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J. Seckbach andM. Walsh (eds.), From Fossils to Astrobiology, 249-273. © Springer Science + Business Media B. V. 2009

development of techniques for measuring light elements and applications of the techniques in Geoscience.

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EVIDENCE OF ANCIENT MICROBIAL LIFE IN AN IMPACT STRUCTURE AND ITS IMPLICATIONS FOR ASTROBIOLOGY:

A Case Study

TOMAS HODE1, SHERRY L. CADY1, ILKA VON DALWIGK2 AND PER KRISTIANSSON3

'Department of Geology, Portland State University, Portland, OR 97201, USA

2Department of Geology and Geochemistry, Stockholm University, SE-106 91, Stockholm Sweden

3Department of Nuclear Physics, Lund Institute of Technology, Lund University, P.O. Box 118, S-221 00 Lund, Sweden

1. Introduction

The search for present and past life on Mars has drawn major attention from the scientific community, as well as from national and international space agencies. A major reason for focusing the search for life on Mars is that, apart from being the closest planetary body of major astrobiological interest, Mars may have shared a number of environmental features with Earth during the early phases of planetary history. The atmosphere was denser and probably similar to the Earth's atmosphere in composition, and liquid water was present on the surface (Squyres et al., 2004), either as a stable water body or as frequent flooding events of short duration (Segura et al., 2002). A number of geological processes occurred on the hydrologically active surfaces of both planets early in their history, including intense volcanism and frequent meteorite bombardment, which would have led to the widespread distribution of hydrothermal systems on early Earth and Mars (e.g., Farmer, 1996). Hydrothermal deposits are of particular interest as targets in the search for fossil (and extant) life on the red planet (e.g., Farmer, 1998; Nealson, 1999; Newsom et al., 2001; and references therein), since life on Earth either originated or adapted relatively early to thermal environments (Stetter, 1996). In addition, hydrothermal systems are capable of preserving biosignatures indicative of microbial life (chemofossils, organosedimentary structures, silicified microfossils, biomarkers, etc.) (e.g., Walter and Des Marais, 1993; Farmer and Des Marais, 1993; Simoneit et al., 1998; Renaut and Jones, 2000; Konhauser et al., 2003). Collectively, these findings suggest that a conservative search strategy for evidence of life on Mars could be carried out with the use of the same principles that have been applied to Precambrian paleontology on Earth (Schopf and Klein, 1992; Walter, 1999).

Because early Mars and early Earth in many ways resembled each other, it is reasonable to argue that ancient-Earth studies will aid in the search for life on Mars even though the nature of the problems encountered may differ somewhat. The main problem with early-Earth studies is the dearth of ancient sedimentary rocks and the extensive amount of alteration and metamorphosis of the rocks that did survive on the surface of our tectonically active planet. The surface of Mars, on the other hand, is most likely well preserved because of the lack of plate tectonics, and in some cases, it probably represents some of the most ancient contiguous strata in the solar system. Nevertheless, when an ancient and well-preserved rock from the surface of the Earth is identified, and the search for traces of life is initiated, the problems encountered are similar to those that will present themselves in the search for traces of biologic activity in martian rocks.

Assuming that life did emerge on Mars, the question is where to look for it. Although it may be relatively easy to locate regions that show evidence for a hydro-logical past, such as sedimentary rocks (clastics, calcareous sediments, evaporitic rocks, BIFs, and phosphorites) and hydrothermal deposits, a key factor will be the ability to identify deposits that accumulated in environments that could have supported life and preserved microbial biosignatures (e.g., Cady et al., 2003).

Possible sedimentary traps and paleoenvironments on Mars have been discussed, for example, by Komatsu and Ori (2000) and Farmer and Des Marais (1993). On Earth, paleobiological repositories include carbonaceous cherts, carbonates, and phosphorites. Such rocks either have not been identified or they are not likely to exist in reasonable quantities at the surface of Mars today. On the other hand, possible clastic and evaporitic sedimentary deposits that would have formed in fluvial and lacustrine environments have been observed at several locales on Mars (e.g., Cabrol and Grin, 1999, and references therein; Squyres et al., 2004).

Hydrothermal systems, which are often active mineralizing environments with high preservation potential, have been suggested as promising targets in the search for life on Mars. Microbial preservation has been extensively studied in modern hydrothermal ecosystems (e.g., Cady and Farmer, 1996; Renaut et al., 1998; Konhauser et al., 2001), and ancient hydrothermal deposits have been shown to contain fossilized microorganisms (e.g., Walter, 1996; Rasmussen, 2000; Westall et al., 2001; Logan et al., 2001).

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