Detecting Extinct Life
Experience gained in the decades-long search for evidence of ever more ancient past life on Earth suggests three major stages in the detection of extinct life on other planets:
Identification of specific sites of likely fossil preservation associated with the past presence of liquid water;
Selection of fossiliferous rocks for study in those locations; and
Actual analyses of the rocks themselves for structural, molecular, or isotopic evidence of past life.
On other planets, the choice of sites is based on geological and compositional observations made from orbit, the rocks are selected using chemical measurements made by landers or rovers, and the analyses can be made either in situ or on samples returned to Earth.
Although it appears unlikely that evidence of fossil life can be obtained by orbital observations of an interesting site, the local environment may have preserved the ecological imprint of biological activity. Ecological signatures, possibly subtle and requiring broader spatial coverage for detection with rovers or networks of instruments, may be reflected among local rocks in chemical or mineralogical compositions or in gradients of these properties, as described in the paper by Fogel (see Session 3). On the other hand, depending on spatial resolution, future orbital observations ought to be able to detect mineral formations, analogous to carbonates on Earth, that are either directly or indirectly the result of biological activity. Research on the nature and identification of such signatures is essential. Definitive evidence of fossil life, however, will require more direct and detailed observations of appropriately selected rocks.
Rock selection serves two purposes:
To demonstrate that the sample formed in a sedimentary environment, and
To show that the sample contains organic matter.
Although the absence of organic matter in a sedimentary rock would not preclude a biological imprint, a rock containing organic matter would offer more lines of evidence to follow in establishing the presence of past life. The detection of organic matter can be accomplished by many techniques differing in sensitivity, molecular
structural information, requirements for sample preparation, and degree of reliability in distinguishing biogenic materials from those produced by nonbiological processes.
METHODS FOR DETECTING FOSSILS
Evidence of extinct life can be sought in detail at various levels ranging from macroscopic stromatolitic structures to microfossils to the intramolecular distribution of carbon isotopes in organic compounds. Depending on their depositional environment, fossilization mechanism, and diagenetic history, both macro- and microscale biogenic structures, including biofilms, can be preserved with varying amounts of their original organic contents. In all cases, especially in the absence of organic matter, the field context of the sample site and the texture and fabric of the structures are critical in determining biogenicity. Since the latter also undergo degradation over time, laboratory and field studies of both fossilization and subsequent diagenesis processes are needed to determine the time scales over which biogenic signatures are lost or preserved under various environmental conditions (see the paper by Cady in Session 4).
The paper by D. McKay, also in Session 4, describes a variety of electron beam techniques. Scanning and transmission electron microscopy combined with optical microscopy provide powerful tools for characterization of putative fossil structures, often in three dimensions, with respect to their location within the mineral matrix, morphology, texture, and size. The energy-dispersive x-ray analyzer (EDXA) and electron energy-loss spectrometer (EELS) attachments and electron microprobes (EMs) provide essential chemical and mineralogical measurements that may support a biogenic origin. Similarly, ion-beam techniques such as time-of-flight–secondary ion mass spectrometry (TOF-SIMS) afford high-spatial-resolution imaging of organic matter and even stable isotopic measurements of specific structures or locations within structures.
No one of these techniques, however, can unambiguously address the question of biogenicity. In light of the continuing controversy surrounding martian meteorite ALH84001, it remains unclear whether the use of an array of these methods can provide unequivocal proof of biological structures (see the paper by Kirschvink in Session 3). The absence of organic remains within the structures makes the problem even more difficult. Highly relevant in this context are studies aimed at determining what biological morphologies, fabrics, and features cannot be produced by inorganic processes.
MOLECULAR AND ISOTOPIC METHODS FOR DETECTING EXTINCT LIFE
Developments in the chemistry of natural products and organic geochemistry over the past decades have yielded molecular structural, isotopic, and stereochemical attributes that are common features in compounds of biological origin. These properties have been instrumental in establishing the antiquity of life on Earth and in tracking the early evolution of biological innovations in the geological record. The use of these traits for discerning extraterrestrial life hinges on how common they are to all biochemistries, earthly and alien. Studies of the organic chemistry of meteorites and laboratory simulations of planetary chemistry provide criteria for characterizing products of nonbiological processes. In the latter case, the value of abiogenic criteria depends on their absence in all biochemistries. Application of both sets of criteria to the organic matter in extraterrestrial samples holds the promise of distinguishing biological from nonbiological materials, as Becker's paper argues (see Session 4).
A lesson of evolution on Earth is that a small number of universal enzymatic processes govern biosynthesis at the cellular level in all life. To fulfill requirements for structure and function, these processes impose distinctive patterns of restricted variation in molecular structure on the building blocks of membranes, proteins, and nucleic acids, the major components of living systems. Moldowan describes these patterns in his paper in Session 4. For membrane lipids, enzymatic pathways preferentially synthesize a small number of specific carbon chain isomers over a broad distribution of chain lengths. Repeating isopentenyl or acetyl subunits are a structural motif, chirality occurs at quaternary (or tertiary-substituted) carbon sites, and—where branched isomers occur—the branching exhibits positional preference. In peptides, only 20 amino acids among the multitude of possible structures are commonly employed and because of their biosynthetic pathway, all are levorotatory α-amino acids with an α-hydrogen. Enantiopurity, characteristic of virtually all quaternary carbon centers in biology, represents a
stereochemical feature that provides compelling evidence of life (see the Session 4 paper by Bada). In the nucleic acids, only four bases are commonly used, and—with two sugars—sugar-phosphate backbones form linear polyionic species. Signatures expected to be common to all biochemistries, even very “primitive” ones, are the pattern of restricted stereochemistry and structural variation and the limited number of compound classes employed. What will not necessarily be common are the molecular identities of the various building blocks and polymers. In his paper in Session 4, Anbar argues that isotopic ratios of multiple elements beyond carbon, such as iron, could provide supporting and well-preserved indicators of past biological activities.
In contrast to biosynthetic pathways, abiotic syntheses, as manifest in the organic chemistry of meteorites, yield distinctly different arrays of molecular structure across many compound classes. Within each class—for example, amino acids—the abundances of isomers decrease smoothly as carbon number increases, with branched isomers often exceeding linear structures. At low carbon numbers, complete molecular diversity prevails and all possible structural isomers occur. Similar results are produced in simulations of prebiotic planetary chemistry. Small enantiomeric excesses occur among meteoritic amino acids, however, which suggests that chirality must be used cautiously to distinguish biogenic compounds from those of nonbiological origin. How widespread enantiomeric asymmetry may be among other classes of meteoritic compounds remains to be determined (see the Session 4 paper by Chang).
Once deposited in a sedimentary environment, biopolymers and biomembranes undergo degradation relatively rapidly over geologic time scales. On Earth, the molecular biomarkers with greatest longevity are hydrocarbons derived from decomposition and rearrangement of biolipids, some even with retention of chiral quaternary centers. The dominant component of ancient sediments, however, is the insoluble macromolecular “ kerogen.” Martian analogues, if they exist, are likely to persist in environments protected from atmospheric oxidants. Asteroidal debris has been delivered to the surface of Mars (and other solar system bodies) over time, and some of that meteoritic organic matter may also have survived. Given a cooler and less tectonically active surface such as that of Mars, the persistence of other classes of compounds including amino acids is possible, regardless of their origins.
The determination of molecular structures in a mixture of compounds that is required to assess biogenicity can be carried out by existing methods such as combined gas chromatography-mass spectrometry (GC-MS) or capillary electrophoresis alone or in combination with MS. The use of chiral chromatographic substrates adds a stereo-chemical dimension to these analyses. The sample preparation required may range from simple sublimation to more complex combinations of solvent extractions, filtrations, and derivatizations. Although these analyses pose no unusual challenges for samples returned to Earth, carrying them out remotely on Mars requires development of new robust miniaturized instruments with low power requirements. These capabilities appear feasible with emerging technologies, and Bada describes one such approach in his paper.
PROMISING LITHOLOGICAL ENVIRONMENTS FOR THE SEARCH FOR EXTINCT LIFE
As elements in the development of a search strategy for signs of extinct life on Mars and beyond, many of the life detection approaches presented in this chapter were guided by studies of the earliest life signs (e.g., fossils) on Earth. In the early terrestrial record, this would have been microbial life—the most robust form of living organisms even on present-day Earth. However, as we have learned from our own fossil record, identifying extinct organisms and their biosignatures requires careful mapping to identify the appropriate environments and a suite of measurements capable of distinguishing evidence for biological processes on several levels (e.g., morphological, isotopic, and metabolic).
The search for signs of ancient life on Earth indicates that the most suitable locations for the accumulation and preservation of biosignatures are deposits associated with ancient or modern ground or surface water environments or sedimentary rocks usually characterized as fine grained with aqueously derived mineral assemblages. The most promising sedimentary lithologies for preserving signs of extinct life are cherts, carbonates, phosphates, and clays that are usually laid down in aqueous environments (e.g., oceans, lakes, thermal springs, and evaporite deposits). Such biosignatures include microfossils, macroscopic textures (e.g., stromatolites), chemical fossils, and isotopic fractionation patterns. All of these types of evidence will be needed to determine definitively whether life ever
evolved beyond our own planet. Careful consideration of the type of lander, the suite of instruments needed for sample selection, and the appropriate size sample must self-evidently be part of a life detection strategy. In his Session 4 paper, Huntress describes a focused program at NASA's Jet Propulsion Laboratory that seeks to identify the most promising approaches to detecting extinct (or extant) life through in situ measurements.
Although sedimentary rocks are arguably the most promising locations for preserving biosignatures, the fact that some terrestrial life forms exist on or in igneous rocks must also be recognized. Thus, hydrothermal systems associated with volcanic regions on Mars may be or may have been sites for life and, as noted above, must be on the list of potentially interesting sites for sampling.