The search for life on Mars using macroscopically visible microbial mats (stromatolites) in 3.5-3.3 Ga cherts from the Pilbara in Australia and Barberton in South Africa as analogues Frances Westall 1 , Beda Hofmann 2 , André Brack 1 1 Centre de Biophysique Moléculaire, CNRS, Rue Charles Sadron, 45071 Orléans cedex 2, France (Tel: +33-2-38-25-79-12; Fax: +33-2-38-63-15-17; westall@cnrs-orleans.fr ); 2 Naturhistorisches Museum Bern, Switzerland. Abstract Microbial mats from early terrestrial environments, formed in an epoch when life could still have existed on Mars (if it appeared), can be macroscopically visible and represent excellent analogues in the search for life on Mars. Tests using the Beagle 2 camera show that they can be observed by in situ instrumentation. Early Archaean sedimentary analogues There are a number of lander missions to Mars at the moment and more are planned in the next 5 years. The landing sites are chosen in areas where there is evidence for liquid water in the past and where sedimentary rocks could have formed. Based on the study of analogue rocks from the 3.5-3.3 Ga Early Archaean terrains of Barberton (South Africa) and the Pilbara (Australia), the sediments would be predominantly of volcanic composition. Precipitates of various origins would also be present, such as hydrothermal deposits (e.g. the hematite site, Christensen et al. [1] but most commonly silica), the products of hydrothermal/diagenetic alteration of volcanic rocks, and evaporite deposits. The Early Archaean sediments were deposited in basinal settings whose water depths ranged from below wave base to the beach environment (littoral). They form thin, bedded horizons sandwiched between generally much thicker sequences of effusive volcanic rocks. Similar deposits on Mars would occur in impact and volcanic crater lake environments. Most of the Early Archaean lithologies were affected by contemporaneous and/or post-depositional, hydrothermal fluids. Such situations would also have been common on early Mars where hydrothermal activity would have been associated with volcanism and impact events. The rocks of the Early Archaean terrains therefore represent ideal analogues for studying strategies in the search for life on Mars. Early Archaean microbial mats The Early Archaean environment offered a variety of habitats for life. The surfaces of sediments deposited in shallow water to littoral environments appear to have been a particularly favourable habitat [2-8]. Studies of microbial mats in silicified sediments from the Early Archaean terrains (3.3-3.5 Ga) of Barberton in South Africa and the Pilbara in Australia document microbial mat formation at the surfaces of the volcaniclastic sediments. These surfaces represent periods of cessation of sediment deposition that allowed the development of microbial biofilms. Such could microbial biofilms/mats could have potentially formed on Mars. Microbial mats can form very rapidly, within days, and thus suitable surfaces become rapidly colonised [9]. Mat structure and robustness depends on the environmental conditions, as well as the age of the mat. Delicate biofilms are formed in quiet water conditions whereas thick, robust biofilms form in stressful environments, such as exposed beaches or where thete is strong current flow. The delicate biofilms are cryptic and not directly visible at the macroscopic level. Biofilms that form thick mat layers are, on the other hand, macroscopically visible. Examples of macroscopically visible microbial mats in Early Archaean formations include: (1) Low, domal stromatolites occurring in the Barberton greenstone belt and described by Byerly et al. [2]. They have wavelengths ranging from 2-5 cm and low amplitudes of the order of 1 to 2 cm; (2) 5-10 cm high conical stromatolites described by Hofmann et al. [4] from the Pilbara ; (3) Small, hummocky, domal stromatolites in the Pilbara described by Westall et al. [6]. Figure 1 shows one of these stromatolites in a 3.47 Ga formation. This example is characterised by contorted layering (arrow). It is sandwiched between layers of precipitated silica of hydrothermal origin (the domal stromatolite formed at a distance of 50 cm from the orifice of an hydrothermal vent)[6]. The contorted fabric indicates plastic deformation of the layers in the stromatolite. This demonstrates that the layers originally consisted of soft, pliable material, such as a polymer-rich microbial mat, before mineralisation. The deformation was probably due to the weight of the precipitated hydrothermal silica, which literally squashed the soft, plastic microbial mat layers. Figure 1. Domal stromatolite (S) with originally soft laminae distorted (arrow) by the weight of the precipitated hydrothermal chert (C). Lunar and Planetary Science XXXV (2004) 1077.pdf