Introduction to the special paper collection: methodologies and techniques for detecting extraterrestrial (microbial) life.

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487 THE DETECTION OF EXTRATERRESTRIAL LIFE is one of our greatest challenges in science. It seeks to answer one of the most fundamental questions of humankind: Are we alone? In this paper collection we attempt to take a step forward in this task by exploring the question of how we would detect the most basic form of life, microbial life, elsewhere. The detection of life obviously requires an operational definition appropriate for the task. While a generally accepted comprehensive definition of life is problematic (Schrodinger, 1944; Lwoff, 1962; Koshland, 2002), certain characteristics are generally agreed to be associated with life that mark its presence, wherever it occurs (Schulze-Makuch et al., 2002a). These include: (1) the sequestration within bounded microenvironments of complex, polymeric chemistry, (2) transformation of energy to maintain a complex state in thermodynamic disequilibrium with its surroundings, and (3) the capacity to replicate nearidentical multiples of itself. A consequence of these properties collectively is the profusion of life in favorable environments, to a point where the chemistry and the collective organisms themselves, or at least their impact on the environment, can be detected. The search for extraterrestrial life everywhere but on our closest planetary neighbors (Mars and Venus) is limited for the foreseeable future by our inability to obtain physical samples. Therefore, information that can only be obtained by remote sensing, and robotic probes will for now provide the only clues concerning the existence of life elsewhere. While it may be impossible to detect a single microbe on a planetary body, even if we would be able to land there, we know from our experience on Earth that a single organism would not exist by itself, but would form a community that would impact its environment (SchulzeMakuch and Irwin, 2004). A typical example of such a community effect of life on Earth is the presence of both molecular oxygen and methane in disequilibrium in the atmosphere, which can only be explained by the constant biogenic production of both gases. This is an example of a biosignature of life. Other signatures of life can be geological like the rocks and sediments produced by biogenic processes such as the banded-iron formation and stromatolite deposits of early Earth, and a rate and type of erosion consistent with biological processes. Signatures of life are direct consequences of the activities of living processes. Some biosignatures are microscopic in scale and therefore difficult to detect remotely, while others, such as atmospheric compositions, or the collective accumulation of biogenically generated macromolecules, are within the exploratory capabilities of current robotic technology. In addition to biosignatures, we have