Mem. S.A.It. Suppl. Vol. 11, 164c© SAIt 2007

Memorie della

Supplementi

Physical characterization of biominerals ofMartian interest

A. Blanco, M. D’Elia, D. Licchelli, V. Orofino, S. Fonti

Physics Department, University of Lecce, Via Arnesano, 73100 Leccee-mail: armando.blanco@le.infn.it

Abstract. In order to obtain mineralogical information and to determine if life ever existedon the surface of Mars, next space missions have the purpose to perform in-situ analysesor recover samples of the Martian ground, and to bring them back to Earth. The aim isdetermining the mineralogical and chemical composition of these samples for studying theMartian geology and possible past or present biology. In this framework, laboratory researchon biotic and abiotic minerals of exobiological interest can allow to characterize possiblediscriminating factors useful to distinguish the biological origin from the mineral one. Wehave investigated the physical properties of calcium carbonate (CaCO3), which could beexpected on Mars. The polymorphs of CaCO3, aragonite and calcite, are interesting becauseon Earth these compounds are produced by abiotic process as well as by biological activity.We performed infrared transmission spectroscopic analyses aimed to examining the be-haviour after heating biotic and abiotic particulate samples composed of calcium carbonate(mineral, fresh biotic and recent fossil aragonite), in order to discriminate their origin. Herewe present the results of our spectroscopic and thermal studies focused also on carbonateslinked to primitive terrestrial living organisms (fresh stromatolites) to understand whetherand how the different biomineralization can influence the structure and composition of thesamples.

Key words. exobiology – Mars – spectroscopy

1. Introduction

The environmental conditions on Marsmay have been Earth-like (Carr, 1996).Observations show now a cold and dry planetwith a very thin atmosphere and no liquid wa-ter on the surface. However, geomorphologicalevidences indicate that liquid water once wason the Planet and therefore simple forms oflife could have developed on the surface or inthe underground (McKay and Stoker, 1989).

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The future in-situ and sample-return spacemissions have the aim to determine whetherlife ever existed on Mars, to characterize andstudy in detail the climate and the geology ofthe planet, and to prepare for human explo-ration.

In this framework Cabane et al. (2001)have proposed the SAM (Sample Analysis atMars) experiment, devoted to in-situ analysesof the Martian soil for the next NASA MarsScience Laboratory, to be launched in 2009.One important part of this experiment has theaim to search, on the surface and underground,for traces of a past prebiotic chemistry and, op-

Blanco: Physical characterization of biominerals 165

Fig. 1. Reference spectra: abiotic aragonite and calcite (data from Salisbury et al., 1991) and pure CaOgrains analysed in our laboratory. In this figure, as well as in all the following figures, the spectra have beenarbitrarily scaled for sake of clarity.

timistically, for extant life. A mass spectrom-eter (MS) associated to a gas chromatograph(GC) will be used as detector to differentiatebetween abiotic minerals and biominerals, byDifferential Thermal Analysis (DTA).

Planning the scientific instruments for in-situ analyses and for experiments on the pos-sible specimens of sample-return missions re-quires preliminary laboratory studies able totest the proposed techniques on terrestrial min-eral samples that are analogous to the Martianones.

Waiting for the final confirmation of thepresence of carbonates on Mars, which mayhave important implications regarding the pos-sibility that life developed there, we studied amineral that may have either biotic or abioticorigins: aragonite, a polymorph of calcium car-bonate (CaCO3). We performed transmissionspectroscopy, using a FT-IR spectrometer, inthe infrared wavelength range (2-25 µm) fo-cused on examining results of different thermaltreatments starting from tested temperatures inDTA experiments (420◦C, 485◦C and other notshown in this work). As a matter of fact, anendothermic transformation occurs from themetastable aragonite to the more stable formof calcite (another polymorph of CaCO3) and

then, at higher temperatures, from calcite intosolid calcium oxide (CaO) and gaseous carbondioxide (CO2).

Fig. 1 reports the reference spectra of arag-onite and calcite (Salisbury et al., 1991) and alaboratory spectrum of pure CaO.

We have also used an optical microscopeand a Scanning Electron Microscope (SEM)coupled with an Energy Dispersive X-raySpectrometer (EDX), in order to morpholog-ically and compositionally characterize thesamples.

2. Measurements and results

Sample preparation, thermal treatments andtypes of measurements are described in Blancoet al. (2005) and in Orofino et al. (2006).Blanco et al. (2005) demonstrated that thespectroscopy and DTA should be helpful indistinguishing minerals from recent biominer-als. Comparing the infrared spectra of recentbiomineral aragonite and crystals of mineralaragonite, it is evident that the thermal process-ing at 420◦C and 485◦C induces different phys-ical changes depending on mineral or biologi-cal origin of the samples; as a matter of fact,the process of transformation is faster for ”re-

166 Blanco: Physical characterization of biominerals

cent” biotic samples compared to the abioticmineral.

Fig. 2. Aligned crystal of aragonite in an Oligocenefossil.

Fig. 3. Mineral aragonite grain morphology.

It would be unreasonable, however, re-stricting the laboratory experiments to modernbiotic samples because, if water and life ex-isted in the past on the surface of Mars, it ismore likely to find ancient living forms. Forthis reason, we studied aragonite fossils datingfrom Oligocene (23-30 million years ago).

It is known that the organic macro-molecules control crystal growth and orienta-tion and we verified that the fossilization pro-cess, in the considered samples, did not modifythe structure of the biominerals which main-

tain their microscopic characteristics. Lookingat the morphology of analysed fossil biominer-als with a Scanning Electron Microscope, it isevident that the crystals are arranged in com-plex architectures compared with the compactstructure of the mineral crystals (Fig. 2 and Fig.3 respectively). Obviously, SEM analysis is notsufficient to distinguish the biotic samples fromthe abiotic ones but, in this case, this techniquegave us a characterization of the analysed fossilbiominerals that helped to distinguish the dif-ferent physical properties.

The aragonite fossils (Ampullinopsis cras-satina and Cerithium sp. dating back 23-30million years) were collected in a layer of fos-sil clay. The very specific environmental condi-tions allowed preserving the original structureof aragonite so that spectroscopic analysis be-fore the heat treatment shows the characteristicfeatures of this mineral (compare the ”T ambi-ent” spectrum in Fig. 4 with that of aragonitein Fig. 1).

After heating at 420◦C, the fossil bioticsamples change from the metastable form ofaragonite to the more stable form of calcite(”T=420◦C” spectrum in Fig. 4 shows thesame spectral features of calcite in Fig.1). At485◦C the transformation process from CaCO3to CaO is activated via CO2 release from thesample; as a matter of fact, looking at the”T=485◦C” spectrum in Fig. 4, it is evident theformation of the CaO band that has its max-imun at about 30 µm (see CaO spectrum in Fig.1). As indicated by the spectra, the thermal pro-cessing of fossil aragonite induces the same be-haviour of ”recent” biotic aragonite (Blanco etal., 2005; Orofino et at., 2006). In fact, bothfresh and fossil analysed shells have spectradiffering considerably from those of the corre-spondent abiotic specimens heated at the sametemperatures. This means that, also in the caseof biomineral fossil aragonite, the transforma-tion process takes place at lower temperatures.An explanation could be that the rapid devel-opment of the crystalline structure under bioticconditions, makes it less resistant than the abi-otic mineral. We note that the fossilization pro-cess did not modify, from this point of view,the physical characteristic of the consideredbiomineral so that the infrared spectroscopy

Blanco: Physical characterization of biominerals 167

Fig. 4. Spectra of fossils of aragonite (Ampullinopsis crassatina and Cerithium sp. fossils have the samebehaviour) before and after processing at 420◦C and 485◦C.

is still able to differentiate between the anal-ysed biotic and abiotic samples (Orofino et at.,2006).

Since on Earth there are microfossils al-most four billion years old, life must have ap-peared within one or two hundred million yearsafter Earth cooled enough to host liquid water.Life may have developed readily, perhaps in-evitably, given the physical and chemical con-ditions on the planet. Such conditions couldhave occurred also on Mars, before it dried andfroze. If Mars was once wet and warm, withseas and hydrothermal vents present in someareas of its surface, then we might hope to findprimitive life form, particularly in such laca-tions (McKay and Stoker, 1989).

On Earth, stromatolites are the oldestknown fossils, dating back more than 3 billionyears. They are colonial structures formed byphotosynthesizing cyanobacteria and other mi-crobes. They were the dominant life form onEarth for over 2 billion years. For this reason,while waiting for some fossil stromatolites, wespectroscopically studied fresh stromatolitesfrom Shark Bay in Australia, that have an arag-onite structure. The spectrum at T=ambient(i.e. before the thermal process), shown in Fig.5, indicates that they are composed of arago-nite. The feature around 9 µm is due to traces of

magnesium sulphate (MgSO4). The SEM mor-phological analysis confirms the presence ofseaweeds while the EDX compositional studyreveals the presence of Magnesium, Sulphurand Oxigen. Also in this case the heating pro-cess transforms aragonite first into calcite andthan into calcium oxide (see the formation ofthe characteristic feature beyond 20 µm in thespectrum at T=485◦C in Fig. 5).

3. Conclusions

Many different areas of investigation may beimportant in studying Mars, but one of themain goal is the search for a possible micro-bial activity. For this reason, scientists are ac-tively preparing in-situ instrumentation for fu-ture landing platforms and they are studyingpossible techniques for the analysis of samplesbrought back from Mars. The future missionsto Mars, taking advantage of a rich legacy ofremote sensing observations, will be able totarget a pre-selected site of scientific interestand will address the issue of life on the planet.

In the meantime, laboratory research on bi-otic and abiotic minerals of exobiological in-terest can be used to find possible discriminat-ing factors, useful to distinguish the biologicalorigin of a sample from an abiotic formation.

168 Blanco: Physical characterization of biominerals

Fig. 5. Spectra of fresh stromatolites before and after processing at 485◦C. The band around 9 µm is due totraces of magnesium sulphate.

Cabane et al. (2004) proved that the massspectrometer coupled with a gas chromato-graph (GC), using the Differential ThermalAnalysis, can differentiate between abioticminerals and fresh biominerals. In our workwe demonstrated that also the infrared spec-troscopy can be a diagnostic tool for discrim-inating samples of biotic and abiotic origin.This is true not only for minerals and ”recent”biomineral (fresh shells and stromatolites) butalso for fossils of aragonite. All the organo-genetic samples behave in the same way: thechanges occur at lower temperatures than thosefor the abiotic specimens. Indeed, studyingthe physical properties and examining the be-haviour after heating, the endothermic trans-formation from aragonite to calcite and thefollowing modification of calcite into CaO, isfaster for biotic samples than for the abioticmineral. As a next step, it will be interest-ing to analyse fossil stromatolites as well asfossils of intermediate geologic epochs (50 -1000 Myr ago), not only of aragonite compo-sition but also of calcite structure. As a matterof fact, the fossilization process can transform

their original composition and we aim to studyin depth the changes in spectral behaviour af-ter heat processing of differently preserved bi-ological structures. In future we plan to repeatthe measurements (again in the infrared wave-length range) using reflectance spectroscopy,which is more easy to be performed directlyin-situ on the surface of Mars.

References

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