Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..848w&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Mathematics
Logic
Scientific paper
Abstract Environmental conditions on early Mars, from a microbial point of view, were largely similar to those on the early Earth. The oldest, well-preserved rocks on the early Earth (~3.5 Ga) host a wide range of morphological and geochemical traces of life, including chemolithotrophic, heterotrophic and photosynthetic anaerobic microorganisms. These microorganisms evolved in a tectonically evolving geological context, including carbonate platform formation. This scenario did not exist on Mars. Moreover, Mars was outside the habitable zone and standing bodies of water were probably ice-covered. Evolutionary advancement of martian life (if it appeared) would have been curtailed very early and it is unlikely that photosynthesis could have evolved. It is therefore unlikely that martian life will leave visible traces that can be detected with in situ instrumentation (no biolaminites or stromatolites). Geochemical detection of organic components will be possible but it is unlikely that the results will be conclusive. The return of suitable rocks from Mars is advocated. Early life on Earth and Mars The oldest, well preserved rocks on Earth, including both sedimentary and volcanic lithologies, contain abundant morphological and geochemical traces of life [1]. Evidence of borings into basalt lavas [2] and microbial colonies within volcanic sediments [3,4] testify to microbial utilisation of chemolithotrophy. Microscopic tunnels, tens of microns in length, containing traces of biologically important elements, such as C and N, in the vitreous rinds of pillow lavas are identified in petrographic thin section (Fig. 1) [2]. Similar 5-10 μm-sized tunnels have been channelled into the surfaces of detrital volcanic grains [4]. They contain the remains of microbial polymeric substances (EPS) but can only be identified in petrographic thin section and using the high magnification of a scanning electron microscope (SEM). Furthermore, volcanic sediments deposited in water contain crytic but abundant evidence of past life [3] in the form of fossilised microbial colonies on the surfaces of detrital volcanic grains, in fine volcanic dust deposits, and in the pores of scoriaceous pumice, etc (Fig. 2). Again, these traces can be identified only through petrographic thin section and SEM study. The bulk organic carbon contents of these rocks is very low, ~0.01-0.05% and their C-isotope signature (~ - 25 ‰), although indicative of life, could also be produced through abiological processes [5]. Only the combination of multiple analytical techniques, of which high resolution microscopy is one of the most fundamental, permitted a biogenic origin to be attributed to these structures. Biolaminated sediments, including domal stromatolites, in Early Archaean terrains are the result of anaerobic photosynthetic activity [6-9]. Photosynthesis is a relatively evolved metabolism. Evidence of photosynthetic activity is preserved in the rhythmic laminations found in sediments deposited at the edges of shallow basins due to the growth of photosynthetic microbial mats on the sediment surfaces. These laminations, ranging from a few tens of microns to packets up to a couple of millimetres in thickness, are macroscopically and microscopically visible (Fig. 3). Given sufficient tectonic stability of the shallow water, carbonate platform environments in which they form, photosynthetic microorganisms on the early Earth formed domical stromatolites. In the case of biolaminated sediments, bulk organic carbon contents are again low (0.01 %) but the individual biolaminae have a higher carbon content (0.07%). Certain highly carbonaceous biomaminated cherts have carbon contents ranging up to 0.5% [10]. Photosynthetic organisms, however, are not only restricted to stable substrates and may also be planktonic, living free in the upper layers of water bodies. Evidence of planktonic microorganisms on the early Earth has been suggested by [10]. Whether floating in the ocean or forming mats in the littoral environment, photosynthesisers need unlimited access to sunlight. The very existence of these microorganisms at ~3.5 Ga demonstrates that, although the Earth was at the cold edge of the habitable zone, the oceans were not frozen. Ice would have limited the penetration of light to the microorganisms (depending on thickness). From the microbial perspective, the lack of a global ocean on early Mars is irrelevant [11,12], as is the probability that standing bodies of water would have been covered with ice [13]. There would have been sufficient amounts of liquid water to support chemolithotrophic and heterotrophic microbial life beneath the ice covered basins (N.B. Lake Vostok on Antarctica has liquid water beneath 3 km of ice that is also fed from hydrothermal sources. Genetic traces of these hydrothermal microorganisms are frozen into the base of the ice [14]). Hydrothermal activity associated with volcanism and impact events would provide sufficient heat for liquid water (and life) to be present. However, the presence of ice on the early water bodies would have hindered the evolutionary development of martian life. Since photosynthetic microorganisms require access to sunlight, and since the earliest photosynthesisers were probably substratebound, the presence of ice on the surfaces of water bodies and, especially at their edges in the littoral environment, would have inhibited the development of photosynthesis. Moreover, the early degradation of the environmental conditions on Mars (between 4.2- 3.5 Ga [12]) would have been a further detriment to the evolution of martian life. Conclusions It is most likely that life appeared on Mars independently of the Earth since the planet had the same ingredients and probably the same starting conditions as the Earth. However, the early degradation of its environmental conditions and the fact that the planet was outside the habitable zone with probable ice-covered water surfaces would have limited the development of photosynthetic microorganisms. On the basis of studies of the fossil traces of microorganisms in analogue early terrestrial rocks, it is clear that the in situ search for life on Mars will be even more difficult, since the traces left by eventual chemolithotrophic and heterotrophic martian life will be very subtle and below the visible resolution of in situ instrumentation. Moreover, geochemical identification may not be conclusive. Thus a series of Mars Sample Return mission are essential for detecting whether Mars had or still has life. [1] Westall, F. & Southam, G. 2006. AGU Geophys. Monogr., 164, 283-304.. [2] Furnes, H. et al., 2004. Science, 304, 578-581 [3] Westall, F. et al. 2006. Geol. Soc. Amer. Spec Pub., 405, 105-131. [4] Foucher et al. 2008. ESPC Abstract # 273. [5] van Zuilen, M., et al. (2002), Reassessing the evidence for the earliests traces of life, Nature, 418, 627-630 [6] Westall F, et al. (2006) Phil. Trans. Roy. Soc. Lond. Series B., 361, 1857-1875 [7] Allwood, A.C., et al.2005: Stromatolite reef from the Early Archaean era of Australia, Nature, 441, 714- 718. [8] Westall et al. 2007. Abstract-EANA, Türku, October, 2007. [9] Noffke, N., et al., 2006. Earth's earliest microbial mats in a silicilastic marine environment (2.9 Mozaan Group, South Africa), Geology, 31, 673-676 [10] Walsh, M.M., and D.R. Lowe (1999), in Geologic evolution of the Barberton greenstone belt, South Africa, eds. DR. Lowe, G.R. Byerly, Geol. Soc. Am Spec. Paper, 329, 115-132. [11] G. Southam, L. Rothschilde, F. Westall, 2007. in Geology and habitability of terrestrial planets, Space Science Reviews, 1-28. [12] Southam, G. and Westall, F., 2007. in T. Spohn, (Ed.) Treatise on Geophysics - Vol. 10 Planets and Moons, Elsevier, Amsterdam. 421-438. [13] Clifford, S. M., and Parker, T. J. Icarus, vol. 154: 40-79, 2001. [14] Bulat, S. et al., 2006. Geophys. Res. Abstracts, Vol. 8, 07109.
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