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OCR for page 123
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
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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
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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,
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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 collection—to 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);
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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
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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
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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.
Representative terms from entire chapter:
studies related
