[PCR]). They also include more general approaches that could be useful in characterizing a broad range of life forms very different from those of Earth, but for which the interpretation could be ambiguous (e.g., time-of-flight mass spectroscopy). In this sense some of the papers could arguably have been included in the session on extinct life (e.g., see the Session 3 paper by Kirschvink on magnetic biomineralization), and indeed a corresponding paper on iron biomarkers is given in Session 4 (see the paper by Anbar). These papers have been split between the two different sessions, along with the papers on microbe-mineral interactions, to emphasize the utility of such techniques in the search for either extant or extinct life. The distinction between extant and extinct life detection becomes arbitrary for the most general techniques that rely on mineral biomarkers or other morphological characterizations.

Most of the participants who presented techniques suitable for assessing the degree of sterilization also emphasized their application to extraterrestrial life detection. In large measure, this reflected the emphasis of the workshop on the search for life elsewhere. To correct this imbalance the committee prepared Table 5.2 (see Chapter 5), which lists techniques discussed at the workshop and gives their sensitivity (typically, when applied in terrestrial laboratories rather than extraterrestrial environments), limitations, and some indication of development needed. Finally, the committee's narratives in this chapter and in Chapter 4 do not attempt a detailed discussion of each technique presented in the sessions. In particular, it was not the purview of the committee to rank in some way the applicability of the techniques. The workshop discussions and the session papers represent a snapshot of the kinds of techniques and their capabilities available at a time when interest in finding life beyond Earth has reached a new crescendo.

BIOMOLECULES AND MOLECULAR TECHNIQUES

New technologies originating in the biomedical community will be available for use in detecting extant life on other planets. Testing these methods on Earth, where many biological communities can be studied with sophisticated laboratory setups, is crucial. The ambiguities associated with metabolic tests for extant life, demonstrated in the prolonged (some would say continuing) controversy over the Viking lander results, demand a broader suite of sensitive detection approaches. The evolution of molecular techniques is toward much higher specificity, in the sense that one can look at all the major components of cellular structures. At issue, of course, is how to generalize from the characteristics of these structures to those that might differ from the terrestrial. Imaging techniques, although less reliant on specific biochemical assumptions, are more subject to misinterpretation.

To apply molecular techniques to the search for life elsewhere requires deciding how geocentric one must be. Must one look for a different set of biochemical molecules, perhaps based on compounds that are structured differently from those of terrestrial life? The Session 3 papers by Stahl; Meller and Branton; and Ruvkun, Finney, Gilbert, and Church present approaches that will work for life forms that use the basic biopolymers found in all terrestrial life—specifically, RNA and DNA. Stahl focuses on the use of molecular methods coupled with microscopic techniques, which provides a powerful combination of sensitivity and generality. The paper by Meller and Branton and that by Ruvkun, Finney, Gilbert, and Church offer two techniques capable of single-molecule detection that are miniaturizable for in situ deployment: nanopore technology and robotic PCR detection. An alternative imaging approach, described in the Session 3 paper by Jacobsen, allows one to image hydrated organic samples in an organism, thus potentially obtaining the structure of the ribosomes in the organism, and simultaneous structural and compositional data.

Highly sensitive techniques for measuring a variety of organic molecules, such as proteins, complex lipids, and carbohydrates, are active areas of investigation. Although there are simple monomeric organic molecules that occur both in terrestrial biology and in abiotic organic phases of meteorites, the macromolecular nature of biologically produced versus abiotically produced molecules should be diagnostic. Cotter describes (see Session 3) improvements in time-of-flight (TOF) mass spectrometry, for which significant technological advances have been made. TOF mass spectrometers can now be miniaturized to allow in situ analysis of biomolecules on an extra-terrestrial surface. The versatility and sensitivity of this technique are high. Protein sequencing has been demonstrated in terrestrial laboratories using this technique.



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