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The Cosmic History of the
Biogenic Elements and Compounds
INTRODUCTION
From our terrestrial perspective it is difficult to conceive of life forms in
which the elements hydrogen, carbon, oxygen, nitrogen, sulfur, and phos-
phorus do not play a predominant role. That they do indeed play such a
role throughout the universe seems highly probable, in part because (apart
from phosphorus) these are the most abundant elements throughout the cos-
mos and they occur in significant quantities among the building blocks of
terrestrial planets as represented by the primitive chondrites and comets.
Moreover, their chemistry is particularly well suited to the development of
the complex structures and functions characteristic of living systems. Since
the Sun and planets formed only some 4.6 billion years ago in a universe
whose age is perhaps 15 billion years, it is clear that these "biogenic ele-
ments" experienced a long and complex chemical history before being in-
corporated into terrestrial biochemistry. At present it is not known whether
this prior history played a direct role in the origin of life on Earth. What is
clear is that astrochemistry is to a large extent the chemistry of the biogenic
elements and that understanding the nature and evolution of chemical com-
plexity throughout the universe is crucial to understanding both the early
chemical state of our own solar system and the frequency with which simi-
lar or related conditions exist elsewhere in our galaxy and other galaxies.
There is, in addition, increasingly suggestive evidence for the survival of
interstellar molecular material within objects present in the solar system
today. Such evidence comes from studies of the isotopic compositions of
the carbonaceous components of certain meteorites and from the inferred
chemical composition of cometary nuclei, the latter supported by models
derived from recent spacecraft encounters. Moreover, some current models
21
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22
THE SEARCH FOR LIFE'S ORIGINS
of the solar nebula suggest that the bulk of the Earth's volatiles would not
have condensed at 1 AU from the Sun, implying that they were provided by
a bombardment of the Earth by volatile-rich cometary and meteoroidal de-
bris, which may well have contained interstellar components.
At the least, these ideas imply links between the chemistry of primitive
objects in the solar system and the interstellar environment in which the
Sun and planets formed and that such links involve the chemistry of the
elements necessary for the origin of life on Earth. Certainly, a knowledge
of the chemistry and physics of both interstellar clouds and the solar nebula
will provide crucial information on how and from what materials the solar
system was formed.
The cosmic history of the biogenic elements and their compounds thus
becomes a critical field of study for exobiologists. Apart from hydrogen,
which is for all essential purposes primordial, these elements are formed in
the interiors of stars and returned to the interstellar medium either in the
violent events accompanying the late stages of evolution of a massive star
(supernova explosions) or in the even larger amounts of processed material
expelled continuously or episodically from stars in late stages of their life
cycles. The subsequent of chemical complexity is a complicated and still
poorly understood story, involving condensation of particulate material
("dust") in the outflowing envelopes around evolved stars, gas-phase reac-
tions that build complex organic molecules in dense interstellar clouds of
gas and dust, and interaction of the particulate and gaseous phases with the
interstellar radiation field and cosmic rays.
In circumstellar and interstellar regions, astronomers have unequivocally
identified gaseous organic molecules with up to 13 atoms and molecular
weights twice that of glycine, the simplest amino acid. Although the pres-
ence of an interstellar "dust" component has been known for more than 50
years, its composition, structure, and special variations are still subjects of
heated controversy. Evidence is accumulating, however, that the size of
these dust particles may well overlap that of large molecules, and their
composition, in terms of biogenic compounds, quite likely ranges from water
and amorphous carbon or graphite to complex, heterocyclic organic poly-
mers.
It is within the denser interstellar clouds that new stars and planetary
systems form. The details of this process are not well understood. None-
theless, it is generally accepted that the physical and chemical properties of
the biogenic compounds play a crucial role in the thermodynamics of star
formation, because radiative energy loss from these molecules allows the
cloud to cool and hence to collapse. Moreover, the trace biogenic constitu-
ents provide critical probes of the physical, chemical, and kinematic states
of both interstellar clouds and protostellar systems, by way of their rotational
and vibrational transitions observable at radio and infrared wavelengths.
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS AD COMPOUNDS 23
How much of this interstellar chemistry is preserved as the parent mo-
lecular cloud collapses to yield the protosun, as the accretion disk develops
into the solar nebula, and as the building blocks of the planets accrete, is
uncertain. The investigation of possible links between the chemistries of
these stages in solar-system formation, and the determination of physical
and chemical conditions during this process by studying both primitive
objects and molecular clouds, are fascinating and crucial areas to be ex-
plored in coming years. It is clear that the solar nebula was not in chemical
equilibrium. Can local kinetic processes mimic those that occurred under
interstellar conditions? What new organic compounds might have formed
in the solar nebula or on the primitive bodies of the solar system? Can the
processes operating on primitive bodies give insight into chemical evolu-
tion on Earth? Detailed analysis of the chemistry and structure of com-
pounds and phases containing the biogenic elements in surviving primitive
material, including comets, can probe such questions.
The single overriding goal of this phase of evolutionary history is stated
below. Five major objectives contributing to the achievement of this goal
follow.
GOAL: To understand the history of physical and chemical transforma-
tions undergone by the biogenic elements and compounds, from nucleosyn-
thesis to their incorporation and subsequent modification in preplanetary
bodies.
OBJECTIVE 1: To determine the extent and the evolution of molecular
complexity in interstellar and circumstellar environments.
Almost 20 years have passed since the first gaseous polyatomic mole-
cules ammonia (NH3) and water (H2O) were discovered in the interstel-
lar medium via the technique of radio astronomy. Since that time, more
than 80 different molecular species and numerous isotopic modifications
have been identified unambiguously in the gas phase. Most of these species
have been detected through their rotational transition frequencies at radio
wavelengths (the term "radio" is often used to apply to wavelengths of 1
mm or less), although a few molecules, especially in circumstellar sources,
have been characterized by their vibrational spectra in the infrared. The
detected molecules range in complexity from diatomics such as hydrogen
(H2) and carbon monoxide (CO) to a 13-atom unsaturated linear nitrite
HEWN and include many simple organic molecules (Table 2.1~. Typically,
molecules involving the biogenic elements carbon, nitrogen, and oxygen are
trace constituents of a gas dominated by molecular hydrogen. Nevertheless,
the large mass of "dense" interstellar clouds implies that there is substan-
tially more organic matter in a typical cloud than on the Earth. In addition
to the existence of organic molecules, dense interstellar clouds have other
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24
TABLE 2.1 Identified Interstellar Molecules
THE SEARCH FOR LIFE'S ORIGINS
Simple Hydrides, Oxides, Sulfides, Halides, and Related Molecules
H2 CO NH3 CS
HC1 SiO SiH4X SiS
H2O so2 CC H2S
OCS CH4X PN
HNO (?) SiCX
Naclx
Alclx
KClx
AlFX
Nitrites, Acetylene Derivatives, and Related Molecules
HCN HC_C CN H3C C_C CN H3C CH2 CN H2C=CH2X
H3CCN H(C_C)2 CN H3C C-CH H2C=CH CN HC_CHX
CCO(?) H(C-C)3 CN H3C- (C_C)2 H HNC
CCCO H(C_C)4 CN H3C (C_C)2—CN? HN=C=0
CCCS H(C_C)s CN HN=C=S
HC_CCHO
H3CNC
Aldehydes, Alcohols, Ethers, Ketones, Amides, and Related Molecules
H2C=0 H3COH HO CH=0
H2C=S H3C—CH2 OH H3C O CH=0
H3C CH=0 H3CSH H3C O CH3
NH2 CH=0 (CH312CO (?) H2C=C=0
Cyclic Molecules
C3H2 SiC2 C3H
ions
CH+ HCO+ H3O+ (?)
H2D+ (?) HOCO+ HCNH+
HN2+ HCS+ SO+
HOC+ (?)
Radicals
CH C3H CN HCO C2S
OH C4H C3N NO SO
C2H CsH H2CCN NS
C6H
H2CNH
H3CNH2
H2NCN
NOTE: The superscript x indicates detection only in the envelopes around evolved
stars. A question mark (?) indicates molecules claimed but not yet confirmed.
features in common, such as a preponderance of gas-phase matter (with
perhaps 1 percent of the material in the form of solid dust grains), tempera-
tures well below those on Earth (10 to 100 K), gas densities quite low by
terrestrial standards (103 to 106 molecules/cm3), and chemically "reducing"
environments in which H2 is the dominant molecular species but in which
oxygenated molecules also exist. The most massive objects, "giant" mo-
lecular clouds, are larger, hotter (100 K versus 10 K), and show more
evidence of past and present massive star formations than the much smaller
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS AND COMPOUNDS 25
"dark clouds." Circumstellar sources appear to exhibit a significant organic
chemistry only if, unlike most objects, they are carbon- rather than oxygen-
rich. In such stellar envelopes, the gas density and temperature are severe
functions of the distance from the center of the star.
Although the spectra of many gaseous molecules have been detected in
interstellar and circumstellar sources, most astronomers have been less inter-
ested in the chemical composition of these sources than in utilizing the
spectra of abundant species such as CO and NH3 to probe the prevailing
physical conditions. High priority in the next decade should therefore be
accorded to a systematic study of the chemical composition of interstellar
and circumstellar clouds. Of particular interest in the context of exobiology
are studies of the degree of molecular complexity that can be attained and
of the diversity of chemical compositions that are produced as a result of
evolutionary effects and different physical conditions.
SYSTEMATIC STUDIES OF INTERSTELLAR CLOUDS
Although much has been learned about individual interstellar and cir-
cumstellar sources, a systematic study of the gas-phase chemistry of any of
these sources has not yet been achieved, even though portions of such
studies are available. A systematic study would entail determination of the
following: the chemical state of the major elements, including the biogenic
ones; isotopic abundances and isotopic fractionation effects; the way in
which abundances of major constituents vary as functions of position and
physical conditions within the cloud and possible cloud history; and the
extent of molecular complexity (see below).
Consider oxygen as an example of how little is known about the domi-
nant repositories of the major elements. It is argued indirectly that oxygen
(O2) and water (H2O) are probably the most abundant oxygen-containing
species after CO in the gas phase of interstellar and circumstellar clouds,
and yet it is difficult to study these species from the ground or even from
aircraft because of atmospheric absorption. A strategy for determining the
abundances of these important gas-phase species via their millimeter and
submillimeter transitions requires space-based instrumentation, perhaps ini-
tially of the Explorer class, but ultimately employing the higher angular
resolution of the proposed Large Deployable Reflector (LDR) and the Space
Infrared Telescope Facility (SIRTF) spacecraft (Space Science in the Twenty-
First Century, SSB, 1988a,c). Consider, as a second example, the case of
carbon. A significant fraction of carbon abundance in the gas phase is in
the form of CO. However, it is unclear how much is in the form of carbon
dioxide (CO2) or simple hydrocarbons such as methane (CH4) and acetylene
(C2H2), because these nonpolar species do not possess strong rotational
spectra. To determine their importance, infrared techniques will have to be
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26
THE SEARCH FOR LIFE'S ORIGINS
utilized to observe vibrational transitions. It is clear from atmospheric ab-
sorption at these wavelengths and from the currently limited sensitivity of
ground-based infrared telescopes that significant progress will be made by
high-spectral-resolution detectors in space, including those employing hetero-
dyne techniques.
Another subject of considerable interest is that of isotopic fractionation,
which may provide the most accurate "fingerprints" of interstellar processes
that are preserved in comets and primitive asteroids. Low-temperature inter-
stellar clouds lead to strong fractionation effects, especially with regard to
deuterium/hydrogen (HD/H2) abundance ratios in trace species. For ex-
ample, although the interstellar abundance ratio HD/H2 is approximately
1-2 x 10-5, the abundance ratio between other deuterated species and their
hydrogen analog can be higher than 0.01 in cold clouds. This effect is un-
derstood theoretically and occurs because the reactions between molecular
ions and neutral molecules that dominate the chemistry at low temperature
(ion-molecule reactions) can only proceed rapidly in exothermic directions.
More systematic observations of selected fractionation ratios as functions of
cloud temperature and density are required to refine current theories fur-
ther. Once these theories have become more quantitative, theoretical treat-
ments of how these isotopic ratios can be preserved as the interstellar cloud
becomes a protosolar nebula will be most useful.
Some studies of the variations in abundance of selected species as func-
tions of position and physical conditions within clouds are under way. For
example, radio astronomers have begun to probe selected regions in the
Orion nebula, a prototype giant molecular cloud. A variety of chemically
unusual regions, associated to a greater or lesser extent with star formation,
have already been delineated. It would be of interest to devote similar
attention to lower-mass clouds such as TMC-1 and L183, because such
smaller and colder regions may lead upon collapse to solar-type stars.
Complementary to the studies discussed above are broad surveys of the
radio line spectra of interstellar sources: knowledge of the radio frequency
spectra of most molecular clouds is extremely patchy. Systematic maps of
the spectra have thus far been partially accomplished for only two giant
interstellar clouds, that in Orion and one near the galactic center. These
surveys, which detected on the order of 1000 emission lines, have resulted
in a significant increase in the amount of chemical information available
(see Figure 2.1~. A similar survey of the dark cloud TMC-1 would be most
worthwhile because it is a precursor of solar-type stars. Because clouds
such as TMC-1 are so cool (10 K) and have little turbulence, spectral line
widths are very narrow, making a survey much more difficult than in the
giant clouds. To survey TMC-1 over a wide range of frequencies, a broad-
band high-resolution spectroscopic capability is required. Some of the in-
strumentation being developed in the SETI program may be useful here
OCR for page 27
THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS ED COMPOUNDS 27
1 1 1 1 1 1 1 1 1 1 1
Orion Molecular Cloud
2.0
1.0
-
~:
o.o
0.6
0.4
0.2
0.0
SOo
_ CH3OH ~ CH3OH
_~` _c ~ ~
1
IRC + 1 0216
C4H C3H
w~,/~
1 1 1 1 1 1 1 1 1 1 1
76000 76100 76200 76300
Frequency (MHz)
76400 76500
FIGURE 2.1 Portions of the millimeter wavelength spectra of a dense interstellar
cloud (Orion) and the envelope around an evolved star (IRC+10216~.
(Astronomy and Astrophysics for the 1980s, National Research Council,
1982, 1983a,b).
In the long run it is essential to broaden such studies to include a large
sample of clouds. Will other abundance patterns be found? Will evolution-
ary effects on chemical composition emerge? Then, eventually, can the
analogous chemistry be probed in external galaxies?
COMPLEX MOLECULES
Biochemistry is clearly the chemistry of large, complex, organic mole-
cules. The largest molecule unambiguously observed in the gas phase of
interstellar and circumstellar clouds is PECAN. Although infrared spectra
provide evidence for far larger species (such as polycyclic aromatic hydro-
carbons; see discussion following objective 2), specific molecules have not
been identified from the existing low-resolution spectra. To extend gas-
phase high-resolution radio astronomical methods to search for molecules
considerably larger than 13 atoms will require continually improving elec-
tronics and a strategy involving laboratory studies. The laboratory work is
necessary because many species more complex than 13 atoms have not been
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28
TlIE SEARCH FOR LIFE'S ORIGINS
studied spectroscopically in the gas phase, especially in the radio and milli-
meter wave regions where their characteristic rotational transitions lie. The
larger a molecule, the higher is its density of rotational levels, so that the
intensity available in a single transition diminishes. Thus, to observe single
rotational transitions of complex molecules in interstellar sources will re-
quire more sensitive instrumentation and large amounts of searching time.
In addition, as molecules grow in complexity, their most intense spectral
lines shift toward lower frequencies. Current plans to enhance the capabili-
ties of the large (300 m), low-frequency radio telescope at Arecibo to in-
clude the 1- to 8-GHz frequency range would seem to be a boon for com-
plex molecule studies.
Another important component of a strategy for determining the extent of
molecular complexity in interstellar and circumstellar sources involves chemi-
cal modeling. Such modeling can tell astronomers what likely molecules
may be found in a given environment and what intensities can be expected.
From successful models involving smaller molecules, it is safe to say that
much of the chemistry of the cold interstellar gas is accounted for by schemes
based on gas-phase ion-molecule reactions which, because they typically
possess no activation energy, can occur rapidly even at low temperature.
Although models of interstellar clouds involving small gas-phase mole-
cules are in good agreement among themselves and with observation, they
differ significantly in their predictions of complex molecule abundances.
These differences derive at least in part from lack of laboratory data on
important ion-molecule reactions. Thus, an additional component of the
strategy emerges the need for laboratory work on important ion-molecule
reactions to aid modelers in calculating the expected abundances of com-
plex molecules. Nor is this the final element of such a strategy: chemical
models cannot be based entirely on laboratory studies of relevant reaction
rates. Reaction systems with rate coefficients that are highly temperature
dependent, which have only been studied in the laboratory at approximately
room temperature, may occur at unsuspected rates under interstellar condi-
tions. In addition, some classes of reactions are not easily studied in the
laboratory. An example is the low-pressure process called radiative asso-
ciation, which is thought to be critical in gas-phase syntheses of complex
molecules. To examine this and other processes requires theoretical studies
of rate coefficients. Such studies are then another integral part of a strategy
aimed at determining the limits of molecular complexity in interstellar and
circumstellar sources.
Although the tenor of the discussion on interstellar chemistry has been
concentrated on gas-phase processes, the influence of dust particles cannot
be ignored. These particles are sites of molecular adsorption, Resorption,
and possible reactions, and they can protect complex molecules from stellar
ultraviolet radiation. Further discussion of particulate matter is given after
objective 2 (see below).
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS AND COMPOUNDS 29
In general, modeling of circumstellar sources has lagged somewhat be-
hind that of interstellar ones. However, within the last few years, several
circumstellar models have become available. The picture of carbon-rich
circumstellar sources such as IRC+10216 that emerges is one in which
chemical equilibrium at high temperature is achieved as material is ejected
from the star, only to be reprocessed by an active photochemistry and ion-
molecule reactions as the material proceeds further from the stellar photo-
sphere. Significant amounts of complex molecules may be produced by
these processes.
STAR-FORMING REGIONS AND
THE SUBMILLIMETER SPECTRAL RANGE
How is the chemistry of an interstellar cloud affected by the process of
star formation? Virtually nothing is known in this regard for isolated solar-
mass stars. For more massive stars, however, some evidence has been
obtained from study of the Orion nebula. Astronomers have thus far de-
tected at least three unusual regions in which the abundances of gas-phase
molecules are quite different from more normal values. Suggested causes
include chemical reactions driven by shock waves, molecules desorbed from
the interstellar grains by temperatures exceeding 100 K, and interactions
between species so produced and the "normal" constituents of the ambient
cloud. As more information becomes available concerning the unique chem-
istry of star-forming regions, it should be possible to develop models of
their chemistries with some predictive power. Indeed, primitive models of
the star-forming regions in Orion are currently being formulated.
Detailed observational studies of small regions warmer than the ambient
interstellar medium will require very high angular resolution, which must
be obtained by interferometric techniques. Expansion of existing facilities
and eventual construction of instruments such as the Millimeter Array and
the Submillimeter Array Telescope being discussed by the National Radio
Astronomy Observatory (NRAO) and the Smithsonian Astrophysical Obser-
vatory (SAO), respectively, will be required. In addition, frequencies higher
than those normally used, particularly in the submillimeter region, are im-
portant. Because, as the temperature rises, the dominant rotational line
emission of most smaller molecules shifts into this wavelength region.
Moreover, some light molecules such as simple hydrides can be observed
only in the submillimeter spectral region; some of these species are critical
to a quantitative understanding of chemical processes in interstellar clouds
(e.g., the ion H3+ and metal hydrides such as MgH). Unfortunately, severe
problems are associated with submillimeter observations. Ground-based
observation is extremely difficult because of atmospheric water. Although
a first generation of ground-based submillimeter telescopes is currently being
constructed in high, dry locations, the advantages to observing this spectral
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30
THE SEARCH FOR LIFE'S ORIGINS
region from space are enormous (e.g., with LDR and SIRTF). An equally
important problem, however, is the small laboratory data base on which
submillimeter astronomy can draw. Very few gas-phase molecules have
been examined in the submillimeter region; many more studies are needed.
Thus, a necessary component of a strategy aimed at using this spectral
range to study star-forming regions involves laboratory spectroscopy.
To achieve the recommendations listed below, it will be necessary for
exobiologists to interact closely with the astronomical and planetary science
communities. The committee supports the major recommendations of the
Astronomy Survey Committee (Astronomy and Astrophysics for the 1980s,
Volume 1, National Research Council, 1982) to construct an LDR in space
to carry out spectroscopic and imaging observations in the far-infrared and
submillimeter wavelength regions of the spectrum that are inaccessible to
study from the ground. Such an instrument, in the 10-m class, will offer
unprecedented opportunities for studying the molecular and atomic pro-
cesses that accompany the formation of stars and planetary systems. The
committee also concurs with the recommendation from A Strategy for Space
Astronomy and Astrophysics for the 1980s (SSB, 1979) that development of
a meter-class, cryogenically cooled, infrared telescope be actively contin-
ued, with the option of its construction as a free-flying spacecraft being
retained until the Shuttle environment has been demonstrated to be suffi-
ciently free of contaminants (SIRTF).
OBJECTIVE 2: To determine the composition, structure, and interrela-
tionships among circumstellar, interstellar, and interplanetary dust.
Interstellar grains constitute an important component of the interstellar
medium. They play a crucial role in the heating and cooling of interstellar
clouds through the absorption of visible and ultraviolet photons and the
ejection of energetic photoelectrons. They also influence the gas-phase
composition of molecular clouds directly by providing surfaces for reac-
tions and indirectly by locking up some elements, as well as by shielding
molecules from the dissociative ultraviolet interstellar radiation field. Ob-
servations have shown that these dust grains have a size distribution rang-
ing from approximately 3000 ~ down to perhaps molecular sizes and that
they lock up a large fraction (290 percent) of some heavier elements such
as silicon, iron, calcium, and aluminum, as well as a substantial fraction of
the available carbon, nitrogen, and oxygen.
The life history of interstellar grains is a complex interplay of many
different competing processes, including nucleation and condensation around
stars and accretion, chemical modification, and shock processing in the
interstellar medium. Some interstellar grains ("star dust") originally con-
densed in the high-density, high-temperature environment (n ~ 108 cm~3; To
1000 K) of the circumstellar envelopes of red giants, planetary nebulae, and
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS AND COMPOUNDS 31
novae and have subsequently been expelled into the interstellar medium
along with gaseous species. Other possible dust components may originate
in the interstellar medium itself by accretion, reaction, and photolysis of
gaseous species on preexisting grain cores. Laboratory experiments suggest
that a C60 spherical molecular species "fullerene" may also be a component
of interstellar dust. Table 2.2 contains a summary of current knowledge of
the composition of interstellar dust. There is at least some evidence for all
of the dust components shown in circumstellar or interstellar environments,
mainly through low-resolution infrared spectroscopy (see below).
Silicate grains are a ubiquitous component of the dust in the diffuse (n <
102 cm~3) interstellar medium, and this star dust component may actually
make up about half the interstellar dust volume (cf. Table 2.2~. On the
basis of elemental abundances and stability, the remainder of the dust vol-
ume has to consist of species containing predominantly carbon. However,
it is still an open question whether this carbon is in the form of graphite,
which would likely be a star dust component, or in the form of refractory
organic grain mantles, which might form via processes in the interstellar
medium. Although small graphite particles (~200 A' may be the carriers of
the ubiquitous 2200-A bump in the ultraviolet spectra seen toward stars,
large graphite grains (1000 ~) do not possess any currently detectable infra-
red or ultraviolet absorption features; thus, we can only guess at their con-
tribution to the interstellar dust volume.
Icy grain mantles, consisting of simple molecules such as H2O, CO, and
perhaps NH3 and CH3OH (methanol), are an important component of inter-
stellar dust inside dense molecular clouds, but they have never been ob-
served in the diffuse interstellar medium. Traces of more complex organic
molecules (e.g., aldehydes, ketones, and nitrites) have also been reported in
some objects. Icy grain mantles are presumably formed by the accretion of
gas-phase species onto preexisting cores inside molecular clouds. In the
less dense interstellar medium, these volatile materials would be efficiently
destroyed by photodesorption and subsequent photodestruction in the inter-
stellar ultraviolet radiation field and by shock waves. Inside dense molecu-
lar clouds the much lower ultraviolet flux from embedded newly formed
stars or from cosmic-ray excitation of molecular hydrogen may be suffi-
ciently high to transform the simple icy molecules into more complex mole-
cules, which are more refractory. This process may also be the source of
the more refractory grain mantles possibly observed in the diffuse interstel-
lar medium.
GRAIN INTERRELATIONSHIPS
The evolution of biogenic elements in the interstellar medium prior to
the formation of the solar system has gained additional interest with the
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS AND COMPOUNDS 45
organic compounds would have been destroyed and carbonaceous grains
would have been converted to graphite. Indeed, these expectations are
largely borne out: metamorphosed ordinary chondrites lack organic com-
pounds but do contain graphite. Perhaps the most important outcome of
internal metamorphisms in the parent bodies would have been the expulsion
and delivery of volatiles through overlying layers to near-surface regions by
diffusion or volcanic activity. Such outgassing or transport of fluids could
have been accompanied by mineral-catalyzed synthesis of organic com-
pounds.
Future Investigations
Little is known about how the physical structure and chemical composi-
tion of individual grains influence their growth under putative nebular con-
ditions. To fill this knowledge gap, several types of investigations should
be carried out. Calculations should be conducted to determine how the
rates of formation or destruction of grain aggregates vary with particle
hardness, porosity, and composition for metallic, silicate, organic, and icy
grains. Theoretical studies should be complemented by laboratory experi-
ments, some of which might be appropriate to carry out under microgravity
conditions on the Space Station. From simulations of grain collisions under
nebular conditions it should be possible to determine the relative "sticking
efficiencies" of materials composed of the biogenic elements as compared
with those of the rock-forming elements. The structures of aggregates pro-
duced in these investigations will provide useful models against which to
compare grain aggregates obtained from meteorites, IDPs, and comets. For
the experimental studies, facilities capable of accelerating small particles to
a range of pertinent velocities would be very valuable.
Experiments should be conducted in which organic compounds and grains
within inorganic matrices are subjected to laboratory simulations of phe-
nomena presumed to have occurred on planetoids. For the biogenic com-
pounds and phases used as starting materials in these experiments, modifi-
cations of physical, chemical, and isotopic properties as a function of envi-
ronmental conditions must be determined.
Deeper understanding of the conditions of aqueous alteration, the identi-
ties of the precursor phases, and the nature of the resulting hydrous phases
should be sought in petrographic and mineral-chemical studies of carbona-
ceous chondrites and IDPs. The fact that prebiotic compounds such as
carboxylic acids and amino acids appear to occur only in these altered
objects is particularly noteworthy, and elucidating the relationship between
the origins of these inorganic and organic components is a research problem
of high priority.
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46
THE SEARCH FOR f IFE' S ORIGINS
OBJECTIVE 5: To determine the distribution, structure, and composi-
tion of presolar and nebular products in existing primitive materials in the
solar system.
Previous sections have considered, more or less chronologically, the
evolution of chemical complexity in interstellar clouds, in the solar nebula
that resulted from the collapse of such a cloud, and in solid objects that
were formed in this nebula. Some end products of this evolution continue
to exist today in asteroids, meteorites, comets, and IDPs and may be studied
to elucidate this overall process.
ASTEROIDS
The asteroids are a large collection of small bodies that orbit the Sun,
predominantly at distances of 2 to 3.5 AU in the "main belt" between Mars
and Jupiter, residing in a transition region between the rocky terrestrial
planets and the gas-rich outer planets. Dynamical calculations of asteroid
orbits suggest that most of the asteroids have remained near their present
relative positions in the solar system since their formation. Thus, one of the
most important reasons for studying the asteroids is that they might pre-
serve valuable information about the chemical and physical processes (e.g.,
condensation and accretion) operating in this transition region during the
formation and early evolution of the solar system.
Our present knowledge of asteroids is based primarily on determination
of their orbits and study of the temporal variability and spectral distribution
of the reflected and emitted radiation from unresolved starlike images. Spec-
troscopic observations show that the asteroids vary in their surface mineral-
ogical compositions and fall into broad classes that parallel, in a general
fashion, some of the meteorite classes. The primitive nature of the bulk of
asteroidal material is reflected by the predominance (by mass) of dark car-
bonaceous material (C-type asteroids) in the main belt.
Likewise, Ceres, which is the largest asteroid and contains approximately
one-third of the total mass in the main belt, has a density of 2.6 + 0.7 g/cm3.
This low density suggests that Ceres is far more volatile-rich than any of
the terrestrial planets. Similar densities are in fact observed for the CI and
CM2 types of carbonaceous chondrites; these meteorites are generally thought
to be some of the most primitive early solar-system materials for which we
have samples. The density of Ceres is also consistent with the predicted
density of nebular condensates forming in the region of 300 K.
The study of primitive material in meteorites has provided valuable in-
formation about the chemical composition of the solar system and the chemi-
cal and physical processes operating in the solar nebula and early solar
system. However, the enormous advantage in studying primitive asteroidal
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS AND COMPOUNDS 47
materials is that the observed properties can be identified with a specific
location in the solar system.
Future Investigations
Determination of the chemical composition of primitive asteroidal mate-
rial with sufficient accuracy to make meaningful comparisons with the chemi-
cal composition of meteoritic material is of prime importance. If such a
direct link can be made, then the large number of meteorite samples can be
used as probes of the main belt region and of specific locations in the solar
nebula. To this end the committee endorses the recommendation made by
COMPLEX (Committee on Planetary and Lunar Exploration, Strategy for
the Exploration of Primitive Solar-System Bodies Asteroids, Comets, and
Meteoroids: 1980-1990, SSB, 1980) that "the principal chemical elements
present in asteroids to more than 1 percent abundance by atom be measured
to an accuracy of about 0.5 atom percent. It is expected that these will
include the elements H. C, O. Na, Al, Si, S. Ca, Ti, Fe, and Nil" The
recommended measurement accuracies should be sufficient to permit infor-
mative comparisons with the known meteorite classes, to determine the
oxidation state of major elements and to assess the degree of hydration of
surface minerals.
Measurement of these elements should be made at one location on the
surface at least; however, it is very desirable to make measurements at
different locations to determine the scale and extent of surficial heterogene-
ity. Similarly it is also of interest to determine the scale and extent of
radial heterogeneity by making measurements at depth or around craters
where samples of the interior may have been exposed. Another important
endeavor is determination of the bulk content and the chemical form of the
major biogenic elements (H. C, N. O. P. and S). These may be present in a
variety of molecular components that would be diagnostic of the primitive
nature and degree of subsequent alteration of the asteroid. The distribution
of carbon among various carbon-bearing volatiles (CO, CO2, CH4), carbon-
ates, graphite, and organic polymers is of particular interest in these mea-
surements.
A third area of investigation, which may take a longer time for imple-
mentation, is the measurement of the D/H, i3C/~2C, ~sN/~4N, 180/~60, and 170/
i60 isotopic ratios on at least one sample of an asteroid. The carbon iso-
topic ratios are of interest because of the carbonaceous nature of many
asteroids and the observed variability of ~3C/~2C ratios in primitive meteorite
components, whereas the oxygen isotopic ratios are important for compari-
son with the ratios in various meteorite classes.
Different types of scientific instrumentation and different means of in-
vestigation and research will have to be involved in these investigations.
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48
THE SEARCH FOR LIFE'S ORIGINS
Two useful techniques are X-ray fluorescence and gamma-ray spectros-
copy. X rays are excited in surficial materials by solar radiation and pro-
vide information on the light elements (e.g., Mg, Al, and Si) in the topmost
few micrometers of a surface. Gamma rays are emitted by long-lived natu-
ral radionuclides such as potassium, thorium, and uranium and also by shorter-
lived nuclides formed by cosmic-ray and solar particle interactions with the
surface. Both X and gamma rays can provide qualitative and semiquantita-
tive analyses for a large number of elements.
Both nondestructive mapping techniques and destructive analytical tech-
niques may be required to measure the abundances and chemical forms of
the major biogenic elements. Spectral reflectance measurements in the
ultraviolet, visible, and near-infrared region can be used to determine the
mineralogy and composition of surficial materials and to map the spatial
extent of different classes of materials (e.g., carbonaceous matter, hydrated
phases). Thermal emission spectroscopy in the mid-range of the infrared
region has similar applications. Because these two techniques are sensitive
to different mineral phases present on the surface, they provide information
complementary to the elemental analysis techniques, which are not sensi-
tive to different phases.
Detailed characterization of the various molecular components in which
the biogenic elements might be present will be considerably more difficult.
Pyrolysis or combustion of carbonaceous material with analysis of the evolved
vapors by gas chromatography/mass spectrometry has been used for meteor-
ite samples and may also be used on a soft-lander. Morphological charac-
terization of carbonaceous phases can be made by scanning electron micros-
copy; this would be possible on a returned sample or in situ by using a
specially developed instrument for spaceflight.
Finally, the committee notes the suggestion that the Martian moons Pho-
bos and Deimos may be captured asteroids, so their characterization is di-
rectly relevant to this objective.
METEORITES
Meteorites are interplanetary objects that survive passage through the
terrestrial atmosphere as discrete bodies or associated fragments. Ranging
in size from a few grams to several tons, meteorites are grouped into two
broad categories, depending on whether they are undifferentiated or differ-
entiated. The undifferentiated meteorites, or chondrites, have generally not
been melted; consist of a mixture of small spheroidal objects (chondrules)
and finer-grained, heterogeneous material (matrix); and have close resem-
blance in elemental abundances to the Sun. In fact, the nonvolatile ele-
ments are generally present in solar proportions, whereas the volatile ele-
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS AND COMPOUNDS 49
meets are depleted to variable extents. The most primitive of the chondrites
are the carbonaceous chondrites.
On the other hand, the differentiated meteorites, which include the irons,
stony-irons, and chondrites, have been subjected to melting and fractiona-
tion events, do not consist of the simple chondrule-matrix duplex structure,
and do not closely resemble the chemistry of the undifferentiated meteorites
or the Sun. In many instances, the compositions of the differentiated mete-
orites (e.g., the chondrites) suggest chemical fractionations similar to those
produced by igneous activity on the Earth and Moon. However, unlike the
continuing igneous activity on the Earth, isotopic dating shows that most of
the igneous fractionations reflected in the differentiated meteorites occurred
4.5 billion years ago, shortly after the formation of the solar system. A1-
though the differentiated meteorites are important sources of information
about the thermal histories of small planetesimals in the early solar system,
they provide much less information than do the chondrites on presolar and
nebular phases in primitive materials.
The carbonaceous chondrites, which are generally thought to be among
the most primitive early solar-system materials for which samples exist, are
the best candidates for preserving presolar and nebular phases or their sig-
natures (e.g., "fossil" elemental abundance patterns or isotopic anomalies).
Indeed, rubidium-strontium (Rb-Sr) dating of the CAIs in the allende carbo-
naceous chondrite has identified some of these inclusions as the oldest
known solids in the solar system. The antiquity of the CAIs, and their
resemblance (at least to a first approximation) to the chemistry and mineral-
ogy of solid assemblages predicted as vapor-solid condensates at high tem-
peratures from a solar composition gas, have led to intensive study of CAIs
in the allende and other carbonaceous chondrites. However, to date no
pristine nebular phases (i.e., vapor-solid condensates) have been identified
unambiguously in any components of the chondritic meteorites.
A similar situation prevails in the search for presolar phases in primitive
meteorites. The canonical model for the formation of the solar nebula
envisioned a homogeneous, totally vaporized swirling cloud of gas that
became a mixture of gas and dust upon cooling. In this scenario, a well-
defined sequence of mineral phases, which became progressively more vola-
tile-rich, formed from this homogeneous cloud with decreasing temperature.
The end products of this sequence were postulated to be the oxidized iron-
and H2O-rich minerals observed in the carbonaceous chondrites.
However, the discovery of non-mass-dependent isotopic anomalies for
oxygen and subsequently for titanium in CAIs showed that this viewpoint
was fundamentally incorrect. A wide range of other non-mass-dependent
and mass-dependent isotopic anomalies in refractory elements (Mg, Si, Ca,
Cr. Ba, Nd, Sm) have since been observed in CAIs. Although the non-
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so
THE SEARCH FOR LIFE'S ORIGINS
mass-dependent isotopic anomalies have been interpreted in terms of mate-
rial from different nucleosynthetic sources, no presolar grains have been
unambiguously identified in the CAIs. Widespread isotopic anomalies are
also observed in the biogenic elements hydrogen, carbon, and nitrogen and
in the noble gases neon, krypton, and xenon.
The observed isotopic anomalies in the biogenic elements reinforce the
notion from the refractory element isotopic anomalies that presolar material
from a variety of environments was incorporated into chondritic meteorites
relatively unaltered and without being thoroughly homogenized in the solar
nebula. Large deuterium enrichments are observed in the insoluble organic
matter that forms the bulk (70 to 80 percent) of all carbon in the CI and
CM2 carbonaceous chondrites. These enrichments, which cannot plausibly
be explained by mass fractionation in the solar nebula, are believed to
indicate that these meteorites contain remnants of material from dark inter-
stellar clouds. Isotopically heavy carbon found in CM2 chondrites may
indicate the incorporation of carbon grains from red giant stars into these
meteorites. Isotopically light nitrogen in components of the Allende mete-
orite may indicate incorporation of almost pure ON into this meteorite. At
present, the complex picture described by the collective isotopic variations
is incompletely understood but strongly suggests the preservation of pre-
solar material from different nucleosynthetic sources and a variety of astro-
physical environments.
Future Investigations
Observational studies of meteorites can be expected to continue to yield
important results and to influence thinking on the chemical and physical
processes responsible for shaping our solar system. In its 1980 report
Strategy for the Exploration of Primitive Solar-System Bodies Asteroids,
Comets, and Meteoroids: 1980-1990 (SSB, 1980), COMPLEX recommended
that "a vigorous program of laboratory and theoretical investigations of
meteorites be maintained" and also stated that "to realize the full promise of
meteorite research it is necessary to maintain laboratory capabilities at the
highest level of evolving technology and to encourage the development of
even more sophisticated analytical methods." The committee endorses both
these statements. In addition, a range of complementary studies should be
pursued.
Some topics are exceedingly important. Laboratory studies are required
of the molecular and isotopic compositions and yields of organic molecules
produced by ion-molecule reactions, ultraviolet-pumped photochemical re-
actions, and high-temperature nucleation-condensation processes in a vari-
ety of astrophysical environments such as dark molecular clouds and cool
stellar outflows. It is of utmost importance to conduct simulation experi-
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS AND COMPOUNDS 51
meets under realistic conditions of pressure, temperature, composition, and
energy flux or to perform the experiments in such a fashion as to permit
meaningful extrapolations to these conditions.
Laboratory studies should also be made of the survivability of artificially
induced and natural isotopic anomalies in refractory carbonaceous materials
such as the insoluble organic polymer found in carbonaceous chondrites.
Of particular interest is the change in a deuterium-enriched sample during
heating in a solar composition gas for varying time periods. The resistance
of t3C-enriched graphitic grains to pyrolysis and isotopic exchange during
heating in Hz-CO gas mixtures with solar '3C/~2C ratios is also of interest.
Such studies should be designed to provide kinetic data that can be applied
to solar nebular models of the survivability of infalling interstellar grains.
Concerted observational studies of primitive meteorites should be made
to determine unambiguously the nature, amount, and distribution of deu-
terium-enriched carrier phases. The use of in situ techniques such as the
ion microprobe should be exploited fully in these efforts. Although the
selective chemical dissolution techniques used in studies of noble gas and
deuterium, TIC, or ON carrier phases have provided invaluable information,
these techniques are ultimately limited by their destructive nature, which
renders observation of the carrier phases in the host meteorite impossible.
COMETS AND INTERPLANETARY DUST PARTICLES
Comets
Comets occupy a special place in the cosmic history of the biogenic
elements and compounds: they hold promise of containing the most vola-
tile-rich relics of processes that occurred in stars, interstellar clouds, and
the protosolar nebula, while at the same time bearing evidence of their own
formation and evolution as building blocks of planetary materials. Not only
are they thought to contain grains and gas inherited from the interstellar
cloud that spawned the solar system, they are also expected to have ac-
creted both refractory and volatile material formed in cold regions of the
protosolar nebula. In addition, a role as carriers of volatile and biogenic
elements to the terrestrial planets where, at least on Earth, life arose and
evolved is attributed to them.
These expectations arise from theories that comets formed by cold accre-
tion of interstellar dust and gas or solar nebular condensates, or mixtures of
these materials, into small planetesimals whose size, composition, and or-
bital distance from the Sun precluded subsequent differentiation. In turn,
the theories are based on estimates of the relative abundances of the major
elements in comets as inferred from a long history of ground-based and
airborne observations of species in their comae and tails and from in situ
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52
TlIE SEARCH FOR LIFE'S ORIGINS
studies of Comet Halley. The relatively high ratio of volatile (e.g., water)
to involatile (e.g., silicates) substances observed in comets signifies that
they were accreted at great distances from the Sun and then never heated
for long to temperatures much above the sublimation point of water ice in
space. Although resembling primitive carbonaceous chondrites in exhibit-
ing approximately solar atomic ratios of the metallic elements, comets more
closely approximate the Sun and interstellar frost in the relative abundances
of the volatile elements. These abundances, coupled with the putative lack
of internal differentiation, place comets among the most primitive solid
objects in the solar system and the likeliest to have preserved intact the
gases and grains that accreted to form them. For these reasons, comets
assume the highest priority among solar-system objects for study of the
cosmic evolution of compounds and phases containing the biogenic ele-
ments.
Many major scientific questions can be addressed by the study of com-
ets. These questions should be kept in mind during present and future inves-
tigations. They include the following: possible relationships among bio-
genic compounds and phases in cometary, meteoritic, and interstellar mat-
ter; similarities between cometary and interstellar organic chemistry; and
the insertion into, and stability of, interstellar material in cometary nuclei.
Prior to the return of Comet Halley, the nucleus of a comet had never
been directly observed, and inferences about its composition relied on re-
constructions based on the abundances of radicals, ions, and atoms ob-
served in the coma and tail. Reconstructions of unobservable "parent"
molecules in the nucleus from observable "daughter" species are fraught
with uncertainties. Nonetheless, H2O and HCN had previously been identi-
fied as parent molecules.
Exciting new data pertinent to the gas phase have been obtained from
Comet Halley by the Giotto and Vega spacecraft, as well as from related
ground-based and airborne observations. Some of these new findings point
to CO, CO2, and perhaps H2CO as additional parent molecules. In particu-
lar, the gases released from the nucleus were composed of about 80 percent
water, 10 to 20 percent CO, a few percent CO2, and smaller amounts of
other gases. In addition, analyses of the coma gas phase by neutral and ion
mass spectrometers revealed a surprising abundance of peaks attributable to
hydrocarbons and other organic compounds. Although the identities of these
compounds are presently controversial, their occurrence strongly underscores
the complexity of the organic chemical content of comets. Other especially
noteworthy findings were the discovery of jets of cyanide associated with
the emission of dust from active regions of the nucleus and the observation
that the source of much of the CO was extended in the coma. This raises the
novel possibility that the dust may contribute species directly to the gas
phase.
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS AD COMPOUNDS 53
Perhaps the most significant and exciting new insights into comets arose
from direct observations of the nucleus of Comet Halley and its solid dust
component. Fine-grained dust composed of dark, apparently carbonaceous,
matter was found covering inactive regions of the comet surface and ejected
in plumes from active regions into the coma. Spacecraft analyses of dust
grains in the coma by impact mass spectrometry revealed a variety of com-
positional types. In addition to silicatelike particles and inorganic grains of
chondritic composition, several populations were found to be composed of
various combinations of the biogenic elements carbon, hydrogen, oxygen,
and nitrogen exclusively, as well as mixed with inorganic elements. The
size and composition of the particles are consistent with our knowledge of
interstellar dust, but no conclusions can yet be drawn about their origin.
Clearly these particles and their counterparts or analogues in meteorites and
IDPs provide fascinating new targets for study.
In principle, the isotopic composition of hydrogen, carbon, nitrogen, sul-
fur, and other elements in comets could provide clues to their origin. Iso-
topic measurements obtained at Comet Halley for carbon, nitrogen, and
sulfur, although still imprecise, appear to fall within the range of solar-
system materials. Similarly, the bulk ratio of deuterium to hydrogen is
compatible with that of terrestrial materials and bulk meteorites. A detailed
study of dust at Comet Halley to probe the possible existence of inclusions
with large D/H ratios was not possible.
Interplanetary Dust Particles
Interplanetary dust particles (IDPs) are extraterrestrial particles, typi-
cally less than 1 mm in diameter, that survive entry into the upper atmos-
phere and are currently collected by high-flying aircraft. Their contents of
solar wind noble gases and cosmic-ray tracks attest to their extraterrestrial
origin. Among the variety of particle types that have been identified, the
most common ones exhibit the solar pattern of relative abundances for ma-
jor and minor elements that typifies primitive, chemically unfractionated,
chondritic materials. Often called "cosmic dust," these IDPs constitute a
unique collection of samples that complement meteorites as "fossils" of the
earliest history of the solar system. Some may be of interstellar origin, but
the bulk are presumably cometary or asteroidal.
Most of the chondritic particles that have been examined are in the 5- to
50-,um size range. They are typically black, and semiquantitative analyses
show them to contain 2 to 5 weight percent, or higher, of carbon. Abun-
dances of hydrogen and nitrogen have not yet been measured. Two popula-
tions make up these particles: one contains only anhydrous phases; the
other is composed largely of hydrated minerals, among which the most
abundant are layer lattice silicates (clays).
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54
THE SEARCH FOR LIFE'S ORIGINS
The anhydrous particles are unique in several distinctive ways. They
contain much larger amounts of carbon than comparably anhydrous meteor-
ite samples. Their extreme porosity suggests previous filling by ice and
structural fragility, the latter being consistent with physical properties of
materials in cometary meteors. They are composed of extremely small
grains, ranging from micrometers down to tens of angstroms in size, as-
sembled in a highly porous, three-dimensional structure. Especially note-
worthy among the minerals found as grains are carbides, graphite, and sul-
fides, along with olivine and enstatite.
Carbonaceous material appears to be ubiquitous as amorphous coatings
and clumps and as a medium for the embedment of other inorganic grains.
The lack of any counterpart for materials with these characteristics in the
meteorite collections argues strongly for a different, probably cometary,
. .
Orlgln.
Recent measurements performed on individual IDPs with the ion micro-
probe revealed anomalously high D/H ratios associated with organic carbo-
naceous material. Similar findings were obtained on both anhydrous and
hydrous IDPs. In the case of carbonaceous chondrites, such high ratios have
been interpreted as indicating the presence of interstellar organic matter.
The commonality of this organic matter among several types of primitive
materials may reflect a common interstellar source.
Some of the IDPs composed of hydrous phases may also be related to
comets. Although the clay minerals in some cases closely resemble those of
carbonaceous chondrites, in other cases they are distinctly different. The
degree of compactness exhibited by these particles may reflect the influence
of liquid water on the origin of the hydrous phases. If such were the case,
and if the particles were determined to be cometary based on other criteria,
the implications for cometary thermal evolution, physical properties, and
solution-phase organic chemistry would be far-reaching.
Future Investigations
For the foreseeable future, IDPs will provide the only prospect for direct
study of comet samples. Therefore, a vigorous program of ground-based
studies should be pursued to characterize them according to physical prop-
erties and chemical, isotopic, and mineralogical composition, with primary
emphasis on the phases and structures containing the biogenic elements.
Furthermore, to expand the size of the existing inventory and perhaps ob-
tain particles not captured in the stratosphere, opportunities should be ex-
ploited to collect IDPs in relatively unaltered form in low Earth orbit, as for
example on the Space Station.
The so-called primitive IDPs appear to have no analogues in the meteor-
ite collections and, therefore, are most likely to be cometary in origin. In
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS AND COMPOUNDS 55
contrast, those "chondritic" particles that contain layer lattice silicates grossly
resemble samples of carbonaceous chondrites whose chemistry and mineral-
ogy have been altered by liquid water, but the detailed similarities that
would confirm a meteoritic rather than a cometary origin remain to be
established. For this reason, the latter particles must be included along with
the "primitive" ones in future investigations, and parallel studies at very
high spatial resolution of the finest-grained material in carbonaceous and
unequilibrated ordinary chondrites must be exploited to establish the neces-
sary comparative data base.
For comets, an approach is needed to address via in situ investiga-
tions scientific questions about the elemental, isotopic, molecular, and
mineral composition of the comet nucleus, as well as its physical properties
and geological characteristics. Included in any mission package should be
instruments designed to determine (1) the identities and abundances of the
volatile organic compounds at depth in the nucleus as well as in the gas and
dust of the coma, (2) the physical structure of the coma dust, (3) the abun-
dances of the biogenic elements in the dust, and (4) the isotopic composi-
tions of the biogenic elements in the gas phase.
The proposed Comet
Rendezvous Asteroid Flyby (CRAP) mission would provide such an instru-
mental complement and would thus be the next major advance in our scien-
tific understanding of comets. Furthermore, this mission would serve as a
necessary precursor to a comet nucleus sample return mission (Strategy for
the Exploration of Primitive Solar-System Bodies—Asteroids, Comets, and
Meteoroids: 1980-1990, SSB, 1980; A Strategy for Exploration of the Outer
Planets: 1986-1996, SSB, 1986b).
Over this same time frame, returning short-period comets and new com-
ets will provide occasions for ground-based and airborne observations. These
opportunities should be exploited to address new questions raised by the
recent studies of Comet Halley.
For the longer term, however, highest priority for the study of the cosmic
evolution of the biogenic compounds and phases must be given to the return
of a comet nucleus sample. Under carefully controlled laboratory condi-
tions, the full range of state-of-the-art analytical instruments and techniques
could be brought to bear. Perhaps most important, the ingenuity of an
international community of scientists would be released from the constraints
of preprogrammed experimental approaches imposed by the requirements of
remote analyses. With the expectation that such a sample will be available
some time near the turn of the century, it is timely now to begin developing
the analytical and sample manipulation techniques required to operate at
subambient temperatures on a micrometer scale on samples likely to be
dominated by ices and volatile components. The committee strongly en-
dorses the recommendation for a comet sample return mission in Space
Science in the Twenty-First Century (SSB, 1988a,b).
Representative terms from entire chapter:
solar nebula
