kaolinite, halloysite) or as a result of marine diagenesis (illite). The ma in areas characterized by non-swelling clays in the region are those underlain by the marine sediments of the Tununk and Blue Gate Members of the Mancos Shale (Figure 6b). Clay plains often exhibit patchy efflorescence of sulfate and halite, especially when the surface dries out after rain or in areas of groundwater seepage or surface water ponding. During weathering there has been only limited formation of large clear fragments of regolith gypsum, largely through remobilization from diagenetic veins and beds. Salt efflorescences are present, and vegetation is sparse. Dissected plains and rises of non-cracking clays. This regolith landform unit mainly occurs of Factory Bench in the vicinity of Coal Mine Wash and forms the Lower Blue Hills. In this area it is developed on the Tununk Member, although this unit is also locally developed in other areas where the stratigraphy is dominated by non swelling clays (Figure 6c). Fragments of clear gypsum are common in surface exposures of this unit and have formed during weathering by formation of sulfate by oxidation of sulfides such as pyrite and marcasite disseminated throughout the host shale. Salt efflorescences are present, and vegetation is sparse. Dissected plains of weathered non-cracking clays. Significant deve lopment of weathered profiles are rare in the MDRS field area, occurring only to the north of Factory Butte and in the Coal Mine Wash area where there was thick development (more than 1 m) of saprolitic material. They have formed on Tununk Member. Dissected plains of gypcreted weathered non-cracking clays. The gypcrete is locally present as duricrust cap on weathered Tununk Member on the dissected plains north and west of Factory Butte. Dissected slopes of cracking clays. This regolith landform unit is very well developed in the North Pinto Hills. The main stratigraphic unit on which this regolith landform unit occurs is the Brushy Basin Member of the Morrison Formation, although local, small-scale slopes of cracking clays are also present in the Salt Wash Member (Figure 6d). In the MDRS field area the montmorillinite and nontronite clays are mostly oxidized and therefore lacking in diagenetic pyrite. During weathering there has been only limited formation of large clear fragments of regolith gypsum, largely through remobilization from diagenetic veins and beds, or through precipitation of cyclic salts in groundwater. Minor salt efflorescences are present, and vegetation is sparse. Dissected slopes of non-cracking clays. Such surfaces are very well developed in the vicinity of Factory Butte, North Caineville Mesa, and the foot of Skyline Rim. The first two examples are developed on the Blue Gate Member, while the third is developed on the Tununk Member. Local dissected slopes are developed on the relatively thin interbeds of non-cracking clays in the Salt Wash Member. Fragments of clear gypsum are common in surface exposures of this unit and formed during weathering by formation of sulfate by oxidation of sulfides such as pyrite and marcasite disseminated throughout the host shale. Minor salt efflorescences are present, and vegetation is sparse. Dissected sandstone slopes. This regolith landform unit is characterized by very steep slopes and escarpments. Dissected sandstone slopes form the prominent landforms of the study area Skyline Rim (Ferron Member), Caineville Mesa and Factory Butte (both Emery Member). Visually impressive washes and canyons are also developed in sandstone, including Salt and Neilson Washes (Ferron Sandstone), and White Rock Canyon (Salt Wash Member).
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The importance of this regolith landform unit is under emphasized on maps because of the tendency of sandstone slopes to form vertical or near-vertical escarpments that form caps to dissected clay slopes (Figure 6e). The skeletal lithic soils support sparse vegetation of perennial and ephemeral shrubs, herbs, and grasses. Dissected and smooth sandstone plains (Figure 6e). These features have developed on Factory Bench (Ferron Sandstone) and east of the North Pinto Hills (Salt Wash Member). They are interpreted to form the top of Factory Butte and the Caineville Mesas. The topography of sandstone plains is often quite rough and may be variably mantled by eolian sand. Washes. Ephemeral streams flowing across the MDRS field area have eroded a number of deep washes. These include the formally named White Rocky Canyon, Salt Wash, Tank Wash and Neilson wash and a number of other similar features (Figure 7a). Sediments in washes are typically various mixtures of sand, gravel, and cobbles. They may be moderately vegetated, possibly because of more abundant water, although gypsum, carbonate, and anhydrite are commonly found in them, presumably from water evaporation. Colluvial fans (Figure 7b) are closely associated with alluvial fans but are dominated by gravity flow processes, especially mud flows. Gravity falls and slides and fluvial processes are minor components. As a result they tend to be steeper than alluvial fans and richer in finegrained material so that the sediments are matrix supported. Colluvial and alluvial fans are end members of a spectrum of fan deposits. No medium or large sale alluvial fans were found in the MDRS field area, although small (meter scale) alluvial fans are common at the foot of rills, gullies, and washouts. Colluvial fans are common along the edges of washes, canyons, mesas, buttes, and rims. They are typically of limited extent in the MDRS field area. Talus deposits are formed by rock fall and slides and are dominated by very coarse (cobble to boulder sized) debris with very little matrix. They occur at the foot of escarpments such as Skyline Rim, and may form talus cones, gully fills, and aprons. Like colluvial fans, they are important process indicators, despite their limited extent. Eolian plains. A sandy source is needed to develop deposits of windblown sand, such as the sand-dominated stratigraphic units of the Emery and Ferron Members, Dakota Formation and the Salt Wash Member (Figure 7c). Although in principle sand can be transported by wind for 10’s or 100’s of km, in practice the sand is usually derived from close to source. There are no major deposits of windblown sand in the MDRS field area, although thin cover of sand and small dunes are present in the eastern (Salt Wash Member) and northern (Dakota Formation) parts of the field area. To the north east of Hanksville there are extensive sand dunes, both crescentic and barchanoid, in the San Rafael Desert. These sands are derived from the Entrada Formation, a Jurassic eolian sandstone. In the MDRS field area eolian sand deposits consist of sand ripples, mounds and small dunes or thin topography mantling sheets over sandstone. Local windblown deposits also develop along channels and washes, which are also rich in sand. Deflationary lags are common between these small eolian deposition features. Eolian sands are moderately vegetated by perennial and ephemeral shrubs, herbs, and grasses, possibly because of better drainage and lower salt content to the soil. Channels. Washes are characteristic of fluvial systems cutting through dissected areas of moderate to high relief. Where these debouche onto plains less confined channel systems ten to develop. In the MDRS are the largest fluvial systems, the Freemont River to the south and Muddy Creek to the north east, are still partly confined. As is common in arid environments, these major streams show strong seasonal variation in flow and have a braided morphology. Smaller streams are ephemeral and are also braided, with well-developed bars.
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Sediments in channels are typically various mixtures of sand, gravel, and cobbles. Channel sediments are often sparsely vegetated by perennial and ephemeral shrubs, herbs, and grasses, possibly because of reasonable soil drainage (Figure 7d). Table 2 SITE SCALE REGOLITH-LANDFORM UNITS LANDFORMS Unit
REGOLITH MATERIALS Code
Unit
SURFACE CRUSTS
Code
Unit
Code
Erosional rise
er
Clay-rich bedrock (undifferentiated)
C
None
0
Rills
el
Non-cracking clay-rich bedrock (kaolinite, illite)
K
Carbonate
1
Gullies
eg
Cracking clay-rich bedrock (smectites)
S
Sulfate
2
Smooth slope
ss
Siltstone-sulfate-rich bedrock
E
Iron
3
Dissected slope
ds
Quartz rich bedrock
Q
Halite
4
Wash
ew
Silty sediment
L
Cryptogamic
5
Unconfined stream
ch
Sandy sediment
A
Manganese
6
Pockets (depressions 10 m)
lp
Silt+sand
LA
Channel bar
ab
Silt+gravel
LG
Erosional stream terrace
et
Silt+boulders
LB
Alluvial plain
ap
Sand+gravel
AG
Talus deposit
ta
Sand+boulders
AB
Colluvial fan
fc
Polymict sediment
PM
Groundwater outflow pipe
po
Organic-rich
O
Groundwater downflow pipe
pd
volcanic bedrock
V
Dunes
wd
Prefix to indicate significant weathering
W
Relict sediments
rl
Floodplains. Floodplains are not well developed in the study area, except along the Freemont River (Figure 7e) and Muddy Creek. Localized flood plains are developed in both channels and washes, typically along the insides of meanders. Relict gravel deposits on rises. These consist of locally derived or lags formed by deflation. Others are local patches of coarse gravel and cobbles that contain exotic clasts composed of a range of igneous rocks, limestones, and chert. These are very well founded and bear no relation to current topography. They are interpreted as dissected remnants of high level terraces or streams, either from an ancestral Fremont River, alluvial fans from the Henry
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Mountains to the south, or Muddy Creek (Figure 7f). Relict gravel deposits are moderately vegetated by perennial and ephemeral shrubs, herbs, and grasses, possibly because of better drainage and lower salt content to the soil.
Figure 5. Site scale regolith landform description (see Table 2 for legend)
Site scale regolith landform systems in the MDRS field area In addition to the above units, which are also represented at individual sites, there are a multitude of smaller regolith landforms at the site scale. Most of these will not be described in detail, as they represent local variations. The more important features are outlined below. Pockets, pans, and basins on cracking clay slopes. These are small to medium sized features that form smooth flat areas on gentle to smooth slopes and are surrounded by raised surfaces of cracked clay (Figure 8a). The pockets, pans, and basins are floored by silty clay and have ephemeral pools of water after rain. Small rills may lead into and from them (Figure 8b). A few have downflow pipes within them. Their genesis is obscure, and may be related to the formation of terraces on clay slopes. Volcanic material, commonly exposed on slopes. Green tuffaceous sediments form lenticular beds in both the Brushy Basin Member and the Dakota Formation. These tend to be more resistant to erosion than the enclosing sediment and shed talus down slope. Organic rich bedrock. Coals are found in the upper part of the Ferron Sandstone and crop out along the margins of the appropriately named Coal Mine Wash. Weathered coals are characterized by abundant sulfate, both clear gypsum and yellow jarosite, and by low soil pH. This contrasts with other areas of the MDRS field area which have alkaline soils. Sulfate rich bedrock, commonly exposed on slopes. The Brushy Basin Member and the Sumerville-Curtis Formation (Figure 8c) contain beds and nodules of gypsum and anhydrite that formed during early diagenesis. Some of these beds are quite large and are significant on the site scale as a source of sulfate for sulfur bacteria and for remobilization through the host rock and regolith. Gypsum and anhydrite that formed during deposition and diagenesis is typically cloudy or translucent and is easily distinguished from the clear gypsum formed during weathering. Efflorescences are thin surface layers of white salts. In the MDRS they are mostly gypsum or anhydrite, with secondary halite and calcite (Figure 8d). Efflorescences are especially common on the cracking clays of Brushy Basin Member. The source of the salts in the Member is unclear, but the oxidised nature of the parent sediment suggests it is probably of depositional or early diagenetic origin, rather than from weathering of sulfides. Piping is a groundwater phenomenon characteristic of cracking clays. Inflow or downflow (Figure 8e) and outflow pipes are very common in the brushy Basin member of the Morrison Formation that forms the North Pinto Hills. Outflow pipes are of all sizes (Figure 8f). Table 3 STRATIGRAPHY OF THE MDRS AREA Formation
Member
Age
Lithology
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Mancos Shale
Emery
Late Cretaceous
Yellow fluvial to marginal marine sand
“
Blue Gate
“
Carbonaceous and pyritic marine shale
“
Ferron
“
Yellow fluvial to marginal marine sand, local coal in upper part
“
Tununk
“
Carbonaceous and pyritic marine shale
“
Calcareous cross-bedded channel filling sandstone, conglomeratic sandstone, and conglomerate with local oyster reefs
Dakota
Morrison
“
Brushy Basin
Late Jurassic
Salt Wash
Sumerville
Entrada
Red brown clays and shales with lesser white and green beds, minor green tuffs, red-brown sandstones, and anhydrite or carbonate cemented nodules
“
White, cross-bedded sandstone and conglomeratic sandstone
“
Thin bedded red -brown shales with beds of nodular gypsum and cross cutting gypsum veins. Thin sandstone lenses towards top
Early Jurassic
Thickly bedded cross-bedded brown sandstone with lesser interbedded brown shales
APPLICATION – MDRS AREA Trafficability One of the immediate applications of regolith studies is the relationship between the regolith type and the ability of vehicles and pedestrians to traverse the surface. This is important not only for safety reasons but also because of damage to maintained tracks and to sensitive rangeland soils. In the MDRS field area trafficability varies depending on the regolith type. Clay slopes and plains are easily disturbed to form dust when dry, and when wet are difficult to traverse on foot and impossible in a vehicle. Conversely, sandstone surfaces can be traversed under almost all conditions. Depositional surfaces pose complex challenges. Areas of windblown sand and alluvium pose bogging hazards even when dry, to two wheel drive vehicles. In addition, the vegetation of dunes is easily disturbed. Sediments in washes pose the additional risk of sometimes appearing superficially dry, but they may overlie wet sediment or mud, providing traps for the unwary driver.
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A
B
C
D
E
F Figure 6. a) Dissected plains of cracking clays. b) Smooth plain of non-cracking clays (illitekaolinite). c) Erosional rise. d) Dissected slopes of cracking clay, gullies ~2 m deep. e) Dissected sandstone escarpment. f) Dissected sandstone plain.
Waste water disposal and drainage Drainage properties of the immediate MDRS site are very important for disposal of wastewater through the leach field. The smectite-rich regolith of the Brushy Basin Member of the Morrison Formation poses major challenges to wastewater disposal because of its swelling properties and low permeability. An understanding of the regolith geology at the time of site selection would have allowed better design of the wastewater management system.
A
B
C
D
E
F
Figure 7. a) Wash. b) Colluvium at foot of escarpment of Sumerville Formation. c). Eolian plain with small dunes developed on Salt Wash Member. d) Small unconfined channel. e) Floodplain of Fremont River. f) Relict gravel deposit on rise.
Biology Distribution of biota is strongly controlled by regolith factors, especially salts and moisture. The near absence of higher plants from the slopes and plays of cracking clays developed on Brushy Basin Member is related to the high salt content in this unit. White efflorescences of halite and gypsum were more common on these units than any other, and were especially evidence after moisture from rain or snowmelt had evaporated. Soil temperature also varied markedly according to moisture content, In situ temperature measurements showed that soils that contained moisture detectable by the PIMA were up to 5 degrees cooler than dry soils. It is also likely that the microbiota will vary according to the chemistry, mineralogy, moisture content, soil temperature and landscape context of regolith units, and further investigation into the correlation should prove fruitful.
A
B
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C
D
E
F
Figure 8. a) Pan developed on slope of cracking clay b) Small rills, 1 cm across, developed on smectitic slope. c) Nodular anhydrite in Brushy Basin Member. d) Sulfate and halite efflorescence. e) Down flow pipe. f) Large (2 m high) outflow pipe in gully wall developed on Brushy Basin Member
MARTIAN REGOLITH Although “regolith” was defined first in by Merill in 189719 it gained prominence in the 1960’s when applied to lunar surface materials and then extended to other extraterrestrial bodies. Martian regolith, such as that documented by Stewart et al. 20 has both similarities and differences to terrestrial regolith. As well as water-laid, windblown and gravity sediments, volcanic materials, and weathering horizons similar to those on earth, Martian regolith also includes impact breccias and ejecta, materials rare in terrestrial regolith. Martian regolith also appears to contain abundant water ice, again rare in terrestrial regolith expect at the poles and extreme altitudes. In addition, Martian regolith may lack the biota that plays a key role in terrestrial regolith processes. Following lunar practice, the term “megaregolith” has been used on Mars21, referring to the uppermost 10-15 km of the planet’s crust. The megaregolith includes both thick volcano-sedimentary sequences and the upper part of the primordial Martian crust that consists of has undergone brecciation and/or fracturing because of largescale early impacts. On earth the record of large -scale early impacts has been lost, and the thick volcano-sedimentary successions of basins are not normally considered regolith. In this paper “regolith” will refer to the uppermost part of the Martian crust, the surficial sediments, weathered rock and associated materials. Despite these differences, Martian regolith can be studied using similar tools to terrestrial regolith. For example, Moore et al22. described the engineering and physical properties of regolith at the Viking 1 and 2 landing sites, while Toulmin et al.23 described the mineralogy and chemistry of the same sites largely in terms of weathered minerals. More recently the mineralogy and chemistry of the Pathfinder landing site has been described by Ward et al. 24 and Moore et al.25 as have the physical properties26 27. Furthermore, studies of terrestrial regolith and landforms can provide important insights into Martian processes 28 29. In the remainder of the paper we give brief examples of the application of techniques such as those used at the MDRS site to representative sites on Mars. REGOLITH-LANDFORM MAPPING ON MARS Data sets Numerous useful data sources already exist that are of considerable value to Mars regolith-landform mapping. Data sets from the ongoing Mars Global Survey and Mars Odyssey 2001 probes include: gamma-ray spectroscopy (GRS) 30, and both thermal infrared spectrometry (TES) and imaging thermal, near thermal, and visible multispectral data (THEMIS)31. In addition there is high-resolution imagery from the Mars Orbital Camera (MOC) and Mars Orbiting Laser Altimeter (MOLA)32, and from surface neutron emissions (HEND)33. Older missions also supply useful data including Viking34 and Phobos 235. However, the equivocal and often conflicting interpretations of surface mineralogy obtained from these data sets means that on mars, as on earth, there is no substitute for detailed field work on the surface.
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Examples of orbital mapping There is an extensive literature on interpretations of the Martian surfaces using remotely sensed data. Some of these have incorporated aspects of what we refer to as regolith-landform mapping. For example, Baker 36 contains numerous examples of geomorphic maps of Martian channel and valley complexes. The geology map (their plate 1) of Greeley et al. 37 includes both interpreted surface materials and geomorphology, but is mainly a geomorphology or landform unit map. The study of Guest et al. 38 contains a surface materials map (their Figure 1). Appendix C of the seminal work by Mutch et al. 39 contained numerous examples, albeit at a very regional scale, of geomorphic maps. However, more recent maps, such as those of Tanaka40, are more akin to terrestrial geological maps rather than regolith maps in that they focus on stratigraphic relationships rather than landform relationships. Age relationships are not required in regolith-landform maps because of the difficulty in ascribing meaningful ages to regolith-landform units. An example of the application of terrestrial regolith-landform mapping to methodologies to Mars is shown in Figure 9. This is based on MOC image E03-0255041 of a small crater within the Newton Basin located at 42.4°S, 158.2°W. In this figure, we subdivided the area on the basis of landforms and surface material characteristics using methods essentially similar to those used for mapping regolith-landforms on Earth. Surface regolith characteristics are correlated with landscape position, allowing us to compile a map of regolith-landform units. The only missing element is ground validation of the units, The regolithlandform units are as follows:
Figure 9. Regolith-landform map on Mars based on satellite image E01-01839 (Edgett and Malin 2002). See text for legend. Image width 2.82 km
Unit 1: Smooth plain. Bright areas may be finer-grained materials. Because of the lack of images showing the context of this unit, it is difficult to assess its regolith-landform characteristics. Unit 2: Irregular plain. Surface regolith probably consists of loose particles of various sizes. Again, because of the lack of context, this unit is difficult to interpret. Unit 3: Steep, rough slopes in layered sediments. Unit 2 may be formed on the same material. Steep valleys have been cut into this unit. Unit 4: Smooth steep slopes covered with fine-grained sediment. Parts of this unit show the underlying material showing though. Unit 5: Steep densely gullied slopes. The distinct sub-horizontal boundary between this and unit 3 suggests this unit is formed on the “bedrock” underlying unit 3. Gullies are particularly well developed on the right-hand occurrence of this unit, and have a form very similar to gullies formed on steep partially weathered bedrock on Earth. Unit 6: Colluvial fans with incised channels in the upper parts, and moderately rough un-dissected surfaces at the lower ends. There are also minor scarps at the lower ends. Unit 7: Dune field with small slightly arcuate dunes with eolian sand. Unit 8: Bedrock outcrops. Unit 9: Dune field with large low transverse dunes with eolian sand.
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What is missing from this map is information on the surface composition, which would require integration with information from instruments such as the GRS, HEND, and TES.
Figure 10. Regolith-landform sketch on Mars based on Viking 1 lander image. 1. Smooth to slightly surface with sandy regolith, probably eolian in origin. 2. Irregular but low relief surface with abundant boulders and cobbles scattered on a sandy matrix. 3. Irregular surface with dense cover of coarse boulders and cobbles. Bedrock may outcrop in places (a). The hill on the horizon (b) may be similar to unit 2.
Ground surveys Only three surface locations have be described in detail, those of the Viking 142, Viking 243 (Mutch et al. 1977), and Pathfinder 44 lander sites. Research on these sites has been essentially of regolith, as, with the possible exception of the Viking 1 site, no bedrock has been exposed. Studies relevant to regolith geology have included morphology45 46 47, as well as physical48 49 50 51 and chemical52 53 properties. With these data, especially the photographic imagery, it is possible to carry out sitelevel regolith-landform description analogous to those carried out it Utah. Figure 10 illustrates such an approach using a Viking 1 lander image. Interestingly, Binder et al54. describe a duricrust from the Viking 1 landing site (their Fig. 6). The exposed surface may or may not be a duricrust, but it exhibits all the features of dirt cracking. Dirt cracking, described by Ollier 55, occurs when rocks on the surface crack as a result of rapid changes in temperature, a process called insolation weathering. Once the rock is cracked, dirt is able to get into the cracks during cold periods when the rock shrinks and the cracks open. The dirt holds the cracks open, and when warmer temperatures cause the rocks to expand, the cracks are widened. This process, common in deserts on Earth, would seem likely to be common on the surface of Mars as well, especially given the diurnal temperature range at the Viking 1 site of almost 50 degrees K56. Application The application of regolith landform mapping to Martian science and technology are as legion as terrestrial applications. Five examples will suffice.
1. They provide an alternative to existing surface materials, landform, and stratigraphic maps. Regolith landform maps will allow new correlations to be drawn between surface materials, landscape position, and surface texture. 2. Regolith landform mapping is particularly suited to locate possible habitats for putative microorganisms, including areas with abundant moisture, nutrients, and solar energy. 3. We would agree with the statement of Hoffman57, quoting Eppler, that sampling is not sufficient, it is vital to understand the context of samples. This requires detailed field working, including mapping, site description, and understanding the architecture of the local geology. This is especially important in regolith geology, where, in many cases geoscientists from other disciplines, such as petrology, geochemistry, and geophysics, are ill equipped to understand its complexities.
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4. Regolith terrain mapping can play a role in selecting areas most suitable from engineering perspectives for landing space probes and rover traverses. 5. Should crewed bases be planned, a detailed understanding of the regolith, obtained from mapping and understanding its composition, distribution, and architecture, will be vital. Such applications and the knowledge necessary to achieve it is assumed in the popular and semi-popular literature, such as Zubrin and Wagner’s book The Case for Mars 58. This last point is worth developing further.
Implementing such plans in practice, whether preparing sites for instrument stations and roads 59, obtaining water supplies from permafrost60, extracting materials for making concrete 61 bricks, and other materials 62, preparing growth medium for plants63, or quarrying material for radiation shielding 64 is going to need considerable detailed knowledge of the regolith. It is not sufficient to note the presence of minerals suitable for cement or ceramics in Martian regolith. These minerals must occur in the right proportions, correct abundance, and lack impurities. Ice is likely to exist in permafrost, however how that water can be extracted and in what quantities will depend on the nature of the frozen regolith, as will the composition of the extracted water. Martian soil may well contain all the chemical elements for plant growth, however it may also contain high levels of salts, as well as being possible highly acid65. Other elements such as aluminum may also be present in toxic quantities and the Martian regolith may need extensive treatment plants can be grown in it. Possible hazards posed by regolith include trafficability of different surfaces, airborne dust and chemical toxicity66. Damage to vehicles or EVA suits from rocks and the risk of getting bogged is soft surfaces are well known to terrestrial explorers. They are almost impossible to predict from remotely sensed data without extensive ground experience. Airborne dust can damage through abrasion while magnetic particles, known to exist on Mars can cause damage to electrical systems. If the dust enters the interior of habitats, rovers, or suits it may pose a hazard if respired. Potential chemical hazards of the Martian surface include the known presence of abundant salts and the inferred presence of strong oxidizing agents such as peroxides, low pH soils and surface water, and relatively abundant toxic chromates. The combination of salts and moisture will be as damaging to machinery on Mars as it is on earth and, if the Martina regolith does contain chemically toxic components, care must be taken to keep crew exposure to within acceptable limits, especially to respired limits. This will be a challenge for in dusty environment. Achieving these goals will require extensive, detailed and sophisticated understanding of Martian regolith and its terrestrial analogs.
CONCLUSIONS Regolith-landform mapping is important part of any field study on earth of surface materials, landscape, biota, and hydrology. It is also an essential platform on which to base more specific applications, such as civil engineering, understanding landscape and regolith evolution, resource extraction, landscape ecology, or geophysical and geochemical research. Techniques of regolith-landscape mapping developed in Australia in recent years proved readily applicable to the MDRS field area in Utah and appear applicable to the surface of Mars. Although aspects of regolith-landform mapping, such as morphology and nature surface materials have been a part of Martian studies since the earliest missions, to our knowledge no study to date has routinely integrated surface materials and surface morphology into regolith landform maps. Given the utility of such an approach, it is our recommendation that regolith landform mapping and site description be an integral part of studying terrestrial analogs as well
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as of Mars itself. Furthermore, the complex, unpredictable nature of regolith and the difficulty in extrapolating from remote sensing to the 3-D reality means that there will be no substitute to ground mapping and description of the regolith if it to be understood and utilized. ACKNOWLEDGEMENTS The authors would like to thank the following people for their comments and input. On Expedition One: M. Battler, J. Butler, F. Karouia, V. Nguyen, R Persaud, S. Rupert-Robles, and N. Wood. In Australia M Thomas, while figures were drafted by XXX. David Bushman, NASA Dryden Flight Research Center, who was also a participant in Expedition One provided the ER-2 data. Another participant in Expedition One, Jean Legarde, scanned and also prepared the DEM. The field expedition was funded through Mars Society Australia.
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