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2 Exobiology WHAT IS E:XOBIOLOGY? Throughout history, humanity's creation myths appear to re- flect 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 ex- panse 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 Dar- winian evolution in the solar system on planet Earth. Prom 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 mul- tidisciplinary 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 devel- opment of the solar system and its planets led to the origin of 8

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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) cosmuc evolution of biogenic elements and compounds; (2) pre- biotic 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- l990s, 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 elements the elements that make up all life and 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 cornrnonly 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 in- fluenced the course of events during the formation of the solar system. Answers to these questions will develop, however, as astro- physicists 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 protosteliar regions. In ground-based laboratories and by remote spacecraft, studies of interstellar dust and samples of other relict materialmeteorites, comets, asteroids, and interplanetary dust- will continue to help

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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 ~ 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 evo- lution is inseparable from planetary evolution, and the path that leacis 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.

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11 In this epoch the major questions concern the relationship be- tween 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 mucrobial evolu- tion 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 supplest biochemical mechanisms and biophysical struc- tures 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 iden- tify 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 mul- ticellular plants and animals and intelligent species evolved. Exo- biological 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 ac- count for major extinctions in the course of biological evolution, particularly events that may have placed advanced life on its evo- Jutionary 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.

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12 PLANETARY E::gP[ORATION AND TEE 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 environ- ments 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 explo- ration and study for exobiology. Although the Viking mission fount} no evidence of extant life, the search was Ignited and not directed at optimum sites. In recent years, dicoveries 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 pro- viding 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

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13 at depth. These measurements should provide further analysis of the chemistry and mineralogy of the surface and subsurface rocket 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 ~ mlmlons. The unusual chemistry of the martian surface soils, manifested in the biom~metic responses elicited by the Viking biology experi- ments, 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 Anion priority for exo- biology. By the m-1990, 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 com- position 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 unanswered- their ori- gin, the identity of higher molecular weight organic compounds, the nature of material contributed by interstellar and solar new ular 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 "fossils record of the materiab and processes involved in the

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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 increas- ing the probability that life could have arisen elsewhere in the galaxy. Searches for cometary and asteroidal matter associated with other stars or with protostelIar objects will provide a basis for determining the frequency of occurrence of preplanetary mat- ter 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 mode! for Earth's prebi- otic 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 relation- ship 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 oW served 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. v ~ 7 Venus, with its atmospheric water, and the Galilean satel- lites Callisto, Ganymede, and Europa, with their water ice, are

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15 objects that contain clues to the history of water in planetary en- vironments. 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 plan- ets 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 wall be pertinent targets of exobiological interest weD into the next century. RESEARCH TOPICS Formation and Evolution of Biogenic Elements and Compound 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 included- e.g., iron, magnesium, calcium, sodium, potassium, and chlorinebut 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) protosteliar collapse, (4) chemical evolu- tion ~ 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.

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16 Nucleosynthesis and Ejection to the Interstellar Medium This stage begins the cosmic evolution of the biogenic ele- ments. Not only ~ 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, no- vae, late type giant stars, circumstelIar shells, and planetary new ulae 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 primor- dial solid condensates survive the transit from sites of stellar ori- gin to interstellar clouds. Especially useful will be high-resolution measurements at millimeter and infrared wavelengths. These mea- surements will require the Large Deployable Reflector (LDR) and the Space Infrared Telescope Facility (STRTF), which should be in operation by the turn of the century. Observations of planetary nebulae and the circumstelIar sheds of late type carbon stars indicate the growth of carbonaceous grains from the gas phase. The mechanisms by which the con- densation processes occur are unknown and require elucidation. Although future theoretical simulations of these processes will be undertaken, the Space Station should offer microgravity condi- tions highly suitable for experimental investigations. Provisions for m~croparticle research should be included in the Space Sta- tion's capabilities. Chemical Evolution in the Interstellar Medium Interstellar clouds serve both as the collectors of atomic and dusty debris from stars in terrn~nal 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 {ow-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 under- stood. The telescope facilities mentioned in the preceding section should be used to characterize the biogenic elemental composition

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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 car- bon dioxide, key starting materiab 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 chern~stry and to allow mechanisms for the formation of these molecules to be determined. With the availability of suitable microwave spectra, sensitive searches for glyc~ne and adentne, which are among the supplest molecular building blocks of proteins and nucleic acids, can be carried out. Computer modeling of interstellar grain growth and ga~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 attain- able on the Space Station could over advantages over terrestrial environments. The occurrence of molecules predicted on the bash of model studies could be tested by telescopic searches. The acquisition of intact interstellar grains for detailed labo- ratory 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 inter- stelIar 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. Protosteliar Collapse This stage in cosmic evolution encompasses the transition from interstellar cloud to the nascent solar system. During proto- stellar collapse, while temperatures remain at or below 20K, the concentration of gas and dust undergoes enormous change over approx~nately 7 orders of magnitude from the highly diffuse con- ditions of interstellar clouds to the considerably denser state of the solar nebula.

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18 As more and more regions of protostar formation are oW served, biogenic compounds e.g., CO, CS, H2CO, and HCN- should 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. Fur- ther developments In astrophysical theory coupled with these oh servations are expected to yield self-consistent models of collapse dynamics. These models should be used for assessing the ex- tent 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 proto- stelIar collapse from an interstellar cloud. ~ this stage, temper- atures increase, gas-solid interactions occur readily, energy fluxes increase, turbulent mass transport of matter between environ- ments that differ in temperature and composition can occur, and solid objects larger than interstellar grains begin to accumulate. Observations and theoretical understanding of protosteliar systems in the mid-199Os should yield more tightly constrained models of the solar nebula. Computer models should be devel- oped 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-scare electric discharges, ion-molecuTe reactions in a partially ionized nebula, and reactions of gases on grains.

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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 life especially fossil life and 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 vir- tually 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 lanclers 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 rn~ssion 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 pene- trators 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 ex- ample, if the dark material associated with some of the ~cracks"

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49 on the surface indeed connote of organic compounds as some have suggested. Plans for a sample return minion or a subsurface probe could then be developed after the results from this precursor mid sion were analyzed. It should be evident that the specific mmsions being consid- ered here involve techniques for gathering data that can easily be applied to other targets as well. Thus a comet nucleus sam- ple return ITussion, 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 de- ployed on asteroids and other satellites. Phoebe and the dark side of lapetus would both be fruitful targets for such rn~ssions. 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 wiD concentrate on the Mars and comet nucleus sample return missions. The SSEC stud- ies will form the background for this discussion, which win em- ph~ize 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 1977-1987~ (Strategy for Exploration of the Inner Planets: 1977-1987, National Academy Press, 1978~. It is now clear that this recommendation will not be satisfied

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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 terres- trial 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 vol- canic 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 sIow-moving water is thought to have covered the sur- face. 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 indige- nous 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

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51 we Alight expect primitive organic material to be preserved, pro- tected from the hostile surface environment by layers of ice. To reach this material, it will be necessary to drill through the per- manent 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 unportant 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.

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52 Comet Sample Return Comets potentially offer the leasLaltered samples of solid ma- terial 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, un- changed 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 planets sometimes catastrophicallybut sometimes yielding starting materials for new chemical reactions. It is by no means established that organic material from comets contributed to the famous prunordial 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 enthusia~- tically endorses the recommendation of the SSEC that "the return of a sample from the nucleus of a comet is one of the highest pri- orities 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

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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 ~ 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 tone 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 wait 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 ON, C2, and Cat 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 ma- terial, 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 ad comet nuclei presumably contain. Thus, it may be con- cluded 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

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54 (1) the comet IS still actively giving a* 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 par- ticular comet that ~ 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, aidehydes, and the other more complex substances that are re- sponsible 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 se- lecting 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 environ- mentally 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

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ss 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 CRAP 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 ele- ments and identifications and abundances of volatile organic com- pounds. 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 im- pact site. (This may be no more than a test of whether the surface

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56 is solid or liquid, but it might include some information about the composition of that solid or liquid). 6. A variety of other information about atmospheric winds, the inorganic composition of the surface, constraints on internal structure, and so on of 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 thesedetermining their composition, perhaps looking for evidence of optical activity in the organic molecules dissolved in them, scouting their shores for con- centrations of material deposited from the atmosphere. If there is a global ocean the need for surface mobility ~ 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 Voy- agers ~ 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 nitrites. 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 pay- load. The main thrust of this mission will be to understand the

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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 rele- vance, if any, does this history have to the organic chemistry on the primitive Earth? Attempts to answer these questions (and others likes 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 ~ CO~IENDATIONS ~ 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. 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. ~ 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. ~ Some specific recommendations for continued planetary exploration are as follows:

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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 characteri- zation of the compositional and behavioral heterogeneity of comet nuclei and evaluation of the dust hazards in their immediate vicini- ties. This should be followed by sample return missions that will bring unaltered samples to Earth for detailed chern~cal 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 orbit- er-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.