Astrobiological Significance of Minerals on Mars Surface

Rev Environ Sci Biotechnol (2006) 5:219–231 DOI 10.1007/s11157-006-0008-x

REVIEW PAPER

Astrobiological signiﬁcance of minerals on Mars surface environment

Jesus Martinez-Frias Æ Gabriel Amaral Æ Luis Va´zquez

Received: 7 December 2005 / Accepted: 25 May 2006 / Published online: 14 July 2006 Springer Science+Business Media B.V. 2006

Abstract Despite the large amount of geomor- mineral parageneses on earth (in particular in the phological, geodynamic and geophysical data context of some selected terrestrial analogues), obtained from Mars missions, much is still un- and (3) to show that their differential UV known about Martian mineralogy and parage- shielding properties, against the hostile environ- netic assemblages, which is fundamental to an mental conditions of the Martian surface, are of a understanding of its entire geological history. great importance for the search for extraterres- Minerals are not only indicators of the physical– trial life. chemical settings of the different environments and their later changes, but also they could (and Keywords Mars minerals Æ Extreme do) play a crucial astrobiological role related with environment Æ Astrobiology Æ UV radiation Æ the possibility of existence of extinct or extant Jarosite Æ Gypsum Æ Sulfates Martian life. This paper aims: (1) to present a synoptic review of the main water-related Mar- tian minerals (mainly jarosite and other sulfates) Introduction discovered up to the present time; (2) to emphasize their significance as environmental geomar- Over the last half century, Mars has been ex- kers, on the basis of their geological settings and plored with telescopes, spacecrafts and robotic rovers. All the information obtained from these J. Martinez-Frias (&) Æ L. Va´zquez different sources, along with the results obtained Centro de Astrobiologıá (CSIC-INTA), 28850 by the study of SNC meteorites and terrestrial Torrejo´ n de Ardoz, Madrid, Spain analogs, is starting to reveal the geological e-mail: [email protected] diversity of the planet and provides data for the- G. Amaral orizing about how the different Martian envi- Departamento de Quı´mica Fı´sica I, Facultad de ronments evolved. Although it is well known that Ciencias Quı´micas, Universidad Complutense, 28040 liquid water is not stable at the surface under Madrid, Spain e-mail: [email protected] today’s atmospheric conditions (e.g., Ingersoll 1970; Hecht 2002), there is significant evidence L. Va´zquez that Mars once had a thicker atmosphere, that Departamento de Matema´tica Aplicada, Facultad de liquid water may have been much more abundant Informa´tica, Universidad Complutense de Madrid, 28040 Madrid, Spain on the surface and in the subsurface earlier in e-mail: [email protected] Martian history, that it has at least sporadically

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flowed on the Martian surface, and that it may Martian environment and its possible association even still be present in the subsurface today (e.g., with other liquid water-related minerals (e.g. Sagan and Mullen 1972; Carr et al. 1977; Cess gypsum) indubitably stresses its astrobiological et al. 1980; Squyres et al. 1992; Mckay and Stoker interest. 1989; Malin and Edgett 2000; Feldman et al. 2002; This paper aims: (1) to present a synoptic Boynton et al. 2002; Mitrofanov et al. 2002; Cos- review of the main water-related Martian minerals tard et al. 2002; Noe Dobrea et al. 2003; Squyres discovered till the present; (2) to emphasize their et al. 2004; Klingelho¨ fer et al. 2004; Madden significance as environmental geomarkers, on the et al. 2004; Christensen et al. 2004; Orofino et al. basis of their geological settings and mineral par- 2005; Glotch and Christensen 2005, among oth- ageneses on earth (in particular in the context of ers). However, despite the huge amount of geo- some selected terrestrial analogues), and (3) to morphological, geodynamic and geophysical data show that their differential UV shielding proper- obtained: (a) there is a clear ambiguity in inter- ties, against the hostile environmental conditions preting certain geological features of the Martian of the Mars surface, are of a great importance for surface, and (b) much is still unknown about Mars the search for extraterrestrial life. mineralogy and paragenetic assemblages, which is fundamental to an understanding of its whole geological history. Minerals are not only indicators Mineralogy and UV radiation on the surface of the physical–chemical settings of the different of Mars environments and their later changes, but also they could (and do) play a crucial astrobiological role The Martian regolith is made up of an apparently related with the possibility of existence of extinct homogenized dust having (broadly) basaltic or extant Martian life. If thirty years ago Viking composition, with admixed local rock compo- landers provided the first elemental analyses of nents, oxides (e.g. hematite), water-bearing Martian surface materials, the detection of an iron phyllosilicates and salts (mainly sulfates). Quar- mineral (gray crystalline hematite) by the Mars tzofeldspathic materials also have been identified Global Surveyor Thermal Emission Spectrometer (Bandfield et al. 2004). Information from scien- (MGS-TES) (Christensen et al. 2000, 2001; Pear- tific literature about past Mars missions, together son et al. 2000) led to the selection of Meridiani with recent reviews and new findings (see for in- Planum as one of the landing sites of the two stance Souza et al. 2004; McSween 2004; Vani- NASA’s Mars Exploration Rovers (MERs). In man et al. 2004; Lane et al. 2004; Squyres and 2004, the Mars Exploration Rover Opportunity’s Knoll 2005; Clark et al. 2005; Poulet et al. 2005; Moessbauer spectrometer obtained new straight- Yen et al. 2005; Hutchinson et al. 2005) indicate a forward evidence that, at least in Meridiani mineralogical composition of the Martian surface, Planum, the formation of hematite involved an which displays, in broad terms, the following aqueous mechanism. Hematite at Meridiani general distribution: silicates and oxides (mainly Planum consists essentially of spherules inter- olivine (Mg2SiO4 to Fe2SiO4), pyroxenes (Ca preted as concretions that have weathered out of a (Mg, Fe, Al)(Al, Si)2O6) and plagioclases (Na, sulfate-rich outcrop. In addition, hematite is also a Ca)(Si, Al)4O8 (87–79%)); hematite, Fe2O3, component of the outcrop matrix material. It also goethite, FeO(OH), sulfate salts (jarosite, indicated the presence of an iron-bearing mineral KFe3(SO4)2(OH)6, kieserite, MgSO4 Æ H2O, and called jarosite in the set of rocks dubbed ‘‘El very possibly also some polyhydrated sulfates: Capitan’’ (Squyres et al. 2004; Klingelho¨ fer et al. epsomite, MgSO4 Æ 7H2O, hexahydrite, 2004; Madden et al. 2004; Christensen et al. 2004; MgSO4 Æ 6H2O, pentahydryte, MgSO4 Æ 5H2O, Glotch and Christensen 2005). ‘‘El Capitan’’ is starkyite, MgSO4 Æ 4H2O, (12%) [Zhu et al. located within the rock outcrop that lines the inner (2006), Bibring et al. (2006), as well as, possibly, edge of the small crater where Opportunity szomolnokite and ferricopiapite] (Lane et al. landed. The exciting discovery of jarosite indicates (2004); and carbonates (Banfield et al. 2003) the existence of an ancient extreme (acidic) (0–4%), chloride salts (1%), nitrates (0–1%),

123 Rev Environ Sci Biotechnol (2006) 5:219–231 221 water (>1%, may be much higher). Poulet (2005) variably enriched in bromine relative to chlorine, detected the presence of phyllosilicates in the indicating a past interaction with water (Fan and ancient Martian highlands. These authors suggest Schulze-Makuch 2005). Generally, Martian bas- that Earth-like conditions existed well before alts are composed of plagioclase, feldspar, clino- 3.5–4 billion years ago. During later martian his- pyroxene, olivine, plus/minus sheet silicates and tory, it seems that the surface became more occur primarily in the equatorial to mid-latitude acidic, suppressing the formation of phyllosili- southern highlands regions (Banfield 2002). Ma- cates and carbonates, and leading to the haema- jor surface geological units of the ancient crust tite and sulfates spectacularly observed at consist of pyroxenes and plagioclase, with varying Meridiani by Opportunity. Very recently, Zhu proportions of olivine and alteration minerals. et al. (2006) suggest the existence of other min- Moreover, Martian (SNC) meteorites display eral phases, such as calcopyrite, covellite, garnet small amounts of secondary minerals (clays, (uvarovite, almandine) and thenardite. Marion carbonates, halides, sulfates) probably formed by et al. (2006) developed a model, parameterized reaction with subsurface fluids. for the Na–K–Mg–Ca–Fe–H–Cl–SO4–NO3–OH– In accordance with Patel et al. (2004) the study HCO3–CO3–CO2–CH4–H2O system, which of solar ultraviolet (UV) radiation is of extreme includes 81 solid phases. Their simulation sug- importance in a wide range of scientific disci- gests the possible existence, among others, of plines, with UV radiation playing an important melanterite, rozenite, mirabilite, szomolnokite role in organic and chemical evolution and also as and schwertmanite. a major constraint in biological evolution. Unlike Infrared observations display evidence for Earth, there is a significant amount of UV flux on igneous diversity and magmatic evolution on Mars, mainly due to the influence of the shorter Mars (Christensen et al. 2005). wavelengths UVC (100–280 nm) and UVB The very recent MEX-OMEGA results have (280–315 nm). On the surface of Mars solar shed light on the discussion about the mineral- radiation which penetrates the thin atmosphere at ogical and petrological characteristics of Mars wavelengths between 200 and 400 nm is capable surface. However, in accordance with Wyatt and of interacting directly with biological structures McSween (2002) we agree that controversy still and causing severe damage. Various works on the remains (and probably it will be necessary to biological effects of UV radiation (Cockell 1998; improve ‘‘in situ’’ analysis’’) about the existence Cockell et al. 2000; Ronto´ et al. 2003; Patel et al. of andesite versus chemically weathered basalts, 2003, 2004) have documented that even the cur- as the basalts of the northern plains on Mars are rent Martian UV flux would not in itself prevent more andesitic and weathered than the basalt of life. Nevertheless, it is a fact that this UV flux the southern highlands. They appear to be well contributes, together with the absence of liquid represented by the Bounce Rock at the Meridiani water and extreme low temperatures, to both Site, which is dominated by pyroxene (clinopy- possible mineralogical alterations (e.g. possible roxene ~55%, orthpyroxene ~5%) and plagio- dehydration) and to the biologically harsh nature clase (~20%), and is poor in olivine (~5%). of the Martian surface. In this sense, UV radia- Oxides are accounting for ~10%. The chemical tion induced dehydration was already suggested composition of Bounce Rock is more evolved around 30 years ago (Huguenin 1976; Huguenin than the basalts in the Gusev crater. It has a high et al. 1977). According to these works, photons

P2O5 content of 0.95 wt%, a Fe/Mg ratio of 36, a with wavelengths shorter than 280 nm release low Mg number (molar MgO/MgO + FeO) of H2O (g) from FeO(OH) (goethite) by ejecting 0.42 and a high Ca/Al ratio of 1.7, a lower FeO OH-ligands which subsequently combine with H+ (15.6%), and a higher CaO (12.5%) content from nearby sites. Morris and Lauer (1981), (Squyres et al. 2004; Klingelho¨ fer et al. 2004; however, repeated the experiments and found no Squyres and Knoll 2005; Christensen et al. 2005; UV dehydration effects on goethite (a-FeOOH) Clark et al. 2005). Broadly, the basalts in the or lepidocrocite (g-FeOOH) in exposures equiv- northern plains are in general rich in sulfur and alent to 10–100 years on the Martian surface.

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More recently, Yen et al. (1997) indicated that preferentially developed in a particular sub- exposure to the Martian environment over geo- surface microenvironment able to protect it from logic time scales could have removed the initial the harsh conditions on the surface. Thus, the water content of the hydrated minerals modeled study of the mineralogical and petrologic features to be present. These authors placed iron oxide of the Martian surface is crucial. Terrestrial samples into an ultra high vacuum (UHV) endolithic communities that live in the subsurface chamber evacuated to 10–8 torr using an ion layers of rock that provide appropriate microen- pump. One sample was exposed to 254 nm radi- vironments against extreme external conditions ation from a mercury vapor lamp through a sap- have been proposed (Friedmann 1982; McKay phire window while the other sample was held as 1993; Wynn-Williams and Edwards 2000; Villar a control. The samples were then heated to et al. 2005) as possible analogs to life on Mars. In 500C, and the evolved water was measured as a this context, Cockell et al. (2003) point out that in function of temperature. The results obtained natural terrestrial environments, there are a (Yen et al. 1997) indicated that, although more variety of speciﬁc substrates (rocks, snow and ice, conﬁrmation is required, ultraviolet radiation was soils, dust), that can cover microbial communities. capable of enhancing the rate of dehydration of Some microbial species inhabit the underside of goethite in high vacuum conditions. rocks as ‘‘hypolithic’’ organisms (Broady 1981a, From the astrobiological point of view, it is b) or they live in cracks in rocks as ‘‘chasmoen- extremely important to note that the present-day doliths’’ (Broady 1981b). The petrologic and DNA-weighted irradiance on the surface of Mars mineralogical composition of the substrate is also is similar to the weighted irradiance on the sur- important. In fact, where the geological features face of Archean Earth (Cockell 1998). The of the substrate allow, they can inhabit the inside amount of dust in the atmosphere has a non- of rocks as ‘‘cryptoendolithic’’ micro-organisms trivial effect and its UV shielding role must not be (Friedmann and Ocampo 1976). Therefore, ex- underestimated. Patel et al. (2004) indicate that tant Martian life would require strong UV high wind speeds, dust devils and local/global shielding, which, in accordance with some min- storms can raise particulate matter from the Mars eralogical studies presented here, could be per- surface and inject it into the atmosphere, where fectly accomplished, at the surface, by certain the dust can remain for long periods of time minerals (e.g. sulfate minerals) already discov- playing a major role in global circulation and ered on Mars. But if it is important to identify and atmospheric dynamics. As indicated by Cockell understand the mineralogical assemblages of the and Knowland (1999), for 3.8 billion years evo- surface, it is also critical to determine the set of lution, the development of strategies to attenuate geochemical reactions which can modify them, UV radiation has been an omnipresent issue for altering the original settings. life, mainly for photosynthetic organisms that With regards to the geochemical processes of require solar radiation for their energy needs. The Mars’ surface, Burns (1987) suggested the possi- authors illustrate a selection of UV shielding bility of formation of the ferric oxyhydroxysulfate methods found on present-day earth which may mineral schwertmannite in equatorial regions of have been relevant on early earth: iron com- Mars, where acidic permafrost melts and is oxi- pounds, sulfur, solid NaCl, water column, dized by the Martian atmosphere and, more re- sediments, different types of rocks and minerals. cently, Lammer et al. (2003) evaluated the Even microbial communities themselves can formation of ferric oxy-hydroxides and sulfates. protect other communities from UV radiation. They indicated that the oxidation of iron may The surface of microbial mats has been shown to schematically be described in terms of the change provide protection against UVC radiation for the of the ferrous component of iron-bearing pre- microbiota beneath (Margulis et al. 1976). cursor phases into a ferric oxide. Gomez et al. There is a general agreement regarding the (2003) studied the growth of prokaryotic and exploration and detection of life on Mars: any eukariotic microorganisms after UV irradiation living organism, as we know it, should have with and without ferric iron as a protection agent,

123 Rev Environ Sci Biotechnol (2006) 5:219–231 223 concluding that ferric iron is an effective protec- related with volcanic or postvolcanic activity tive agent for both cell systems. It is proposed that (alteration of volcanic rocks in acid fumaroles, or the ferric oxide on Mars may be hematite and/or hydrothermal activity with or without implication maghemite, which are chemically identical and of bacterial activity, and (c) evaporitic processes. that the oxidation process itself is independent of Some common chemical reactions leading the the transformation of ferric oxides into oxyhy- formation of iron sulfates, which are widespread droxides. A signiﬁcant aspect that they stress is in the gossan areas of many hydrothermal mineral that the formation of sulfates may be as important deposits, involve the oxidization of pyrite, mar- as rusting, and for the oxidation process itself, the casite, or other sulﬁdes by the atmosphere and type of sulfate is unimportant. Under the oxidiz- water (Jerz 2002): ing conditions on the Martian surface, any sulfur in the soil should be bound in sulfatic weathering FeS2ðsÞðpyrite, marcasiteÞþ7=2O2ðaqÞ þ H2OðaqÞ phases (Lammer et al. 2003). ¼ FeSO4ðsÞ þ H2SO4ðaqÞ

Minerals as environmental geomarkers: Fe1xSðsÞðpyrrhotiteÞþ8 2x=4O2ðaqÞ þ xH2OðaqÞ astrobiological signiﬁcance of water- ¼ð1 xÞFeSO4ðsÞ þ xH2SO4ðaqÞ related sulfates

In accordance with Farmer (2004), in defining a A third reaction occurs when sulfur is burned site-selection strategy to explore for a Martian and the gas is released, which slowly reacts with fossil record, a key concept is contemporaneous free oxygen in humid air to form: chemical precipitation, or mineralization. On S þ O þ heat ¼ SO Earth, geological environments where microor- ðsÞ 2ðgÞ 2ðgÞ ganisms are often preserved in this way include, 2SO2ðgÞ þ O2ðgÞ þ 2H2OðgÞ ¼ 2H2SO4ðgÞ among others: (1) mineralizing systems (subaer- ial, subaqueous, and shallow subsurface hydro- A well known and very interesting example is thermal systems, and cold springs of alkaline represented by the acidic waters of the Rio Tinto, lakes), (2) ephemeral lacustrine environments and the associated deposits of hematite, goethite, (sabkhas), or terminal (evaporative) lake basins, jarosite and other sulfates, which have been rec- (3) duricrusts and subsoil hard-pan environments ognized as an important chemical analog to the formed by the selective leaching and re-precipi- ‘‘Sinus Meridiani’’ site on Mars (Fernandez- tation of minerals within soil profiles, and (4) Remolar et al. 2004, 2005; Fairen et al. 2004). The periglacial environments ground ice or perma- Mars Analog Rio Tinto Experiment (MARTE) frost (frozen soils) have captured and cryopre- (Stoker et al. 2003, 2005, 2006) has been investi- served microorganisms and associated organic gating the hypothesis of a subsurface microbial materials. Thus, if we want to identify potential ecosystem based on the metabolism of iron and biomarkers regarding the type of microbes which sulfur minerals. Reduced iron and sulfur might lived (or still live) at the surface of Mars, we will provide electron donors for microbial metabolism previously need to use the minerals as geomar- while in situ oxidized iron or oxidants entrained kers to understand the geological and environ- in recharge water might provide electron accep- mental context. tors. The results obtained indicated that geo- Sulfates are indeed to be present in the Mar- chemical resources are available in the Rio Tinto tian soil as indicated by the sulfur measured and subsurface to support several kinds of anaerobic other mineralogical determinations at the Viking, chemolithotrophic metabolism (Stoker et al. Mars Pathfinder (MPF), and Mars Exploration 2005, 2006). Rover (MER) (Lane et al. 2005). Sulfates are Likewise, sulfate deposits related with evapo- widespread minerals in nature, mostly linked with ritic processes are also important indicators of different formation mechanisms (O’Connor their depositional environments, including cli- 2005): (a) alteration of sulfides; (b) genetically mate and the hydrochemistry of the water from

123 224 Rev Environ Sci Biotechnol (2006) 5:219–231 which the minerals precipitated (Spencer 2000). tures. Murad and Rojı´k(2003) believe that Spencer and Hardie (1990) calculated the pre- these changes in mineralogy and the associated cipitation sequence for the evaporation of mod- color variations are direct indicators of the ern seawater, and discovered that precipitate environments in which the minerals were minerals would form in the following order: cal- formed. cite, gypsum, anhydrite (CaSO4), halite, glaube- As previously deﬁned, some water-related rite (Na2Ca(SO4)2), halite, polyhalite minerals (e.g. gypsum, jarosite) were recently (K2Ca2Mg(SO4)4 Æ 2H2O), epsomite (MgSO4 Æ 7- discovered (Squyres et al. 2004; Klingelho¨ fer H2O), hexahydrite (MgSO4 Æ 6H2O), kieserite et al. 2004; Langevin et al. 2005) on Mars’ sur- (MgSO4 Æ H2O), carnalite (KMgCl3 Æ 6H2O), and face. Both sulfates can be used as idoneous bischoﬁte (MgCl2 Æ 6H2O) (Spencer 2000). How- examples which represent very well their envi- ever, caution is needed in the extrapolation of ronmental and astrobiological applications as these ‘‘precipitation patterns’’ to possible evapo- potential geomarkers. ritic Martian systems as very big differences be- Gypsum, CaSO4 Æ 2H2O, is a very common tween modern locations of evaporite mineral terrestrial evaporitic sulfate. It has essentially a deposition and those in the rock record can exist. layered structure bound by hydrogen bonds.

Quinn et al. (2005) examined the dry acid Zig-zag chains of CaO8 polyhedra, running par- deposition and accumulation on the surface of allel to c, are bound together by similar chains of 2– Mars and in the Atacama desert and proposed isolated (SO4) tetrahedra, forming a double that the recent discovery of the Martian jarosite, sheet perpendicular to (010). Each Ca2+ ion is which forms in strongly acidic-sulfate rich envi- surrounded by six oxygen atoms belonging to the ronments, increases the importance of under- sulfate groups and two oxygen atoms belonging to standing the chemical state of the Martian the H2O molecules. These H2O molecules form a surface material and its behavior in aqueous layer binding the polyhedral sheets together with systems. These authors concluded that the ex- weak hydrogen bonds. The H2O molecules are tremely low pH resulting from acid accumula- significantly distorted and are oriented such that tion, combined with limited water availability the hydrogen bond H2 O1 acts almost entirely and high oxidation potential, will result in acid- along b (Schofield et al. 1996). It has been sug- mediated reactions at the soil surface during gested (Moore and Bullock 1999) that evaporite low-moisture transient wetting events (i.e. thin deposits may represent significant sinks of mobile films of water) (Quinn et al. 2005). These soil cations (e.g., those of Ca, N, Mg, and Fe) and acids are expected to play a significant role in anions (e.g., those of C, N, S, and Cl) among the the oxidizing nature of the soils, the formation materials composing the Martian surface and of mineral surface coatings, and the chemical upper crust. modification of organics in the surface material. Some well known gypsum-rich evaporitic areas In this context, it is important to stress that (e.g. Sorbas area, SE Spain) have been proposed Murad and Rojı´k(2003) found that some sulfate (Martinez-Frias et al. 2001a) as possible Mars precipitates showed color and mineralogy vari- analogs to study paleogeographic, paleoclimatic ations depending on the pH. In initial acidic and mineralogical problems associated with conditions, which have a pH of about 2.3, the catastrophic evaporitic processes. The Sorbas dominant mineral is jarosite. A pH between 3 basin contains one of the most complete sedi- and 4 yields precipitates that are orange in mentary successions of the Mediterranean (gyp- color, and the most predominant mineral is sum karst) reflecting the increasing salinity during usually schwertmannite. Ferrihydrite and goe- the Messinian salinity crisis (desiccation of the thite, brownish-red in color, formed at a more Mediterranean Sea) (Fig. 1) (Martin and Braga neutral pH between 5 and 7. Jarosite, schwert- 1994; Riding et al. 1998; Krijgsman et al. 1999) mannite, ferrihydrite and goethite are all ferric and showing a complex paleogeographical evo- (hydr)oxysulfates, meaning that the minerals all lution, being a signature of its progressive contain Fe3+ and OH- in their chemical struc- restriction and isolation.

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Jarosite is a mineral of the alunite-jarosite family. In accordance with Scott (2000), the alunite–jarosite minerals are deﬁned as having the

general formula AB3(XO4)2(OH)6, where A is a large ion in 12-fold coordination (e.g., K, Na, Ca,

Pb, and REE), B is usually Fe or Al, and the XO4 anions are usually SO4,PO4 or AsO4. Jarosite was first characterized on Earth, in 1852, in the ‘‘Jar- oso Ravine’’ at Sierra Almagrera (Fig. 2), in the Cuevas del Almanzora natural area (Jaroso Hydrothermal System, Almeria province, Spain), which is the world type locality of jarosite (Amar de la Torre 1852; Martinez-Frias 1999). The Jar- Fig. 1 Gypsum crystals from the Sorbas evaporitic basin, oso Hydrothermal System (Martinez-Frias et al. ´ Almerıa province, SE Spain. Sulfate-rich layers reflect the 2004) is a volcanism-related multistage hydro- increase of salinity during the Messinian crisis (desiccation of the Mediterranean Sea) thermal episode of Upper Miocene age, which includes oxides and oxy-hydroxides (e.g. hema- Fishbaugh et al. (2006) propose that north tite, goethite), base- and precious-metal sulfides polar gypsum deposit of Mars was formed as an and different types of sulfosalts. Hydrothermal evaporite deposit in the unique conditions pro- fluids and sulfuric acid weathering of the ores vided at the north pole. Water from the Chasma have generated huge amounts of oxide and sulfate Boreale melting event (and possibly a nearby minerals of which jarosite is the most abundant impact into ice) pooled beneath the ice and (Martinez-Frias et al. 1992; Martinez-Frias 1998; evaporated, precipitating gypsum. The ice has Rull et al. 2004). Very recently, hallotrichite since retreated, exposing the gypsum source re- (FeSO4Æ Al2(SO4)3 Æ 22H2O) also has been found gion, allowing gypsum to be eroded from this and characterized at the Jaroso area (Frost et al. source by the wind. Sand sized gypsum particles 2005). It is important to note that this area of SE are now saltating and are intimately mixed with Spain had already been proposed as a relevant the dark, mafic sands. geodynamic and mineralogical model (Martinez- Recent studies (Parnell et al. 2004) of micro- Frias et al. 2001a, b; Martinez-Frias et al. 2004; bial colonization in impact generated hydrother- Rull et al. 2005; Rull and Martinez-Frias 2006)to mal crystalline gypsum deposits in the Haughton follow for the astrobiological exploration of Mars. Crater, Devon Island, Canadian High Arctic, have demonstrated the presence of cyanobacteria in endolithic habitats up to 50 mm from the crystal margins. The crystalline gypsum was found to exist in the clear selenite form. These authors indicate that the propensity for sulfates to form clear crystals makes them an advantageous habi- tat for photosynthesisers. In accordance with Parnell et al. (2004) and Edwards et al. (2005), the gypsum colonisation in the Haughton Crater has a particular astrobiological relevance with the recent discoveries of sulfate minerals on Mars. The authors consider interesting to speculate that the colonisation of gypsum deposits on Mars could be a geological niche of microbial activity Fig. 2 Typical alteration crust rich in jarosite at EI Jaroso ravine, Sierra Almagrera, Jaroso Hydrothermal System, from periods when there was significant moisture Cuevas del Almanzora Natural Area, Almerıá province, at the Martian surface. SE Spain

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Microorganisms typically are involved in the Haughton selenite gypsum exposed to the oxidation of sulfides to sulfates in terrestrial acid environmentally relevant range 290–400 nm, mine drainage sites. Hence, outcrops on Mars exhibited a mean absorbance of 0.12 (transmis- which are rich in acid sulfate minerals (e.g. sion of 0.88). Recent results obtained by Amaral jarosite) may be a good location to search for et al. (2005) showed that whereas gypsum showed evidence of life on that planet (Colmer and Hinkle a much higher transmission percentage, jarosite 1947; Bishop et al. 2005). An astrobiologically samples, with a thickness of only 500 lm, pre- significant aspect linked with sulfates was recently vented transmission. It is well known that, iron described by Aubrey et al. (2005). These authors and iron-bearing compounds can provide an UV studied concentrations of organic matter along screen for life (Sagan and Pollack 1974; Olsen and with amino acids in natural terrestrial sulfate Pierson 1986; Pierson et al. 1993; Kumar et al. mineral samples. They found that sulfate minerals 1996; Allen et al. 1998; Phoenix et al. 2001; Go- contain between 0.03 and 0.69% organic carbon mez et al. 2003; among others). The results ob- as well as high ppb to low ppm abundances of tained by Amaral et al. (2005) fit this working amino acids and their degradation products in hypothesis well and are extremely important for samples ranging from 30 million years old to the search for life on Mars as: (a) jarosite typically contemporary. Thus amino acids and their amine occurs on Earth as alteration crusts and patinas, decarboxylation products are well preserved over and (b) a very thin crust of jarosite on the surface long geological time in the sulfate mineral of Mars would be sufficient to shield microor- matrices on Earth, and, as suggested by the au- ganisms from UV radiation. thors, sulfates should be principal targets in the search for organic compounds, including those of Final remarks biological origin, on Mars (Aubrey et al. 2005, 2006). Jarosite (Fig. 2) has proven to have a great As demonstrated by the MER mission, mineral- astrobiological importance, not only for its rela- ogy provides the most robust means for discov- tion with liquid water, but also because it can act ering ancient aqueous environments and as a sink and source of Fe ions for Fe-related comprises an essential step in selecting the sites chemolithoautotrophic microorganisms, such as that have the best chance for having captured and those encountered in numerous extremophilic preserved a record of ancient life or pre-biotic ecosystems (e.g. Tinto river) (Lo´ pez-Archilla chemistry. A sophisticated spectrometer can et al. 2001; Gonzalez-Toril et al. 2003; Amaral accurately identify a specific water-related min- Zettler et al. 2003; Fernandez-Remolar et al. eral (e.g. jarosite, gypsum, kieserite, etc.) on 2004, 2005). Mars; but, what does it mean? We know that the Considering all previous aspects and the as- same mineral can be formed in different terres- trobiological relevance of both Martian Ca and trial environments; the same sulfate that we can Fe sulfates, Amaral et al. (2005) performed UV find in a dessert can also be the product of a radiation experiments on jarosite and gypsum hydrothermal system. Thus, as previously defined, samples (Figs. 1 and 2) from Jaroso and Sorbas a previous step to detect possible Martian area, SE Spain (Martinez-Frias et al. 2001b) using biomarkers is the utilization of minerals as geo- a Xe Lamp with an integrated output from 220 to markers to understand the geological and envi- 500 nm of 1.2 Wm–2. Samples were flattened to ronmetal context. This implies that if it is different thicknesses (between 0.1 and 1.6 mm) essential to determine what minerals and rocks before being exposed to UV light. The results are significant for the search of life on Mars, the obtained demonstrated a large difference in the appropriate selection and detailed study of the UV protection capabilities of both minerals and different geological and mineralogenetic terres- also confirmed that the mineralogical composition trial settings (Mars analogs) in which such min- of the Martian regolith is a crucial shielding erals occur and evolve, are also of a great interest. factor. In a previous work, Parnell et al. 2004 Unfortunately the number of minerals unambig- had determined that a 1 mm thickness of the uously identified on Mars’ surface is still

123 Rev Environ Sci Biotechnol (2006) 5:219–231 227 extremely scarce and their textural relationships Bandfield JL, Hamilton VE, Christensen PR, McSween are not well understood. The interdisciplinary HY Jr (2004) Identification of quartzofeldspathic materials on Mars. J Geophys Res E100:1–14 study of potential Mars analogues en Earth Bibring JP, Langevin Y, Mustard JF, Poulet F, Arvidson (hydrothermal systems, evaporitic areas, acidic R, Gendrin A, Gondet B, Mangold N, Pinet P, Forget rivers, impact craters, (mineralizing) submarine F, the OMEGA team (2006) Global mineralogical and hydrocarbon vents, etc.) are helping us to and aqueous Mars history derived from OMEGA/ Mars express data. Science 312:400–404 recognize the great variety of geological and Bishop JL, Dyar MD, Lane MD, Banfield JF (2005) mineralogical frameworks and the richness of Spectral identification of hydrated sulfates on Mars environmental settings useful in astrobiological and comparison with acidic environments on Earth. exploration. Int J Astrobiol 3:275–285 Boynton WV, Feldman WC, Squyres SW, Prettyman TH, Bru¨ ckner J, Evans LG, Reedy RC, Starr R, Arnold JR, Acknowledgements This work was supported by the Drake DM, Englert PAJ, Metzger AE, Mitrofanov I, Spanish Centro de Astrobiologia (CSIC/INTA), associated Trombka JI, d’Uston C, Wa¨nke H, Gasnault O, Ha- to the NASA Astrobiology Institute. Thanks to the Rover mara DK, Janes DM, Marcialis RL, Maurice S, Mi- Environmental Monitoring Station (REMS) project. 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