A model for formation of dust, soil, and rock coatings on ...1.1. Composition of Martian Surface Rocks and Soils [3] Characterization of the composition of Mars has been approached

A model for formation of dust, soil, and rock coatings on Mars:

Physical and chemical processes on the Martian surface

Janice L. Bishop,1 Scott L. Murchie,2 Carle M. Pieters,3 and Aaron P. Zent4

Received 31 August 2001; revised 14 February 2002; accepted 18 April 2002; published 6 November 2002.

[1] A model is presented here to explain the generation of surface material on Mars usingchemical, magnetic, and spectroscopic data from Mars and geologic analogs fromterrestrial sites. One basic premise is that the dust/soil units are not derived exclusivelyfrom local rocks, but are rather a product of global, and possibly remote, weatheringprocesses. Another assumption in this model is that there are both physical and chemicalinteractions of the atmosphere with dust particles and that these two processes createdistinctly different products. Physical processes distribute dust particles on rocks and driftunits, forming physically aggregated layers; these are reversible processes. Chemicalreactions of the dust/soil particles create alteration rinds on rock surfaces and cohesive,crusted surface units between rocks, both of which are relatively permanent materials.According to this model the dominant components of the dust/soil particles are derivedfrom alteration of volcanic ash and tephra and contain primarily nanophase and poorlycrystalline ferric oxide/oxyhydroxide phases as well as silicates. These phases are thealteration products that formed in a low-moisture environment. These dust/soil particlesalso contain a smaller amount of material that was exposed to more water and containscrystalline ferric oxides/oxyhydroxides, sulfates, and clay silicates. These componentscould have formed through hydrothermal alteration at steam vents or fumeroles, thermalfluids, or through evaporite deposits. Wet/dry cycling experiments are presented here onMars soil analogue mixtures containing poorly crystalline and crystalline componentsdominated by nanophase to �2 mm diameter particles. Cemented products of these soilmixtures are formed in these experiments, and variation in the surface texture wasobserved for samples containing smectites, nonhydrated silicates, or sulfates. Reflectancespectra were measured of the initial particulate mixtures, the cemented products, andground versions of the cemented material. The spectral contrast in the visible/near-infraredand midinfrared regions is significantly reduced for the cemented material compared to theinitial soil and somewhat reduced for the ground, cemented soil compared to the initialsoil. The results of this study suggest that transient fluvial activity on Mars will have aprofound effect on the texture and spectral properties of the dust/soil particles on thesurface. The model developed in this study provides an explanation for the generation ofcemented or crusted soil units and rock coatings on Mars and may explain albedovariations on the surface observed near large rocks or crater rims. INDEX TERMS: 6225

Planetology: Solar System Objects: Mars; 3934 Mineral Physics: Optical, infrared, and Raman spectroscopy;

5470 Planetology: Solid Surface Planets: Surface materials and properties; 5415 Planetology: Solid Surface

Planets: Erosion and weathering; 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere

interactions; KEYWORDS: Mars, spectroscopy, chemical alteration, aeolian processes, cemented soil, rock

coatings

Citation: Bishop, J. L., S. L. Murchie, C. M. Pieters, and A. P. Zent, A model for formation of dust, soil, and rock coatings on Mars:

Physical and chemical processes on the Martian surface, J. Geophys. Res., 107(E11), 5097, doi:10.1029/2001JE001581, 2002.

1. Introduction and Background

[2] This paper describes analysis of the spectral, chemicaland magnetic data from Mars Pathfinder soil, together withanalog materials and the products of laboratory alterationexperiments, in order to describe possible mechanisms forthe formation of drift, cemented soil and rock coatings onMars. Soil analog mixtures have been prepared, character-ized and tested through wet/dry cycling experiments for

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. E11, 5097, doi:10.1029/2001JE001581, 2002

1SETI Institute/NASA Ames Research Center, Moffett Field, Califor-nia, USA.

2Applied Physics Laboratory, Johns Hopkins University, Laurel,Maryland, USA.

3Department of Geological Sciences, Brown University, Providence,Rhode Island, USA.

4NASA Ames Research Center, Moffett Field, California, USA.

Copyright 2002 by the American Geophysical Union.0148-0227/02/2001JE001581$09.00

7 - 1

changes in binding and spectroscopic properties that arerelated to what could be expected for cemented soil units onMars. Hydrothermal and solfataric alteration of volcanictephra frequently produces reactive sulfate species that mayattack the surfaces of dust particles and rocks to formcemented soil units and rock coatings. This idea is pursuedhere. Recent magnetic, chemical, spectral and imaging datafrom Mars Pathfinder suggest that surface alteration and/orweathering has taken place on Mars and that there may becoatings on the rocks resulting from these processes [Mad-sen et al., 1999; McSween et al., 1999; Bell et al., 2000].Combining these results from Pathfinder with results fromlaboratory studies [e.g., Bishop et al., 1995, 1998a; Bishopand Murad, 1996; Banin et al., 1997; Morris et al., 1997,1998, 2000, 2001] provides the clearest picture of themineralogy and potential alteration processes on Mars.

1.1. Composition of Martian Surface Rocks and Soils

[3] Characterization of the composition of Mars has beenapproached through telescopic observations as well asinstruments on landers and orbiters. The spectral propertiesof bright and dark regoliths on Mars have been summarizedby Soderblom [1992] and Roush et al. [1993] and ofalteration minerals on Mars by Bell [1996]. More recentspectroscopic analyses of Mars have shown that global rockcompositions are basaltic to andesitic [Bandfield et al.,2000; Christensen et al., 2000b], that coated rocks arepresent at the boundaries of bright regions in Arabia andTharsis [Murchie et al., 2000a], and that a large region ofspecular gray hematite is present in Sinus Meridiani [Chris-tensen et al., 2000a, 2001a]. Thermal inertia data measuredby Viking indicate that regional dust deposits are found onMars at Arabia, Tharsis and Elysium [Christensen, 1986].More recent thermal inertia data from the Thermal EmissionSpectrometer (TES) on Mars Global Surveyor (MGS) haveenabled mapping of global dusty regions [Mellon et al.,2000] and a newly reported boundary for much of the dustyregions that may consist of well-indurated duricrust [Chris-tensen et al., 2001b]. These indurated regions identified byChristensen et al. [2001b] as having intermediate inertia andalbedo are consistent with the areas found by Murchie et al.[2000a] to have intermediate near-infrared brightness andband depths, plus a negative slope, which are attributed tocoated rocks.[4] The Viking landers in Chryse Planitia and Utopia

Planitia provided evidence of fine-grained drifts and blockymaterial superposed on a rocky substrate, impact craterrims, slow erosion rates of rocks and surface material, soilredistribution, surface condensate formation, and freeze-thaw processes in some regions on the surface [Binder etal., 1977; Mutch et al., 1977; Jones et al., 1979; Arvidson etal., 1989]. Thin layers of dust were observed to accumulateat both sites during the first two Martian years [Arvidson etal., 1989]. The MPF site is located at Ares Vallis in a rockytransition region between typical bright and dark regions[Golombek et al., 1997]. The chemical and mineralogicalcompositions of the rocks near the Mars Pathfinder (MPF)lander indicate that they are andesitic-basaltic and that manyrocks are covered with alteration rinds [McSween et al.,1999]. Analysis of the Pathfinder soil units has shown thatthey are chemically and mineralogically distinct from therocks, and that they may contain goethite, maghemite or

other iron oxyhydroxides [Bell et al., 2000], but do notshow the expected evidence for crystalline hematite basedon previous telescopic measurements of Mars [Bell et al.,1990]. Chemical mixing models have shown that simplesoil-rock mixing scenarios are inadequate to explain thechemical composition of the Martian rocks and soils[McLennan, 2000; McSween and Keil, 2000] and that theglobal dust on Mars resulted from weathering of basalticrather than felsic rocks [McLennan, 2000; McSween andKeil, 2000; Morris et al., 2000].[5] The magnetic tests on the Viking and MPF landers

indicated the presence of a magnetic component intimatelymixed with nonmagnetic components, such as silicates, inthe dust/soil particles [Hargraves et al., 1977; Pollack et al.,1977; Hviid et al., 1997; Madsen et al., 1999]. Saturationmagnetization, Js, has been estimated from the magneticexperiments on the MPF lander and depends on theassumed particle density. Madsen et al. [1999] estimate Jsat 4 ± 2 Am2/kg, while Morris et al. [2001] estimate Js at2.5 ± 1.5 Am2/kg. A few wt.% Fe2O3 as maghemite[Hargraves et al., 1977; Hviid et al., 1997; Madsen et al.,1999] and magnetite/titanomagnetite [Pollack et al., 1977;Morris et al., 2000] have both been proposed as themagnetic minerals present in the Martian dust/soil basedon these data, leaving the bulk of the iron in the Martiandust/soil in ferrous silicates or nonmagnetic (probablynanophase) ferric oxides/oxyhydroxides.

1.2. IMP Spectra

[6] The Imager for Mars Pathfinder (IMP) has producednumerous spectra of specific rock and soil units near thelander [Smith et al., 1997]. These spectra have beenrecalibrated and compared with both chemistry and viewinggeometry in a recent study [Bridges et al., 2001]. Spectralanalyses of laboratory soil analog materials and several soilunits measured by the IMP showed that there are specificdifferences in the soils observed by Pathfinder that areattributed to differences in mineralogy and composition ofthe soils [e.g., Bishop et al., 1998d; Bell et al., 2000; Morriset al., 2000]. Specifically, many of the soils are character-ized by a smooth upward slope from 0.5 to 0.7 mm and abroad maximum near 0.75–0.80 mm. Some soil units have astronger shoulder feature near 0.6 mm and some exhibit aweak, broad band near 0.9 mm. Variations in the chemicalabundances of these soils indicate a good correlation ofwt.% SO3 with the red/blue reflectance ratios [Bridges etal., 2001]. This suggests that the ferric minerals responsiblefor the reflectance maximum near 0.75 mm are correlatedwith sulfates in the soils. Ratios of numerous spectralparameters have been performed in order to characterizethe subtle variations in the IMP spectra and are summarizedby Bell et al. [2000].[7] The spectral features observed in the extended visible

region are due to electronic transitions and charge transferbands [Morris et al., 1985; Sherman and Waite, 1985; Burns,1993]. Spectral analysis using second derivatives is usefulfor identifying minerals in mixtures from their specificcrystal field splitting parameters and electronic transitions[Scheinost et al., 1998]. A study involving spectral analysiswith second derivatives of Martian soil spectra and analogsoil spectra enabled determination of the 6A1 ! 4T1g and6A1 ! 4T2g electronic transitions and crystal field splitting

7 - 2 BISHOP ET AL.: MARS SOIL AND ROCK ALTERATION MODEL

parameters [Bishop et al., 1998d]. This enabled separation ofgroups of iron oxyhydroxide-silicate aggregates according toboth chemical conditions during formation and sulfate con-tent. This study showed that the spectral character of manyof the Martian soils measured by IMP includes a reflectancemaximum near 0.8 mm that is consistent with nanophaseferric oxides/oxyhydroxides. It also demonstrated variabilityin the shape of the 0.6 mm shoulder feature that would beconsistent with variable abundance of a sulfate (e.g.,schwertmannite, jarosite) or goethite. A related study ofMartian rocks measured by IMP attributed this �0.6 mmshoulder to the presence of goethite or sulfate minerals in therock coatings [Barnouin-Jha et al., 2000].[8] If magnetite is also a soil component it could be

responsible for weakening the �0.9 mm absorption charac-teristic of ferric oxides. Preliminary analyses (unpublishedspectral mixing experiments) indicate that adding the �6%maghemite estimated for the Martian soils near the MPFlander [Hviid et al., 1997; Madsen et al., 1999] would notgreatly alter the spectral character of these samples. Naturalsoils containing magnetite are darker than otherwise similarsamples, exhibit a weaker 0.9 mm band and a broad, shallowabsorption centered near 1.5 mm [Scheinost et al., 1998;Morris et al., 2000]. Morris et al. [2000] further observedthat the finest size fractions of altered tephra become darkerwith increasing magnetic susceptibility due to the presenceof magnetic Fe,Ti-oxides.

1.3. Terrestrial Alteration of Volcanic Tephra/Ash

[9] Recent studies of the fine-grained fractions of vol-canic material indicate that there are multiple alterationprocesses taking place: palagonitic, pedogenic or solfataricalteration of volcanic tephra, ash or lava [e.g., Morris et al.,1993, 1996, 2000; Bishop et al., 1998a, 2002; Schiffman etal., 2000, 2002]. Long-term palagonitic weathering ofvolcanic tephra in a dry environment have been contrastedwith the more rapid and/or intense solfataric alteration in thevicinity of steam vents and cinder cones [Bishop et al.,1998a]. Solfataric (or sulfatetic) alteration indicates thatsulfuric fumes from cinder cones, fumeroles and volcanicexhalations of H2S or hydrothermal waters containingH2SO4 or sulfate ions, are actively participating in thealteration process. Palagonitic weathering involves hydra-tion and vitrification of basaltic glass to form fine-grainedand poorly crystalline ferric oxides/oxyhydroxides andsilicate phases under low-temperature and low-moistureconditions, with the formation of smectite and serpentineclay minerals under higher temperature and higher moistureconditions [e.g., Bates and Jackson, 1984; Bishop et al.,2002; Morris et al., 2001; Schiffman et al., 2002]. Thesolfatarically altered tephra often contain significantamounts of crystalline iron oxides/oxyhydroxides and jar-osite/alunite, while the palagonitic tephra tend to containpoorly crystalline and/or nanophase minerals. Recent stud-ies included chemical, mineralogical and spectroscopicanalyses of jarositic tephra from Mauna Kea [Morris etal., 1996], from a cinder cone in the Haleakala crater basin[Bishop et al., 1998a], and from the Kilauea region [Morriset al., 2000]. Analysis of Martian bright region spectra andspectra of sulfate minerals formed in terrestrial steam ventsor fumeroles found that a number of these alterationproducts are consistent with the visible/near-infrared

(NIR) spectra of Mars [Calvin et al., 1999]. Potential factorscontributing to ferric oxide/oxyhydroxide formation alongwith jarosite and other sulfates in such soils include thetemperature and composition of the volcanic gases, durationof hydrothermal exposure, and oxidation following hydro-thermal alteration.[10] Palagonitic alteration of glassy basalts and andesites

has been characterized using a variety of geochemical andmineralogical techniques [e.g., Colman, 1982; Staudigeland Hart, 1983; Nesbitt and Young, 1984; Fisher andSchminke, 1984]. These studies have shown that volcanicglass and olivine are generally the least stable componentsof basaltic deposits, while Fe,Ti-oxides are the most resist-ant to alteration. According to these and other studies, thetypical low-temperature alteration products of mafic vol-canic rocks are a mixture of short-range ordered alumino-silicates (e.g., allophane and imogolite), amorphous ironoxides/oxyhydroxides, and poorly crystalline and crystallineclay minerals (including smectite and kaolinite). Palagoni-tization and formation of weathering rinds on basalts andandesites are also accompanied by large reductions in Ca,Mg, Na and K, depletion in Si, oxidation of the Fe, andincorporation of water [e.g., Fisher and Schminke, 1984].

1.4. Possible Alteration Scenarios on Mars

[11] Our model is based upon mixing and distribution ofdust/soil components and chemical reaction of selectedspecies in the dust/soil particles. The aeolian processesinvoked for our model are similar to patterns of dustdeposition and removal described by McSween et al.[1999] in order to explain variable rock coatings. Our modelis distinct from others presented recently including ‘‘acid-fog’’ weathering, where atmospheric aerosol particles arethe active force behind chemical alteration of rocks [Settle,1979; Banin et al., 1997] and hydrothermal fluids (neutralchloride and acid-sulfate) associated with impact events orvolcanism as a source of the mobile elements in the soil[Newsom et al., 1999]. Our model involves chemicalreactions of minerals and alteration phases in the dust/soil.Other alteration scenarios that have been described aspotentially contributing to soil formation on Mars includechemical alteration of Martian meteorite-like rocks [Dreibuset al., 1998; Bruckner et al., 1999; Wanke, 1999], partialpalagonitization of volcanic ash [Morris et al., 2001] andphysical alteration of rocks [Bridges et al., 1999; Ruff andChristensen, 1999]. Impacts have also been suggested asmechanisms of producing glassy alteration products con-taining ferric oxides on Mars [Morris et al., 1995; Schultzand Mustard, 1998; Minitti and Rutherford, 2000; Yen,2001].[12] These potential alteration scenarios are built upon

observations of Mars and evidence for water, condensates,dust and duricrust on the surface. Early studies of Marssuggested the possibility of water and/or ice on the surface[Salisbury, 1966; Anderson et al., 1967; Wade and de Wys,1968; Masursky, 1973; Baker and Milton, 1974]. Analysisof the channels in Viking images led Masursky et al. [1977]and Carr [1979] to suggest the possibility of fluvial activity.Seasonal reservoirs of water and ice have been described interms of interactions with the regolith, ice caps and atmos-phere [Jakosky, 1983a, 1983b; Hart and Jakosky, 1986;Kahn, 1990; Svitek and Murray, 1990]. Squyres et al.

BISHOP ET AL.: MARS SOIL AND ROCK ALTERATION MODEL 7 - 3

[1992] provide a summary of ground ice activity on Marsprior to Mars Global Surveyor. More recent evidence ofwater/ice activity has been provided by analysis of MOCimages [e.g.,Malin and Edgett, 2000b; Costard et al., 2001;Gaidos, 2001; Mustard et al., 2001] and MOLA data [Smithet al., 2001]. Recent spectral observations and analysis ofH/D ratios in the Martian atmosphere [Krasnopolsky andFeldman, 2001] and analysis of H/D ratios in a Martianmeteorite [Leshin, 2000] are consistent with even morewater on early Mars than previously considered.[13] Evidence of condensates on Mars were reported in

analyses of Viking images [Jones et al., 1979; Wall, 1981]and laboratory studies suggest that these condensates mayinclude interactions between ice and the dust particles[Gooding, 1986; Saunders et al., 1986]. Sulfate aerosolcalculations indicate that they may have formed globally onMars from surface volcanism and that these sulfate aerosolswould be suspended with ice condensates and dust particlesin the atmosphere for long periods [Settle, 1979]. Dust haslong been observed to be active on the surface of Mars [e.g.,Martin et al., 1979; Pollack et al., 1979; Colburn et al.,1989] and analysis of Viking images produced evidence ofoptically dense dust clouds and much smaller features thatmay include dust devils [Briggs et al., 1977, 1979]. A largenumber of dust devils were later identified on Mars byevaluating selected Viking orbiter images for transientsmall, elongated clouds with nearly vertical orientation[Thomas and Gierasch, 1985]. Cemented soil or duricrustunits were first observed on the surface of Mars near theViking landers [Binder et al., 1977; Mutch et al., 1977].Analysis of radar and thermal inertia measurements on Marswere best explained by a global distribution of duricrust in astudy by Jakosky and Christensen [1986]. They cite thepresence of water and migration of salts as contributing tothe duricrust formation.

2. Model Outline

[14] A model was developed that describes potentialmechanisms of rock coating and duricrust formation onMars [Bishop et al., 1999]. We acknowledge that the dataavailable at this time are insufficient for unique determi-nation of any given surface model and we are proposing thisas one of many possibilities for consideration. This modelstates that (1) the �2 mm sized ‘‘dust’’ particles that aresuspended in the atmosphere on Mars are redeposited on thesurface of Mars, and are compositionally similar to the‘‘soil’’ covering the tops of rocks and the surface in betweenrocks, (2) physical processes (e.g., wind, dust devils)primarily govern distribution, aggregation and disaggrega-tion of the dust, soil and dust layers on rocks, and (3)chemical processes (e.g., chemical reactions of the sulfateswith the ferric oxide and silicate surfaces in the presence ofwater and/or redox reactions of Fe2+/Fe3+) are primarilyresponsible for the formation of rock coatings and duricrustand that once formed, these chemical layers cannot besimply disaggregated by the wind. This model has beenrefined and expanded into the present form and has theadvantage that it incorporates both chemical and physicalprocesses observed on Mars in previous studies and, further,that it builds on alteration processes observed in the fieldand laboratory.

[15] In this model for surface alteration on Mars volcanicsteam vents and hydrothermal activity associated withvolcanism are primarily responsible for the production ofcrystalline, Fe and S-bearing minerals such as hematite,maghemite/magnetite, and jarosite/alunite. Evaporite depos-its may be another source of sulfates and salt minerals.These minerals form in high concentrations (� half of thecomposition of the altered ash/soil) in specific locations –abundant, yet not ubiquitous. In contrast, palagonitic alter-ation of volcanic ash is more common and prevalent, andresults in nanophase Fe oxide phases, poorly crystallineclays and/or protoclays (e.g., allophane and imogolite),poorly crystalline grains of feldspar and pyroxene, and tinyglass particles. Aeolian mixing of these two alterationmaterials produces a nonhomogenous but regionally similar,fine-grained material. This explains the subtle differences indust/soil composition measured at the Viking and Pathfinderlander sites [Clark et al., 1982; Bell et al., 2000; Bridges etal., 2001; Foley et al., 2001].

2.1. Physical Interactions

[16] Mars sustains periodic large-scale dust storms thatcan vary in size and intensity up to complete global cover-age. A persistent background optical depth of 0.3–1.0 wasmeasured at the Viking Lander sites for visible wavelengths[Pollack et al., 1979]. Changes in atmospheric pressure,temperature and wind direction measured by Schofield et al.[1997] were consistent with dust devils passing the Path-finder lander. These events were described as small-scaleconvective vortices lasting for a few minutes or less at thelander position and accompanied by a pressure drop of�0.03 mbar in the seconds preceding a local temperaturemaximum, followed by a return of the atmospheric pressure.Wind tails have been observed at the Viking and MPFlanding sites and suggest that aeolian processes have beenactive [Arvidson et al., 1989; Greeley et al., 1992, 1999].Localized dust devils have been identified on Mars inimages taken by Viking [Thomas and Gierasch, 1985],Pathfinder [Metzger et al., 1999] and MOC [Edgett andMalin, 2000]. A mean dust particle size at the MPF landersite of �1.6 mm in radius was determined from pressurechanges and calculations of dust loading [Smith and Lem-mon, 1999; Tomasko et al., 1999], which was also observedat the Viking sites [Pollack et al., 1995]. Evidence foraeolian distribution of dust particles on Mars has beenobserved in nearly every measurement/experiment suggest-ing that it is a pervasive surface process [Greeley et al.,1999]. Metzger et al. [1999] estimated that a dust devillocated on the southern flank of the South Twin Peak had avortex 14 m wide by 230 m high and a ground speed of 0.5m/s. This indicates that dust transport is a dynamic process,even at the small spatial scales of the MPF landing site andthe small temporal scales of the three month MPF mission.[17] According to our model dust particles settle on the

rocks to form physical dust coatings (a thin layer sprinkledon the surface) and cover the surfaces in between rocks toform drift deposits on Mars. The physical coatings and driftunits in this model are compositionally similar to theatmospheric dust particles; the primary difference is thatthe drift deposits and rock coatings contain larger aggre-gates of dust particles, held together by electrostatic orphysical forces. We suggest that these local dust devils


could be disaggregating and suspending soil particles andredistributing them as 1–3 mm sized dust particles. Wefurther suggest that there is a reversible interaction betweenthese dust particles suspended in the atmosphere and the soilparticles covering the surface. As long as the particles arekept in motion and are prevented from forming chemicalbonds, they can be readily disaggregated and/or redistrib-uted. A diagram of this process is shown in Figure 1, whereparticles can be transformed from atmospheric dust to driftand from atmospheric dust to physical rock coatings andback to dust again with the next dust devil or dust storm.[18] Spectral and chemical evidence from Mars Path-

finder revealed that many of the surface soil units arecompositionally similar, and that spectral differences formany units are thought to be due to grain size andcompaction [Bell et al., 2000]. The ‘‘brown’’ soil unit isamong those that are compositionally distinct from theprimary soil type near the MPF lander. The spectral proper-ties of dust particles suspended in the atmosphere near MPFappear different from spectral properties of the drift depos-its, especially in the blue wavelengths [Thomas et al., 1999;Bell et al., 2000]. However, the spectral properties of

suspended particles are not well characterized because theyare difficult to measure and would include scatteringthrough greater distances than for dust deposited on asurface, so this is not necessarily inconsistent with ourmodel.[19] McSween et al. [1999] suggest based on laboratory

studies that coatings of drift material on rocks near the MPFsite may be responsible for some of the rock spectralvariability [Fischer and Pieters, 1993; Johnson, 1999;Morris et al., 2000]. Rock coatings were suggested inearlier studies as well to explain the Viking lander multi-spectral observations [Adams et al., 1986; Guinness et al.,1987]. Fischer and Pieters [1993] showed that an opticallythin coating of fine-grained ferric particles on a dark neutralsubstrate (e.g., basalt) induces an increase in the visible/NIRregion reflectance and the red/blue ratio, and produces ferricabsorption bands. Johnson and Grundy [2001] found thatthe NIR spectral properties of thin dust layers on a basaltsubstrate are difficult to model because of the wavelengthdependence of the single-scattering phase function, thussupporting the need for additional laboratory studies. Thincoatings of fine-grained, altered volcanic ash were alsoobserved to influence the mid-IR reflectance and emissionspectra of basaltic rocks [Crisp and Bartholomew, 1992;Johnson, 1999]. Crisp and Bartholomew [1992] found thata layer 25 mm deep of fine-grained ash on basalt suppressedthe silicate bands near 10 and 19 mm by about 50%. Basedon these studies a layer as thin as a few microns is sufficientto induce measurable changes in the visible/NIR and mid-IRspectral properties of basalts. Analyses of the effect of thindust coatings on the rock chemistry showed that the rock‘‘Wedge’’ would have a silica content of 56% and a Scontent of 0.1% after removing a thin layer of A5 compo-sition dust [Crisp, 1998].

2.2. Chemical Interactions

[20] In contrast to physical aggregates of particles, chem-ical interactions of dust particles in the soil or on rockcoatings form more permanent, cemented products. Theseare shown as irreversible pathways in Figure 1 and thisprocess requires the formation of chemical bonds. Thesebonds can be broken again, but this would require a moreactive process than simply a dust devil. Sand abrasion thatforms ventifacts on rock surfaces [Bridges et al., 1999], forexample, would be sufficient to remove rock varnishes.Shown in Figure 2 are examples of the kinds of bonds thatmay be formed when the physical dust layer is transformedinto a chemically bound cemented soil or rock varnish.Sulfates or ferric oxides/oxyhydroxides are likely to be thebinding agents in these chemically bound materials.[21] Sulfate species are particularly reactive because of

their polar nature, as are Fe3+ bearing phases because theyreadily accept electrons. For these reasons dust/soil particlescontaining ferric oxides/oxyhydroxides and jarosite/alunitewould be particularly reactive even on the dry surface ofMars. If these dust particles are allowed to remain undis-turbed on the surfaces of rocks sufficiently long to enablechemical reactions between the ferric, sulfate and saltspecies and the primary rock minerals, then alteration rindsare expected to form that may be resistant to dust devils orwind storms. Although interactions of the dust particles inthis manner may explain observations of cemented soil units

Figure 1. Model of Martian dust reactions. Dust particlesare shown near the top of this diagram. These are distributedvia aeolian processes and settle on top of rocks and betweenrocks on Mars forming drift deposits of dust/soil particlesand physical layers of dust as rock coatings. Aeolianprocesses may redistribute these drift and rock coatingdeposits back into the atmosphere as dust particles orelsewhere on the surface. Chemical interaction of the dust/soil particles in the drift deposits forms a hardened,cemented soil that can no longer be freely redistributed byaeolian processes. Chemical interaction of the dust/soilparticles deposited on rock surfaces forms a hardened rockvarnish above a chemically leached layer of rock.


and rock coatings near the Viking and Pathfinder landers,this process, if it indeed occurs, would be expected to bemore widespread on the surface of Mars. Only minimalwater is required for these chemical reactions (given longtimescales), and this could be provided via atmosphericwater ice particles. Evidence for these exists through diurnalvariations in the atmosphere observed using data from theViking orbiters [Farmer et al., 1977], telescopic observa-tions [Sprague et al., 1996], ISM on the Russian orbiter[Titov et al., 1995] and MPF lander [Smith and Lemmon,1999]. Hart and Jakosky [1986] estimated an ice condensatethickness of 10 mm near the Viking lander 2 site andestimated stability of this condensate for much of the colderseason through cycling of daily sublimation and nightlyrecondensation. Models of atmospheric water vapor, icehaze and water in the soil layer on Mars suggest that dustparticles may be nucleating water frost condensation and,further, that ice hazes appear to play a major role in theseasonal water cycle on Mars [Kahn, 1990]. Laboratoryexperiments and theoretical analysis of dust-ice nucleationindicate that molecular structure, chemical bonding and thenature of active sites are important factors [Gooding, 1986].This study suggested that a number of clay minerals wouldbe good nucleators of water ice on Mars and that glass andpalagonitic material would be poor ice nucleators. Saunderset al. [1986] performed laboratory experiments on mont-morillonite-ice condensates and sublimates, and found thatsublimated montmorillonite–ice particles became stronger,more flexible and more elastic. Calculations on the mont-morillonite-water system under current Martian conditions[Anderson et al., 1967] indicate that a monolayer of

adsorbed liquid water would be present on montmorillonitegrains, followed by ice at higher hydration levels.[22] Chemical reactions would have progressed much

faster on Mars in the presence of liquid water than undercurrent conditions. Although there is no direct evidence ofpast liquid water on Mars, hypotheses for liquid waterabound. As described earlier, Carr [1979, 1981] and othershave suggested that abundant flowing water best explainsthe many runoff and outflow channels depicted in Vikingimages of Mars. Analyses by Baker et al., [1991], Clifford[1993], and Parker et al. [1993] support the presence ofaqueous processes and perhaps sedimentary deposits and anancient ocean on Mars. Recently observed erosional featureson Mars have been explained by groundwater seepage andsurface runoff [Malin and Edgett, 2000b] and sedimentarydeposits [Malin and Edgett, 2000a]. Nonaqueous explana-tions have also been submitted for these Martian surfacefeatures [e.g., Hoffman, 2000;Musselwhite et al., 2001], anda number of scenarios involving aqueous and subaqueousprocesses on Mars are possible; thus, consideration ofpotential chemical alteration mechanisms is warranted.

3. Laboratory Alteration Studies

[23] A set of laboratory experiments was designed todemonstrate the dust/soil ! cemented soil portion of thismodel. Rock coating experiments are part of a future study.Two soil analog mixtures were prepared that are a combi-nation of synthetic and natural materials. The goal indesigning these two samples was not to exactly recreatethe soil chemistry measured by MPF, but rather to prepareanalog materials that are, in general, consistent with thechemical, magnetic and spectroscopic properties of theMartian soil to be used in experiments testing the feasibilityof soil alteration reactions on Mars.

3.1. Mixture Samples

[24] Samples for this study were prepared by combiningpowders of sulfate-bearing volcanic soils and magnesiumsulfate with either iron oxyhydroxide-montmorilloniteaggregates or fine-grained altered volcanic ash. All end-members were dry sieved to <45 mm prior to mixing.Mixing was performed by combining the specified amountof each component, shaking the particles, and dry sievingthe mixture to <125 mm particle size. The sulfate-bearingsoils are examples of products forming from solfataricalteration and the magnesium sulfate is an example of whatcould form through an evaporite deposit on Mars. The firstmixture (Mix-1) contains 70 wt.% of sample jb11 (syntheticiron oxyhydroxide-montmorillonite aggregate soil, fromBishop et al. [1995]), 10 wt.% of sample jb250 (sulfate-bearing volcanic soil, collected on the rim of a cinder cone,Haleakala crater basin [from Bishop et al., 1998a]), 10 wt.%of sample jb182 (sulfate-bearing volcanic soil, collectednear an active steam vent, Santorini, from Bishop et al.[1998a]), and 10 wt.% of hydrated magnesium sulfate(MgSO4.7H2O from Fisher Scientific). The second mixture(Mix-2) contains 70 wt.% of sample jb248 (fine-grainedaltered volcanic ash, Haleakala crater basin, from Bishop etal. [1998a]), and 10 wt.% each of the other three samples asdescribed for Mix-1. The ferric oxyhydroxide-montmoril-lonite aggregate soil was produced in the laboratory by

Figure 2. Model of surface reactions. Possible chemicalreactions occurring on the surfaces of Martian dust/soilparticles and rocks are shown here. Iron oxyhydroxidespecies and sulfate salts are examples of reactive compo-nents that could bind together chemically neutral silicateparticles in the dust/soil particles.


exchanging the Ca/Na interlayer cations with Fe using astandard cation exchange procedure; these hydrated Fecomplexes react further to form oxyhydroxides [Bishop etal., 1995]. This is a chemical mixture of nanophase tomicron-sized particles bound together. The volcanic soilsamples included in these mixtures contain a variety ofpoorly crystalline, altered phases including nanophase ferricoxyhydroxide particles, fine-grained sulfates, and silicates.These soil samples include both chemical and physicalmixing of nanophase to micron-sized particles. Shown inFigure 3 is an example of how these chemically and physi-cally mixed particles may appear.[25] Chemical compositions of the Fe-montmorillonite

and volcanic soil end-members were measured previously.The approximate chemical composition of each mixture wasdetermined by summing the relative proportions of themixture components. This is given in Table 1. The chemicalcomposition for the magnesium sulfate is based on theassumption that this salt was uncontaminated. The magneticsusceptibility of the mixtures was measured using a Bar-tingtin Model MS2 magnetic susceptibility meter as inprevious studies [e.g., Morris et al., 2000, 2001] and foundto be on the order of what was observed for the dust at the

MPF site, and slightly higher for Mix-2 than for Mix-1 (R.V. Morris, personal communication, 1999).

3.2. Reflectance Spectroscopy

[26] Visible-infrared reflectance spectra were measured asdescribed in previous experiments [Pieters, 1983; Bishop etal., 1995]. Bidirectional visible/NIR spectra were measuredrelative to Halon under ambient conditions at the Reflec-tance Experiment Laboratory (RELAB) at Brown Univer-sity. Biconical reflectance spectra were measured relative toa rough gold surface using a Nicolet 740 Fourier transforminfrared interferometer (FTIR) in a H2O- and CO2-purgedenvironment. Composite, absolute reflectance spectra wereprepared by scaling the FTIR data (�1 nm spectral reso-lution NIR and �2 cm�1 spectral resolution mid-IR) to thebidirectional data (5 nm spectral resolution) near 1.2 mm.

3.3. Hydration/Dehydration Cycling Experiments

[27] Wet/dry cycling experiments were performed on foursamples: the two newmixtures described here plus each of thedominant end-members for these mixtures. The samples wereexposed to repeated cycles of dehydration and rehydration inthe laboratory. The samples were hydrated by adding about500 ml of distilled, deionized water to the sample surface;rehydration was performed by adding 100–300 ml H2O.Dehydration was performed by drying the samples in air, ina desiccator and by heating at low temperature in an oven.These experiments were performed by pouring the freshly drysieved (fluffy) mixtures into sample dishes 10 mm in diam-eter, where the samples remained for the duration of theexperiment. The specific hydration history for the samples inthis study included several steps: dehydration in a desiccator,hydrationwithH2O, drying in air, dehydration in a desiccator,rehydration with H2O, dehydration in a desiccator, tapping ofthe dishes in order to break the surface tension of the crust,rehydration with H2O, heating in an oven for 1 hour at 50 �C.The degree of moisture used in these experiments is morerepresentative of past climatic regimes on Mars, when liquidwater is thought to have been present [e.g., Carr, 1981], thanthe current one. Chemical reaction of sulfates and oxides inthe dust/soil particles are expected to occur more slowly fordrier conditions. The timescale for crust formation wouldvary depending on available water and would require much

Figure 3. Model of aggregate particles. This diagramshows a physical mixture on the left of iron oxide/oxyhydroxide, sulfate and silicate grains as well as clayaggregate particles. These clay aggregate particles areexamples of chemical mixtures. A chemical mixture ofintergrown, nanophase-sized grains is shown on the right.

Table 1. Chemical Composition of Mars Soil and Laboratory Samplesa

Mars SoilMPF Average

Mix-1(364)b

Mix-2(365)c

Fe-Mont(11)

Hal Soil(248)

Sant Soil(182)

Hal Soil(250)

Mg-Sulfate(366)

SiO2 40.6 43.7 28.0 54.3 31.9 33.2 23.2 0.0TiO2 0.9 0.4 3.8 0.1 5.0 1.3 2.1 0.0Al2O3 9.6 13.9 19.3 16.2 23.9 15.0 10.4 0.0Fe2O3* 22.3 13.6 18.7 12.0 19.3 9.0 43.2 0.0MgO 8.3 3.1 3.1 1.9 1.9 0.1 1.1 16.4CaO 5.8 0.5 4.0 0.1 5.2 0.4 3.4 0.0Na2O 3.5 1.3 0.8 1.6 0.8 1.0 1.1 0.0K2O 0.9 0.5 0.6 0.3 0.4 1.6 1.1 0.0SO3 6.1 4.9 5.0 0.0 0.2 10.8 5.8 32.5LOI n.d. 17.9 14.8 14.2 9.9 20.9 7.5 51.2

Sum 98.0 99.8 98.1 100.7 98.5 93.5 98.9 100.1aNote: Data are in wt.%, Fe is shown as Fe2O3* and includes FeO and Fe2O3, n.d. is not determined, LOI is lost on ignition

at 850 �C, mixture chemistries are calculated from their components, Mars Pathfinder (MPF) soil average is determined fromdata reported by Foley et al. [2001], iron oxyhydroxide-montmorillonite aggregate (Fe-mont) sample chemistry is from Bishopet al. [1995], and volcanic soil chemistries are from Bishop et al. [1998a].

bMix-1(364): 70wt% (11) + 10wt% (182) + 10wt% (250) + 10wt% (366).cMix-2 (365): 70wt% (248) + 10wt% (182) + 10wt% (250) + 10wt% (366).


longer using actual Martian diurnal water variation levels andwould thus be impractical for laboratory experiments.

4. Results: Composition and SpectroscopicProperties of Mixtures

[28] Shown in Figures 4 and 5 are visible/NIR and mid-IRreflectance spectra of the two new mixtures and end-membercomponents used in this study. The spectrum of the hydratedmagnesium sulfate (MgSO4.7H2O) is brightest in the visi-ble/NIR region and the altered volcanic soils are the darkest.Soil Mix-1 (364) is darker than its dominant iron oxyhydr-oxide-montmorillonite end-member (Fe-mont, 11) and boththe reflectance maximum near 0.75 mm and minimum near0.88 mm are shifted toward shorter wavelengths from thoseof the iron oxyhydroxide-montmorillonite sample. Also thecharacteristic montmorillonite bands near 1.42, 1.91 and2.20 mm are weaker and broader in spectra of soil Mix-1.Soil Mix-2 (365) is less noticeably changed from its dom-inant palagonitically altered volcanic soil end-member(248). Although the spectral features for soil Mix-2 are weakin this region, soil Mix-2 exhibits more pronounced features

than palagonitic soil 248, including: the reflectance max-imum near 0.78 mm, the reflectance minimum near 0.86 mm,and sheet silicate absorptions near 1.4, 1.9 and 2.2 mm.Hydrated magnesium sulfate also has narrow bands in thisregion near 1.44 and 1.94 mm, as well as broad bands near2.9–3.1 and 4.4–4.6 mm. Although the hydrated magnesiumsulfate makes up only 10 wt.% of each mixture these arestrong absorption features, so this component is likelycontributing to the NIR spectral properties of the mixtures.[29] The mid-IR spectra are shown in Figure 5 in terms of

wave number, in cm�1, in order to best show the spectralfeatures and facilitate comparison with TES and miniTESspectra. The hydrated magnesium sulfate spectrum has thestrongest contrast in this region and the altered volcanicsoils exhibit the least spectral contrast. All of the spectrahave a water band near 1630–1650 cm�1 (�6.1 mm). Themagnesium sulfate powder spectrum contains a strong bandnear 950 cm�1 (�10.5 mm) as shown in Figure 5 which isshifted to 1150 cm�1 (�8.7 mm) for cemented magnesiumsulfate [Cooper and Mustard, 2002]. Silicate bands areobserved near 1075–1150 cm�1 (�9 mm) and 470–540cm�1 (�20 mm) for Mix-1, the iron oxyhydroxide-mont-morillonite (11) and the Santorini soil (182).

Figure 4. Reflectance spectra from 0.3 to 5 mm of the newmixtures and their end-members. Each mixture contains 10wt.% each of hydrated magnesium sulfate (MgSO4.7H2O),and sulfate-bearing volcanic soils from both Santorini andHaleakala. Mix-1 contains 70 wt.% of an iron oxyhydr-oxide-montmorillonite aggregate soil and Mix-2 contains 70wt.% of palagonitically altered ash from Haleakala.

Figure 5. Reflectance spectra from 400 to 2000 cm�1 (5to 25 mm) of the new mixtures and their end-members. Themixtures are as described in Figure 4 and in the text.


[30] Comparison of the chemistry, mineralogy and spec-tral properties of the two soil mixtures prepared for thisstudy shows that although they each are a mixture of relatedcomponents, there are major differences. Mix-1 contains alarge amount of smectite, while Mix-2 contains a very smallamount of smectite. Mix-1 has a higher Si abundance andlower Al and Fe abundances compared to Mix-2, and Mix-1also has a greater LOI value which is consistent withsmectites in this sample retaining more water than in Mix-2. Mix-1 has brighter reflectance in the visible/near-IRregion and stronger ferric and sheet silicate absorptionbands, while Mix-2 has a more broad and rounded waterband near 2.9–3.1 mm. In the mid-IR region Mix-1 exhibitsmore spectral contrast than Mix-2.

5. Results: Influence of Wet/Dry Cycling onSample Texture and Spectroscopic Properties

[31] Hydration/dehydration experiments were performedin order to test the effect of compositional differences on theformation of cemented crusts and to examine the effects of

the resulting textures on the spectral properties of thesecemented crusts. Images of the samples taken during thisexperiment are shown in Figure 10. The pair of smectite-bearing samples (soil Mix-1 and the iron oxyhydroxide-montmorillonite aggregate sample, ‘‘Fe-mont’’) exhibitedswelling and cracking through the hydration/dehydrationprocess as expected for smectites. Little difference wasobserved due to the addition of a sulfate component forthis sample pair. For the palagonitic soil samples thepresence of sulfates produced changes in the sample behav-ior as a function of hydration and dehydration. The pala-gonitic soil sample (Hal. Soil) showed a small amount ofcracking and bubble formation, while the palagonitic soilplus sulfates mixture exhibited additional cracking andbubbling, as well as separation of some of the sulfatematerial from the sample mixture. The dehydration/rehydra-tion cycling of sulfate-bearing soils appears to inducemigration of the sulfate components to the surface. Alighter-colored rim is observed in Figure 10 for Mix-2,image 3, and small white flakes crystallized on the surface

Figure 6. Reflectance spectra from 0.3 to 5 mm of twodust/soil analog mixtures (A) particulate samples of fine-grained aggregates passed through a <125 mm sieve, (B)cemented samples hardened into crusts by adding H2O anddrying, and (C) ground crusted samples that were passedthrough a <125 mm sieve.

Figure 7. Reflectance spectra from 400 to 2000 cm�1 (5to 25 mm) of two dust/soil analog mixtures (A) particulatesamples of fine-grained aggregates passed through a <125mm sieve, (B) cemented samples hardened into crusts byadding H2O and drying, and (C) ground crusted samplesthat were passed through a <125 mm sieve.


for Mix-2, image 4. These white sulfate flakes becamemore apparent after the final dehydration cycle, but are notshown in Figure 10. This was not observed for the othersamples in this experiment. That sulfates precipitate out ofmixtures with silicate upon dehydration was observed inanother study [Cooper and Mustard, 2002] where thesamples were heated for 20 min at 150 �C in order toretain the sulfate within the cemented crusts. Comparisonof the cemented crusts formed from these two differentmaterials showed that a harder, thicker crust (throughoutthe �4 mm sample depth) formed for the smectite-domi-nated material, while a thinner (upper �1 mm of thesample) and more friable crust resulted for the palagoniticsoil (low smectite) based material.[32] Reflectance spectra are shown of the soil Mix-1 and

Mix-2 samples in Figures 6 and 7 for the visible/NIR andmid-IR regions. In each case spectra were measured (A) ofthe particulate samples of the fine-grained aggregates thatpassed through a <125 mm sieve prior to the hydrationexperiments, (B) of the cemented samples hardened intocrusts following the hydration/dehydration experiments, and(C) of the ground crusted samples that were passed througha <125 mm sieve. The smectite-based sample, Mix-1,exhibited much larger differences in NIR brightness as afunction of sample texture than the palagonitic soil-basedsample, Mix-2. For both samples the spectrum of the initialpowder is the brightest, the reground sample is darker andthe crusted sample is the darkest. The NIR and mid-IRabsorption bands are significantly enhanced in the spectrumof the crusted Mix-1 sample compared to the other textures.The influence of sample texture and the presence of sulfatesin soils observed here compare well with the recent work byCooper and Mustard [2002] that explores these parametersin detail. Anhydrous magnesium sulfate was used in thatstudy and strong sulfate bands were observed in thepalagonitic soil crusts containing as little as 12.5 wt.%MgSO4. Weaker sulfate bands are observed in our studybecause the total sulfate abundance is less than this. Thespectrum of the crusted Mix-1 sample includes significantlyenhanced Si-O smectite bands near 1075–1150 cm�1 (�9mm) and 470–540 cm�1 (�20 mm) compared with thesebands in the initial and final powders (Figure 7). Thissuggests that if smectite clays are present in the dust/soilon Mars, miniTES would be most likely to identify them incemented soil units. The silica bands near 800 cm�1 (�12mm) are weak in the particulate Mix-1 spectra, and disappearin the crusted spectrum. The water band near 1630–1650cm�1 (�6.1 mm) in the particulate Mix-1 and Mix-2 spectrabecome weaker in the crusted spectra and nearly disappearsfor the Mix-1 crusted spectrum (Figure 7). This decrease inintensity of the �6 mm water band in the mid-IR region is incontrast with the great increase and broadening of the �3mm water band in the NIR region for the crusted Mix-1 andMix-2 spectra (Figure 6).[33] TES data of Mars include atmospheric and surface

components. Three distinct surface types have been identi-fied to date and are attributed to basalt, andesite and a grayhematite [Bandfield et al., 2000; Christensen et al., 2000a,2000b]. The Martian basaltic and andesitic componentshave bands near 900–1100 and 300–500 cm�1 that com-pare well with bands observed in laboratory spectra ofbasaltic and andesitic rocks [Hamilton and Christensen,

2000; Hamilton et al., 2001]. The silicate bands observedin spectra of Mix-1 and Mix-2 in this study fall in the sameregion as the silicate bands in the Martian basaltic andandesitic components and would be difficult to identify inthe 3–5 km TES spot size on Mars if present as crustsscattered on the surface between rocks. Cooper and Mus-tard [2002] found that the sulfate band near 1150 cm�1

varies with both sulfate concentration and cementation intheir laboratory experiments. They determined thatcemented soil units would need to comprise at least 10%of the area of a pixel in order to be identified in TES data.Recent investigations by Cooper and Mustard [2001] withTES data indicate that sulfate cemented crusts may bepresent on Mars in several medium albedo regions includingLunae Planum, Isidis, parts of Arabia and others. Theyargue against evaporation of large-scale bodies of water asthe formation mechanism for the sulfate crusts becausethese units are not found in topographic lows. Their studysuggests that the sulfate crusts are widely distributed on theplanet, which is consistent with the results of Jakosky andChristensen [1986] and our model presented here.

6. Results: Comparison With IMP Spectra ofMartian Soils and Rocks

[34] The bidirectional reflectance data were convolved toPathfinder Multispectral data using the filter positions andband passes of Smith et al. [1997]. These resampledlaboratory spectra of the initial particulate Mix-1 andMix-2 samples and the ground, cemented crusts of Mix-1and Mix-2 are shown in Figure 8. The ground cementedcrust spectra for both the smectite-based Mix-1 and thepalagonitic soil based Mix-2 are darker than their respectiveparticulate spectra measured prior to hydration/dehydrationexposure. This suggests that sample texture and the degreeof binding of the individual aggregates in the dust/soilparticles contribute to the spectral brightness in this region.Particle size and soil texture contribute well-known effectson the spectral properties of soils [e.g., Pieters, 1983]. Whatis new in this study is a change in spectral propertiesresulting from repeated hydration/dehydration of soils.The bright, dark and disturbed Mars soil spectra are eachan average of multiple IMP point spectra from bright, darkand disturbed soil deposits reported by Yingst et al. [1999].The spectral character and composition of soils near theMPF lander have been described in detail by Bell et al.[2000] andMorris et al. [2000]. The Fe and S ratio of Mix-2fall very close to the correlation line for Pathfinder soilsreported by Morris et al. [2000, Figure 39] and that of Mix-1 falls somewhat below this line.[35] Resampled laboratory spectra are also shown in

Figure 8 for the two solfataric volcanic soils used as end-members in the mixtures. The sulfate-bearing Haleakala soil(250) spectrum contains a strong band minimum near 0.86mm due to hematite and is less consistent with the MPF soilspectra. The character of the sulfate-bearing Santorini soil(182) spectrum includes the stronger curvature (0.6–0.67–0.75 mm) of the disturbed soils shown in Figure 9. Bridgeset al. [2001] analyzed the spectral properties of rocks andsoils measured by the IMP and compared spectral ratioswith Si, Fe, Cl and S abundance of MPF soils and foundmultiple trends that are best explained by homogeneity in


the soil units. Comparison of trends in many elements withS for soils at the MPF and Viking sites [McSween and Keil,2000; Morris et al., 2000] shows variations in the soilchemistry at the MPF site and between the MPF and Vikingsites. Chemical analyses of MPF [Bell et al., 2000; Foley etal., 2001] and Viking [Clark et al., 1982] soils in otherstudies also suggest compositional differences between thetwo sites.[36] Resampled laboratory spectra of the Mix-1 and Mix-

2 cemented crusts and ground crusts are shown in Figure 9along with IMP spectra of rocks near the MPF lander (fromBarnouin-Jha et al. [2000] and resampled laboratory spectraof a Martian meteorite (from Bishop et al. [1998b, 1998c].The gray rocks, i.e., ‘‘Shark’’, exhibit fairly typical spectralproperties in this region for the MPF site, and a fewanomalous rocks such as ‘‘Black’’ have also been identified[Murchie et al., 2000b; Bell et al., 2002]. Additional red orpink rocks and others, such as ‘‘Maroon’’ and ‘‘Orange’’,exhibit unique spectral character and probably containcoatings. ‘‘Orange’’ has an �0.6 mm shoulder and reflec-tance maximum near 0.75 mm that have been attributed tothe presence of ferric oxide or sulfate coatings and

‘‘Maroon’’ is brighter and thought to contain an additionalferric component [Barnouin-Jha et al., 2000]. The spectraof Martian meteorite ALH 84001 contain a strong band near0.93 mm due to low-Ca pyroxene [Bishop et al., 1998b,1998c]. This band is much weaker in spectra of the MPFrocks and may be shifted toward longer wavelengths, whichwould be more consistent with a high-Ca pyroxene[McSween et al., 1999].[37] The spectrum of a ground portion of Martian mete-

orite ALH 84001 is brighter than the ALH 84001 chipspectra and has a strong band near 0.93 mm and relativelyhigh reflectance near 0.45 mm. The spectral character of thisparticulate ALH 84001 sample near 0.45 and 0.93 mm arevery unlike the Martian soil spectra shown in Figure 8.Typical MPF rocks such as ‘‘Shark’’ may exhibit different

Figure 8. Extended visible region reflectance spectra ofdust/soil analog mixtures in comparison with Martian soilspectra. Versions A and C of Mix-1 and Mix-2 are the fluffyand reground particulate samples as described in the textand Figure 7. These data were convolved to Pathfindermultispectral data using the filter positions and band passesof Smith et al. [1997]. The bright, dark and disturbed soilspectra are averages of multiple IMP point spectra frombright, dark and disturbed soil deposits reported by Yingst etal. [1999].

Figure 9. Extended visible region reflectance spectra ofanalog mixtures in comparison with Martian rock andmeteorite spectra. Versions B and C of Mix-1 and Mix-2 arethe cemented crusts and reground particulate samples asdescribed in the text and Figure 7. These data wereconvolved to IMP filter positions and band passes. The IMPspectra of Martian rocks are from Barnouin-Jha et al.[2000] and the Martian meteorite spectra are from Bishop etal. [1998b, 1998c]. Spectra of Martian meteorite ALH84001 are given for both chip surfaces (splits 92 and 271)and a powder (split 92 only).


spectral properties from Martian meteorites because they aremineralogically dissimilar or the rocks on Mars may bealtered. Pyroxene and other minerals in these rocks may beweathered or the rocks may all contain coatings. Rocks like‘‘Black’’ are more similar spectrally to Shergottites such asALH 84001, although the spectrum of ‘‘Black’’ shown inFigure 9 is darker and has a weaker band near 0.9–1.0 mmcompared to the ALH 84001 chip spectra. Differencesbetween the typical gray rocks (e.g., ‘‘Shark’’) and thecoated rocks (e.g., red, pink, ‘‘Maroon’’, or ‘‘Orange’’),suggest that rock coatings are responsible for brighterreflectance in this region and/or stronger features near 0.6,0.75–0.8, and 0.9–1.0 mm in the spectra [Barnouin-Jha etal., 2000; Murchie et al., 2000b].[38] Spectra of the Mix-1 and Mix-2 cemented crusts

(‘‘B’’ in Figure 9) are darker in this region than spectra ofthe respective ground crusts (‘‘C’’ in Figure 9). A cementedrock coating formed from something similar to Mix-1,comprising 10–15 wt.% Fe2O3 as nanophase Fe3+ oxy-hydroxide in a silicate-sulfate matrix, could be responsiblefor the spectral features observed for the rock ‘‘Orange’’. Acemented rock coating containing schwertmannite or jaro-site would also be consistent with the spectral properties ofthe rock ‘‘Orange’’ [Barnouin-Jha et al., 2000]. A cemented

rock coating created from dust similar to Mix-2, comprising15–20 wt.% Fe2O3 as both Fe2+ in poorly crystallinesilicates or glass and nanophase Fe3+ oxyhydroxides in asilicate-sulfate matrix, could be responsible for the spectralfeatures observed for the rock ‘‘Maroon’’. Experimentsinvolving abrasion of rock coatings by Kraft and Greeley[2000] found that varnishes composed of amorphous silicaare resistant to abrasion and if present at the MPF site wouldbe protecting the rocks. They further noted that dustaggregate particles would be ineffective abrasive agentson the surface of Mars. The results of Kraft and Greeley[2000] support the formation of rock varnishes on Mars inaccordance with our model of dust aggregates containingferric oxides and sulfates in a poorly crystalline silicatematrix.[39] These chemically hardened coatings covering por-

tions of the MPF rock surfaces are expected to be tens tohundreds of microns thick as observed for desert varnish onEarth. Because the spectral properties of altered rocks aredistinct from those of cemented soil and appear to containcomponents due to the rock and the coating [McSween etal., 1999; Morris et al., 2000; Bridges et al., 2001] it isassumed that these rock coatings are not optically thick.McSween et al. [1999] showed that the rock ‘‘Scooby Doo’’

Figure 10. Images of samples in wet/dry cycling experi-ments. Mix-1 is shown at the top, followed by the ironoxyhydroxide-montmorillonite aggregate sample (Fe-mont)that makes up a substantial portion of Mix-1. Mix-2 isshown next followed by the palagonitically alteredHaleakala ash sample (Hal. soil) that makes up a substantialportion of Mix-2. The samples were (1) dried in adesiccator, (2) hydrated with H2O, (3) dehydrated in thedesiccator, and (4) dehydrated in the desiccator following asecond hydration cycle.

Figure 11. Drift and cemented soil deposits from (a) arocky plain in western Iceland in the Laugarvatn mountainrange (the red bar is �1 m), and (b) a cemented deposit ofaltered volcanic ash in the SW region of the island ofHawaii (the red bar is �1 m). Fine-grained dust, rockyoutcrops and crusted soil were observed in both regions.The surface in Figure 11b was scraped with a plastic trowelto reveal dark, fine-grained material below, shown in Figure11c. The image of the broken crust is expanded (the red baris �10 cm) and includes the region within the white box inFigure 11b.


could be coated by as little as 50 mm of dust and chemicalanalyses of ‘‘Scooby Doo’’ indicate that it is intermediatebetween rock and soil. Fischer and Pieters [1993] foundthat viewing geometry, as well as composition and thicknessof dust coatings, influences the spectral properties of fine-grained particles on a rock surface. Spectra measured of a225 mm thick layer of ferric oxide particles on basalt atincidence and emergence angles of 30� and 0�, respectively,produced a negative slope in the NIR region, while thespectra measured under specular configurations exhibited amuch smaller positive slope [Fischer and Pieters, 1993].The illumination geometry of the rocks at the MPF site,coupled with unknown thicknesses of dust layers or rockcoatings, increases the challenge of spectral analysis ofthese rocks. In general a decrease in reflectance has beenobserved with increasing phase angle for these rocks[Johnson et al., 1999; Bridges et al., 2001].

7. Results: Physical Evidence in VolcanicEnvironments

[40] Volcanic ash is observed on Hawaii and Iceland atgreat distances from the active volcanic centers and isfrequently observed as drift and cemented soil deposits.Shown in Figure 11 are examples of the terrain observed inthese volcanic regions. Figure 11a is an image taken of arocky plain in western Iceland in the Laugarvatn mountainrange. Fine-grained dust covered most surfaces in thisregion and multiple, rocky outcrops and crusted soil depos-its were also observed. Figure 11b is an image of an orange-colored, cemented deposit of altered volcanic ash in thesouthwestern region of the island of Hawaii. The ash in thisregion ranged from orange to light brown to black and atthis location the orange crusted material was fixed and theblack ash was loosely covering the surface and transportedoccasionally in the wind. A portion of this crusted materialwas tapped with a plastic trowel. The crust broke with asmall force and revealed slightly darker, loose orange-brown ash underneath as shown in Figure 11c.[41] Drift deposits on Mars near the MPF lander were

described byMcSween et al. [1999] and analyses of the drift

bedding by Metzger [2000] indicated deposition from multi-ple wind regimes. Three images of Mars near the MPFlander are shown in Figure 12. A super-resolution imagecontaining Barnacle Bill in the lower right corner is shownin Figure 12a. This image was provided by T. Parker and issimilar to that used by Metzger [2000]. The super-resolutionimages are prepared by deconvolving individual imageframes and co-registering the group of images whichenables improved resolution [Parker, 1998]. Directionaldrift deposits are clearly visible in this image. The materialcontained in these drift deposits is the fine-grained dust/soilparticles transported by aeolian processes. Some of this driftmaterial is blown up onto the upper portions of rocks andmay be trapped in crevices or ventifacts where it is less freeto be transported away by aeolian forces.[42] The cemented crusts shown in Figures 11a–11c are

terrestrial examples of what may be happening to theundisturbed ash deposits on Mars. Hardened crusts formedin laboratory experiments using fine-grained silicate-sulfatecomponents shown in Figure 10 and reported by Cooperand Mustard [2002] may also explain the ‘‘duricrust’’ and‘‘hardpan’’ soil units observed by the Viking and Pathfindercameras [Binder et al., 1977; Greeley et al., 1992; Smith etal., 1997]. Shown in Figures 12b and 12c are images ofhardened or cemented soil units near the MPF landing site.Shown in Figure 12b is the rock ‘‘Scooby Doo’’ whichappears to be covered by cemented material and has spectraland chemical properties intermediate between the rocks andsoil [McSween et al., 1999]. Shown in Figure 12c aredisturbed soil in the rover tracks and the rock ‘‘Poptart’’,which is also thought to be covered by cemented material.Sulfate transport through the ash particles during diurnalmoisture cycling on Mars may be responsible for formationof the cemented soil units. Sulfate efflorescence to thesample surface can be seen in Figure 10 for soil Mix-2after slowly dehydrating the sample in air and then in adesiccator. The lighter-colored ring along the outer surfaceof this sample is due to a higher concentration of sulfates.This was not observed for faster dehydration at 150 �C[Cooper and Mustard, 2002]. These observations suggestthat a small amount of sulfate in dust/soil particles would

Figure 12. Images of drift and cemented soil on Mars near the Pathfinder lander site. Shown in Figure12a is a super-resolution image of Barnacle Bill provided by T. Parker and modified afterMetzger [2000].The rock ‘‘Scooby Doo’’ is shown in the foreground of Figure 12b and is covered by hardened soil thatcould be a cemented dust/soil composite as described in the model presented in this study. The large rock‘‘Yogi’’ is shown in Figure 12c. Below the rover is ‘‘Poptart’’, another rock covered by a hardened soilunit, and to the lower left are rover tracks showing additional possible regions of cemented soil.


become more concentrated at the sample surface throughmild/slow moisture cycling. Therefore, the diurnal moisturecycling on Mars may force these sulfate components tosample surfaces where they would be more readily able toreact with other particles. Recent results by Zent et al.[2001] evaluating the H2O adsorption kinetics on smectitesfound that the equilibration timescale for water vapor insmectites is too slow under Martian early morning con-ditions. Their results suggest that the moisture is beingadsorbed by another soil component. Possibly sulfates ornanophase ferric oxyhydroxides can adsorb H2O faster thansmectites and could account for adsorption of the earlymorning moisture.

8. Model Summary

[43] Observations from the Viking and MPF landers haveshown that many of the rock surfaces have coatings/alter-ation rinds and that the particles suspended in the atmos-phere have many properties similar to those of the cementedsoil and drift. This model was designed to explain thechemical, magnetic and spectroscopic properties of thesurface material on Mars and is based on a combinationof palagonitic and solfataric alteration processes associatedwith volcanism and may include sulfates and other saltsfrom evaporite deposits. This model involves both physicalinteractions, such as aeolian winds that distribute particlesand electrostatic charges that bind the fine-grained, dryparticles, and chemical interactions including reaction ofthe sulfate and/or ferric phases and salts with water pro-vided either by the diurnal moisture variations over longtime periods or by melting ice flows or other more spec-ulative forms of water.[44] The idea presented here is that 1–3 mm diameter

dust/soil particles on Mars contain fine-grained silicateparticles, formed through palagonitization or other alterationprocesses, that are mixed via aeolian processes with reactivesulfate or ferric oxyhydroxide phases, produced on Mars viasolfataric or evaporitic processes. The products formedthrough these processes are not required to be located nearpalagonitically altered volcanic material. The dust storms onMars are continually transporting mobile fines across thesurface and would, according to this model, actively mixnanophase to micron-sized particles across vast regions ofthe surface of Mars. Over time these dust particles becomesomewhat homogenized, although it is assumed that therewould be regional differences. This dust is then transportedand converted into the drift and cemented soil unitsobserved on the surface, as well as rock coatings andvarnishes on horizontal rock surfaces. The corrosive agentsin the dust particles react over time forming the cementedsoils/crusts and rock varnishes. These processes would alsomodify the chemistry and mineralogy of the surface crustsand rock alteration rinds as some elements are leached outof the silicate matrices and less resistant minerals arealtered.[45] Cemented crusts in this study were formed using a

mixture of silicates, sulfates and nanophase ferric oxyhydr-oxides that is dominated by smectites and another that isdominated by palagonitically altered volcanic soil. Thesmectite-based Mix-1 exhibited significantly greaterchanges in both its binding properties throughout the

hydration/dehydration cycling experiments and in its spec-tral properties following grinding of the hardened crust thandid the palagonitic soil-based Mix-2. The hydration/dehy-dration experiments showed that a smectite component isnot required for cementation of potential Mars analog soilsinto crusted material. The less reactive palagonitic soil Mix-2 allowed transport of water and sulfate to the surfaceforming a thin crust at the upper �1 mm of the sample,while the more reactive smectite soil Mix-1 formed acemented crust throughout the �4 mm sample depth. Thehigh smectite crust was also much harder than the low-smectite crust. Future experiments on cemented soils arerequired in order to better characterize the influence ofspecific silicate and sulfate minerals on crust formationmechanisms.

[46] Acknowledgments. Support for JLB from NASA’s Mars DataAnalysis Program is much appreciated. Reflectance spectra were measuredat RELAB, a multiuser, NASA-supported facility (NAG5-3871). Thanksare due to R. V. Morris for thoughtful discussions of the ideas presentedhere, to T. Hiroi for assistance with the bi-directional spectra, to L. Grundlerwith the Hawaii images and to T. Parker for donating the MPF super-resolution image. This manuscript has benefited from the helpful commentsof W. Calvin and an anonymous reviewer.

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��J. L. Bishop, SETI Institute/NASA Ames Research Center, MS-239-4,

Moffett Field, CA 94035, USA. ( [email protected])S. L. Murchie, Applied Physics Laboratory, Johns Hopkins University,

Laurel, MD 20723, USA.C. M. Pieters, Department of Geological Sciences, Brown University,

Box 1846, Providence, RI 02912, USA.A. P. Zent, NASA Ames Research Center, MS-245-3, Moffett Field, CA

94035, USA.


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