limits on both biological and abiological pattern generation is incomplete, making it difficult to understand the dimensions of the “gray zone” of overlap. Recognition of biological pattern in extraterrestrial samples will require the identification of structures or molecules that reside in the biological field, but not in the zone of overlap. On the other hand, there is no assurance that terrestrial life exhausts the possibilities of biological pattern generation; therefore, knowing the limits of pattern formation by physical processes may provide the best yardstick for evaluating martian or other extraterrestrial samples. Dr. Benner suggested an alternative depiction of the Venn diagram in which the biological field is completely encompassed within the physical field—the point being that biological processes are a subset of a larger and more inclusive set of physical processes. As a depiction of process, this view is unimpeachable; nevertheless, the patterns generated by biological processes include structures and molecules not known to form under strictly physical conditions. Bones, radiolaria, and red algal thalli are examples of biologically diagnostic morphologies; cholesterol is a biologically diagnostic molecule.

Lessons from Earth

Panel 3 members agreed that our collective experience with Earth's geological record provides an important guide to fossil recognition and interpretation on Mars. Dr. Farmer demonstrated that processes of mineral precipitation can preserve biologically interpretable microfossils and sedimentary fabrics. This increasing knowledge of fossilization processes not only sheds light on the postmortem information loss that attends fossilization, but also focuses attention on martian environments most likely to preserve a biological record. Preservation of terrestrial remains is selective, with some organisms—and some parts of organisms—more likely to escape decay than others. During fossilization, cells can also shrivel or collapse, resulting in fossils that are much smaller than the organisms from which they are derived.

Dr. Schopf summarized experience in interpreting Earth's early fossil record, stressing the early phase of discovery, when reports of objects that proved to be abiological outnumbered those of genuine fossils. Not everything that is small and round is biological, and the rigorous criteria for biogenicity developed over the past three decades by paleontologists can be useful in the evaluation of extraterrestrial microstructures. Dr. Schopf emphasized the need to conduct interdisciplinary studies of petrology, micropaleontology, isotopic geochemistry, and molecular organic geochemistry, developing multiple lines of evidence for interpreting potentially biological patterns.

Although most of Panel 3's discussion focused on micron-scale structures, the distinctive macroscopic structures known as stromatolites were also considered. Stromatolites are laminated structures found in chemical sedimentary rocks, especially but not exclusively carbonate rocks. These structures, which can be flat-laminated, domal, columnar, or conical, are commonly interpreted as the products of sediment trapping, binding, and/or precipitation by microbial communities; however, it is apparent that comparable structures can be generated without the need for microbial templates. Indeed, such structures actually occur in the early geological record. This being the case, images of laminated precipitates that may be transmitted by a Mars rover cannot be construed as unambiguous evidence for biological activity (Farmer, Knoll). Micro- and mesoscale fabric studies on returned samples will be necessary to confirm or reject hypotheses of biological origin. Dr. Farmer suggested that specific microscopic textures may provide the biological fingerprint needed to be confident of stromatolite biogenicity in the terrestrial or martian rock record.

Dr. Bradley focused attention on abiologial pattern formation, suggesting that non-biological processes may be sufficient to explain a number of micro- and nanoscale features sometimes interpreted as biological, including those reported from Mars meteorite ALH84001. Dr. McKay vigorously disputed



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