Life Sciences Space Science in the Twenty-First Century -- Imperatives for the Decades 1995 to 2015 (1988) / Chapter Skim
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2. Exobiology
2. Exobiology
Pages 8-58
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From page 8... ...
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.
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From page 9... ...
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.
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From page 10... ...
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.
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From page 11... ...
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.
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From page 12... ...
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.
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From page 13... ...
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.
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From page 14... ...
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 satellites Callisto, Ganymede, and Europa, with their water ice, are
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From page 15... ...
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)
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From page 16... ...
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 gassolid interactions occur between water, organic, and inorganic compounds.
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From page 17... ...
Protosteliar 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 approx~nately 7 orders of magnitude from the highly diffuse conditions of interstellar clouds to the considerably denser state of the solar nebula.
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From page 18... ...
Chemical Evolution in the Solar Nebula The solar nebula corresponds to the terminal state of protostelIar collapse from an interstellar cloud. ~ 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.
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From page 19... ...
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 · — Cy grains. Accumulation and Thermal Processing of Planetoids In our own solar system, small bodies were assembled and clistributed around the solar system during this stage and their contents altered to varying degrees by the accretion process itself.
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From page 20... ...
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.
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From page 21... ...
Pervasive voIcan~m must have injected dissolved minerals as well as gases into the ancient Bear. The early atmosphere must have had little oxygen, as evidenced by the apparent ubiquity of ferrous iron in the surface environment.
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From page 22... ...
3. To assess the role of biogenic elements in influencing specific geophysical and geochernical processes that established, maintained, and altered physical-chemical conditions in these regions over time.
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From page 23... ...
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.
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From page 24... ...
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.
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From page 25... ...
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 maxtures of activated ribonucleotides that condense to form an RNA polymer,
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From page 26... ...
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.
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From page 27... ...
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.
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From page 28... ...
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.
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From page 29... ...
organic molecules. Clays may have been unportant 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.
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From page 30... ...
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.
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From page 31... ...
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.
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From page 32... ...
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 chern~cal and tectonic evolution of our planet.
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From page 33... ...
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.
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From page 34... ...
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 ~rnportance to this research of NASA's continued strong intellectual leadership.
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From page 35... ...
A short description of each area follows: 1. Extraterrestrial Bodies and Biological Evolution.
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From page 36... ...
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 modem of biosphere function developed as part of NASA's Mission to Planet Earth.
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From page 37... ...
states, fit is hard to imagine a more exciting astronomical discovery or one that would have greater unpact 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
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From page 38... ...
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.
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From page 39... ...
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.
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From page 40... ...
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.
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From page 41... ...
Mapper Phobos U.S.S.R. 1988 1989 Mare Intensive study of Phobos; includes laser vaporization of surface samples.
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From page 42... ...
Numerous other remote investigations of Phobos are planned, and inclusion of a penetrator that lodges in the surface of the satellite is under consideration. The United States is planning a Mars Orbiter Mission (MOM)
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From page 43... ...
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.
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From page 44... ...
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.
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From page 45... ...
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 wall have in 1995 can be summarized as follows: Spacecraft will have visited every planet in the solar system except Pluto.
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From page 46... ...
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.
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From page 47... ...
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 m~ssionssample return from Mars and from a comet nucleus will be considered in some detail.
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From page 48... ...
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.
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From page 49... ...
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.
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From page 50... ...
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.
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From page 51... ...
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.
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From page 52... ...
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.
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From page 53... ...
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.
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From page 54... ...
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.
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From page 55... ...
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)
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From page 56... ...
It seems highly probable from our current perspective that Titan will have some liquid hydrocarbons on its surface. We will surely want to examine these—determining their composition, perhaps looking for evidence of optical activity in the organic molecules dissolved in them, scouting their shores for concentrations of material deposited from the atmosphere.
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From page 57... ...
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?
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From page 58... ...
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.
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