Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
7 Major Research Recommendations The recommendations in this chapter fall into two general categories: those that require observations and experiments to be conducted on either planetary missions or facilities in Earth orbit and those that include obser- vations, experiments, and theoretical modeling studies that can be carried out in ground-based facilities. In view of the generally large differences in cost and complexity between these two categories, the committee assigns priorities within the two groups separately. Within each group, lists have been priority ordered on the basis of a combination of near-term feasibility and scientific importance. In making recommendations involving space flight, it should be recognized that the principal impetus for these missions is most likely to stem from the astrophysical and planetary scientific communities and that the resources involved in performing exobiological studies thereon will typically be very much less than overall mission expenses. It should also be noted that obtaining the full potential from such missions will re- quire the participation of scientists with interests in planetary biology and chemical evolution from the inception of planning. RECOMMENDATIONS REQUIRING FLIGHT OPPORTUNITIES Mars The highest priority in the category requiring flight missions is accorded to studies of Mars. It is hard to imagine more exciting and fundamental questions than those concerning the early surficial environment and the possibility of chemical or even biological evolution on the early surface of our neighboring planet. Furthermore, Mars is the only other object in the solar system on which an earlier origin of life could have left a well- 123
24 THE SEARCH FOR LIFE' S ORIGINS preserved, exposed record. Sedimentary rocks on Mars may contain a rec- ord of the interval in chemical evolution that is nowhere preserved on the Earth and may thus contribute to understanding the processes that led to the origin and early evolution of organisms on this planet. Thus, investigations of Mars can contribute to the elucidation of objectives discussed previously in connection with early planetary environments and the origin of life- both on the Earth and, possibly, on Mars as well as with the course of biological evolution on this planet. (More complete discussions of these issues can be found in Chapters 3 [pp. 71-77] and 5 [pp. 102-1031~. The committee therefore recommends studies to · conduct chemical, isotopic, mineralogical, sedimentological, and paleontological studies of Martian surface materials at sites where there is evidence of hydrologic activity in any early clement epoch, through in situ determinations and through analysis of returned samples; of pri- mary interest are sites in the channel networks and outflow plains; highest priority is assigned to sites where there is evidence suggestive of water-lain sediments on the floors of canyons as in the Valles Marineris system, particularly Hebes and Candor chasmata; and · reconstruct the history of liquid water and its interactions with surface materials on Mars through photogeologic studies, space-based spectral reflectivity measurements, in situ measurements, and analysis of returned samples. Comets and Asteroids Critical information about the chemical nature, and early processing, of materials containing the biogenic elements (i.e., the evolution of organic complexity in the solar nebula) can be obtained from the study of these relatively unmetamorphosed materials of the solar system. These issues are more fully discussed in Chapters 2 (pp. 46~8, 51-53, and 55) and 3 (pp. 61-62~. Such studies can lead to an understanding of the role of these bodies in supplying the primitive Earth with the organic constituents and volatiles necessary for the origin of life on the planet. Furthermore, these bodies are also of interest as projectiles that may have had significant ef- fects on the course of biological evolution by impacting the Earth. The committee therefore recommends that · measurements be made, by remote spectroscopic observations, and in situ, of the elemental and isotopic composition of cometary comae and nuclei, and of the principal asteroid types, including determination of the molecular composition of components containing the biogenic elements hydrogen, carbon, nitrogen, oxygen, phosphorus, and sulfur in comets and primitive asteroids; such measurements should be made at
MAJOR RESEARCH RECOMMENDATIONS 125 various surface locations and depths to determine the degree of homo- geneity; and · a cometary sample be obtained for detailed laboratory analysis of atmospheric, surface, and subsurface materials. Titan and the Giant Outer Planets The outer planets, in contrast to the inner, represent bodies with atmo- spheres dominated by hydrogen and containing organic constituents. Study of these objects can yield considerable insight about the processes involved in the formation of organic compounds under natural conditions in a hydro- gen-rich environment. Much interesting chemistry must also be taking place in the strongly reducing atmosphere of Titan. Thus, investigations of these objects can be expected to shed much light on one model for the formation of life on the Earth, in which a reducing atmosphere has been invoked. The committee therefore recommends studies to · identify the compositions, and measure the abundances and distri- butions, of gaseous organic compounds and organic haze particles in Titan's atmosphere, using atmospheric entry probes and remote astro- nomical observations (see Chapter 3, pp. 59-61~; and · elucidate the distribution, with altitude, of organic matter, carbon monoxide, and phosphine in the atmospheres of Jupiter and Saturn by using atmospheric entry probe measurements and astronomical obser- vations (Chapter 3, pp. 58-59~. The Interstellar Medium and Cosmic Dust Particles The earliest stages of chemical processing involving the biogenic ele- ments are taking place in molecular clouds and protosolar nebulae. Studies of these objects can therefore answer fundamental questions about the early history of organic chemical evolution. For investigation of the interstellar and protostellar regions, significant advances in the understanding of early organic chemical evolution can be realized by opening up those portions of the infrared- through millimeter-wavelength spectrum for which the atmo- sphere is opaque. Additional opportunities to increase understanding of pro- cesses and events in the evolution of volatiles and organic materials in the early solar system can be attained by the study of extraterrestrial dust par- ticles. The two recommendations below follow from discussion in Chapter 2 (pp. 34~1 and 25-341. For effective probing of these scientific issues, the committee · strongly supports the development of high spectral resolution,
26 THE SEARCH FOR LIFE'S ORIGINS Earth-orbital facilities for astronomical observations at infrared, sub- millimeter, and millimeter wavelengths; and · recommends Earth-orbital collection of interplanetary (and poten- tially interstellar) dust particles including, ultimately, nondestructive methods of collectionto allow their detailed chemical and isotopic analy- sis. RECOMMENDATIONS REQUIRING GROUND-BASED STUDIES Chemical Evolution and the Origin of Life Scientific developments over the past decade that bear on the processes leading to the origin of life have resulted in an expansion in emphasis from prebiotic chemistry into biochemical evolution as well. One consequence of this expansion is that work of high interest to the exobiology community, and supported by NASA, has increasingly come to overlap studies sup- ported by other federal agencies such as NIH and NSF. NASA's continuing support is critical, however, because only it provides the programmatic integration that promotes the necessary cross-fertilization of the various disciplines relevant to exobiology. As in the past, NASA programs in this field should strive to avoid duplicating the efforts of other agencies and should complement the work of these agencies by focusing on issues that directly concern interactions between the physical and chemical environ- ments that led to the development and evolution of organisms on this planet. Accordingly, the committee recommends: · the reexamination of biological monomer synthesis under primi- tive Earthlike environments, as revealed in current models of the early Earth, and the synthesis and study of simple model systems for funda- mental biological processes such as polynucleotide replication, seques- tration of biomolecules, coenzyme functions, and elements of the trans- lation system in protein syntheses (this recommendation is based on con- siderations discussed in Chapter 4 [pp. 80-903~; · the development of improved data on the biological and physical development of the Earth by modeling the geochemistry of the prebiotic and earliest biotic oceans to obtain their composition and their physical and chemical responses to large impacts, and by careful sedimentologi- cal, geochemical, and paleontological analysis of ancient sedimentary basins; local environments favorable for the origin of life should be identified and characterized geophysically and geochemically: geologi- cal research should be aimed not only at the elucidation of environ- mental evolution, but also at understanding the cosmic influences on terrestrial environments and evolution (see Chapters 3 [pp. 63-71] and 5 [pp. 93-102] for discussion of these points);
MAJOR RESEARCH RECOMMENDATIONS 127 · studies designed to recognize extraterrestrial signatures in sedi- mentary successions and research to evaluate temporal patterns in the composition of the biota (as recorded in the fossil record) in light of recognizable extraterrestrial signals (see Chapter 5 [pp. 100-1021~; · the continued search on Earth for igneous and sedimentary rocks formed prior to 3.8 billion years ago (the background for this is dis- cussed in Chapter 3 Epp. 65-691~; and · the development of robust phylogenies relating living organisms, through the comparison of sequences in informational macromolecules, especially small subunit ribosomal RNAs, and the elucidation of the biochemical and ultrastructural characters of microorganisms in order to relate patterns of phenotypic diversity to phylogeny (these points are discussed in Chapter 5 [pp. 93-983~. Mars-Related Studies Ground-based studies, discussed in Chapter 3 (pp. 71-77), are necessary to understand present environmental conditions on Mars, as well as the history of the evolution of this environment in order to plan effective ex- ploratory investigations related to exobiology. The committee therefore rec- ommends that . laboratory and theoretical model studies be carried out of photo- chemical and weathering processes on Mars that will determine the nature of inorganic carbon, nitrogen, sulfur, and iron-bearing phases in Martian surface soils; will indicate the geochemical cycles of these ele- ments during an earlier aqueous epoch; and will characterize the na- ture of the oxidants revealed by the Viking experiments; and · scenarios be developed for chemical evolution and the origin of life on Mars, based on our knowledge of these processes on Earth, but bounded by existing data and theory on the accretionary, tectonic, geo- logic, and climatic history of Mars. Studies Related to Comets and Asteroids These bodies of the solar system are of interest to the field of exobiology from many points of view: as projectiles impacting the planets, as possible sources for the biogenic elements and volatiles on the terrestrial planets, and as reservoirs of information about the early history of the solar system. In relation to these issues, the committee recommends · maintenance of a vigorous program of research on the chemical, isotopic, mineralogical, and petrographic properties of meteorites, and laboratory studies of the molecular and isotopic compositions and yields
28 THE SEARCH FOR LIFE'S ORIGINS of organic molecules produced in realistic simulations of those astro- physical environments within which presolar constituents of carbona- ceous meteorites may have been produced (see Chapter 2 [pp. 48-5111; and · theoretical studies on the physics of comet formation to determine the maximum size of comets accreted in the solar nebula, as well as thermocalculations of the composition of atmospheres produced by large impacts of cometary and various asteroidal-type bodies (see Chapters 2 [pp. 51-53] and 3 [pp. 61-621 for a discussion of this topic). Studies Related to Titan and the Giant Outer Planets Theoretical modeling and laboratory studies are required to elucidate the organic chemistry in the atmospheres of Titan and the giant planets, as well as to effectively interpret relevant data obtained from missions to these objects, which are discussed in Chapter 3 tpp. 58-611. The committee therefore recommends that . simulations be carried out of organic synthesis resulting from the deposition of electrons, photons, and cosmic rays into Titan's atmo- sphere and that similar experiments, as well as computer simulations, be conducted that will yield predictions of the molecular compositions and abundances of organic matter produced by processes operating at various levels in the atmospheres of Jupiter and Saturn. Studies Related to the Interstellar Medium and Dust Data from laboratory investigations and from theoretical modeling are necessary to prepare for, understand, and extend the results obtained from space-borne experiments aimed at studying the interstellar medium and dust particles of interstellar and interplanetary origin. For these purposes, the background of which is given in Chapter 2 (pp. 25~5 and pp. 53-54), the committee recommends · study of the spectra of, and chemical processes involving, potential gas and grain constituents of molecular clouds that are the sites of star and planetary formation, as well as the study of gas and grain reactions under conditions consistent with realistic models of the solar nebula, including a variety of nonequilibrium processes, and of the growth and destruction of grain aggregates; · utilization of ground-based telescopic facilities to probe the chem- istry and physics of star-forming regions in detail, and development of the instrumentation necessary to maximize the scientific return from
MAJOR RESEARCH RECOMMENDATIONS 129 space-based, laboratory, and telescopic measurements, including broad- bandwidth, high-resolution spectrometers and microanalytical tech- n~ques; · maintaining a vigorous program of research on the chemical and isotopic properties of dust particles of extraterrestrial origins; and · theoretical modeling of chemical and physical processes, including grain growth, in the solar nebula and in interstellar, circumstellar, and protostellar environments. Studies Related to the Search for Life Outside the Solar System Two parallel avenues of research should be pursued in attempts to detect life beyond the solar system: searches for evidence of biological modifica- tion of an extrasolar planet and searches for evidence of extraterrestrial technology. These separate approaches can conceivably influence each other. For example, if a nearby solar-type star is found to have a planetary system, it would become a prime target for a SETI-type search; similarly, if an "SETI signal" were detected from the direction of some nearby star, inten- sive efforts would undoubtedly be made to image and study the host planet. Because both lines of investigation can proceed simultaneously, the overall priorities listed below are those suggested naturally by the existing maturity of the requisite instrumentation. For these studies, the committee recom- mends · continued support for ground-based and Earth-orbital searches for extrasolar planets (for discussion, see Chapter 6 [pp. 106-108, 109-112, and 1 13-1 171~; · commencement of a systematic ground-based search through the low end of the microwave window for evidence of signals from an extra- terrestrial technology (see Chapter 6 [pp. 111-112 and 117-1223~; and · studies leading to the development of future technologies for these investigations, including large-scale optical, infrared, and submillime- ter arrays or monoliths in orbit or on lunar farside for imaging extraso- lar planets and protoplanetary nebulae; a dedicated SETI facility with RFI protection in high Earth orbit or lunar farside; advanced data- processing techniques; and substantive original or unconventional ap- proaches to the detection of other technological civilizations (this topic is discussed in Chapter 6 [pp. 117-12211.