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Icarus 171 (2004) 20–30www.elsevier.com/locate/icaru

Meridiani Planum hematite deposit and the search for evidence of lifMars—iron mineralization of microorganisms in rock varnish

Carlton C. Allena,∗, Luke W. Probstb, Beverly E. Floodc, Teresa G. Longazod,Rachel T. Schelblec, Frances Westalle

a NASA Johnson Space Center, Houston, TX 77058, USAb Rice University, Houston, TX 77005, USA

c University of Southern California, Los Angeles, CA 90089, USAd University of Arizona, Tucson, AZ 85721, USA

e Centre de Biophysique Moleculaire, Orleans 45071 cedex 2, France

Received 18 June 2003; revised 26 January 2004

Available online 19 June 2004

Abstract

The extensive hematite deposit in Meridiani Planum was selected as the landing site for the Mars Exploration Rover Opportunity becausthe site may have been favorable to the preservation of evidence of possible prebiotic or biotic processes. One of the proposed mfor formation of this deposit involves surface weathering and coatings, exemplified on Earth by rock varnish. Microbial life, includingcolonial fungi and bacteria, is documented in rock varnish matrices from the southwestern United States and Australia. Limited evthis life is preserved as cells and cell molds mineralized by iron oxides and hydroxides, as well as by manganese oxides. Such minof microbial cells has previously been demonstrated experimentallyand documented in banded iron formations, hot spring depositsferricrete soils. These types of deposits are examples of the four “water–rock interaction” scenarios proposed for formation of the hematdeposit on Mars. The instrument suite on Opportunity has the capability to distinguish among these proposed formation scenarios and, pobly, to detect traces that are suggestive of preserved martian microbiota. However, the confirmation of microfossils or preserved biowill likely require the return of samples to terrestrial laboratories.Published by Elsevier Inc.

Keywords:Rock varnish; Mars; Exobiology; Hematite; Meridiani Planum; Microfossil

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1. Introduction

The hematite deposit in Meridiani Planum is the laing site for Opportunity, one of the two Mars ExploratiRover (MER) spacecraft (Fig. 1). This site was chosen because it shows “strong evidence for surface processevolving water and appear(s) capable of addressing the scence objectives of the missions, which are to determine thaqueous, climatic, and geologic history of sites on Mwhere conditions may have been favorable to the presetion of evidence of possible prebiotic or biotic process(Golombek et al., 2003). We are investigating the potentifor hematite, as well as otheriron oxide and hydroxide min

* Corresponding author. Fax: 281-483-5347.E-mail address:[email protected] (C.C. Allen).

0019-1035/$ – see front matter Published by Elsevier Inc.doi:10.1016/j.icarus.2004.04.015

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erals, to preserve microfossils and physical biomarkeactual evidence for life on Mars.

Christensen et al. (2000), using data from the MarGlobal Surveyor Thermal Emission Spectrometer (TEidentified gray crystalline hematite [α-Fe2O3] in a 350 kmby 750 km region near Meridiani Planum. This deposit cresponds closely to the low-albedo highlands unit “smapped as a wind-eroded, ancient, subaqueous sedimedeposit(Edgett and Parker, 1997). Christensen et al. (2001interpreted the deposit to be “anin-place, rock-stratigraphisedimentary unit characterized by smooth, friable laycomposed primarily of basaltic sediments with appromately 10 to 15% crystalline gray hematite.”

Christensen et al. (2000)discussed five possible scnarios for the formation of the martian hematite depo(1) direct precipitation from standing, oxygenated, iron-rwater; (2) precipitation from iron-rich hydrothermal fluid

http://www.elsevier.com/locate/icarus

Rock varnish and the Meridiani Planum hematite deposit 21

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(3) low-temperature dissolution and precipitation throumobile groundwater leaching; (4) surface weathering ancoatings; (5) thermal oxidation of magnetite-rich lavas. Tfirst four of these scenarios involve the interactions of rwith water, and thus have implications in the search fordence of martian life.

Christensen et al. (2001)assessed the scenarios for foring the hematite deposits and argued for chemical precition from aqueous fluids, under either ambient or hydrothmal conditions.Newsom et al. (2003)reported a chain opaleolake basins and associated layered deposits alonmargin of the hematite region.Hynek et al. (2002)suggestedthat the hematite may have been formed by precipitafrom circulating fluids within layered volcanic materiaCatling andMoore (2003)favored a hydrothermally chargeaquifer as the original setting for developing coarse-graicrystalline hematite. They also recognized that hematitethe observed crystallographic signature could be produfrom sedimentary deposits subsequently buried to a depseveral kilometers, in agreement with the formation hypoesis described byLane et al. (2002).

Iron oxide and hydroxide minerals, including hematitecan mineralize and preserve microfossils and physicalmarkers. Previous research by ourselves and others,marized below, has demonstrated such mineralizationpreservation in deposits from three of the four water–rinteraction scenarios listed byChristensen et al. (2000).

The current study is focused on the fourth scenarimineralization of microorganisms in iron-rich surface weaering and coatings, using the specific example of rocknish from desert sites in the United States and AustraThis work also addresses the capabilities and limitationsMER spacecraft in the search for evidence of life at Merani Planum.

2. Iron mineralization of microorganisms andbiosignatures in rock varnish

2.1. Previous studies

Rock varnish, also known as desert varnish, is a dhard coating that forms on rocks in many arid enviroments(Dorn, 1998). Varnish coatings can develop on eposed rock surfaces in a matter of decades, and noticedarken on a time scale of centuries(Dorn, 1991; Krinsley etal., 1990). Rock varnish is a complex combination of clawith iron and manganese oxides and hydroxides. The dinant clay minerals are illite, smectite, interstratified illitsmectite, and chlorite(Potter and Rossman, 1977). Hematiteis the major iron oxide phase in rock varnish, and birnes[Na4Mn14O27·9H2O] is the major manganese oxide pha(Potter and Rossman, 1979; McKeown and Post, 2001).

Many rock varnish samples contain living microorgaisms. The most common rock varnish inhabitants are slgrowing, melanin-pigmented microcolonial fungi alo

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with typical soil-inhabitingactinomycetes and nonmotiendospore-forming gram-positive cocci(Krinsley and Dorn,1991; Nagy et al., 1991; Staley et al., 1992; Sterflinger, 20Gorbushina et al., 2002; Gorbushina, 2003). Dozens ofculturable strains of varnish microorganisms oxidize mganese and/or iron. These microorganisms include memof the bacterial generaMicrococcus, Arthrobacter, Bacil-lis, and the actinomycetesGeodermatophilis(Hungate et al.,1987; Adams et al., 1992; Staley et al., 1992).

Several studies have indicated limited mineralizationpreservation of the remains of microbial life in rock varniMineralized microcolonial fungi have been recognizedvarnish layers(Taylor-George et al., 1983; Dragovich, 199Gorbushina et al., 2002; Gorbushina, 2003). Nagy et al.(1991) reported textures resembling “microstromatolitewithin varnish layers.Dorn (1991)found botryoidal struc-tures in varnish samples. He interpreted the structurebacterial colonies, which served as nucleation centers durinvarnish formation.Krinsley (1998) and Krinsley and Rus(2000)used high-resolution TEM techniques to study vnish layers on rocks from deserts in California, Hawaii, Peand Antarctica. They reported micrometer-scale coccoidaand granular structures within the varnish that had muchigher concentrations of iron and manganese than therounding matrix. They suggested that these structuresbacterial casts, hyphae, buds, or bacterial precipitates.

2.2. Samples

2.2.1. Sonoran DesertSamples of varnish coatingson granite were collected fo

the present study from a road cut off Interstate Highwayjust north of Camp Verde, AZ. Additional samples were clected from outcrops at South Mountain Park in Phoenix,from an outcrop in a wash just east of Gates Pass adjacethe Tucson Mountain Park. Initial results were reportedProbst et al. (2002).

2.2.2. PilbaraWe also examined varnish coatings on foliated rocks

granitoid composition from the Pilbara region of WesteAustralia. These rocks have been exposed to weathesince the Permian glaciation, approximately 280 myr athough the age of the varnish layers is unknown. Initialsults were reported byFlood et al. (2003).

2.3. Laboratory methods

The Sonoran Desert samples were collected with flasterilized tools and immediately placed in sterile plastic bagfor transport. The Pilbara samples were collected and trported in a non-sterile manner. All samples were presein a laboratory desiccator at room temperature. Subsamplwere prepared and mounted in open-face hoods to minimsurface contamination.

The varnish samples were initially examined in refleclight using a Nikon ME 600 binocular microscope with

22 C.C. Allen et al. / Icarus 171 (2004) 20–30

Fig. 1. Meridiani Planum landing site viewed by the Mars Exploration Rover Opportunity. NASA image.

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digital imaging system. Based on this examination, subsples were selected for additional study by scanning elecmicroscopy (SEM) and transmission electron microscop(TEM).

Exterior surfaces and interior regions of Sonoran Devarnish samples were mounted on SEM stubs and cowith 5 to 10 nm of conductive platinum. These sampwere examined with a JEOL 6340F SEM and an IXRF (Gsham Scientific Instruments, Ltd.) light element energypersive X-ray spectrometer (EDS). Other subsamples wexamined uncoated in an FEI XL-30 environmental S(ESEM), operated at a pressure of 1.7 torr. Several varchips, approximately 20 µm in size, were prepared for Tstudy. These were embedded in epoxy, ultramicrotomedexamined with a JEOL 2000 FX TEM equipped with a dicated Link EDS system using a windowless detector.

Chips of the Pilbara samples were mounted on SEM susing carbon paste and coated with 100 Å of platinum. Pliminary electron microscope investigations of these samwere conducted utilizing a JEOL 5910 SEM equipped wIXRX EDS. The primary investigation utilized the JEO6340F SEM and IXRF light element EDS described abo

2.4. Structure

2.4.1. Sonoran DesertVarnish coatings on the Sonoran Desert samples w

uniformly hard and dark, similar to material from the saarea previously studied byAllen (1978) and Nagy et a(1991). Characteristic varnish thicknesses ranged from 5100 µm.

2.4.2. PilbaraThe dark, smooth varnish from the Pilbara samples

lamellate and typically 75 to 150 µm thick (Fig. 2). The dark

Fig. 2. Pilbara sample cross section showing varnish layer (black), alterrind (orange), and underlying rock (white); reflected light optical micrograph.

layer coated heavily weathered, orange rinds 200 to 300thick.

2.5. Composition and mineralogy

2.5.1. Sonoran DesertThe predominant clay mineral in the Sonoran Desert

nish samples was illite [KAl2(Si3AlO10)(OH)2], identifiedby EDS elemental ratios and a regular basal spacing oproximately 1.0 nm in high-resolution TEM images. Tother dominant mineral was a sheet-like manganese ophase containing trace concentrations of barium, likelynessite (approximately 0.6 to 0.7 nm basal spacing). Birsite is known to accommodate water and readily undecation exchange reactions to accommodate a variety ofcations of elements including potassium, sodium, calci


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Fig. 3. Sonoran Desert varnish layer showing lath-shaped illite; SEMondary electron micrograph.

Fig. 4. Hyphae on the surface of Sonoran Desert varnish layer; Sback-scattered electron micrograph.

and barium(Post, 1999). Illite occurred as large regionof well-crystallized laths (Fig. 3) and as individual packetmixed with the fine-grained, poorly-crystalline, birnessitelike phase. Discrete anhydrous mineral grains, tentatiidentified by their EDS spectra as hematite, ilmenite, quand rutile, were embedded in the clay minerals.

2.5.2. PilbaraThe surface layer consisted mainly of clays with low co

centrations of potassium and iron. Platy iron-rich minerand discrete sub-micrometermanganese-rich minerals wedistributed throughout the clay matrix.

2.6. Microbiology

2.6.1. Sonoran DesertThe Sonoran Desert samples were partially coated

black patches that corresponded in scale and morpholothe microcolonial fungi identified byPalmer et al. (1986).Fungal hyphae were common features of these samplesphae were visible both on the surface (Fig. 4) and within thevarnish layers.

The ESEM images show rod-shaped objects, apprmately 0.5 to 2 µm in length, located within thin continuo

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Fig. 5. Bacterial biofilm with cells (arrows) in Sonoran Desert varniESEM secondary electron micrograph.

layers incorporated in the varnish coatings (Fig. 5). Theseare interpreted as examples of the bacterial biofilms,conglomerations of individual cells and water-rich extracelular polysaccharide (EPS) produced by diverse microorgisms(Costerton et al., 1994). The morphologies of individual cells and EPS layers were preserved from desiccaand deformation by the 1.7 torr pressure in the ESEM sple chamber.

Cells were also recognized in SEM images, though twere characteristically dehydrated and possibly contoby the high vacuum of the microscope chamber (Fig. 6a).An EDS spectrum of one group of cells (Fig. 6b) indicateshigh concentrations of carbon and oxygen, along with misulfur, indicative of cellular material. The spectrum also ctains peaks corresponding to aluminum, silicon, and potassium from the underlying illite, as well as manganese, ironand minor barium from the birnessite.

2.6.2. PilbaraThe Pilbara samples hosted multiple species of b

microcolonial fungi. Some species appeared to be speto the varnish, while the other species inhabited onlynon-varnished substrate. These fungi apparently contribto the weathering of the non-varnish substrate by creaand inhabiting large micropits.Gorbushina et al. (2002) anGorbushina (2003)also reported widespread microcolonfungi on varnished quartzite surfaces from Australia aNamibia.

While fungi were common, confirmation of bacterpresence within the Pilbara samples was rare. One cell, posibly deformed by the SEM vacuum, is shown inFig. 7a.The EDS spectrum of this feature (Fig. 7b) indicates a highconcentration of carbon and oxygen, along with minor sfur, in the cell. The aluminum, silicon, manganese, and ipeaks are attributed to the varnish matrix.

2.7. Mineralization

2.7.1. Sonoran DesertPartial mineralization of a fraction of the fungal popu

tion is apparent on the surfaces and within these sam


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Fig. 7. (a) Deformed cell in Pilbara varnish; SEM secondary electron mi-crograph. (b) EDS spectra of cell and substrate; Pt peak from conductivecoating.

Fig. 8. Hymenium of microcolonial fungi in Sonoran Desert varnish sampSEM secondary electron micrograph.

Figure 8shows a portion of the hymenium (the layer of ceoriginally containing the spore-bearing cells) of one mcrocolonial fungal body. The hymenium is partially coawith interlocking platy minerals, characteristically 3 to 5 µmin width. This relationship closely matches the mineralifungal colonies documented in rock varnish byGorbushinaet al. (2002).

Groups of rod-shaped cavities, each 1 to 2 µm in lenare occasionally observed in various states of degradand mineralization within the varnish layers (Figs. 9a, 10a).These objects, similar in scale and morphology tocarbon-rich cells found in these samples (Fig. 6a), are in-terpreted as mineralized cell molds. EDS spectra of thmolds (Figs. 9b, 10b) demonstrate that most of the organmaterial, composed chiefly of carbon, oxygen, and suhas been lost. The spectra include different ratios of thejor elements correspondingto illite and iron and manganesoxides.

2.7.2. PilbaraThese samples contained small numbers of partia

mineralized microcolonial fungi, very similar to those doumented in the Sonoran Desert material.Figure 11ashowsa portion of the hymenium from one microcolonial fungbody, comparable in size and morphology to the moretensive hymenium inFig. 8. EDS element mapping of thPilbara material (Fig. 11b) demonstrates that the hymeniuwas partially mineralized by platy iron-rich minerals 23 µm across, as well as finer-scale manganese-rich min

The samples also exhibited elevated concentrationmanganese and/or iron in discrete nodules, typically onecrometer or less in length, either within the matrix of the vnish or loosely bound to detrital grains. Some of the nodhad higher carbon contents than the surrounding varsuggesting that they may have been mineralized cells ofragments. These nodules resembled the mineralizedcoidal and granular structures reported byKrinsley (1998)and Krinsley and Rusk (2000). One completely intact bacterial cast, containing a significant concentration of carbwas documented by SEM.


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Fig. 9. (a) Mineralized cells in SonoranDesert varnish; SEM back-scatteredelectron micrograph. (b) EDS spectra of mineralized cells and substrate; Ptpeak from conductive coating.

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Fig. 10. (a) Mineralized cells in Sonoran Desert varnish; SEM secondaryelectron micrograph. (b) EDS spectra of mineralized cells and substrate; Ptpeak from conductive coating.

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Fig. 11. (a) Mineralized fungal hymenium in Pilbara varnish; SEM sondary electron micrograph. (b) EDSmap overlay on SEM secondary eletron micrograph showing concentrations of iron (green) and manga(purple) on and around fungal hymenium in Pilbara varnish.

2.8. Abundance and preservation

Microanalysis of rock varnish samples from wideseparated locations demonstrates similar patterns of malization. Microcolonial fungi become coated with iron- amanganese-minerals. Bacterial forms can be preserveiron- and manganese rich casts of the individual cells. Thpatterns have been reported by previous authors, in samfrom desert regions in the United States, Peru, Australia,Antarctica.

Rock varnish may therefore be a suitable medium fothe preservation of biosignatures indicative of microbial lHowever, it is not yet clear that such preservation is comon, nor that biosignatures are preserved in rock varover geologic time.

While semi-intact microcolonial fungi were commonvarnish samples from the present study, mineralization ofungi and microbial cells proved to be rare. Many hoof SEM traverses yielded only a few clear examplessuch mineralization.Krinsley (1998), in a comprehensivTEM study of varnish samples from four locations, simila


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found very few objects that could confidently be interpreas preserved cells, and no preserved fungal bodies. Minization of microorganisms on rock surfaces in arid envirments may be rare, relative to the total bioloads typical othe varnish layers.

Varnish samples hundreds to thousands of yearsdated by archaeological methods, are common(Dorn, 1991;Krinsley et al., 1990). Varnish has been shown to persin arid-alkaline environments for periods of 100,000 ye(Liu, 1994). However, varnish can be destroyed by a varof mechanisms, including biogeochemical leaching and aelian erosion(Krinsley and Dorn, 1991; Krinsley et al., 1990.Very little evidence of rock varnish is apparent in the sementary record(Krinsley, 1998). The potential for long-termpreservation of evidence of life in rock varnish may thuslimited by erosion of the thin varnish layers.

3. Iron mineralization of microorganisms andbiosignatures—other Mars-relevant scenarios

Christensen et al. (2000)proposed three other scenariobesides surface weathering, by which water–rock intetions could have produced the martian hematite depoThese include: (1) direct precipitation from standing, ogenated, iron-rich water; (2) precipitation from iron-rich hdrothermal fluids; (3) low-temperature dissolution and pcipitation through mobile groundwater leaching. Previouresearch, summarized below, has demonstrated that iroide and hydroxide minerals, including hematite, can minealize and preserve evidence of microorganisms in controexperiments as well as in natural examples from eachthese proposed scenarios.

3.1. Experiments

Laboratory and field experiments have demonstratediron oxides and hydroxides can mineralize living cells,well as the EPS that binds cells into biofilms. Ferrihyd[Fe5O7(OH)·4H2O] has been shown to mineralize spectypes of microbial cells(Thomas-Keprta et al., 1998). Cellwalls were completely coated with this mineral on a tiscale of weeks, in experimental microcosms utilizing baand groundwater.Allen et al. (2000)studied the formation oiron-mineralized, amorphous coatings in hot spring depoat 60◦C. In situ experiments demonstrated that the coatwere in fact biofilms, and that their formation was essentiprevented by passing the water through 0.2 µm filters,excluding microorganisms.

3.2. Direct precipitation from standing, oxygenated,iron-rich water. Example: banded iron formations

Banded iron formations (BIFs), as defined byKlein andBeukes (1992), are “chemical sediments, typically thinlbedded or laminated, whose principal chemical characteris

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tic is an anomalously high content of iron, commonly butnecessarily containing layers of chert.” BIFs, which rangage from 3.8 to 0.8 Ga, are among the most distinctiveimentary deposits of the Precambrian. BIFs are compprincipally of hematite and magnetite [Fe3O4] interbeddedwith chert.

Investigations of many BIFs have revealed evidencancient life. For example, stromatolitic sections of the Gflint Iron Formation, which spans the US/Canada boundthrough Minnesota and Ontario, contain a variety ofcroorganisms that have been preserved in chert for appimately 1.8 Ga(Barghoorn and Tyler, 1965; Awramik anBarghoorn, 1977; Strother and Tobin, 1987). Barghoorn andTyler (1965)originally classified the Gunflint microfossiland documented numerous filamentous and coccoidal formWith the aid of an electron microprobe,Tazaki et al. (1992documented trace quantities of iron within silicified microganisms from the Gunflint.

Allen et al. (2001)investigated microfossils mineralizeby iron oxides in samples from the Gunflint Iron Formtion. This initial study has been continued in more detaiSchelble et al. (2004). Filaments and coccoids were docmented by analytical SEM in samples etched with hydrooric acid vapor. Many of the coccoids and filaments ctained elevated concentrations of iron and carbon, compto the surrounding matrix material. These structures werterpreted as microfossils mineralized and preserved byoxides, though the specific mineral or minerals could noidentified.DeGregorio and Sharp (2002)used TEM electrondiffraction analysis to demonstrate that some filamenmicrofossils in the Gunflint Formation were mineralizedhematite.

3.3. Precipitation from iron-rich hydrothermal fluids.Example: hot spring deposits

Hematite is one of several iron oxide and hydroxide merals that can precipitate when large volumes of hydrotmal fluids move through rocks. Terrestrial hydrothermdeposits range from simple systems (predominantlycite, silica, or iron oxides) to complex assemblages wmineralogies reflecting localchanges in temperature, pand fluid composition(Guilbert and Park, 1986). A studyby Wade et al. (1999)of iron-depositing hot springs iColorado and Yellowstone National Park (48 to 55◦C)showed that iron was partitioned among hematite, ferrdrite, goethite [α-FeO·OH], siderite [FeCO3], and nontron-ite [Na0.3Fe2(Si,Al)4O10(OH)2·nH2O].

Microorganisms are abundant in many hot springs,are often preserved by mineralization. They are most cmonly preserved in hydrothermal systems dominated byica (Cady and Farmer, 1996; Cady et al., 2003), sometimesin association with iron-bearing minerals.Wade et al. (1999noted the remains of rod-shaped bacteria and dehydbiofilm in a silica-dominated iron hot spring that precipitahematite and other iron oxides and carbonates.Ferris et al.


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Microorganisms can also be preserved in a variety ofspring deposits dominated by iron minerals(Allen et al.,2000; Pierson and Parenteau, 2000). Chafetz et al. (1998reported recognizable bacterial remains in iron and mganese oxide deposits within hot spring travertines in Mrocco. These microfossils were found in centimeter-scblack dendrites. Filamentous and coccoidal bacteria werobserved densely packed within the dendrites, but nocrofossils were found in the enclosing aragonite and calaminae.

3.4. Low-temperature dissolution and precipitation throumobile groundwater leaching. Example: ferricrete soils

Ferricretes are formed on Earth when acidic groundters extensively leach soil and rock. Mafic minerals aresolved in low-pH, reducing environments and form colloidoxides and hydroxides. These can be transported by wand redeposited as hydrated silica and iron oxide mine(Guilbert and Park, 1986).

Fortin et al. (1997)reported thatThiobacilluscells canact as substrates for the precipitation of iron oxides in acidicmine tailings.Ferris et al. (1989b)identified hematite, ferrihydrite, and goethite as the dominant iron minerals asciated with bacteria in acidic environments.Furniss et al.(1999)noted rod-shaped structures, apparently bacteriaeralized by ferrihydrite and goethite, in both modern aancient (8840 years b.p.) ferricrete samples from a mindistrict in Montana.

Westall and Kirkland (2002) and Kirkland et al. (200studied a ferricrete sample from Western Australia usSEM and emission spectroscopy. The sample was collefrom an extremely dry soil surface adjacent to Shark BFilamentous microfossils, as well as abundant EPS, wpresent in the ferricrete sample. Some filaments appearecollapsed and split for portions of their lengths. They winterpreted as microbial filaments, bacterial or possibly fgal, embedded in films of EPS. Iron oxide precipitatesgulfed the networks of filaments. The degree of iron oxmineralization varied locally over a scale of micrometeMineralization apparently occurred over a long enoughriod to produce several generations of microfossils.

4. Meridiani Planum hematite deposit and the searchfor evidence of life on Mars

4.1. Martian hematite

The large hematite deposit at Meridiani Planum is imptant for understanding many aspects of early martian hisThis deposit represents the only place on the planet wa specific mineral has been identified and correlated wita mappable geologic unit. On Earth, hematite is formed

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several mechanisms, most of which involve water. The mtian hematite may prove to be the only large-scale minalogical evidence for water–rock interactions on early Mars

If the Meridiani Planum hematite was formed by a meanism or mechanisms involving water, the deposit maysignificant in the search for life on Mars. Liquid watis the one prerequisite accepted by most researcherthe existence of terrestrial life (e.g.,Jakosky, 1998). OnEarth, diverse microbiota have been reported in each ofour “water–rock” scenarios discussed byChristensen et al(2000) for hematite deposition. The present study, as was many others, provides evidence for the mineralizatiomicroorganisms in each of these scenarios.

4.2. Exploring the Meridiani Planum hematite deposit

Opportunity, one of NASA’s twin Mars ExploratioRovers, is exploring Meridiani Planum in 2004. Each rocarries six major science instruments(Squyres et al., 2003).Pancam, a mast-mounted stereo multispectral imagersensitive to visible and near-infrared (0.4 to 1.1 µm) walengths. The instantaneous field of view is 0.28 mradpixel, approximately the resolving power of the human(Bell et al., 2003). This camera is bore-sighted toMini-TES,an infrared (5 to 29 µm) spectrometer that provides medresolution (20 or 8 mrad per pixel) spectra that complemthose from the orbiting TES on the Mars Global Survespacecraft(Christensen et al., 2003). Thermal emissionspectra can identify many rock-forming minerals, includingsilicates and oxides. Each MER rover also deploys anticulated arm carrying four instruments (Fig. 12). An AlphaParticle X-Ray Spectrometer(APXS) can determine abundances of major and minor rock-forming elements, includcarbon(Rieder et al., 2003). The Mössbauer Spectrometecan determine iron valence states and thus help identifiron-bearing minerals in rocks and soils(Klingelhofer etal., 2003). TheMicroscopic Imagerreturns monochromatireflected light images at a resolution of 30 µm per pi(Herkenhoff et al., 2003). TheRock Abrasion Toolexposesinterior surfaces of rocks for analysis by grinding to a deof 5 mm(Gorevan et al., 2003).

Terrestrial hematite deposits produced by various meanisms differ significantly from one another in their meralogies, textures, and spectra. Such differences are distinguishable with the combination of analytical capabilitiesOpportunity. For example, the Pancam and Microscopicager can detect sedimentary layering by color and texon the scale of centimeters to millimeters. The Mini-Tcan distinguish among different iron oxides and can speically identify hematite, in the same manner as its orbitcounterpart. The APXS and Mössbauer Spectrometer, wcombined, can identify and quantify the iron-bearing phain a rock or soil. Rock surfaces can be analyzed byAPXS, Mössbauer Spectrometer, and Microscopic ImagerThe Rock Abrasion Tool can then grind into the rock, afwhich the analyses can be repeated. Comparison of the


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Fig. 12. Mars Exploration Rover arm, carrying the Alpha Particle X-Ray Spectrometer, Mössbauer Spectrometer, Microscopic Imager, and Rock AnTool. NASA image.
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sets of analyses allows characterization of surface alteraincluding possible rock varnish layers. Such in situ obsetions may lead to a consensus on the origin of hematitthe martian surface.

If the Meridiani Planum hematite deposit carries a recof martian life, traces that are suggestive of life may alsorevealed by the instruments on Opportunity. The distinctored banding characteristic of many banded iron format(Klein and Beukes, 1992)should be obvious in MER Pancam images. The fossil record contains many examplemacroscopic fossil biofilms and microbial structures(West-all et al., 2000, 2001, 2004), observable at the resolutioof the Microscopic Imager and in some cases the restion of the Pancam. Many hot springs precipitate minethat fossilize and preserve characteristically colored mibial mats, as well as masses of organisms many centimacross(Allen et al., 2000; Chafetz et al., 1998). Mineralizedmicrobes sometimes contain traces of carbon in abundathat could be detected by APXS analysis, though thisstrument cannot distinguish between organic and inorgcarbon.

The instruments on Opportunity, however, are notsigned to provide unambiguous evidence of life, particulat the microscopic scales characteristically observed inrestrial iron oxide and hydroxide deposits. The mineralimicrofossils and associated biofilms described above havtypical dimensions of micrometers to tens of micrometAlmost all such features are significantly smaller than thespatial resolution of any MER instrument. Most fossil mcroorganisms in terrestrial rocks are only detectable by opcal and electron microscopy(Westall, 1999). The associationof carbon with putative microfossils can only be confirmusing microbeam techniques.

,

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Thus, while evidence suggestive of martian microbialmay be detected in situ, confirmation by direct fossilidence will probably require the return of samples torestrial laboratories. A key function of the next generatof Mars landers will be to discover and certify prime sifor future sample return missions. The Meridiani Planhematite deposit may well be among those prime sites.

5. Conclusions

Microbial life, including microcolonial fungi and bacteria, is documented in rock varnish layers from desert regof the American southwest and Australia. Analytical andvironmental SEM data show that portions of the fungi,well as molds of individual cells, are mineralized by iroxides and hydroxides, as well as by manganese oxThe number of microorganisms thus mineralized is smcompared to the microbiota on and within the varnish cing these samples. Iron oxide and hydroxide mineralizaof microbial cells has previously been demonstrated eximentally and documented in banded iron formations,spring deposits, and ferricrete soils.

These deposits, along with rock varnish, are examof the four “water–rock interaction” scenarios proposedformation of the hematite deposits detected on Mars.extensive Meridiani Planum deposit is the landing siteone of the twin Mars Exploration Rover spacecraft. The lder instrument suite may have the capability to distinguamong these proposed formation scenarios. In addition, thlander instruments may be able to detect spectral, morlogical, or compositional evidence suggestive of presemartian biosignatures. The confirmation of microfossils


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Acknowledgments

We gratefully acknowledge support from the NASA Atrobiology Institute through Principal Investigator DavidMcKay (CA, FW, TL) and the Lunar and Planetary InstituSummer Intern Program (TL, RS, LP, BF). Craig SchwanKathie Thomas-Keprta, and Susan Wentworth providedvaluable assistance in the electron microscope laboratoEverett Gibson generously donated the Pilbara samplethis study. Thoughtful comments by David Krinsley signcantly improved the manuscript.

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