Preservation In Thin Films Covering Rocks

The water history of Mars suggests that rock coatings, desert varnishes and weathering rinds should be present in any sedimentary deposit or embedded inside alteration materials derived from ancient and recent aqueous activity. Spirit MER has indeed provided direct evidences of rock coatings of unequivocal aqueous alteration (Haskin et al., 2005). In this sense, location and analysis of rock coatings of several ages can be essential to trace back the climatic and environmental history (Dorn and Dickinson, 1989; Liu and Broecker, 2000) of the planet since its origin. In fact, lamina accrection and substrate weathering currently work under seasonal climatic regimen that results from the periodic water availability and thermal conditions. Such information can obvioulsy be essential to determine the ancient to recent surface and subsurface environmental water patterns that took place on Mars. <-

Figure 4. (continued) Meridiani Mars area explored by the Opportunity (B) microbial remains embedded in a cryptocristalline matrix composed of SO4 and Fe3+-bearing mineralogies sampled n modern deposits of Río Tinto (Spain). (C) 2.1 million years goetithe layer showing preserved filaments inside having cryptocristalline habit (Río Tinto, Spain). (D) Organic association preserved in neutral and reducing underground areas of the Río Tinto system where hopanoids are present.

In desert varnishes thin aqueous films contacting the rock surfaces are currently oversaturated in ionic species such as silica, manganese or iron remobi-lized from the rocky substrate (Perry et al., 2006), which favors microbial activity (Kuhlman et al., 2006) and later preservation to organic traces when the conditions are favorable (Perry et al., 2006). In rock coatings and weathering rinds oversaturation under aggressive acidic or alkaline conditions is also favorable for inducing high mineralization rates to preserve from microbial structures to organics. As a result, a complex mixing of different mineralogies such as iron and manganese oxides, sulfates, carbonates, opaline silica and different phyllosilicates (Potter and Rossman, 1977) occurs as laminae enveloping weathered rocks.

Several environmental conditions besides aridity areas can be imprinted in the surface rinds. Some volcanic emissions centered in geothermal activity produce SO2-rich acidic fog which acts on volcanic tephra to induce the formation of silica and sulfate laminae (Schiffman et al., 2006). Ancient fluvial deposits are currently composed of conglomeratic materials which pebbles may show coatings depending on the climatic conditions that originated them. The Triassic Buntsandstein fluvial deposits in association to contemporaneous and older lacustrine and aeolian desert-like sedimentary materials, contain rounded pebbles that are covered by iron-rich coatings recording paleoclimatic information of great interest (Fig. 5A). The Río Tinto geological record dating back from Tertiary also shows iron rich coatings which origin is undoubtly associated to seasonal activity of acidic environments (Fig. 5B) and with clear traces of biological activity (Figs. 5C, D).

The importance of these microdeposits as recorders of modern and ancient biological activity, which can be easily detected in many planetary regions of Mars, cannot be overemphsized. As showed by Kuhlman et al. (2006), rock varnishes, as many other film coating environments, are microhabitats inhabited by diverse microbial communities having up to 108 microorganisms per gram of varnish lamina. Such a microbial activity can be traced through biochemical and organic compounds (Perry et al., 2006) that can be the base for the development of an exploration strategy for searching for life on Mars.

4. Conclusions: Mars Preservation Windows and Strategy for Planetary Exploration

Integrative research on preservation of biological information in terrestrial analogs is essential for building a consistent strategy to search for extinct life on Mars. From the scientific point of view, any exploration strategy developed for this compelling objective has to deal with the diverse Mars geological record which shows different preservation potential of biology depending on the paleoenvironment and diagenetic processes that have conformed it. As a result, different paleobiological entities may have potentially persisted and detection demands distinctive explorative procedures, sampling techniques and instrumentation (Farmer and Des Marais, 1999).

Figure 5. Iron-rich coatings on boulders (A) embedded in fluvial Iberian Permotriassic deposits (250 million of years), and (B) inside young Río Tinto terrace materials (1,000 years old). (C) SEM image showing microbial patches covering coated boulders and (D) EDAX microanalysis (spectrum 1 in (C) ) showing a carbon and iron enrichment as expected for microbial films associated to watery environments enriched in iron.

Figure 5. Iron-rich coatings on boulders (A) embedded in fluvial Iberian Permotriassic deposits (250 million of years), and (B) inside young Río Tinto terrace materials (1,000 years old). (C) SEM image showing microbial patches covering coated boulders and (D) EDAX microanalysis (spectrum 1 in (C) ) showing a carbon and iron enrichment as expected for microbial films associated to watery environments enriched in iron.

Therefore, the application of the preservation windows concept can be of great utility to define a specific explorative strategy based on the preservation potential of a given geological unit. The Río Tinto Mars analog can be claimed to illustrate shortly this assumption (Fernández-Remolar et al., 2008c). As mentioned above, whereas the Rio Tinto surface environment favors preservation of morphologies and organics in iron- and sulfate-rich materials, organics are only preserved in the reducing subsurface; however, preservation is reset to simple preservation of morphologies when all these materials are exposed to a 2-million-of-year diagenesis. Under these varying conditions, the detection of extinct life on Mars would require different methodologies going from optical to analytical instrumentation which application strongly drives the exploration strategy of a given area. Finally, the differential preservation observed in those surface and subsurface Río Tinto environments strongly support that automated drilling instrumentation will be essential to sample subsurface regions that can have recorded some traces of extinct life. Same underground areas protected against conditions that may have induce the destruction of biological information through oxidizing, acidic or any other extreme conditions.

5. Acknowledgements

We thank to Prof. Raymond Arvidson, Thomas Stein and Richard Schinteie for providing essential information and very illustrative images. We also appreciate to the suggestions kindly provided by Alix Davatzes and other refeer and editors who have highly improved the work. Special thanks to the USGS Astrogeological Program, NASA/JPL-Caltech, NASA/JPL/ASU and the THEMIS Public Data Releases which have provided the Mars surface images, as well as NASA/JPS/University of Arizona for the HiRiSE images. This paper was supported by the Project ESP2006-09487 funded by the Ministry of Science and Education of Spain.

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1P.O. Box 1132, Efrat 90435, Israel 2The Abdus Salam ICTP, Strada Costiera 11, 34014 Trieste, Italia and Instituto De Estudios Avanzados, Idea, Caracas 1015A, República Bolivariana de Venezuela

3Department of Plant and Environmental Sciences, The Institute of Life Sciences, and the Moshe Shilo Minerva Center for Marine Biogeochemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel

4Lisa Umr Cnrs 7583 Universités Paris 12 & Paris 7, 61 Avenue du General de Gaulle F 94010 Creteil Cedex, France

This volume describes and discusses the oldest, extinct microorganisms from the depth of Earth and possible microbes from the upper spheres above Earth. Even though spacecraft or space Lander vehicles have yet to detect life traces outside of Earth, it is well possible that a record of past life or even currently living forms will be found on the surface or in subsurface areas of some extraterrestrial places.

"Fossil" according the Encyclopedia Britannica means a remnant, impression, or trace of an animal or plant of a past geologic age that has been preserved in the Earth's crust. Fossils are thus mineralized or preserved remains or traces of various organisms, such as microorganisms, plants and animals. Paleontology investigates fossils across geological periods, their formation, and the evolutionary relationships between taxa. When searching for fossil remains, one finds remnants ranging from microscopic single cells (often in large masses forming structures such as stromatolites), plants including petrified wood, and animals: mammals, fish, snakes, turtles, birds, up to gigantic animals such as elephants, mammoths or dinosaurs. Fossils are found ubiquitously in land and marine environments. The common presence of fossilized sea creatures high up in the mountainsides was considered by some people as a proof of the Great Flood described in the Bible or similar stories that appear in folklore worldwide. Fossil fuel is also consists of a class of ancient biological material occurring within the crust of the earth. Usually fossils mainly consist of portions of organisms that were already partially mineralized during life, such as the bones and teeth of vertebrates. Such compounds can be used as biomarkers to specifically detect certain groups of organisms. The age of the rock strata which contain fossils can be determined by radiometric dating method. Some fossil specimens can be estimated to have originated billions of years ago.

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