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EXOBIOLOGY 8 2 Exobiology WHAT IS EXOBIOLOGY? Throughout history, humanity's creation myths appear to reflect each culture's perception of the dimensions of its universe and its place within it. Today, the scope of those perceptions has expanded beyond the reaches of the solar system to the stars and the interstellar clouds that populate the seemingly limitless expanse of space. We see life as the product of countless changes in the form of primordial stellar matter wrought by the processes of astrophysical, planetary, and biological evolution. The science of exobiology attempts to reconstruct the natural history of processes and events involved in the transformations of the biogenic elements from their origins in nucleosyntheses to their participation in Darwinian evolution in the solar system on planet Earth. From this reconstruction will emerge a general theory for the evolution of living systems from inanimate matter. The goal of exobiology is to increase knowledge of the origin, evolution, and distribution of life in the universe. This is a multidisciplinary science, and the conceptual and experimental tools of virtually all scientific disciplines and branches of learning are relevant. In seeking answers to such questions as how the development of the solar system and its planets led to the origin of
EXOBIOLOGY 9 life on Earth, how planetary evolution subsequently influenced the course of biological evolution, and where else life may be found in the solar system and beyond, exobiology brings together life scientists and physical scientists in a common interest. Exobiology is concerned with four evolutionary epochs: (1) cosmic evolution of biogenic elements and compounds; (2) prebiotic evolution; (3) early evolution of life; and (4) evolution of advanced life. Each of these epochs, briefly described below, represents a major arena of research. Between now and the mid-1990s, the task group expects this conceptual framework to become widely acknowledged and to act as a stimulus for interdisciplinary attacks on exobiological research problems. 1. Cosmic Evolution of Biogenic Elements and Compounds. The first epoch encompasses galactic time and distance scales and involves the death and birth of stars. It begins with the synthesis in stars of the biogenic elementsâthe elements that make up all lifeâand their ejection into the interstellar medium; it ends with the distribution of these elements and their compounds throughout our solar system within the planetoids, which became building blocks of planets. Discoveries in carbonaceous meteorites strengthen this perspective, as organic and mineral matter made up of carbon, nitrogen, hydrogen, and oxygen has been found that retains properties traceable to its origins in interstellar clouds and stars. How commonly the aggregation of interstellar dust and gas into small primitive bodies occurs during star formation is not known. What transformations were undergone by the biogenic elements and their compounds during this process remain poorly understood, as are the ways in which the physical and chemical properties of these elements and their compounds may have influenced the course of events during the formation of the solar system. Answers to these questions will develop, however, as astrophysicists and astrochemists take advantage of the capabilities for large-scale modeling, and make use of sensitive space-borne astronomical telescopes with high spatial and spectral resolution to make observations of condensed matter in protostellar regions. In ground-based laboratories and by remote spacecraft, studies of interstellar dust and samples of other relict materialâmeteorites, comets, asteroids, and interplanetary dustâwill continue to help
EXOBIOLOGY 10 reconstruct the nature and chronology of the processes that took place at the time of the solar system's formation. 2. Prebiotic Evolution. This epoch begins with the accretion of planets and ends, in the case of Earth, with the emergence of living organisms after nearly 1 billion years. This is the epoch for which there has been found no geological record on Earth and, therefore, no direct basis on which to reconstruct the conditions of the environment. Yet it is the period in which life emerged from inanimate matter. For any planet this is the period of most rapid environmental change as the energy of accretion and radionuclide decay dissipates, and the planet undergoes a transition to either geological inactivity or to a steady state. Prebiotic chemical evolution is inseparable from planetary evolution, and the path that leads to the origin of life may be terminated if the development of a planet takes the wrong turn. In what ways and how fast were conditions changing on Earth in its first billion years when life originated? Were conditions initially similar on Mars or Venus? What types of geological settings were maintained far from equilibrium and conducive to the origin of life? What processes and reactions involving the biogenic elements were important for the origin of life? Answers to these questions can come from several sources: the application of planetary geophysics and geochemistry to the development of models for Earth's earliest history, the deciphering of the existing geological record among the terrestrial planets, and laboratory simulations of prebiotic reactions. 3. Early Evolution of Life. The third epoch begins with the emergence of living systems prior to 3.5 billion years ago, and carries through to the evolution of multicellular organisms, which appear in the fossil record about 1 billion years ago. The history of life in this period is largely the history of microscopic unicellular organisms. This period is also a time of continued change in environmental conditions, although less intensive or rapid than in the previous epoch, as the physical evolution of Earth moved into a more moderate stage. The Sun gradually became more luminous, the frequency of large-scale accretionary impacts declined, the Moon's separation from Earth increased, volcanism declined, the continents grew in volume and wandered across the face of the planet, oxygen began to appear in the atmosphere, and the geomagnetic poles reversed.
EXOBIOLOGY 11 In this epoch the major questions concern the relationship between the evolution of unicellular life and the climate and changing geology of the Earth. What was the path by which the ancestors of modern microbes evolved from the first living organisms? In what ways did physical and chemical changes in the environment influence the rate and direction of microbial evolution? Over what geological time scales did major events in microbial evolution occur? How did the evolving biota modify and modulate their environment over time? What was the nature of the earliest forms of life, and in what sequence were new attributes acquired? What are the simplest biochemical mechanisms and biophysical structures that can fulfill the functions of living systems, and what irreducible combinations of these constitute a living entity? Did life arise on Mars or on other planets during this time, and if so what changes in those extraterrestrial environments would have led to its extinction? The answers to these questions hinge on our ability to identify and obtain inorganic and organic fossils, and to decipher the record of geological and biological evolution through the layers of alteration and mutation accumulated over several billion years. This means that the right rocks and organisms must be studied; the phylogeny of microbial life must be tied to the chronology of geological change; and the time resolution with which we can discern changes in these two records must become more finely tuned. 4. Evolution of Advanced Life. The fourth epoch deals with the most recent billion years of the history of life, in which multicellular plants and animals and intelligent species evolved. Exobiological interest in this epoch grew out of the realization that collisions of asteroidal-sized objects with Earth produce global changes in its surface environment. That these changes would perturb the biosphere cannot be denied; whether they can account for major extinctions in the course of biological evolution, particularly events that may have placed advanced life on its evolutionary track to intelligence, remains to be determined. What is clear, however, is the importance of gaining more knowledge about the relationship between biological evolution and changes in the environment.
EXOBIOLOGY 12 PLANETARY EXPLORATION AND THE NEED FOR SPACE DATA In this search for knowledge about the origin of life as a natural process, NASA must explore as many extraterrestrial bodies as possible for the relevant information they can supply. The search should include bodies totally devoid of organic chemicals, those conceivably undergoing (or having underdone) organic chemical evolution, and those possibly harboring life. The study of lifeless planets provides examples of environments where chemical or biological evolution ended. On them it may be possible to find the remnants of chemical evolution or of past life and to learn how planetary changes may have broken the thread of chemical or biological evolution. The discovery that there is no life, extant or extinct, or no organic matter on a planet is of high interest because the conditions on the planet and what we can learn of its past history constitute basic data pertinent to a general theory of the origin of life. Mars continues to be an extremely interesting site for exploration and study for exobiology. Although the Viking mission found no evidence of extant life, the search was limited and not directed at optimum sites. In recent years, discoveries about the earliest history of life on Earth have reemphasized the need to examine the geological record of Mars. Compelling evidence for flourishing communities of microorganisms has been found in 3.5 billion-year- old sedimentary rocks of Australia and South Africa. Even if no martian life exists, the evidence of liquid water and a more clement epoch in the first billion years of martian history, at the time when Earth already had a thriving microbial biosphere, has important implications. The possibility that life arose on Mars early on and subsequently became extinct must be kept open and investigated whenever the opportunity to send missions to Mars arises. In addition, Mars exploration may provide a geological record of the first billion years of planetary evolution, for which little trace has yet been found on Earth. The Mars Orbiter Mission (MOM) will be invaluable for providing data that will aid in identifying sedimentary and other types of environments that have resulted from the interaction of liquid water with the planetary surface. Further characterization of such sites should be carried out with landers or penetrators capable of making in situ measurements both at the surface and
EXOBIOLOGY 13 at depth. These measurements should provide further analysis of the chemistry and mineralogy of the surface and subsurface rocks. They should determine the nature and abundances of the forms in which the biogenic elements occur. These explorations should provide a sound basis for site selection for future sample return missions. The unusual chemistry of the martian surface soils, manifested in the biomimetic responses elicited by the Viking biology experiments, continues to be intriguing and inadequately understood; it may contain important clues to the role of minerals and inorganic chemistry in prebiotic evolution. Elucidation of this chemistry may be attained in part by conducting experiments with landers and penetrators, but a full understanding will probably require detailed chemical, mineralogical, and geochronological study in earth laboratories of a sample returned from Mars. It is conceivable that a sample return mission could be mounted early in the first decade of the twenty-first century, in which case it should be given highest mission priority for exobiology. By the mid-1990s, planning for integrated exobiological and geological field investigations should get under way in the event that sample return missions will involve human rather than robotic activity on the martian surface. Microscale methods of analysis for characterizing and determining the origin of chemical and mineral phases composed of the biogenic elements should be developed for use in both remote and terrestrial laboratories. Comet sample return is high on the mission priority list. Earth-based astronomical observations of comets, together with the Halley encounters, have provided a wealth of data on the composition of volatile gases in the coma and the properties of the dust. From these observations conclusions may be drawn about what comets are composed of and how they behave while under the influence of the Sun's radiation, gravitation, and magnetic fields. But major questions about comets remain unansweredâtheir origin, the identity of higher molecular weight organic compounds, the nature of material contributed by interstellar and solar nebular sources, the ages of their components, and their histories of accretion and thermal and dynamical evolution. All of these questions are pertinent to exobiology, insofar as comets are composed largely of water, organic matter, and other materials containing the biogenic elements. Comets provide a "fossil" record of the materials and processes involved in the
EXOBIOLOGY 14 transition from the diffuse realm of gas and dust represented by interstellar clouds to the highly condensed realm of planetary objects. As with carbonaceous meteorites, which are thought to have been derived from either primitive asteroids or "burnt out" comets (or both), this record can best be studied with samples. The maximum scientific return will be obtained from samples that have been collected and preserved in a state as close as possible to that of their original storage in a comet. Comets and asteroids are the remnants of star formation in a system in which planets formed and life arose. Detection of planets orbiting other stars would represent a major milestone in establishing the generality of planetary solar systems and increasing the probability that life could have arisen elsewhere in the galaxy. Searches for cometary and asteroidal matter associated with other stars or with protostellar objects will provide a basis for determining the frequency of occurrence of preplanetary matter containing the biogenic elements. By the turn of the century, capable astrometric and infrared telescopes should be available, and searches for planetary and preplanetary objects should have broadened. In the outer solar system, Titan will continue to be a target of interest for exobiology; it represents a natural laboratory for the study of planetary organic chemistry. Titan provides a Urey-Miller (in contrast to a Rubey) type model for Earth's prebiotic atmosphere, one which contains nitrogen and hydrogen and in which methane is the predominant carbon source. The variety of minor atmospheric constituents identified by the Voyager missions and earth-based astronomical observations already testifies to the organic chemistry taking place in Titan's atmosphere. However, the origin of the major gases in the atmosphere, their relationship to the accretionary and outgassing history of the satellite, the full range of complexity in the organic chemistry occurring in the atmosphere, and what process is responsible for which observed minor compounds will remain poorly understood. A Titan atmospheric probe and a lander, capable of both characterizing the molecular and isotopic compositions of materials composed of the biogenic elements as well as measuring the abundances and isotopic compositions of the noble gases, would address many of these questions. Venus, with its atmospheric water, and the Galilean satellites Callisto, Ganymede, and Europa, with their water ice, are
EXOBIOLOGY 15 objects that contain clues to the history of water in planetary environments. Whether Venus has lost most of its initial aqueous endowment, which may have been comparable in size to that of Earth, or whether it was always strongly depleted in water is an issue pertinent to understanding the mechanisms that distributed and preserved the biogenic elements among the terrestrial planets and perhaps made Earth unique among them. The Galilean satellites, if built up from planetesimals resembling carbonaceous chondrites and comets, may also contain a frozen record of the organic matter that survived planetary accretion. Venus and the Galilean satellites will be pertinent targets of exobiological interest well into the next century. RESEARCH TOPICS Formation and Evolution of Biogenic Elements and Compounds The biogenic elements are those that compose the bulk of life and are generally thought to be essential for all living systems. Primary emphasis is placed on the elements hydrogen, carbon, nitrogen, oxygen, sulfur, and phosphorus. The compounds of major interest include water and those normally associated with organic chemistry. The essential elements usually associated with inorganic rather than organic chemistry are also includedâe.g., iron, magnesium, calcium, sodium, potassium, and chlorineâbut they are given secondary emphasis. Water plays a central role in the development of life as we know it, and therefore in the environments in which chemical evolution could have occurred. Thus, special importance is attributed to the cosmic history of water and its interaction with other substances of either organic or inorganic nature. It is useful to identify six stages in the cosmic history of the biogenic elements and compounds: (1) nucleosynthesis and ejection to the interstellar medium, (2) chemical evolution in the interstellar medium, (3) protostellar collapse, (4) chemical evolution in the solar nebula, (5) growth of planetesimals from dust, and (6) accumulation and thermal processing of planetoids. In each of these stages there are major scientific questions to be answered.
EXOBIOLOGY 16 Nucleosynthesis and Ejection to the Interstellar Medium This stage begins the cosmic evolution of the biogenic elements. Not only is it responsible for the origin of the elements, it also initiates the condensation of solid matter out of the gas phase. Astronomical observations should be made of supernovae, novae, late type giant stars, circumstellar shells, and planetary nebulae where biogenic elements are being produced and ejected into the interstellar medium. Whereas emphasis in the past has been placed on gases, future studies should focus on characterizing the dust and grains and determining the extent to which these primordial solid condensates survive the transit from sites of stellar origin to interstellar clouds. Especially useful will be high-resolution measurements at millimeter and infrared wavelengths. These measurements will require the Large Deployable Reflector (LDR) and the Space Infrared Telescope Facility (SIRTF), which should be in operation by the turn of the century. Observations of planetary nebulae and the circumstellar shells of late type carbon stars indicate the growth of carbonaceous grains from the gas phase. The mechanisms by which the condensation processes occur are unknown and require elucidation. Although future theoretical simulations of these processes will be undertaken, the Space Station should offer microgravity conditions highly suitable for experimental investigations. Provisions for microparticle research should be included in the Space Station's capabilities. Chemical Evolution in the Interstellar Medium Interstellar clouds serve both as the collectors of atomic and dusty debris from stars in terminal stages of evolution and as the spawning grounds of new stars. In the course of cosmic evolution they provide the first environments in which gas-gas and gas-solid interactions occur between water, organic, and inorganic compounds. By the mid-1990s, the gas phase chemistry of low-molecular-weight compounds in interstellar clouds should be reasonably well known. For lack of observational tools, however, the chemistry and the role of grains in interstellar processes will still be poorly understood. The telescope facilities mentioned in the preceding section should be used to characterize the biogenic elemental composition
EXOBIOLOGY 17 and organic chemical content of interstellar dust and, thereby, to begin closing this large gap in knowledge. These same facilities should also be employed to search for interstellar methane and carbon dioxide, key starting materials for organic chemical evolution in the solar system. Knowledge of the properties of high-molecular-weight organic compounds in the gas phase will continue to be important in the next century. This information is necessary to determine the complexity of interstellar chemistry and to allow mechanisms for the formation of these molecules to be determined. With the availability of suitable microwave spectra, sensitive searches for glycine and adenine, which are among the simplest molecular building blocks of proteins and nucleic acids, can be carried out. Computer modeling of interstellar grain growth and gas-grain interactions should be used to explore the organic chemistry that could occur on or in various types of grains. For experimental approaches to such research, the microgravity conditions attainable on the Space Station could offer advantages over terrestrial environments. The occurrence of molecules predicted on the basis of model studies could be tested by telescopic searches. The acquisition of intact interstellar grains for detailed laboratory studies of their structure and composition is a very exciting prospect, and should be set as a goal for the turn of the century. Especially important would be application of dating techniques to determine the chronology of the presolar processes that produced the interstellar grains. Throughout this time, substances of interstellar origin should continue to be sought in comets, interstellar dust particles (IDP), and carbonaceous meteorites to establish more firmly the continuity of cosmic evolution from interstellar cloud to solar nebula. Protostellar Collapse This stage in cosmic evolution encompasses the transition from interstellar cloud to the nascent solar system. During protostellar collapse, while temperatures remain at or below 20K, the concentration of gas and dust undergoes enormous change over approximately 7 orders of magnitude from the highly diffuse conditions of interstellar clouds to the considerably denser state of the solar nebula.
EXOBIOLOGY 18 As more and more regions of protostar formation are observed, biogenic compoundsâe.g., CO, CS, H2CO, and HCNâshould come into increasing use as molecular probes to reveal the physical conditions and the variations in chemical composition that occur in these environments. Future observational studies of small-scale structures in dark interstellar clouds should be made with instruments currently unavailable such as a millimeter-wave, very-large-array (VLA) radio telescope, or an orbiting LDR. Further developments in astrophysical theory coupled with these observations are expected to yield self- consistent models of collapse dynamics. These models should be used for assessing the extent to which observed inhomogeneous distributions of the organic compounds and water in interstellar clouds are preserved during collapse. As knowledge of collapse kinetics and conditions grows, calculations of increasing sophistication and reliability should be carried out to determine the accompanying chemical and isotopic fractionations (redistributions) that occur between the gas phase and dust. These calculations could provide theoretical bounds on the amount and distribution of interstellar matter composed of biogenic elements and compounds that become incorporated into solar system dust and which survive homogenization processes in the solar nebula. Chemical Evolution in the Solar Nebula The solar nebula corresponds to the terminal state of protostellar collapse from an interstellar cloud. In this stage, temperatures increase, gas-solid interactions occur readily, energy fluxes increase, turbulent mass transport of matter between environments that differ in temperature and composition can occur, and solid objects larger than interstellar grains begin to accumulate. Observations and theoretical understanding of protostellar systems in the mid-1990s should yield more tightly constrained models of the solar nebula. Computer models should be developed to simulate the processes that may have contributed to the chemistry and governed the distribution of biogenic elements and compounds within the nebula over time. Among the processes yielding organic compounds and carbonaceous grains that should be studied are photochemistry due to starlight and the early Sun, large-scale electric discharges, ion-molecule reactions in a partially ionized nebula, and reactions of gases on grains.
EXOBIOLOGY 19 Confirmation of the relevance of putative nebular processes for cosmic evolution should be sought by direct astronomical observation of species with predicted molecular and isotopic compositions in protostellar systems. In addition, identification of predicted compounds and condensed phases uniquely attributable to nebular sources should be sought in comets, asteroids, and carbonaceous chondrites, all of which are composed in part of components that originated in the nebula. The acquisition of samples from a cometary nucleus for examination in terrestrial laboratories is a major goal (see the section on space missions, below). Cometary material is especially interesting because it holds the greatest promise of containing primordial material from the earliest history of the solar system. Growth of Planetesimals from Dust In this stage of cosmic evolution in the solar nebula, aggregation and accretion of dust occur as particles settle to the mid-plane of the nebula, where they further agglomerate into kilometer-sized bodies. For this stage as well as the preceding two, self-consistent models should have been developed by the mid-1990s that describe the evolution of temperature and other state variables as a function of radial and vertical position over time in the nebula. These models should allow the calculation of the rates of destruction and the rates of growth and coagulation of particles composed of the biogenic elements and compounds, particularly carbonaceous and icy grains. Accumulation and Thermal Processing of Planetoids In our own solar system, small bodies were assembled and distributed around the solar system during this stage and their contents altered to varying degrees by the accretion process itself. Other energetic events, the nature and origin of which remain unclear, further influenced their thermal histories. If the parent bodies of meteorites are any indication of the range of possibilities, these histories ranged from very mild, as in the case of carbonaceous meteorites, to extreme, as in the case of differentiated meteorites. By the 1990s, the study of meteoritic samples in terrestrial laboratories should have identified those components that were
EXOBIOLOGY 20 preserved intact since accretion, those that were subsequently altered, and those that were produced as a consequence of thermal processing. The emphasis should then shift to determining what processes could have caused thermal alteration of small bodies in the early solar system, and how widespread these processes were. It is known from the study of carbonaceous meteorites that reactions between water and minerals in at least one parent body yielded clays, carbonates, and other products usually associated with weathering; in these same objects organic compounds and carbonaceous phases occur in abundance. To determine how widespread such processes and products were, mapping the nature and present distribution of the biogenic elements and compounds in the solar system inward of the giant planets should be completed using the spectral reflectance or other spectroscopic properties of the asteroids. The resulting data will help us to understand how these ingredients were distributed in the solar system as a consequence of the dynamics of nebular evolution and planetary accretion. The particular distribution of these elements may have been important for the origin of life on Earth. Prebiotic and Chemical Evolution Knowledge of the conditions on the prebiotic Earth can provide bounds on the nature of the environments and the range of chemical systems and mechanistic pathways that can be fruitfully explored as appropriate models for the development of life. The history of environments and conditions in which life evolved is preserved, albeit incompletely, in a geological record that extends 3.5 billion years back in time. In sharp contrast, the corresponding record for chemical evolution in the first billion years of Earth history is virtually nonexistent. As a consequence, the conditions on the prebiotic Earth must be inferred by extrapolation either backward in time from the existing geological record or forward in time from the initial conditions associated with planetary accretion. Either approach is fraught with uncertainties. The 3.8-billion-year-old Isua metasediments of Greenland testify to the existence of bodies of liquid water, carbon dioxide in the atmosphere, volcanism, higher heat flow, relatively stable differentiated crust, and the emergence of continental shelf environments, from which the occurrence of weathering, a hydrologic cycle, and a carbon geochemical cycle can be inferred. The 3.5-billion-year-old
EXOBIOLOGY 21 rocks from South Africa and western Australia record a marine environment dominated by volcanic islands. The microfossil-bearing black cherts in these formations were apparently deposited during relatively quiescent periods between cycles of volcanic eruptions. Pervasive volcanism must have injected dissolved minerals as well as gases into the ancient seas. The early atmosphere must have had little oxygen, as evidenced by the apparent ubiquity of ferrous iron in the surface environment. Beyond these general characteristics, the record of environmental conditions is mute. The lack of information about environmental conditions is intimidating. The prevailing temperature of the oceans is not known, although the range of local variations could have run from 4Â°C to the highs of deep-sea hydrothermal vents as they do today. If, as solar evolution theory asserts, the Sun was 70 to 80 percent as luminous then as it is now, the atmosphere must have been better able to retain incoming solar radiant energy or else the ocean would have frozen. Recent theoretical models indicate that the early atmosphere could have contained much carbon dioxide, and that this could have provided the necessary greenhouse effect; however, the actual solution to the problem of the dim Sun, and the atmospheric structure, composition, and pressure 3.8 billion years ago, remains uncertain. The extent and volume of the oceans at that time are unknown, as are the intensity of tidal activity, the day length, and the amount and spectral distribution of sunlight reaching Earth's surface. Details of the chemistry of seawater and the chemistry and mineralogy of sediments await improvements in the ability to see through the overlay of metamorphic and diagenic alteration. The extent to which the geological settings preserved in the rock record are statistically representative of their actual frequency of occurrence is still poorly understood. In general, detailed reconstruction of environmental conditions from the earliest existing record remains a monumental task that is as important for the biological sciences as it is for the earth and atmospheric sciences. Although progress will be made in some areas in the next 10 years, the overall task will remain a challenge for decades to come. Even if a satisfactory reconstruction could be achieved, it is possible that the environmental conditions at the time of the origin of life differed substantially from those of 3.8 billion years ago. The lack of a record or the inability to extrapolate far enough back in time from the existing record suggests that we use the
EXOBIOLOGY 22 earth and planetary sciences to construct a theoretical model for the evolution of Earth's prebiotic environment instead. Ideally, the theoretical approach should yield a model that, when extrapolated forward, converges with that inferred by extrapolation of the geological record backward in time. The outcome of the theoretical approach, however, depends on assumptions for the initial boundary conditions for planet formation, most of which are not known with certainty. If this method is to be useful, we will need more accurate estimates of the rate of planetary accretion, the timing and mechanism of core formation, and the nature of early mantle convection, all of which govern the thermal evolution of the planet. This may allow calculation of the time of origin and the composition of the early atmosphere, oceans, and crust. The third approach that can be used to bridge the gap in the geological record is to examine the martian geologic record, which may preserve evidence from this period. Although the martian record will speak most effectively for Mars, comparative planetology may allow extrapolation to the history of Earth. (See also the section on space missions.) Conditions far from equilibrium certainly occurred throughout the history of Earth, as they do today, at solid-liquid-gas phase boundary regions. These include fumarolic and volcanic vents on continents and continental shelves, deep-sea plate spreading centers, submarine and island-arc volcanic vents, the land surface, and the sea surface. It seems likely that among these environments were the spawning grounds for the first living systems. The exobiological objectives of research in early planetary evolution should be fourfold: 1. To develop models of specific geophysically active boundary regions in which chemical evolution could have occurred during the prebiotic epoch. 2. To provide limits for the range of variations in temperature, pressure, nature, and intensities of fluxes of energy and matter, and the chemical and mineralogical compositions of the boundary regions, all as a function of time. 3. To assess the role of biogenic elements in influencing specific geophysical and geochemical processes that established, maintained, and altered physical-chemical conditions in these regions over time.
EXOBIOLOGY 23 4. To determine the occurrence and history of disequilibrium processes involving the biogenic elements and compounds on other planets in the solar system and beyond. Energy Harvesting, Storage, and Transduction An energy gradient was essential for the origin of life on the primitive Earth. The Sun was the principal source of this energy, as it is on the Earth today. This energy was manifested directly in the form of solar radiation and indirectly in the form of lightning and thermal energy. Other energy sources included the shock waves resulting from the impact of comets and meteorites in the atmosphere, the heat release resulting from tectonic processes, and heat and radiation resulting from radioactive decay. It is likely that many of these sources were more powerful when the Earth first formed than they are today. For example, it was discovered recently that young stars have more than a hundredfold stronger flux in the short wavelength region than do stars the age of our Sun. The intense ultraviolet flux would have been effective in photolyzing the small organic and inorganic molecules present in the atmosphere of the primitive Earth. This energy flux would have been intense at the surface of the Earth since there was little or no ozone shield in the primitive atmosphere to absorb this radiation. Energy from shock waves would have been greater because of the high rate of meteoritic and cometary impact on the primitive Earth. This intense meteoritic bombardment would have warmed the Earth's crust as would the heat resulting from the decay of the highly radioactive elements present there. Some of these energy sources would have been still near their maximum intensity at the time life originated (about 4 billion years ago) and certainly would have fueled the conversion of the simple constituents of the Earth's atmosphere into more complex structures. What is not clear is how the larger organic structures were protected from destruction. For example, the photochemical transformation and destruction of larger organic molecules proceeds at a faster rate, in general, than that of the smaller ones from which they are formed. Laboratory studies have shown that reactive compounds such as hydrogen cyanide and nitriles are among the reaction products when these energy sources act on mixtures of compounds that simulate the atmosphere of the early Earth. These compounds
EXOBIOLOGY 24 store energy that can be used to drive other chemical reactions. For example, nitriles spontaneously condense to form polymers, or the energy stored in the nitrile grouping can be used to drive an otherwise energetically unfavorable reaction via coupled chemical processes. Similar reaction coupling has been observed with the pyrophosphate derivatives formed by thermal processes. In the past 30 years, there have been extensive studies of the conversion of simple organic compounds to more complex molecules using a variety of energy sources. The next decade will see increasing use of computer simulations to predict the chemical effects that energy sources exerted on the atmosphere of the primitive Earth. The capability that the current generation of computers possesses to deal quantitatively with the rates of hundreds of reactions, coupled with the more precise determination of reaction rates, will permit the analysis of complex scenarios for the possible chemical processes that may have taken place on the primitive Earth. Experimental studies will continue that will be designed to answer specific questions concerning the photochemical generation of complex organic molecules. For example, the photochemical transformation of organics absorbed on clays and minerals will be investigated. Little information is available concerning the effects of surface adsorption on the course of photochemical reactions, yet this may have been an important process on the primitive Earth. In addition, the possible use of clays and minerals for the storage and transduction of energy in chemical evolutionary processes will be studied. Computer simulations of the possible atmospheric and crustal processes will continue into the decade of 1995 to 2005. Early Life An energy gradient was required to maintain life once it evolved, as it is required to maintain life on Earth today. The energy that primitive life forms used to drive their biochemical machinery may have come from the breakdown of abiotically formed compounds with a high energy content. Alternatively, primitive life processes may have been driven by direct solar radiation. If the first life forms were not autotrophic, it seems likely that this photosynthetic capability evolved rapidly because the oldest known fossils (3.5 billion years old) appear to represent the products and remains of simple photosynthetic organisms.
EXOBIOLOGY 25 Primitive heterotrophs would have utilized the same energetic organic compounds, which were essential for the initial formation of living systems. The pyrophosphate bond incorporated into organic or inorganic compounds would likely have been the energy carrier in the breakdown of these compounds. Significantly, energy carriers of this type are still functional in organisms today. To utilize solar energy, heterotrophs were required to develop a light gathering system. Porphyrins, formed by abiotic processes, may have been incorporated into these primitive life forms for the absorption of long- wavelength solar radiation (visible light). These compounds are reasonable initial light-gathering compounds because they are formed readily from simpler compounds (pyrroles) and absorb light effectively. Research will be undertaken in the 1985 to 1995 decade on possible abiotic processes leading to pyrroles and porphyrins. In addition, the light-promoted electron transfer properties of porphyrins embedded in clays and lipid membranes will be investigated using sulfide, ferrous ion, and other reducing agents that were likely to have been prevalent on the primitive Earth. In conjunction with these abiotic approaches, investigations will be conducted to elucidate biochemical processes of primitive photosynthetic and heterotrophic organisms still extant. The next stage of research, to be undertaken in the 1995 to 2015 time period, may be directed toward devising systems that reduce carbon dioxide and carbon monoxide to formaldehyde and simple carbohydrates. Since these gases are likely components of the primitive atmosphere, the first autotrophs must have had the capacity to carry out reductive transformations of this type. The ability to demonstrate such systems in the laboratory will support the postulate that photochemical processes were an important source of energy for primitive forms of life. As mentioned above, knowledge of the photochemical capabilities of primitive organisms still extant will indicate the research areas to be emphasized in the laboratory studies conducted in the 1995 to 2015 time period. Replication and Transcription One of the more dramatic advances in the field of chemical evolution in the past 10 years is the template-directed synthesis of RNA in the laboratory. This system involves mixtures of activated ribonucleotides that condense to form an RNA polymer,
EXOBIOLOGY 26 complementary to the RNA template present in the reaction mixture. With the proper design of both template and monomer, the incorporation of both purine and pyrimidine nucleotides with the complementary RNA can be accomplished. The polymerization is strongly influenced by small changes in the constituents of the reaction solution. In one example, the use of Pb2+ as the catalyst results in the almost exclusive formation of RNA oligomers with bonding that is not present in living systems, while the substitution of Zn2+ for Pb2+ gives the 3',5'-linked RNA found in contemporary life. The fidelity attained with Zn2+ is close to that observed with some RNA polymerase enzymes. The formation of the ''natural'' 3',5'-linked oligomers was observed also when the 2-methylimidazolide was substituted for 2-imidazolide derivative of the nucleotide. Metal ion catalysis was not required for the oligomerization of these 2-methylimidazolides at 0Â°C. The effect of other reagents on this RNA oligomerization reaction is currently under investigation. Possible effects of the addition of polypeptides to the system or the incorporation of amino acids and peptides into the RNA monomers are also being studied. It has also been demonstrated that DNA templates can be used in place of RNA templates in this RNA synthesis. In one case that has been reported, RNA oligomers twice as long as the template were among the products of the DNA- directed synthesis of RNA. These findings suggest that "sliding" along the template may provide a route from smaller oligomeric templates to large polymeric products. Over the past 5 years, it has been discovered that RNA can have catalytic properties. Contrary to accepted dogma, the RNA of some RNA-containing proteins was observed to be the site of catalysis, and it was shown that the RNA alone could carry out the reaction normally carried out by the combined RNA and protein subunits. The role of the protein in these RNA-protein complexes is apparently to facilitate the binding of the RNA of the enzyme to the substrate being cleaved, as well as to maintain the RNA in a conformation that possesses optimal catalytic properties. This discovery suggests the exciting possibility that RNA may have served as the site of both catalysis and information storage in the first life forms and that DNA and protein evolved later. Template-directed RNA polymerization will continue to be
EXOBIOLOGY 27 refined in the next 10 years. By 1995, we should have determined whether simple peptides can exert the same control on the specificity of the reaction as has been observed for metal ions or substituents on the imidazolide. Other activated nucleotides, which are more likely to have been present on the primitive Earth, should also have been synthesized. In addition, systems may be developed by 1995 that can use templates containing equal amounts of purines and pyrimidines. Methods should also have been devised to separate the complementary RNA oligomer from its template so that RNA synthesis is not terminated by this tight association. At the same time we should have a much greater insight into the mechanism by which RNA catalyzes the cleavage of another RNA molecule. With this information, studies will begin on the RNA-catalyzed cleavage and formation of polypeptides, since there is no reason to believe that such processes will not be feasible. An efficient system for the formation of RNA and DNA from the appropriate monomers and templates should be in place by 1995. The factors that govern the efficiency and specificity of the synthesis should be well understood. Research on the integration of RNA and polypeptide synthesis can then begin as a start toward understanding translation and the origin of the genetic code. At this point, the understanding of RNA catalysis will have proceeded to the stage where it may be possible to design RNA oligomers with well-defined catalytic properties. It should then be possible to ascertain the minimal structural requirements for RNA molecules that may have exhibited both information storage and catalytic properties in the first forms of life. Translation In the contemporary cell, the synthesis of a single protein molecule requires the participation of many agentsâgenetic DNA, messenger RNA, ribosomal RNA, ribosomal proteins, amino acids, transfer RNA, GTP, ATP, and various enzymesâtogether with initiation, elongation, and termination factors. The overall reaction results in the sequence of bases in a "coding" region of DNA giving rise to a specific sequence of amino acids in the protein, through the intermediary formation of a molecule of messenger RNA. This translation process is complex, and we may reasonably assume that it evolved from a simpler system. Research into the nature of this simpler system is currently being intensified, for the
EXOBIOLOGY 28 origin of translation represents one of the most significant single steps along the pathway that led to life as we know it. A simple system that carries out translation should show not only recursive formation of the peptide bond, but also relate the sequence of nucleosides along a molecule of RNA to the specific sequence of amino acids in the polypeptide produced. Such a system has not yet been identified, although this subject has captured the imagination of scientists in many fields, and the literature contains many theoretical suggestions. Weak, but quite specific interactions have been found between individual amino acids and combinations of their anticodon nucleotides. In spite of the advances in modern methods for laboratory synthesis of the peptide bond, prebiotic versions of this reaction are not yet known, except for cases that seem unlikely to have been part of a coded system. The plausibility of a system depends both on the type of chemical compounds required and on the complexity of the system. The less complex the system, the more likely it is to have arisen spontaneously over geological time. It is likely that by 1995 more examples of specific interactions of amino acids and nucleosides will have been reported, although these are likely to be "static" or equilibrium interactions. It is also likely that an efficient peptide bond formation reaction will be known. Based on the rate of progress in the past 15 years, however, it is probable that a working model of a prebiotic translation scheme will not exist until sometime between 1995 and 2015. Such a discovery would have a profound effect on the field of exobiology, and on our view of our own origin. Clays and Other Minerals as Catalysts It is likely that minerals had an important role as catalysts for the formation of biological molecules on the early Earth, since we know of many cases where minerals adsorb and catalyze the reaction of organic compounds. Current research on mineral catalysis is focused on clays. Clays are formed by the weathering of igneous rock and are found abundantly on the Earth. They have also been detected in the 3.8-billion-year-old Isua rock formation as well as in some meteorites, suggesting that clays were also prevalent on the early Earth. In addition, the chemical transformations observed in the Viking mission suggest clays are prevalent on Mars.
EXOBIOLOGY 29 Clays consist of aggregates of platelets of aluminosilicates that contain Mg2 +, Fe2+, and Fe3+ as occasional substitutions for the aluminum. This substitution can result in a net negative charge on the aluminosilicate lattice, which is neutralized by exchangeable cations. In the contemporary environment these cations consist mainly of Na+, K+, and Ca2+. There are some examples of significant quantities of transition metals serving as the exchangeable cations on the clays. The concentration of dissolved transition metal ions was probably much higher in the environment of the primitive Earth, so they are likely to have been associated with clays in greater amounts at that time. The adsorption of organic compounds causes the clay platelets to separate a few angstroms to accommodate small molecules, and 10 Ã or more to bind high-molecular-weight polymers. The extent of adsorption is governed by the charge on the organic molecules, the charge density on the clay surface, the exchangeable cations associated with the clay, and the ionic strength of the medium. Adsorption may result in the hydrolysis, oxidation, reduction, or polymerization of the organic molecule due to the presence of activated water molecules, catalysis by exchangeable cations, surface catalysis, or the high local concentration of the adsorbed organic molecules. Clays may have been important on the primitive Earth since they are able to adsorb organic molecules, catalyze their reactions and desorb the reaction products much in the same way an enzyme catalyzes a chemical transformation. Because clays have the capacity to bind a variety of organic compounds, they may have catalyzed a number of different reactions on the early Earth. Oligomers of amino acids have been prepared in laboratory studies using clays by cycling a clay-amino acid mixture through a wet / dry-heat cycle several times. A degree of polymerization of 2 to 5 has been reported. Catalysis by the dipeptide histidylhistidine and an enhancement in the proportion of the higher molecular weight oligomers by RNA homopolymers has been observed. Much larger polymers (degree of polymerization up to 60) have been prepared by the reaction of amino acid adenylates on clays. Apparently this polymerization proceeds by the alignment of the monomer on the clay surface; no heat or other activation is required. However, the starting material is a highly reactive compound, and it is not clear that it would have formed in appreciable amounts on the primitive Earth.
EXOBIOLOGY 30 RNA oligomers have been prepared by the dry phase polymerization of 2',3' cyclic nucleotides in the presence of clays. Low yields of oligomers with a degree of polymerization as high as 12 have been obtained. These oligomers contain 62 percent of the natural 3',5'-nucleotide linkage and 38 percent of the 2',5'-linked material. A limited number of studies have been performed using other minerals. The calcium phosphate derivative hydroxylapatite binds higher molecular weight RNA oligomers more strongly than smaller ones. In this way it facilitates the template-directed synthesis of RNA oligomers by adsorbing the larger oligomers while the smaller oligomers remain in the solution phase until they grow to sufficient size to be strongly adsorbed. Studies have also been performed on the iron mineral akaganeite, iron hydroxide soils, and volcanic basalts. There should be a significant increase in the understanding of the mineral- catalyzed formation of oligonucleotides and oligopeptides by 1995. New findings should include the discovery of polymerization reactions that are more consistent with conditions believed to have been prevalent on the primitive Earth, and the stereospecific formation of polymers, in which the monomer units all have the same chirality. The properties of minerals will have been more thoroughly investigated by 1995, so we will probably have a better understanding of how they may have catalyzed the transformations of prebiological molecules. The accomplishments of the 1985 to 1995 decade will provide the foundation for investigation of more complex scenarios for the origins of life. Of prime concern is the possible role of mineral catalysis in a system in which the synthesis of polypeptides is coupled with oligonucleotide synthesis. The surfaces of minerals may have served to bind and catalyze the reactions of the primitive translation process. The possible role of minerals in adsorbing and thereby segregating the molecular species essential for life's origins from the complex mix of nonessential organic compounds can then be studied. In other words, it will be possible to investigate experimentally whether minerals served as the skeletal framework on which the origins of life took place. Lipids and Compartments A cell is a compartment, and even a rudimentary cell has
EXOBIOLOGY 31 an enclosing membrane. We do not know the chemical nature of the barrier membrane of the earliest cells, but some form of lipid is a reasonable candidate, and has the conceptual attraction of continuity; lipid is a major component of the membrane in today's cells. A selectively permeable membrane would confer several benefits on the protocell that used it. It would allow the concentration of essential compounds, energy trapped by the cell could be retained until needed; the internal environment of the cell could be buffered against sudden changes "outside," and, if the cell possessed a unique polypeptide or polynucleotide that, for example, catalyzed more rapid translation or replication than was achieved by the surrounding cells, it would help the cell retain this selective advantage over competitors. In addition, a membrane would protect a set of interacting molecules from dilution by rain or the incoming tide. Early life was probably based on compartments. This has been realized for some time. Only recently has experimental work indicated that some form of lipid was a likely component of the membranes of the earliest living cells. Thermal cycling of a mixture of fatty acids, glycerol, and inorganic orthophosphate has been found to yield simple phospholipids. Under the conditions of the experiment, the lipid-bilayer compartments known as vesicles formed spontaneously. Fatty acids are not easy to make under simulated prebiotic conditions, but short-chain fatty acids are found in meteorites. Indeed, vesicle-like structures have been seen in aqueous dispersions of nonpolar extracts of the Murchison meteorite. Thus, lipid-like molecules are attractive materials from which to construct primitive membranes: they readily form compartments that are self-healing, their membranes can be made selectively permeable, and suitable molecules could reasonably be expected to have been present on the prebiotic Earth. By 1995, new pathways for the prebiotic synthesis of fatty acids will probably have been defined. More work will probably be done over the next 10 years on the interactions between lipids and several other classes of compounds. For example, more will be done on encapsulating oligonucleotides and oligopeptidesâdriven partly by the pharmaceutical industry's interest in using lipid vesicles to deliver compounds to the inside of cells. The effect of lipids on the polymerization of nucleotides and of aminoacids will have been further studied. This work will require detailed analysis with modern analytical instruments. There is present interest in the
EXOBIOLOGY 32 controlled "replication" of a vesicle. Under what conditions will a vesicle pinch off and divide in two? Could the contents of a vesicle control its size? What happens if additional phospholipid is supplied continuously to the inside of a vesicle? Speculation on how to achieve selective permeability of lipid vesicles will be replaced increasingly by new experimental work. Other wall materials will also be investigated, although they appear inferior to lipids at present. The use of lipid vesicles in exobiological research is relatively new. Consequently, the status of this area of interest in 1995 is more difficult to predict than for some more established areas. However, the task group expects that the following will be unsolved problems, of great interest to researchers in exobiology: 1. The design and encapsulation of prebiotic light-transducing pigments. 2. The use of these molecules to generate pH or potential gradients, which can then be used to drive chemical reactions. 3. Sophisticated schemes for the polymerization of biomonomers inside a vesicle. 4. Controlled replication of a vesicle, perhaps coupled to polymerization reactions that are taking place inside. Early Biological Evolution A major goal of NASA's life sciences program is a greater understanding of early biological evolution. Essential components of this understanding are the historical course of early metabolic and structural diversification, and the ways in which this course was constrained or influenced by the chemical and tectonic evolution of our planet. Conversely, we also want to know how biological evolution has influenced the development of the Earth's atmosphere, hydrosphere, and rocky surface. Recent research in Precambrian paleontology has shown a hitherto unsuspected degree of interpretable pattern in the early fossil record. Microbiological research has revealed a complementary pattern of phylogenetic relationships among living bacteria and unicellular protists. Our present state of knowledge gives us confidence that continuing research along established and developing lines will result in an extensive geological, paleontological, and microbiological data base by 1995. New techniques developed
EXOBIOLOGY 33 during the next decade will ensure continued progress during the period 1995 to 2015. These include microchemical techniques for the in situ analysis of individual fossils, microbiological techniques for the culturing and analysis of bacteria that are at present difficult to study; new empirical means of establishing phylogenetic relationships among species, new geochronological tools and the refinement of existing methods for accurate dating of ancient sedimentary rocks, and new discoveries of ancient as well as living microbes. The task group anticipates that the following research agenda will be important during the early decades of the next century: 1. The Development of Terrestrial Environments. Biological evolution on Earth took place in the context of a changing earth surface environment. Indeed, at least some of the major steps in the evolution of the physical environment were biologically induced. Careful sedimentological and geochemical studies of ancient sedimentary regimes will be needed to establish with greater precision the environmental context of early evolution. Such studies should include work on the oldest unmetamorphosed sedimentary terrains, but should not be restricted to them. Analysis of younger Precambrian terrains is necessary to trace the path of evolution and the continuing development of environments, as well as to establish the validity of techniques and approaches before they are applied to geologically difficult Early Archean sequences. Of primary importance will be the elucidation of the early history of atmospheric oxygen and its relationship to major events in crustal and biological evolution. 2. An Integrated Tripartite Approach to Early Biological Evolution. Current data obtained through studies of the geological record, microbial phylogeny (based on the sequencing of informational macromolecules), and comparative microbial physiology must be integrated to provide a picture of early evolution. This cannot be obtained from any one of these above sources. 3. The Development of Models to Relate Data Produced by Agenda Items 1 and 2. A major goal of NASA is to produce a working model of the biosphereâthe interacting geochemical cycles that relate the biota and its environment. Simplified versions of this model have great potential to illuminate early biological and physical evolution on Earth. As a single example, it should be
EXOBIOLOGY 34 possible to use this simplified model to explore the effects of continental growth on productivity and, hence, on atmospheric oxygen levels. Biological evolution on Earth cannot be understood until the constraints and influences of our planet's physical evolution are elucidated. The task group stresses the importance to this research of NASA's continued strong intellectual leadership. No other agency can provide the interdisciplinary framework necessary for biological research on early evolution. 4. The Search for Evidence of Past Life on Extraterrestrial Bodies. In addition to the search for extant life in the cosmos, investigations specifically designed to detect evidence of past life on places other than Earth are needed. For example, although it is unlikely that the Mars of today has any life, it is possible that life could have evolved on Mars during its early geological history. On Earth, it is only during the last 650 million years that large, complex life forms appeared, following approximately 3 billion years of microbial dominance. If life evolved on other planets, it seems likely (using Earth, our only known example, as a model) that early stages in the evolution of such life included a long period limited to microbial ecosystems. There is evidence that 4 billion years ago Mars had surface liquid water and a thicker carbon-dioxide-rich atmosphere. That is, 4 billion years ago the surface environments of the Earth and Mars may both have been suitable for the formation of living systems. High priority must be placed on exobiological missions to Mars that call for paleontological and geochemical analyses of samples of ancient sedimentary rocks. The Evolution of Complex and Higher Organisms Unlike most other NASA scientific programs, research on the evolution of complex life is not tied to specific space missions or technological developments slated for the next decade. Thus, it is difficult to determine with any degree of accuracy where the field will stand in 1995. For several decades, NASA has played a major role in research on the origin and early evolution of life on Earth; more recently, the agency has assumed a position of leadership in integrated geological, oceanographic, atmospheric, and biological studies of our planet in its present stateâthe projected "Mission to Planet Earth" (discussed in a separate volume). A sophisticated understanding of our planet's developmental history will require
EXOBIOLOGY 35 the maturation of both these programs, plus the development of new research programs on the evolution of morphologically complex organisms. In essence, it is this last discipline that links research on the early Earth and the present workings of the planet into an integrated picture of biological evolution through geological time. There is at present a considerable body of research being funded by the National Science Foundation (NSF) on the evolution of plants and animals. NASA should not duplicate or annex this program. Rather, NASA should seek to develop areas of research that specifically relate to its initiatives in global habitability, the early evolution of terrestrial life, and the possible effects of extraterrestrial phenomena on terrestrial evolution. The task group recognizes three areas of research that fit this criterion. All are poorly funded by other agencies at present, and all represent fields in which NASA has unique capabilities to forge exciting programs of research. A short description of each area follows: 1. Extraterrestrial Bodies and Biological Evolution. Traditionally, evolutionary patterns documented in the fossil record have been interpreted in terms of earth-bound processes; however, recent paleontological and geochemical research indicates that there is a high probability that some, perhaps all, mass extinctions were linked to impacting comets or asteroids. Patterns of extinction among marine invertebrates have been interpreted as indicating a 26-million-year periodicity in mass extinction events. Thus, we have the fascinating possibility that animal evolution over the past 650 million years has been controlled in no small measure by periodic mass extinctions whose period is defined by events or processes in the solar or galactic environment. This hypothesis is divisible into several component ideas, each of which is the subject of lively current debate. The outcome of the debates cannot be predicted at the moment, but it is clear that the coming decade will see the development of an extensive body of new data on taxonomic and ecological patterns of extinction during and between mass extinction episodes, as well as on the time scales on which individual extinction events occur. These data will be complemented by astronomical research on extraterrestrial phenomena that could produce periodic patterns. By 1995, it will be time to develop a new generation of models to explain extinctions,
EXOBIOLOGY 36 their role in the evolution of complex life, and the role of extraterrestrial events in determining these biological patterns. NASA can and should assume a leadership role in this interdisciplinary effort. 2. The Origins of Multicellular (tissue-grade*) Organisms. NASA has led research efforts on the Precambrian origin and evolution of microbial life on Earth; NSF has sponsored much research on the evolution of multicellular life during the Phanerozoic Eon. At present, no coordinated research program exists that is aimed at the interface between the twoânamely, the origins and diversification of tissue-grade algae and animals. Such a research program will require the integrated efforts of specialists in a wide spectrum of disciplines, including cell biology, developmental biology, organismic evolutionary biology, molecular phylogeny, paleontology, sedimentary geology, and geochemistry. Questions of importance include the evolution of cellular mechanisms that permitted and facilitated the emergence of organisms with differentiated tissues; the course of early plant and animal diversification as determined from comparative biological and paleontological studies, the impact of new grades of biological organization on ecosystem function; the composition of the atmosphere and hydrosphere; the possible effects of extraterrestrial perturbations; and the possible role of physical Earth evolution on the timing of animal and plant evolution. This last question is of interest with regard to inquiries about the possible evolution of intelligent life elsewhere in the universe (SETI). Is the tempo of biological evolution on Earth controlled biologically, or is it constrained by rates of planetary development? The answer to this question is of immense importance to exobiologists. 3. The Evolution of the Biosphere. Major events in biological evolution affect the physical composition and operation of the biosphere, and conversely, important physical changes in the crust, oceans, or atmosphere influence the subsequent course of biological evolution. The geological record provides what is really our only testing ground for models of biosphere function developed as part of NASA's Mission to Planet Earth. Equally important, once global biosphere models become sufficiently sophisticated, it * The term ''tissue grade'' is used to denote a level of structural complexity characterized by the differentiation of tissues composed of morphologically and physiologically distinct cell types. This is distinct from the multicellularity seen in some algae and prokaryotes in which there is either no cellular differentiation or only the differentiation of individual cells.
EXOBIOLOGY 37 should be possible to "run them backward" through geologic time to formulate an integrated view of earth history. How has life persisted on this planet for nearly 4 billion years? How have biological innovations such as the origin of unicellular and multicellular photoautotrophs, skeleton-forming animals, or wood-producing vascular plants influenced the composition of the atmosphere or rates of continental weathering? What, if any, long-term effects have mass extinctions had on biogeochemical cycles? How have physical events such as the Archean growth of large stable continents, plate tectonic movements, ice ages, or sea-level changes influenced the biosphere? The answers to these and related questions are fundamentally important to understanding the Earth's unique status in the solar system. They are also of tremendous practical importance in that the answers to these questions will determine how and where future exploration for fossil fuels and economically important minerals will take place. The task group's recommendations are consistent with those presented in the NASA workshop report The Evolution of Complex and Higher Organisms (1985), which deals with the establishment and immediate needs of a research program on the evolution of morphologically complex and intelligent life. That document can be consulted for voluminous background information. NASA is uniquely qualified to lead and coordinate research in this area. Indeed, NASA is uniquely responsible for such coordination in that an integrated model of earth history will be the crowning gem for several of its major initiatives in biology and earth science. Search for Extraterrestrial Intelligence (SETI) Are we alone in the universe? The interest in this fundamental question permeates many other areas of space research. Indeed, as Astronomy and Astrophysics for the 1980's (National Academy Press, 1982) states, "It is hard to imagine a more exciting astronomical discovery or one that would have greater impact on human perceptions than the detection of extraterrestrial intelligence." At present, there is a consensus that the best way to try to detect extraterrestrial intelligent life is through a coordinated search for radio signals from technologically advanced civilizations. A complementary approach is based on the technology that has led
EXOBIOLOGY 38 to the discovery of possible planetary systems beyond the solar system. This includes Van Bisbroeck V-8, a red hot planet or brown dwarf, and the infrared- emitting matter orbiting around beta-Pictoris. This complementary approach uses highly sophisticated infrared detectors and will continue to be pursued for its own sake by planetary astronomers. However, a further refinement and extension of its methodology would be of synergistic value to SETI. Detection of any planets, even if the size of Jupiter, increases the statistical odds that earth- like planets, life, and advanced civilizations may be relatively abundant in the galaxy. If techniques reach the point at which light from an earth-like planet can be investigated spectroscopically, it would be easy to test for the presence of a surplus of atmospheric oxygen. This would constitute evidence for the existence of life, even if that life were very primitive. But all of these intermediate steps can be bypassed if radio contact with an advanced civilization is achieved. As we write this (January 1986), a proposal is being submitted for a 10-year radio frequency SETI search with state-of-the-art technology. This program will consist of two parts: an all-sky survey in the frequency range 1 to 10 GHz, and a targeted search in the 1-to 3-GHz range. The difference between the two is simply that the targeted search will be a high- sensitivity narrow-band study of selected Sun-like stars. In other words, this search makes the additional assumption that the most likely locations for advanced civilizations are earth-like planets that orbit Sun-like stars. It is the only assumption for which we have any scientific support at present, since we are the only advanced civilization we know. The all-sky survey is free of this assumption. Should a detection be achieved in this mode, the full sensitivity of the system could be brought to bear for confirmation and study. The enabling technology that is making these two approaches practical for the first time is a multichannel spectrum analyzer and a computer software system that can analyze the flood of data this device will produce. It will now be possible to detect and analyze signals with 107 channels simultaneously. This significantly speeds up the work while increasing the sensitivity. The multichannel spectrum analyzer is followed by a pattern detector that searches for regular pulse trains or drifting signals in an array of spectra. This approach will provide a search billions of
EXOBIOLOGY 39 times more comprehensive, in terms of sensitivity and frequency coverage, than the sum of all previous searches. Further improvements can be envisaged. Depending on the outcome of this 10-year program, we can anticipate some interest in using very large, filled arrays to pursue the search with still greater sensitivity. We should also be open to other ingenious suggestions for ways to detect advanced civilizations. These might involve searches in other frequency domains of the electromagnetic spectrum. There seems to be a strong feeling among the space research community that support of such programs by U.S. funding agencies is a legitimate scientific activity, and that choice of programs within each agency should be made through the normal process of peer review. As a number of other nations have initiated steps toward SETI programs, the opportunities for international collaboration are substantial. The Search for Extraterrestrial Intelligence (SETI) is a program of exploration currently in an early stage of technological development. Once the engineering design and construction phase of the signal processing equipment is completed, the search, or operational phase, can begin. Funding of an appropriate magnitude will be required to carry out the microwave observing program. This program, while managed by NASA's Life Sciences Program Office, should not compete for funding from other life sciences research resources, but should be funded as a new NASA initiative. SPACE MISSIONS Introduction This section of the report describes the interest within the life sciences community in some highly sophisticated missions to selected objects in the solar system. These missions will advance our knowledge about environments that can test ideas about the origin and evolution of life on Earth, and help to define the qualities that make our own planet so uniquely habitable. Hanging in the balance is the question of the prevalence of life in the universe. To place these advanced missions in perspective, we must first try to estimate as best we can with scheduling uncertainties, what we will know by the year 1995. An approximate idea of the perspective we will have by that time can be gained simply by
EXOBIOLOGY 40 reviewing the planetary missions that are already under way or scheduled for the future. Optimistically, the task group has included several missions in this section that are still in the planning stages. These are listed in Table 2.1 and summarized in the next section. The summary is given by target object, with a stress on research goals relevant to the life sciences. It is important to stress that this suite of missions is largely uninfluenced by the interests of the life sciences community. With a few exceptions, there is little emphasis on the detection and identification of large organic molecules. While the further characterization of planetary environments is obviously necessary and desirable from a biological perspective, the key measurements that will tell the level of complexity reached by chemical evolution or the processes involved in that evolution are usually slighted. All of this adds up to a requirement for dedicated or sample return missions, which are described in a later section. Pre-1995 Planetary Missions Venus The U.S.S.R. has deployed (June 1985) two spacecraft from its Vega project. These involve atmospheric probes with surface sampling capabilities, and the release of balloons to study atmospheric circulation. The United States plans a radar mapping orbiter to Venus in the late 1980s or early 1990s. This spacecraft will provide a topographic map of the entire planet, with surface resolution approaching 150 m in selected areas. For the life sciences, Venus functions as a control experiment. What would Earth be like if it were closer to the Sun? But is solar proximity the only reason for the extraordinary difference between Earth and Venus? These are two sets of questions these missions are trying to answer. Mars The U.S.S.R. has approved a mission called Phobos that is currently scheduled to reach Mars in 1989. While making some reconnaissance observations of the martian surface, this mission, as its name implies, will have the study of the martian satellite Phobos as its main goal. The mission consists of an orbiter that
EXOBIOLOGY 41 TABLE 2.1 Missions--Planned or Under Way Name Agency Launch Arrival Target Objective (Country) Voyager NASA 1977 Jupiter Flyby 1 Saturn reconnaissance-- 2 NASA 1977 1986 Jupiter images, spectra, Saturn magnetic and Uranus other fields, particles, etc. 1989 Neptune Vega 1,2 U.S.S.R. 1984 1985 Venus Atmospheric, cloud, and surface composition; meteorology (includes balloons). 1986 Halley's Composition of Comet inner and outer coma, appearance of nucleus, magnetic, and other fields and particles. Giotto ESA 1985 1986 Halley's As for Vega, but Comet Giotto goes much closer to nucleus. Planet A Japan 1985 1986 Halley's Plasma Comet environment and hydrogen cloud around comet. Galileo NASA ? ? Jupiter Atmospheric probe with in situ composition and structure measurements. Orbiter with complement of remote sensing instruments. Venus NASA ? ? Venus Radar map of Radar surface. Mapper Phobos U.S.S.R. 1988 1989 Mars Intensive study of Phobos; includes laser vaporization of surface samples.
EXOBIOLOGY 42 will match orbits with the satellite, slowly moving past it while irradiating it with a powerful laser and an ion beam. The material ejected from the satellite will be analyzed with mass spectrometers aboard the spacecraft. Numerous other remote investigations of Phobos are planned, and inclusion of a penetrator that lodges in the surface of the satellite is under consideration. Name Agency Launch Arrival Target Objective (Country) Mars NASA ? ? Mars Detailed Orbiter spectrometric Mission mapping of surface; studies of dust and volatile transport. Vesta U.S.S.R. 1992 or 1994 Mars Atmosphere and 1994 or 1996 surface composition, meteorology; additional objectives to be specified. Vesta Characterization by remote sensing of surface composition; evidence of differentiation. Comet NASA ? ? Comet As for Halley's Rendevous/ Temple Comet, but with Asteroid II much more time Flyby and detail. Cassini ESA/ ? ? Saturn Remote sensing NASA of planet rings and satellites, direct study of magnetosphere; probe into Titan's atmosphere. The United States is planning a Mars Orbiter Mission (MOM). This spacecraft will go into a polar orbit about the planet, and
EXOBIOLOGY 43 carry out an extensive series of mapping studies designed to elucidate the surface composition, the general circulation of the atmosphere, and the behavior and transport of dust and volatiles. MOM is an essential precursor for a Mars sample return. The relevance of Mars to the life sciences is well known. While it is extremely unlikely that there is any life on Mars today, it remains the only planet on whose surface we have found evidence for the presence of liquid water sometime in the past. This evidence leaves open the possibility that life may once have originated on Mars, only to die out as the global climate deteriorated. The Phobos mission will provide compositional information on a dark, low- density satellite that is thought to be rich in organic materials, and perhaps representative of asteroidal parent bodies of the carbonaceous chondrite meteorites. The Soviets have also approved a mission called Vesta that will put a 200- kg payload in orbit around Mars in the 1990s. This spacecraft will deploy at least one penetrator, perhaps into one of the polar caps. There will also be two balloons carrying cameras, gamma-ray spectrometers, and an array of meteorological instruments. An auxiliary spacecraft (currently allocated to the French) will fly on to the asteroids. Comets and Asteroids In 1985, ESA launched its Giotto mission to Halley's Comet. It was preceded at Halley by the Soviet Vega spacecraft. Both missions encountered Halley in March 1986. Both of these projects are altering our ideas about comets in profound and fundamental ways. Other spacecraft will study cometary plasmas, but Giotto and Vega have already provided the first pictures of a comet nucleus and will produce the first close-in measurements of composition. The latter will include mass spectrometric studies of the inner coma, searching for the so-called parent molecules of the coma that are expected to include some interesting organic compounds. At this writing (late March 1986), it is already clear that the nucleus of Halley's Comet is covered with a dark, presumably carbon-rich coating, which is probably contributing to the evidence for a high abundance of organic compounds in the inner coma. The United States is planning a Comet Rendezvous Asteroid Flyby (CRAF) mission. If the mission is approved, the spacecraft
EXOBIOLOGY 44 will rendezvous with Comet Temple II in the late 1990s, having encountered one or more asteroids en route. Unlike the Halley fast flybys, this spacecraft will stay in the vicinity of the comet nucleus for months, allowing repeated measurements and studies of temporal variations of the release of gases and subsequent chemical reactions. The composition of the nucleus, including studies of the non-volatile components, is one of the prime mission goals. The Soviet Vesta mission, as its name implies, will go on to the asteroid Vesta in the 1990s, after dropping the aforementioned probes at Mars. The U.S. Galileo mission (see Jupiter below) will also visit an asteroid en route to its planetary target. Outer Planets The U.S. Voyager 2 spacecraft encountered Uranus in January 1986 and will visit Neptune in August 1989. This means that every planet except Pluto will have been visited by spacecraft by the year 1995, the beginning of the 20- year period upon which this study focuses. While the lower atmospheres of both Uranus and Neptune are warm enough for some potentially interesting chemistry to take place, the Voyager spacecraft are not equipped to explore these regions. On the other hand, the satellites and rings of Uranus are suspiciously dark, suggesting that they may be coated with carbon-rich material. Furthermore, the dark material on the surfaces of these objects is distinctly different from the dark coatings on Saturn's satellites Phoebe and Iapetus. There is clearly some variety in the types of organic compounds that are currently stored in the outer solar system. Neptune's largest moon Triton may have oceans of liquid nitrogen on its surface as well as a methane-containing atmosphere. The Galileo mission to Jupiter will arrive sometime in the 1990s. It consists of an orbiter with a probe that will be deployed for measurements in Jupiter's atmosphere. The atmospheric composition will be determined to a sensitivity of better than 1 ppm for many constituents. Without a gas chromatograph, however, these measurements, will just be the first step in an in situ characterization of trace organic molecules. A similar mission for the Saturn system is currently in the planning stage. Called Cassini, it would be a joint project between ESA and NASA, in which the Europeans would build a probe to be sent into the atmosphere of Titan, while the United
EXOBIOLOGY 45 States would construct an orbiter that would include a radar mapping instrument for studying the satellite's surface. Because the atmosphere of Titan is already known to contain a rich variety of organic molecules, the probe is being designed to include a gas chromatograph. Current plans also include the possibility of some post-impact measurements to try to characterize materials on Titan's surface. Planetary Missions After 1995 Sample Return Missions: The Next Step Assuming all of the missions described in the previous section are approved, launched, and successfully fulfill their objectives, the new perspective we will have in 1995 can be summarized as follows: Spacecraft will have visited every planet in the solar system except Pluto. The Space Telescope will have given us our first clear views of Pluto and its unusually large satellite Charon. At least one of the two dark, asteroidal moons of Mars will have received a detailed investigation, and at least three bona fide asteroids will have been surveyed. A wealth of information about Halley's Comet will have been assimilated, and data on the behavior of Comet Temple II may shortly be arriving from a spacecraft poised in its vicinity. This (CRAF) mission will be equipped to deliver much more detailed information about the organic material found in cometary nuclei than the Halley missions could tell us. The existing suite of in situ measurements on Venus may have been extended in number and type. Detailed topographic maps of Venus and geochemical maps of Mars may allow an intercomparison of the physical and chemical processes at work on all three large inner planets at an unprecedented level of detail. The information from the Galileo Jupiter probe may provide a fundamental calibration scale for elemental and isotopic ratios made throughout the solar system. Galileo will also provide the first direct investigation of a highly reducing planetary atmosphere and the chemistry that takes place there. A mission may be under way to bring our knowledge about Saturn's satellite Titan up to this same level of understanding. This mission should provide far more information about organic chemistry in a reducing environment than Galileo could. To move forward past this point, it is necessary to consider
EXOBIOLOGY 46 a much more ambitious class of missions: those that involve the return of samples to Earth. It is sobering to realize that this most difficult enterprise was first accomplished in 1969 when Armstrong, Collins, and Aldrin returned to Earth with material from the Moon. Twenty-six years later, the Moon will remain the only celestial object from which we have collected samples. This circumstance may be attributed primarily to the high cost of such missions. Given the sophistication of modern analytical techniques, is it really necessary to pay this price? Do we really have to bring samples back to our earthly laboratories? There is a clear consensus in the planetary community that the answer to both of these questions is affirmative. Among the unique data that can be obtained from returned samples are those described in the following list (based on the Solar System Exploration Committee (SSEC) report titled Planetary Exploration Through Year 2000 (SSEC / NASA Advisory Council, 1983), describing augmented missions): 1. Independent Ages and Historical Data. These are determined from highly precise measurements of radioactive isotope parent-daughter pairs in selected samples and different fractions of the same sample. Such measurements require large mass spectrometers and ''clean-room'' techniques. The resulting ages permit the timing of planetary events and the time limits on processes during the early solar system. 2. Precise Chemical and Isotopic Data. Remote measurement techniques cannot compete with the abilities of terrestrial laboratories to measure abundances of 60 to 70 elements to levels of parts per billion, including precise isotope ratios. 3. "Ground Truth" for Global Planetary Missions. Once we have detailed information on a few samples, we can use this to calibrate the global mapping data available from precursor orbiters. Among the operational advantages accruing to the analysis of returned samples, the following can be mentioned: 1. There are virtually no limits on the instrumentation that is available to study the samples. Given enough material, virtually any type of analysis can be performed. 2. The analytical instruments being used are state-of-the-art. They do not suffer from the 5-to 10-year lag that is common in the case of space missions.
EXOBIOLOGY 47 3. There is greater planning flexibility. No assumptions need to be made in advance about the nature of the sample to be tested, about its physical state nor its composition. 4. Analyses are open-ended and not time-limited. They can be repeated (even after long time intervals), done in variable series, and repeated by various investigators in different laboratories using a variety of techniques. Given all these advantages, it is not surprising that the community of scientists interested in the planets is actively promoting sample return missions as the next step in planetary exploration in the years 1995 to 2015. In later sections, two of these missionsâsample return from Mars and from a comet nucleusâwill be considered in some detail. Targets and Strategies for Future Missions The purpose of this section is to define the next large steps forward in solar system exploration beyond the year 1995 that would be most important to the life sciences. We have seen that by 1995 it will be possible to design sample return missions to several objects. Yet other targets, especially Titan, will require additional exploration of a highly sophisticated nature before sample return can be attempted. The task group will consider both sample-return and exploratory missions in this report. The distinction between the two is well brought out by the criteria established for targets of sample return missions set out by the SSEC. For sample return missions: 1. Targets must be accessible with the technology available in the relatively near future. 2. Collecting devices and associated hardware must be able to survive and function on target surfaces. 3. Targets must be understood well enough as a result of previous missions that a productive sample return mission can be planned in adequate detail. The task group would add a fourth criterion: 4. Targets must show evidence of chemical evolution relevant to the problem of life's originâeither as processes taking place today or in the form of deposits of products from earlier epochs.
EXOBIOLOGY 48 Using just the first three criteria, the SSEC selected Mars and a comet nucleus for their candidate missions, to be initiated prior to the year 2000. These two targets nicely satisfy the fourth criterion as well. As the SSEC report states in the case of Mars, "Important bioscience goals could also be achieved through a sample return. The presence or absence of indigenous lifeâespecially fossil lifeâand the reasons for the surprising absence of organic matter in the martian soil could be determined directly by the laboratory examination of an unsterilized sample returned to Earth." The report points out that comets ". . . are an entirely new and virtually unstudied class of objects which, because of their primitive nature, are fundamentally important to understanding the origin and earliest history of the solar system." With a returned sample, "a complete characterization of cometary organic material would be possible, with exciting implications for understanding the origin of life. . . ." Mars and the comets are at the top of the task group's list, too, and it strongly endorses the SSEC recommendations that these be the first of the sample return missions. Given that the task group's study of future missions extends 15 years beyond the limit accepted by the SSEC, the task group can legitimately add some missions to the SSEC list. These might include precursors to sample return missions such as soft landers and penetrator networks. Titan is certainly a prime candidate for a soft lander mission, similar to the Viking project that studied Mars in 1976. We will need the results from the Cassini mission before such a mission can be planned in detail. At that point we will know whether we must use a lander that floats or whether a more conventional spacecraft will suffice. It will also be possible to address such issues as the location of the landing site and the type of surface sampling devices. There has also been great interest in a possible mission to Europa, the smallest of Jupiter's Galilean satellites, which may be covered with an ocean of liquid water whose surface is shielded by an icy crust. Here, the next step in exploration after the Galileo mission will be the deployment of some type of seismic network to determine whether the subsurface ocean really exists. The penetrators that carry the seismometers could also be equipped with devices to make some compositional analyses of their immediate surroundings. Thus, it would be possible to determine, for example, if the dark material associated with some of the "cracks"
EXOBIOLOGY 49 on the surface indeed consists of organic compounds as some have suggested. Plans for a sample return mission or a subsurface probe could then be developed after the results from this precursor mission were analyzed. It should be evident that the specific missions being considered here involve techniques for gathering data that can easily be applied to other targets as well. Thus a comet nucleus sample return mission, once it has been successfully carried out for a given comet, will allow the study of many other comets with the same payload and mission strategy. Modifications in the payload would permit the use of the same or similar spacecraft to carry out sample return missions to selected asteroids. The penetrator network described for Europa could be deployed on asteroids and other satellites. Phoebe and the dark side of Iapetus would both be fruitful targets for such missions. Both of these satellites in the Saturn system have surfaces that seem to be covered with carbon-rich material. The discovery by Voyager 2 that the dark material on the satellites of Uranus has distinctly different optical properties from the material found in the Saturn system invites direct exploration of the Uranus system as well. Given the low temperatures in this part of the solar nebula, this organic material could be even less modified than that found in the carbonaceous chondrites. The next two sections of this report will concentrate on the Mars and comet nucleus sample return missions. The SSEC studies will form the background for this discussion, which will emphasize those aspects of these two missions that are especially interesting to the life sciences. The final section will cover the soft lander and penetrator missions to outer planet satellites. Sample Return from Mars In addition to the high priority awarded it by the SSEC, a sample return mission from Mars was also implicitly endorsed by the Space Science Board of the National Research Council 1978: "The study of Mars is an essential basis for our understanding of the evolution of the Earth and the inner solar system. . . . We recommend that intensive study of Mars be achieved within the period 1977â1987" (Strategy for Exploration of the Inner Planets: 1977â1987, National Academy Press, 1978). It is now clear that this recommendation will not be satisfied
EXOBIOLOGY 50 in the stated time interval. As Table 2.1 indicates, the first U.S. mission to follow the success of the Viking project will be the Mars Orbiter Mission (MOM). Nevertheless, sentiment is building to support a Mars-intensive program that would culminate in the return of one or more samples. This might be done as a cooperative project with the Europe or the U.S.S.R., continuing a welcome international participation in planetary exploration. The sample return missions can also be viewed as precursors for the still more advanced idea of manned Mars missions, ultimately leading to the establishment of a base on the martian surface. The return of samples from another planet is obviously a very expensive proposition. In the case of Mars, a large number of different scientific inquiries can be aided by the ability to carefully study preserved samples that are successfully returned to terrestrial laboratories. This means that great care must be exercised in the choice of landing site and sampling strategy in order that the greatest number of scientific objectives can be met. It will be necessary to return a variety of carefully chosen samples taken from sites that have been selected with specific objectives in mind. This means that the sample identification and selection devices must be mobile; some type of intelligent rover will be required. Even with careful planning, there will be conflicts. From a geological point of view, the key units for sampling in order of decreasing priority are young volcanic units, intermediate-age volcanic units, ancient cratered units, layered units, polar units. The last two have the highest priority from a biological perspective. Areas where major sedimentation has occurred may provide the best opportunity for collecting samples that contain evidence for past life on Mars. Of special interest are those regions where standing or slow-moving water is thought to have covered the surface. It may well be possible to combine a sampling expedition to an area of sedimentation with one of the higher ranking geological objectives. But investigation of the polar regions will require a separate mission. The poles are important for several reasons. The north pole is already known to be covered by a permanent water ice cap. The long-term availability of water even in the form of ice at the planet's surface could have been essential for any surviving indigenous organisms. Both poles serve as cold traps on the planet. They are known to be regions of aeolian deposition; layered terrains have been observed in both locales. These are therefore regions where
EXOBIOLOGY 51 we might expect primitive organic material to be preserved, protected from the hostile surface environment by layers of ice. To reach this material, it will be necessary to drill through the permanent cap. The missions being designed for Mars sample return include the capability of landing anywhere on the planet. The rover will be equipped with a drill, a core tube, and a sampling arm, all of which could be put to good use on a polar cap mission. Finally, it is important to stress that many of the geological studies, especially those aimed at an understanding of the history of water and surface- atmosphere interactions on the planet, will be of great interest to the biological community. The troublesome problems of contamination of Mars and back contamination of Earth require additional study. Both biologists and geologists agree that the samples returned to Earth should not be sterilized, since the required heat or chemical treatment could have a seriously deleterious effect on the samples. The Space Station might play a role here by providing a specialized environment in which preliminary analyses of unsterilized samples could be carried out before shipping them to Earth. One way of using this facility would be to sterilize a small, carefully selected portion of a sample that could then be shipped to Earth for detailed study. The major portion of the sample would remain on the Space Station, presumably undergoing tests for dangerous biological or chemical activity. This approach will require careful thought and international agreement on the kinds of tests to be run before the samples are considered "safe." Biological safety could be assured in a two-step process in which a preliminary set of tests are run in orbit, while a more sophisticated set would be carried out in laboratories on Earth. This two-tiered procedure would reduce the cost associated with a fully-equipped laboratory on the Space Station. It is not possible to look beyond sample return missions with much certainty. The discovery of viable organisms in returned martian samples would have a profound impact on strategies for future exploration of the planet. In the more likely case that the samples are sterile, it still seems inevitable that momentum for a manned mission to Mars will continue to grow. The maintenance of the people sent on such a long mission, including their survival on Mars, is another aspect of the problems confronted by specialists in space medicine, as described elsewhere in this report.
EXOBIOLOGY 52 Comet Sample Return Comets potentially offer the least-altered samples of solid material still available in the solar system. This is demonstrably true of substances whose volatility is equal to or less than that of carbon dioxide, and probably holds for still more volatile compounds as well. A comet nucleus has been likened to a dirty snowball, where the snows are now known to consist predominantly of ices of water and carbon dioxide, and the dirt includes carbon-rich compounds in addition to silicates. Here then is an opportunity to examine some of the solid materials from which the planets accreted, unchanged since the solar nebula first formed. Comets represent the solar system's starting conditions, providing examples of the kind of chemistry that went on before there were planets. This includes examples of materials that have been continually delivered to the surfaces of planetsâsometimes catastrophicallyâbut sometimes yielding starting materials for new chemical reactions. It is by no means established that organic material from comets contributed to the famous primordial soup from which life on our planet arose. Yet this is one of several current hypotheses regarding the first steps for the origin of life, and it is certain that comets must have contributed some fraction of the volatile elements that dominate the chemistry of life as we know it: the hydrogen, oxygen, carbon, and nitrogen from which we are mainly composed. These considerations make the investigation of comets of great interest to the life sciences. Accordingly, the task group enthusiastically endorses the recommendation of the SSEC that "the return of a sample from the nucleus of a comet is one of the highest priorities for an augmentation mission and should be undertaken as soon as possible." It is only by bringing samples back to our laboratories on Earth that we can perform the full array of experiments with the finesse required to tell us what we want to know. What compounds are present and in what proportions? Are they really pristine, or is there evidence of some processing during the comet's lifetime? How does this suite of organic compounds compare with those found in carbonaceous chondrites? in the interstellar medium? in laboratory experiments? Are there clues from isotope ratios or rare gas abundances that can let us decide what fraction of terrestrial volatiles were actually brought to Earth by these icy messengers? Do comets represent frozen primordial soup? That is, would a
EXOBIOLOGY 53 melted comet constitute a good starting point for further chemical evolution leading ultimately to life? These are just some of the questions we would like to answer. The Choice of a Suitable Comet. As the news from the Halley observations by spacecraft and Earth-based techniques trickles in, it is clear that early ideas suggesting that comets contain substantial amounts of organic material are indeed correct. The special attraction of comets arriving in the inner solar system for the first time is that they may have been maintained at very low temperatures (less than 20K) since the solar system began. Even a comet like Halley's, which has been trapped in a short-period orbit that forces repeated visits to the vicinity of the Sun, displays the full range of phenomena and the variety of molecular fragments that are found in "fresh" comets. In choosing a comet for a sample return, however, we will have to pick from less active objects that are in orbits of still shorter periods than Halley's majestic 76 years. That means they are, on average, much closer to the Sun and will therefore have lost some of their most volatile constituents. Nevertheless, we know from ground-based spectroscopic observations that there are comets even in this category that are actively emitting gas in which molecules such as CN, C2, and C3 can be identified. Furthermore, the icy nuclei of all comets whose nuclei have been directly observed are found to be very dark. They do not exhibit the high reflectivity associated with the ices we know they contain. For example, the nucleus of Halley's Comet was found to be nearly twice as large as the size calculated from its assumed reflectivity, because it was actually much darker than had been thought. Evidently, these comet nuclei are coated with dark material, which apparently becomes concentrated as the ices sublime away, much as lag gravels are left behind during aeolian erosion. With reflectivities below 5 percent, this material must contain a large amount of carbon. Hence the surface layers of a short-period comet represent a concentrated sample of the organic material that all comet nuclei presumably contain. Thus, it may be concluded that we should be able to answer many of our most pressing questions by sampling one of the small, short-period comets that are relatively common and easy to reach. To make this enterprise worthwhile for scientific purposes, however, we must require that
EXOBIOLOGY 54 (1) the comet is still actively giving off gas, and (b) the gas shows the presence of C3, C2, CH, and CN radicals. Mission Description. "It is now technologically within our reach to collect pristine samples directly from active comets and to return them to Earth for intensive laboratory study." This bold statement from the SSEC report sets the tone for the mission. While bringing back samples from the surface of a comet nucleus is obviously of great interest and must be done, we also want to obtain a deep core sample. This offers the best opportunity to obtain material that has undergone only minimal heating, outgassing, and exposure to radiation that would lead to alteration of the original state of the material. The extent to which this is possible will depend on the particular comet that is sampled. It is likely that we shall have to sample several nuclei of different comets that are at various stages of disintegration (devolatilization) before we will know just how primitive (or representative) is a particular sample. But even if the most volatile substances such as carbon monoxide, molecular nitrogen, methane, and argon are deficient or absent, we shall be very pleased to be able to obtain samples containing cyanides, aldehydes, and the other more complex substances that are responsible for the dark coatings on these icy nuclei. The basic mission profile as currently envisaged is as follows: 1. Rendezvous with the comet. 2. Spend some time observing the comet's behavior and selecting appropriate sampling sites. 3. Obtain at least two separate core samples approximately 1 m long and 6 cm in diameter. 4. Deploy a long-lived surface lander to be left on the nucleus. 5. Return to Earth with the samples contained in an environmentally controlled capsule. This may sound straightforward, but there are a number of serious technical concerns to address before such a mission can be realized. The first of these is the propulsion system. This mission requires a low-thrust capability that is currently (1986) not available. An example is the solar powered ion drive, under study in both the United States and West Germany. An engine of this general type could provide a low, but continuous propulsion that would allow the spacecraft to carry the heavy payload required
EXOBIOLOGY 55 on the round trip to the nucleus and to carry out the necessary rendezvous and reconnaissance maneuvers at the target comet. A second major problem is the hazard presented by dust that is expelled by the comet nucleus along with the subliming gas. Both the Vega and the Giotto spacecraft were damaged by dust from Halley's Comet; this was a much greater problem than had been anticipated. The comets selected as targets for sample return will be far less active than Halley, however, and this problem needs to be reevaluated after the CRAF mission has successfully visited one of these better behaved candidates. Finally, the questions of sample handling and contamination problems will have to be studied carefully. Despite familiarity with extraterrestrial samples through the Apollo program, the low temperature, highly reducing, volatile-rich environment of a comet nucleus will provide new challenges that should be anticipated as part of the mission planning. It would be ironic indeed if the effort and expense of a comet nucleus sample return mission were wasted in the end because of an inability to examine the sample properly on Earth. In Situ Studies of Titan On the assumption that the Cassini mission (or some surrogate that accomplishes the same objectives) will have made a successful study of Titan by the year 2005, we can ask what the next stage of exploration of this satellite would be. This assumption implies that we will know the following: 1. The location and extent of lakes, seas, or oceans of liquid hydrocarbons (principally ethane) on the surface of Titan. 2. The composition of the atmosphere to the level of 1 ppm (better in some cases), including isotope ratios for abundant elements and identifications and abundances of volatile organic compounds. 3. A first-order characterization of the chemical composition of aerosols collected during descent. 4. The temperature and pressure profile of the atmosphere along the descent trajectory and the vertical distribution of clouds along this same path. 5. A crude characterization of the surface at the probe's impact site. (This may be no more than a test of whether the surface
EXOBIOLOGY 56 is solid or liquid, but it might include some information about the composition of that solid or liquid). 6. A variety of other informationâabout atmospheric winds, the inorganic composition of the surface, constraints on internal structure, and so onâof less immediate relevance to the task group's specific concerns. The next step in exploration will require some type of soft lander with a capability to move around on the surface. Depending on the nature of that surface, this could be either a rover, a boat, or some type of amphibious device. An alternative approach would be the use of a powered dirigible with the capability of sampling the surface by means of one or more instrument packages lowered on a tether. With so much uncertainty remaining about the nature of the environment we wish to explore, it is not possible to be much more specific than this. It seems highly probable from our current perspective that Titan will have some liquid hydrocarbons on its surface. We will surely want to examine theseâ determining their composition, perhaps looking for evidence of optical activity in the organic molecules dissolved in them, scouting their shores for concentrations of materials deposited from the atmosphere. If there is a global ocean the need for surface mobility is less obvious, but we would still like to be able to investigate the atmosphere over a wide range of latitudes. The dark polar cap and the hemispherical asymmetry in the reflectivity of the smog layer observed by Voyagers 1 and 2 imply that different chemical reactions are occurring at different locations in the atmosphere. This is borne out by the spectroscopic evidence for latitudinal gradients in the abundances of hydrocarbons and nitriles. At the present time, we have no specific information about the chemical composition of Titan's aerosols. We only know that they must be a combination of condensates of the volatiles detected spectroscopically and polymers produced by the irradiation of these polymers. The Cassini probe is being designed to have some capability for collecting and analyzing these aerosols, but only along the single probe entry trajectory. To investigate latitudinal differences in chemistry, a very ambitious mission will be called for, but it is simply too early to define it. The same impediment interferes with efforts to specify a payload. The main thrust of this mission will be to understand the
EXOBIOLOGY 57 chemical reactions taking place on Titan. What processes are involved and what are they producing? Are there any preferred pathways toward complexity? Are any catalysts available and what role do they play? Do the molecules produced show any specific optical activity? Have there been any changes in these processes over geologic time? Are further reactions occurring at the surface in addition to those in the atmosphere? What relevance, if any, does this history have to the organic chemistry on the primitive Earth? Attempts to answer these questions (and others like them) will require an array of sophisticated instruments operating in a benign environment that is maintained in spite of ambient temperatures of 94K or less. This will be a formidable challenge, but one that surely can be met. The opportunity to explore this natural laboratory (and repository) already seems well worth the effort. The task group anticipates that the results from the Cassini mission will simply increase the appeal of this next step in the exploration of Titan. CONCLUSIONS AND RECOMMENDATIONS â¢ The set of missions described in Table 2.1 implies that significant advances in our understanding of the various members of the solar system will be achieved by the year 1995. â¢ The Task Group on Life Sciences particularly endorses the new missions to Jupiter, to comets, to the martian satellite Phobos, to Mars itself, and to Titan. But the task group is concerned that most of these missions are being planned with little concern for the detection and analysis of organic compounds with high molecular weights. The task group strongly urges the spacefaring countries of the world to give greater consideration to this issue. â¢ Mars, comets, and the satellite Titan are currently the prime targets for intensive exploration from the perspective of the life sciences. The task group strongly supports efforts to learn more about the progress and history of chemical evolution on these bodies. The dark asteroids and the dark material found on the surfaces of icy satellites in the Uranus and Saturn systems also merit further investigation from this same point of view. â¢ Some specific recommendations for continued planetary exploration are as follows:
EXOBIOLOGY 58 1. Mars: The task group recommends intensive exploration of the surface to define regions most suitable for subsequent in situ analysis. These areas include layered terrains such as those exposed on canyon walls, areas of sediment deposit such as the floors of ancient basins that held standing water, and the ground around and under the polar caps. This exploration phase is then to be followed by the deployment of highly instrumented rovers or the return of samples for analysis on Earth. The choice of samples to be brought back from Mars must include an assessment of their relevance to the problem of life's origin. 2. Comets: The task group recommends further characterization of the compositional and behavioral heterogeneity of comet nuclei and evaluation of the dust hazards in their immediate vicinities. This should be followed by sample return missions that will bring unaltered samples to Earth for detailed chemical analysis. Concurrently, there must be development of facilities to handle these low-temperature, volatile-rich samples. 3. Titan: The task group recommends that the initial orbiter-probe mission (Cassini or its surrogate) should be followed by a mobile lander (or a floater with surface-sampling capability) that can carry out sophisticated analyses of the organic materials accumulating on Titan's surface and define the processes that lead to their production.