CP-2156 Life In The Universe

 

Organic Chemical Evolution

SHERWOOD CHANG

 

[21] What were the chemical origins of earthly life, and what was the chronology of major events? A full understanding requires far more knowledge than we now have about the early history of the Solar System and Earth.

 

The study of organic chemical evolution represents a cosmic quest for an understanding of our chemical origins, starting from the Big Bang and proceeding through interstellar clouds, the solar nebula, the formation of the Sun and planets, to the origin of life on Earth. In this context, the exploration of environments across space and time is directed at understanding not only their present state, but also what they can tell us about their past, their origins, and their evolution. During this quest, we will learn more about our own origins on Earth and discover more about the constraints imposed by stellar, solar system, and planetary evolution on the origin and distribution of life in the cosmos. The latter knowledge then helps narrow future searches for life elsewhere in the Universe.

In an evolutionary sense, all life is a product of countless changes in the form and content of primitive matter wrought by processes of chemical and biological evolution. The course of biological evolution can be traced back to common ancestors in the Precambrian period. The prebiotic history of Earth and the Solar System remains much more obscure, however. The components of the Solar System are products of chemical evolution from interstellar matter, but the circumstances of this evolution and even the resulting present state of some bodies are very poorly understood. Fortunately, there are windows, though obscured, that permit us to look into the past and to discern some of the features of past events. These windows are provided by astronomical observations and by geological and geochemical studies of rocks from ancient Earth, the Moon, and meteorites from outer space. Data [22] from these sources then provide bases for formulating working models that attempt to reconstruct environments and to describe the physical-chemical processes that shaped them in the past. These working models, in turn, will be modified by new observations.

 

BEGINNINGS OF CHEMICAL EVOLUTION IN THE SOLAR SYSTEM

 

Just as biological evolution implies that all organisms on Earth have a common ancestry, so chemical evolution implies that all matter in the Solar System has a common origin. Consider the following scenario: An interstellar cloud of dust and molecules collapses, perhaps triggered by the shock wave associated with a nearby supernova, thus beginning the chemical evolution of the Solar System. According to one current model, gravitational collapse of the interstellar cloud led to an enormous disk of gas and dust, the primitive solar nebula, shaped like a flying saucer with the proto-Sun at the center. To a first approximation, this disk has been pictured as having a chemical composition that was spatially uniform and similar to that of the present Sun, at least for the major elements. Detailed studies of meteorites, however, have revealed anomalies in the isotopic composition of some elements, indicating that the solar nebula may not have been as homogeneous as theorists have suggested. Gravitational collapse would have been accompanied by heating and the establishment of a pressure gradient, with pressure highest in the region of the proto-Sun and decreasing with radial distance. Similarly, temperatures would have been highest in the central possibly exceeding 1600 K. Temperatures would have decreased, with radial distance, falling steeply within several astronomical units, then much more gradually to the edges of the solar nebula, beyond the limits of the present orbits of the giant planets, where temperatures would have been less than 100 K.

According to the equilibrium condensation model, as the inner region of the solar nebula cooled, minerals formed from the hot gas, yielding solids at their various condensation temperatures. Some of these primary condensates may have undergone secondary transformations if they continued to react with the nebula gas as the system cooled. In the outer regions of the solar nebula, in and beyond the current orbits of the giant planets, where temperatures would have remained low, condensation and accretion of organic and inorganic interstellar material could have taken place at temperatures near 100 K, allowing volatile materials to be preserved. Because of turbulence and the pressure and temperature gradients in the solar nebula, some mixing of materials with high- and low-temperature histories would [23] have occurred during the accretion of small bodies. Eventually the accretion of fine-grained condensed material led to larger and larger objects and ultimately to the formation of planets. Thus, from the solar nebula came the Sun, the planets and their satellites, comets, meteorites, and asteroids. The Sun continues to contribute matter to various objects in the Solar System through the injection of solar-wind particles. The dashed line in figure 1 suggests that comets may preserve intact material that originated in the interstellar medium. Some cosmochemists believe that comets are products of low-temperature condensation and accretion processes that occurred at the outer edge of the Solar System where the lack of heating permitted survival of interstellar matter.

The stage of condensation from the nebula gas was probably terminated by the so-called T-tauri stage in the proto-Sun's evolution, during which a very powerful solar wind swept the uncondensed gas out of the Solar System and into the interstellar medium. Generally, the material condensed in the...

 


Figure 1. Interrelationships between various bodies in the chemical evolution of the Solar System. Solid arrows indicate contributions of matter from one source to another. The dashed arrow signifies uncertainty regarding direct condensation of comets from interstellar matter. The arrow from <<Life>> implies its eventual dispersal from Earth.

Figure 1. Interrelationships between various bodies in the chemical evolution of the Solar System. Solid arrows indicate contributions of matter from one source to another. The dashed arrow signifies uncertainty regarding direct condensation of comets from interstellar matter. The arrow from "Life" implies its eventual dispersal from Earth.

 

[24] ....inner Solar System had a high-temperature origin and was depleted in volatiles (i.e., materials containing H, C, N, O, and S in volatile form and the noble gases), but the materials that condensed and accreted in the outer Solar System were rich in volatiles.

This very simple model has been criticized, and inevitably, as theory, experiments, and observations progress, it will undergo changes, perhaps so many that a new model will emerge. In the meantime, it provides a useful framework for discussing various aspects of organic chemical evolution. However, some cosmologists believe that the proto-solar system was initially a partially ionized gas, in which case the notion of equilibrium condensation would not be valid.

Sometime within 1000 million years of Earth's birth, life arose on its surface and biological evolution began. Eventually, the death of the Sun may be accompanied by an ejection of matter back into the interstellar medium that spawned it. According to this scenario, the origin and evolution of life on Earth were and will continue to be inextricably bound to the evolution of both the Sun and Earth. It is somewhat ironic that life arose on Earth, a planet that, relative to the Sun, is severely depleted in the volatile elements that make up organic chemistry: hydrogen, carbon, nitrogen (see table 1). On the other hand, the chemistry of the cosmos seems to be dominated by these elements. From this knowledge springs the conviction that organic chemistry constitutes an integral and fundamental part of cosmochemistry, and from this comes the anticipation that, despite the seeming improbability....

 

TABLE 1. RELATIVE ABUNDANCES OF SELECTED ELEMENTS (IN ATOM PERCENT).

Element

Sun

Earth

Biosphere

.

Hydrogen

94

0.08

64

Helium

6

~0

0

Carbon

0.04

0.01

9.1

Nitrogen

0.008

0.00002

0.1

Oxygen

0.07

49

27

Neon

0.004

~0

0

Sodium

0.0002

0.7

0.005

Magnesium

0.004

14

0.02

Aluminum

0.0003

4

0.0004

Silicon

0.004

14

0.03

Argon

0.0001

~0

0

Calcium

0.002

0.8

0.008

Iron

0.003

17

0.002

 

[25] .....of the origin of life on Earth, life may in fact be widely distributed throughout the Universe. With this introduction we shall proceed to a discussion of interstellar clouds, comets, outer planets, asteroids as represented by meteorites, and the primitive Earth, and consider the organic chemistry of these various environments.

 

INTERSTELLAR CLOUDS

 

Interstellar clouds of dust and gas make up about 50% of galactic matter, and if the material in the clouds were spread uniformly over all space, the concentration of matter would amount to something like three hydrogen atoms per cubic centimeter. We consider two basic types of clouds: in very diffuse clouds, which contain little dust, concentrations of gas molecules are very low and hydrogen atoms are the dominant species; in dark, dense clouds, which are abundant in dust, molecular hydrogen is the dominant species and the gas concentrations range from about 103 to about 107 molecules/cm3. A roster of molecules observed in the interstellar medium is given in table 2. The more complex molecules (triatomic or larger) occur in the dense clouds. The bulk composition of the interstellar gas is presumed to reflect cosmic elemental abundances. The dust in interstellar clouds is not well characterized; there is evidence to suggest the presence of ice, silicates, graphite, macromolecular organic compounds, and mixtures of these ingredients. The dust and molecules may have come from several sources: some of it may be a remnant of nebula condensation and solar-system formation, that is, material ejected into the interstellar medium by the T-tauri stage of stars, and some of it may have been ejected from the dense atmospheres of giant stars.

Estimates of the lifetimes of dense clouds before gravitational collapse exceed estimates of the lifetimes of molecules in the gas phase before freezing out on dust grain surfaces. Therefore, the fact that we observe interstellar molecules in the gas phase indicates a continuous production mechanism within the clouds themselves. In formulating a production mechanism, one must consider the environment in which it occurs. The temperatures are very low, 3 to 100 K, which means that chemical reactions in the clouds (except some reactions of hydrogen atoms) must occur with essentially zero activation energy. In addition, the extremely low concentrations of molecules mean that all collisions between them (and therefore chemical reactions) are binary, that is, they involve only two species. These and other constraints have led to a model for the synthesis of interstellar molecules in dense clouds in which reactions are initiated by collisions of ubiquitous, high-energy, cosmic-ray particles with H2 and He. Reactive ionic species are generated.....

 

[26] TABLE 2. INTERSTELLAR MOLECULES (1979).

 

Inorganic

Organic

.

H2

Hydrogen

Diatomic

CH

Methylidine

OH

Hydroxyl radical

.

CH+

Methylidine ion

NO

Nitric oxide

CN

Cyanide radical

NS

Nitrogen sulfide

CO

Carbon monoxide

SiO

Silicon monoxide

CS

Carbon monosulfide

SiS

Silicon sulfide

.

SO

Sulfur monoxide

.

H2O

Water

Triatomic

HC2

Ethynyl radical

HNO

Hyponitrous acid

.

HCN

Hydrogen cyanide

N2H+

.

HNC

Hydrogen isocyanide

H2S

Hydrogen sulfide

HCO+

Formyl ion

SO2

Sulfur dioxide

HCO

Formyl radical

.

OCS

Carbonyl sulfide

.

NH3

Ammonia

4-Atomic

HC2H

Acetylene

.

C2CN

Cyanoethynyl radical

H2CO

Formaldehyde

HNCO

Isocyanic acid

H2CS

Thioformaldehyde

.

.

5-Atomic

CH4

Methane

.

HC4

Butadiynil radical

H2CNH

Methanimine

H2NCN

Cyanamide

HC2CN

Cyanoacetylene

HCO2H

Formic acid

H2CCO

Ketene

.

.

6-Atomic

CH3CN

Methyl cyanide

.

CH3OH

Methyl alcohol

HCONH2

Formamide

.

.

7-Atomic

CH3C2H

Methylacetylene

.

CH3NH 2

Methylamine

H2CCHCN

Vinyl cyanide

H(C2)2CN

Cyanodiacetylene

CH3CHO

Acetaldehyde

.

.

8-Atomic

HCO2CH3

Methyl formate

.

.

9-Atomic

CH3CH2CN

Ethyl cyanide

.

H(C2)3CN

Cyanotriacetylene

CH3CH2OH

Ethyl alcohol

(CH3)2O

Dimethyl ether

.

.

11-Atomic

H(C2)4CN

Cyanotetracetylene

 

 

[27] ....which can enter into binary reaction sequences that require little or no activation energy. Some Several types of reactions are shown in table 3. In addition, chemical reactions may occur on dust grain surfaces that act as "collectors," and these would involve the recombination of free radicals, a type of reaction requiring little or no activation energy. In addition, atoms and free radicals may react with the grain itself.

Examination of the list of compounds in table 2 leads to two important observations. First, the compounds are chemically diverse and structurally complex. Second, many of them are known to be important intermediates in the production of organic matter in abiotic synthesis experiments (e.g., hydrogen cyanide, formaldehyde, and cyanoacetylene).

Clearly, the interstellar environment, as exotic and as seemingly inimical to chemical reactions as it may appear at first consideration, nonetheless exhibits a rich chemistry that manifests itself in the production of organic compounds that, for the most part, are familiar from terrestrial experience.

 

TABLE 3. ION-MOLECULE REACTIONS IN INTERSTELLAR CLOUDS.

Primary ionization:

H2 + CR -> H2+ + e + CR'

................-> 2H+ + e + CR'

He + CR -> He+ + e + CR'

.

Secondary ionization

H2+ + H2 -> H3+ + H

H3+ + A -> AH+ + H2

(A = 0)

He+ + AB -> A+ + B + He

(AB = CO)

...................-> AB+ + He

.

Hydrogenation

A+ + H2 -> AH+ + H

(A = CH)

.

Radiative association

A+ + H2 + -> AH2+ + hv

(A = C)

.

Insertion

AH+ + CO -> ACO+ + H

(A = CH2)

.

Hydrogen transfer

AH+ + CO -> HCO+ + A

(A = CH2)

AH + C+ -> AC+ + H

(A = NH2)

.

Dissociative recombination

AH+ + e -> A + H

(A = HCN)

.

Radical recombination

A + B -> C + D

(CH3 + O -> H2CO +H)

.

CR = cosmic ray

e = electron

hv = photon

 

[28] Although many scientists believe there is little basis for speculating that interstellar organic molecules found their way intact and unchanged to the prebiotic Earth's surface, there is growing interest in the possibility that interstellar molecules may be preserved intact in comets and/or in altered form in carbonaceous meteorites. These possibilities stem from the idea that all solar-system matter had a common origin in an interstellar cloud of dust and molecules. To the extent that comets and carbonaceous meteorites contributed mass to early Earth and arrived at the surface intact, interstellar organic compounds could have survived to take part in subsequent chemical evolution.

 

COMETS

 

Comets occupy an especially interesting place in models of solar-system origin and evolution. They may have been a partial source of planetary atmospheres, and they are believed to have been the building blocks for the rocky cores of the outer planets. Present understanding places the origin of comets in the outer regions of the primitive solar nebula in and beyond the space now traversed by the giant planets. Perturbations of their original orbits by the formation of the giant planets are believed to have sent some into the inner Solar System to collide with the Sun and inner planets and others into orbits extending great distances from the Sun (up to 50,000 astronomical units (AU), where 1 AU = 150 million kilometers).

The components of a comet observable within several astronomical units of the Sun include the nucleus, the coma, and the tail (fig. 2). According to the "dirty ice" model, comet nuclei consist of simple and complex organic molecules and meteorite-like dust and rock embedded in a matrix of frozen water and possibly solid CO2. As the comet approaches the Sun, heating by the Sun occurs and the ices sublime, ejecting volatile "parent" compounds (possibly H2O, CO2, CH4, C2H2, NH3, HCN, CH3CN, etc.) and entraining nonvolatile dust and rock from the nucleus. In the coma, interactions of the parent compounds with solar radiation can lead to physical and chemical processes that result in the partial-to-complete breakdown of the so-called parent molecules. Ion-molecule reactions analogous to those occurring in interstellar clouds probably play an important role. The neutral daughter products are observed in the coma, while the positively charged ones are observed in the tail. According to an alternative view, all the observed species already exist in the nucleus and are simply released directly into the coma by evaporation. These two possibilities arc summarized in figure 3. A third possibility is that both parent molecules and simpler species are released into the coma where they undergo reactions to yield the....

 


[
29]

Figure 2. Major features of a comet. The distance scale is logarithmic. Ions are observed in the tail, neutral species in the coma.

Figure 2. Major features of a comet. The distance scale is logarithmic. Ions are observed in the tail, neutral species in the coma.

 

.....observed ions, atoms, and molecules. In addition to the species indicated in figure 2, metallic elements (Fe, Si, Mg, Ca, Ni, Na, Cr) have been detected in spectroscopic studies of meteor showers associated with comets. The relative abundances of these elements suggest similarities between the chemical compositions of cometary dust and carbonaceous meteorites.

The comet nucleus is thought to be small, typically 1-10 km in diameter, but no direct observations have yet been made. It appears as a small point of light embedded within the bright, large, and extensive coma. The mass of the nucleus could range from 1015 to 1018 gm. The light from the visible coma and tail is emitted by atoms and molecules that have interacted with solar radiation. The size of the coma is remarkable, perhaps greater than 105 km in radius. The tail, composed of dust grains and ionic molecules, is even larger, possibly exceeding 107 km in some cases. When comets become visible in the inner Solar System, they may be spatially the largest objects in the sky.

As mentioned above, comets are believed to be material condensed and accreted at the outer edge of the primitive solar nebula. Thus a relationship may exist between comets and interstellar molecules; if one compares the molecules observed in comets (fig. 3) with interstellar molecules (table 2),.....

 


[
30]

Figure 3. Production of observed cometary molecules by direct evaporation from the nucleus or by evaporation of parent molecules followed by their interactions with solar radiation.

Figure 3. Production of observed cometary molecules by direct evaporation from the nucleus or by evaporation of parent molecules followed by their interactions with solar radiation.


 

....there do seem to be similarities. Both populations contain cyanide derivatives with the CN group, and comet species can be produced by fragmentation of interstellar molecules. It has also been suggested that carbonaceous meteorites, which are rich in various forms of the volatile elements and organic matter, are remnants of volatile-depleted and moribund comets. If comets do not contain relatively unaltered interstellar matter, and if they formed at the outer edge of the solar nebula where temperatures were low enough to condense gases such as carbon dioxide and water, then the presence of parent organic molecules in comets is difficult to understand. There is no widely accepted model for chemical reactions in the solar nebula that could yield the chemistry of comets. Indeed, without direct observations of the nucleus, our knowledge of comet chemistry is exceedingly sparse and model-dependent. Since comets are poorly understood and may represent a chemical evolutionary link between the primitive solar nebula and the interstellar medium, their direct study by space probes constitutes a high-priority objective for many space scientists.

 

JUPITER

 

From the outer regions of the Solar System where comets originated, we move sunward to consider organic chemistry on Jupiter. Spectroscopic observations made by astronomers, theoretical considerations, and, more recently, direct study by space missions provide the basis for current models of Jupiter. The planet has approximately a solar-composition atmosphere consisting primarily of hydrogen and helium with minor to trace amounts of methane, ammonia, water, hydrogen sulfide, ethane, acetylene, phosphine, carbon monoxide, arsine, and the noble gases. The planet's compositional similarity to the primordial nebula makes it a critical object for cosmological study. Planetary processes that prevailed soon after its origin are probably still occurring now.

The model of the environment of the upper (and observable) Jovian atmosphere is depicted schematically in figure 4. Sunlight and extraplanetary particles arc represented as entering at the top of the atmosphere. The changes in temperature (in °K) and pressure (in atmospheres) with depth (not drawn to scale) are shown on the far left-hand scale. The locations of a haze layer and the various observed and postulated cloud layers are also shown, as are the directions of motion of the latter (thick arrows) according to recent meteorological models. The depths (in terms of temperature and pressure) to which sunlight of various wavelengths penetrates into the atmosphere are indicated by the vertical arrows. The presence of gas constituents is also shown by arrows, starting at about the maximum altitude in the atmosphere where they may occur. Thus H2, He, and CH4 occur throughout the atmosphere, while H2O does not occur above the 250 K level. The stratification of some atmospheric components results from the formation of their solid (or liquid) condensates at different temperatures (and altitudes). The jagged lines through the clouds indicate lightning flashes.

The distinct coloration of Jupiter's cloud cover and the variability of its patterns with time have been observed for over a century. These were the first indications of the occurrence of disequilibrium processes in the atmosphere. In 1952, Urey first suggested that complex organic molecules might be responsible for the cloud colors. Since then the hypothesis that Jupiter is at an advanced stage of organic chemical evolution has been widely promulgated. Support for this view has been marshaled from observations that complex organic compounds and colored organic polymers are produced when gaseous components life those in the Jovian atmosphere are subjected in laboratory experiments to electric discharges, thunder shock waves, high-energy proton irradiation, or ultraviolet irradiation. On the basis of theoretical models of Jovian atmospheric photochemistry, however, the contrasting view that the colors are attributable to inorganic substances photochemically....

 


[
32]

Figure 4. Summary of features in a model of Jupiter's atmosphere.

Figure 4. Summary of features in a model of Jupiter's atmosphere.

 

....synthesized in the atmosphere has also been championed. Candidate inorganic species include red phosphorus (P4), ammonium and hydrogen polysulfides ((NH4)2SX and H2SX), and elemental sulfur. No data yet obtained from planetary observations can verify either viewpoint to the exclusion of the other.

The hydrocarbons observed on Jupiter-acetylene (C2H2) and ethane (C2H6) -are believed to be produced by the interaction of sunlight with methane high in the atmosphere. There is also good reason to believe that acetylene is synthesized mainly during thunderstorms. The origin of the carbon monoxide is not clear. Two processes have been suggested which may act separately or jointly: an upwelling of material from deep in the [33] atmosphere, where at high temperatures carbon monoxide is thermodynamically stable, or reactions of atmospheric methane with oxygen atoms injected into the upper atmosphere from extraplanetary sources. Images from the Voyager mission have shown lightning flashes on Jupiter near the ammonia clouds. Although the occurrence of lightning does not prove that the colored material is organic matter, it supports the view that organic matter is produced on Jupiter as it is in laboratory experiments designed to simulate Jovian phenomena. It thus lends plausibility to the mechanism and stimulates interest in providing future space probes of the planet with the ability to detect a variety of organic compounds.

The occurrence of organic compounds on Jupiter has been linked to the possibility of life on that planet. Although available evidence cannot eliminate this possibility, it does make it highly unlikely. The cloud features visible on the planet have a finite lifetime; that is, the various colored belts are observed to disappear with lifetimes ranging from weeks to years. The Great Red Spot has been observed for hundreds of years, but meteorologists believe that the material visible now is not the same material that was visible a hundred or even five years ago. Vertical atmospheric cycling transports material in the upper atmosphere down to some depth in the lower atmosphere where the temperatures and pressures are high. As a consequence, any organic compounds formed higher in the atmosphere by electric discharges, thunder shock waves, ultraviolet irradiation, and other processes are destroyed by hydrogenation and reconverted to the primary ingredients of the atmosphere - hydrogen, ammonia, methane, water, and hydrogen sulfide. Thus atmospheric circulation imposes severe constraints on the time available for simple atmospheric gases to be converted to complex organic macromolecules akin to proteins and nucleic acids, the critical biochemicals of life on Earth For life to arise on Jupiter, the rate of chemical evolution must be extraordinarily fast, that is, fast compared to the rate of circulation of matter to hot regions deep in the atmosphere.

Many uncertainties thus exist about the nature and identity of the colored substances on Jupiter, but on the basis of available knowledge it appears that the dynamics of atmospheric circulation have limited the progress of Jovian organic chemical evolution.

 

METEORITES

 

Sunward from Jupiter lies the asteroid belt, samples of which arc believed to find their way to Earth in the form of meteorites. Application of spectroscopic remote-sensing techniques to asteroids has revealed the presence of mineral assemblages on their surfaces analogous to those found to [34] dominate in known types of meteorites. For our discussion, the most interesting meteorites are the ones called carbonaceous. (Objects with spectroscopic properties similar to those of carbonaceous meteorites appear to be quite common in the asteroid belt.) These objects consist of complex assemblages of relatively fine-grained mineral and organic matter that reflect a broad range of elemental compositions, textures, and petrologies, indicative of wide variations in the environment of origin for the various components.

According to a prevailing model for their origins, some of the mineral ingredients were formed primarily by equilibrium condensation from the cooling gaseous solar nebula. Other minerals resulted from secondary reactions of high-temperature condensates with lower temperature nebula gas while the condensates were still suspended as particles in the nebula. Presumably the diverse ingredients were eventually assembled into rocky material on parent bodies, probably resembling asteroids, where compaction events and the environmental conditions further influenced their chemistry, mineralogy, and petrology. Later, disruption of the parent bodies (perhaps by collision with other bodies) yielded fragments representative of the various parts which, in time, fell under the influence of Earth's gravitational field. Carbonaceous meteorites therefore represent material formed very early in solar-system history and contain clues to the primitive environments and processes that produced them.

Figure 5 summarizes major and minor phases found in carbonaceous meteorites; gives their probable temperature of formation by equilibrium condensation from the gaseous nebula, by secondary alteration, or by non-equilibrium processes, either in a solar-composition gas or on a parent body; and shows their relative abundances and distributions in three types of carbonaceous meteorites. For present purposes, the major differences between the C1, C2, and C3 meteorites are their increasing content of volatile elements and their decreasing content of minerals of high-temperature origin. Accordingly, the amount of organic matter increases in the same order from about 0.5 to 5% by weight. High-temperature mineral phases occur most abundantly in C3 meteorites, along with metals and the mafic silicates, olivine and pyroxene, which comprise the bulk of their mass. These minerals exist only in low to trace amounts in C2 meteorites; all, except traces of mafic silicates, appear to be absent in the C1 meteorites.

A complex, chemically heterogeneous carbonaceous phase, characterized by insolubility in solvents and acids, occurs as the major carbon component in all three types of meteorites but is least abundant in the C3 meteorites is designated the solvent- and acid-insoluble carbonaceous (SAIC) phase. This material is especially noteworthy because it serves as host phase for several noble-gas components of the "planetary" type, the isotopic.....

 


[
35]

Figure 5. Distributions and approximate formation temperatures of minerals and other phases in carbonaceous meteorites. Parentheses indicate low to trace amounts.

Figure 5. Distributions and approximate formation temperatures of minerals and other phases in carbonaceous meteorites. Parentheses indicate low to trace amounts.

 

.....composition of at least one of which cannot be readily explained by solar or solar-system processes. Some of the SAIC material is probably relatively unaltered interstellar matter. The natural mechanism(s) responsible for producing the SAIC phases with their noble-gas contents is unknown, however. When carbonaceous matter is condensed in the presence of noble gases by passage of electric discharges through CH4 and N2 or by laser vaporization of carbon targets, the noble gases become trapped in a "planetary" elemental pattern; but the relevance of these specific mechanisms to the cosmochemical context in which the meteoritic SAIC phases were produced is unclear. In figure 5, the indicated formation temperature of SAIC material was arbitrarily chosen and may be viewed as simply the midpoint in a possible ±500 K range of formation temperatures.

Organic synthesis promoted by Fischer-Tropsch-type (FTT) reactions, electric discharges, ultraviolet photochemistry, or other mechanisms must have occurred at temperatures sufficiently low to permit preservation of the [36] variety of volatile and thermally labile organic compounds found in low abundances in C1 and C2 meteorites. Although it is uncertain about how these compounds were synthesized, some investigators favor production on a parent body rather than on mineral grains suspended in the solar nebula. An interstellar origin is also possible if the building blocks of meteorite parent bodies included material formed in regions of the solar nebula where cometary matter originated.

The predominant minerals in C1 and C2 meteorites (50-80%) are the layer-lattice silicates or phyllosilicates. These minerals resemble terrestrial clays in crystallographic structure, but exhibit elemental compositions remarkably similar to the pattern of cosmic abundances. This similarity suggested a solar nebula origin for this material, but recent observations and analyses indicate that a more likely mode of production involves hydrothermal alteration at about 350 K of previously formed silicates on a parent body. Also found in C1 and C2 (and rarely in C3) meteorites are minor amounts of magnetite, sulfates, and carbonates. Recent findings indicate that these, too, have a parent-body rather than nebula origin.

Table 4 shows the distribution of carbon in the Murchison meteorite, the most pristine and carefully examined carbonaceous meteorite. Note that the volatile organic compounds - the hydrocarbons, carboxylic acids, ketones, aldehydes, alcohols, and amines constitute a small fraction of the total carbon and less than 0.05% of the total mass of the meteorite. Nonetheless, the variety of compounds indicates that the organic chemistry of meteorites is quite complex.

 

TABLE 4. DISTRIBUTION OF CARBON IN MURCHISON METEORITE.

Species

Abundances

.

Acid-insoluble carbonaceous phase, %

1.3 - 1.8

CO3, %

0.2 - 0.5

Hydrocarbons and lipids, %

0.07 - 0.11

Carnoxylic acids, ppm

~350

Amino acids, ppm

10 - 30

Ketones and aldehydes, ppm

~17

Urea and amides, ppm

<2 - 15

Alcohols, ppm

~6

Amines, ppm

~2 - 3

N-heterocycles, ppm

<2 - 40

Sum, %

1.81 - 2.45

Total carbon, %

2.0 - 2.58

ppm = parts per million

 

 

[37] Clearly, organic chemical evolution prior to or on the meteorite parent body yielded substances which on primitive Earth may have constituted the building blocks of the first organisms. An intriguing question is how much the meteorite parent-body environment (or other source regions of meteorite organic matter) resembled that of prebiotic Earth. Continuing studies of the organic chemistry of meteorites integrated with inorganic, mineralogic-petrologic, and other types of investigation should yield new insights into the relationships between the organic chemical evolution represented in carbonaceous meteorites and that of prebiotic Earth.

 

VENUS AND MARS

 

The widespread occurrence of organic compounds in the cosmos and within our Solar System confirms the expectation based on cosmic elemental abundances that organic chemical evolution is a natural consequence of the evolution of matter in the Universe. But organic chemical evolution is inextricably intertwined with the evolution of environments, be they interstellar clouds, meteorite parent bodies, or planets, and its progress toward life may be terminated at different stages depending on the physical and chemical constraints imposed by the environment.

On Venus, Earth's nearest neighbor, the temperatures at the surface and in the lower atmosphere, planetwide, are so high that organic compounds such as amino acids, sugars, and nucleic acids cannot survive. Consequently, the probability that life or organic chemistry exists or could survive on Venus is virtually nil. However, there may have been organic chemical evolution on Venus early in its history, if its global environment resembled that of early Earth. Clearly, planetary evolution and thus chemical evolution followed different tracks on Venus and Earth.

On Mars, local environments exist today which are not totally inimical to life or which would have permitted relics of earlier organic chemical evolution or life to be preserved in ancient rocks and sediments. Since we only sampled a very limited number of Martian environments on the Viking missions, the existence of present and past organic chemical evolution, and even life, remains to be adequately confirmed or denied.

 

PREBIOTIC EARTH

 

In many respects our knowledge of early Earth is much like our knowledge of the early Solar System. It is model-dependent and relies on the reconstruction of an environment by extrapolation from a record preserved [38] in, but deciphered only in fragmentary fashion from lunar rocks, meteorites, remotely discernible features of Venus and Mars, and very ancient rocks and sediments of Earth. As more of the record is unveiled, new evidence leads to new interpretations and revisions of models.

According to the Oparin-Haldane-Miller-Urey paradigm, a highly reducing atmosphere consisting of methane, ammonia, and water prevailed on primitive Earth. Passage of energy in various forms through this hypothetical atmosphere produced the reservoir of organic molecules from which life evolved. The existence of this reducing atmosphere required the presence of metallic iron in the upper mantle and crust, which appears to conflict with some geochemical observations. Recently, a case has been made for a primitive atmosphere composed predominantly of H2O, CO2, and N2.

The background and basic features of this model are depicted schematically in figure 6. It starts with refractory minerals condensing from the cooling nebula and accreting to form the protoplanet. Rapid accretion was accompanied by melting and segregation into a molten metallic core and a fluid silicate mantle. The initial inventory of volatiles was driven to the surface. As the nebula gas continued to cool, metallic iron was converted to the ferrous state. Presumably, when the Sun passed through its T-tauri stage, the powerful solar wind blew the remaining nebula gas out of the inner Solar System, carrying Earth's primitive and possibly highly reducing atmosphere with it. (Because some doubt exists about the efficacy of the T-tauri wind, it is significant that another mechanism has been proposed that could achieve the same result: in a recent physical model of the primitive solar nebula, it has been suggested that tidal stripping of the atmospheric envelope of a giant, gaseous, inner protoplanet by the Sun could have occurred early, leaving behind a core of condensed matter.) Debris remaining from the nebula condensation was steadily accumulated by primitive Earth. This debris, presumably of carbonaceous meteoritic composition, contributed material to form the thin crustal veneer of Earth. Heating of the debris as it passed through the atmosphere, during its impact with the surface, or while it was embedded in the hot surface, released the volatiles to form the secondary atmosphere. As a result of Earth's continued cooling, a thin, solid crust probably existed about 4.1 to 4.0 billion years ago. The crust must have formed by about 3.9 billion years ago, because shortly thereafter aqueous environments and sedimentary processes had begun, as evidenced by the 3.8-billion-year-old metasedimentary rocks of Greenland.

According to the model, H2O and CO2 were the dominant constituents of the secondary atmosphere, N2 occurred in minor amounts, and H2 and CO were present only in traces, if at all. Traces of CH4 and other hydrocarbons are presumed to have been oxidized readily to CO2 by iron oxides in the crust. The composition of this steam atmosphere was determined by the....

 


[
39]

Figure 6. Stages in a scenario for the Earth's early evolution.

Figure 6. Stages in a scenario for the Earth's early evolution.

 

[40] ....redox potential of the silicate crust and upper mantle and would have strongly resembled contemporary volcanic exhalations. Once the temperature of Earth's crust dropped below 373 K, water condensed to begin formation of the oceans, and the weathering of basic igneous rocks by CO2 afforded carbonates. The prebiotic atmosphere that resulted would have closely resembled the present atmosphere minus oxygen.

We remain ignorant of the true composition of primitive Earth's atmosphere, but we can nevertheless explore the potential for abiotic organic synthesis in a variety of possible compositions ranging from highly reducing (H2, NH3, CH4, H2O) to nonreducing (CO2, N2, H2O). Some of the results obtained with different gas compositions and energetic processes are summarized in table 5. Although other types of experiments have been conducted, the table is restricted to those involving electrical discharges, ultraviolet light, and the so-called Fischer-Tropsch-type synthesis. The last experiments involve passage of mixtures of H2, CO, and NH3 over mineral catalysts at 400 to 600 K to produce products.

The brief survey in table 5 shows that in abiotic syntheses the types of organic compounds formed, and probably their relative abundances, depend on both the composition of the reactant gases and the energy source. In a reducing atmosphere it appears to be relatively easy to produce some of the types of compounds that comprise either the molecular building blocks of life or potential precursors for them. For example, many biochemicals have been synthesized starting with the products observed in electric discharge experiments; thus, from simple aldehydes and nitriles it has been possible to produce amino acids, carbohydrates, purines, and pyrimidines. In a CO2-N2-H2O atmosphere, however, only acids and aldehydes have been reported; the production of nitrogen-containing organic compounds appears to be inhibited. The selective absorption of ultraviolet light by various components of the atmosphere imposes limitations on the variety of organic compounds that can be produced by photochemical means. For example, experimental and computational studies have shown that ultraviolet irradiation of a CO2-N2-H2O mixture yields formic acid and formaldehyde but no nitrogenous organic matter. Even when CO2 is replaced by CH4, no nitrogen is incorporated in organic compounds by photochemical means. When N2 is replaced by NH3, however, HCN, amino acids, and, presumably, other compounds are produced photochemically. The Fischer-Tropsch-type reactions produce a variety of organic compounds but require the presence of H2 and NH3.

In other experiments, the coupling of carbohydrates with purines has produced nucleosides of the purines, but it has not been easy to combine pyrimidine bases with carbohydrates to form the pyrimidine nucleosides. Phosphorylation of nucleosides to yield nucleotides has been achieved, as has formation of polynucleotides from mononucleotides. Amino acids have also....

 

[41] TABLE 5. RESULTS OF EXPERIMENTS ON ABIOTIC ORGANIC SYNTHESIS.

Gases

Electrical discharges

Ultraviolet photochemistry

Fischer-Tropsch synthesis

.

CO2, N2, H2O

HNO3, etc (?)

Formic acid, formaldehyde

.

CO2, CO, N2, H2

HCN, amino acids, (?)

.

.

CO, N2, H2O

.

Hydrocarbons, alcohols, ketones

.

CO, NH3, H2

HCN, amino acids, etc.

.

Hydrocarbons, aldehydes, ketones, nitriles, amino acids, purines, etc.

CH4, N2 (NH3), H2O

HCN, Hydrocarbons, aldehydes, ketones, nitriles, amino acids, carboxylic acids, etc.

Aldehydes, alcohols, ketones, hydrocarbons, (HCN),* (amino acids)*

.

* Formed when N2 is replaced by NH3.

 

....been converted to polypeptides of various lengths. Thus the view that protobiopolymers could have been produced with relative case on primitive Earth has been widely promulgated. Underlying these synthesis experiments, however, is the assumption that biomonomers (e.g., amino acids, carbohydrates) or their simple precursors (e.g., hydrogen cyanide, formaldehyde) were readily produced in the atmosphere of primitive Earth. Furthermore, many of the syntheses of the complex biomolecules involve either single reactants or very simple mixtures of reactants and are carried out under laboratory conditions whose geological relevance to and plausibility in the primitive [42] Earth environment are not readily discernible. Despite these limitations, it is clear that, under certain conditions, simple atmospheric gases can be converted in stepwise fashion into complex molecules possibly related to primordial precursors of the proteins and nucleic acids of living systems.

Although production of the organic compounds necessary for chemical evolution would have proceeded readily in a reducing atmosphere (CH4, NH3, H2O), the possibilities in a CO2-N2-H2O atmosphere with traces of H2, CO, and/or CH4 appear to be limited, at least in the types of experiments carried out so far. However, reports have appeared recently of photochemical reactions involving gas-solid and gas-liquid-solid systems in which carbon and nitrogen "fixation" occurs with CO2 and N2 as the source gases. Investigations of organic synthesis in such heterogeneous systems are highly desirable if we intend to understand the possible pathways for production of organic matter in a nonreducing secondary atmosphere.

It is important to keep in mind that, as embodied in the Oparin-Haldane-Miller-Urey paradigm, the fundamental conception of abiotic organic synthesis as a necessary and natural prelude to the origin of life on Earth retains its validity. Changes in the chemical composition of model prebiotic atmospheres must be viewed as challenges to be met with fresh approaches to achieving abiotic organic syntheses compatible with changing environments. Furthermore, the nonreducing (or only slightly reducing) model, like the highly reducing one, remains poorly constrained in a number of ways and undoubtedly will undergo refinement and alteration with time. A realistic model probably lies somewhere between the two, with CO2 and CH4 both present and with nitrogen present predominantly in the form of N2.

 

PROBLEMS AND PROSPECTS FOR FUTURE STUDY OF PREBIOTIC EARTH

 

An adequate understanding of the course of organic chemical evolution on primitive Earth and the chronology of major events involved requires far more knowledge about the early history of the Solar System and Earth than is available now. A listing of major issues is given in table 6.

The course of condensation and accretion of matter in the solar nebula would have been critical in establishing the chemical composition of material that formed the planets and the processes that initiated their geophysical and geochemical evolution. New knowledge of the geophysical and geochemical evolution of early Earth or other planetary bodies (including meteorite parent bodies) would lend insight into the timing of core differentiation and....

 

[43] TABLE 6. SOME MAJOR ISSUES IN ORGANIC CHEMICAL EVOLUTION ON EARTH.

 

Condensation and accretion in the solar nebula
Early geochemical/geophysical evolution
Origin and early evolution of the atmosphere
composition
thermal structure
radiative transmission
transport processes
Production rates for organics
sources: half lives
sinks: recycling mechanisms
condensation mechanisms
Organic-inorganic interactions
catalysis
phosphate utilization
sequestration
Environmental fluctuations
day/night
seasons
tides
Phase separations
membranes
microstructures
Molecular selectivity
biological subset of monomers
genetic code
chirality

 

 

....crustal evolution and the thermal history of crust. From this knowledge could come information about the origin, composition, and early evolution of the atmosphere. The thermal structure would determine the rate at which H2 (and therefore reducing power) was lost from the top of the atmosphere. It would also determine, in part, the atmosphere's composition and the kinds of chemical transformations that could occur in various parts of the atmosphere For similar reasons, it is important to know the radiative transmission in the atmosphere, which determines the energy budget associated with incident sunlight. Transport processes in the atmosphere are also important. For example, if material is produced by ultraviolet light high in the atmosphere, how long does it take for it to act down to the surface of Earth? Is it a short enough time to preserve it against destruction or against conversion [44] to a refractory material incapable of further involvement in chemical evolution? If material is produced on land, how can it be transported to bodies of water where it can encounter other materials? If it is produced in oceans, how is it concentrated?

Possible production rates for organic compounds, even in a reducing atmosphere, are not well known; synthesis could occur in the atmosphere, in the oceans, or at the interfaces between the atmosphere, bodies of water, and land. As mentioned above, there are also chemical reactions that act as sinks for organic matter. For example, amino acids and sugars would react to form water-insoluble products, which are thus removed from the reservoir of organic material that would have been available to continue organic evolution. How could material in sinks of this type have been recycled to make it available?

What were the environmentally plausible pathways available to convert simple monomeric amino acids and nucleotides to the biopolymers necessary for life? One pathway that has recently been receiving attention involves the interaction between inorganic matter (particularly clays) and organic matter. There is little doubt that organic chemical evolution occurred in a predominantly inorganic realm. An important question is how organic chemistry interacted with inorganic chemistry and mineralogy. Did the inorganic world provide catalysts for organic reactions? If so, what were they? How was phosphate utilized to produce nucleotides and polynucleotides? Did inorganic material sequester organic matter, thereby removing it from the realm of chemical evolution?

Environments on Earth are subject to all kinds of fluctuations: day and night, seasons, and tides. In some experiments, environmental fluctuations of temperature and moisture content have been successfully used to produce peptides from amino acids, for example. How important were fluctuations overall for organic chemical evolution? Were they necessary?

At some stage in organic chemical evolution it would have been necessary to achieve a phase separation between an evolving organic system and the external environment. Thus the origin of membranes must be considered. Could the organic microstructures produced in a variety of abiotic synthesis experiments serve as models for early membranes?

Finally, molecular selectivity during chemical evolution must be considered. In meteorites and in the products of model prebiotic organic synthesis experiments, a rich variety of compounds exists. Even within a single class -the amino acids-there are a large number of different molecular structures. How were the very limited number of amino acids now utilized by organisms selected from this larger abiotic set? How did the genetic code arise? And what was the origin of chirality, that is, the handedness prevalent in the amino acids and the sugars of all living organisms?

[45] Partial answers may exist for some of these questions, but much remains to be learned. As a field of active scientific inquiry, the study of organic chemical evolution and the origin of life is still in its infancy, and contributions from a variety of disciplines, including astronomy, astrophysics biology, geochemistry, inorganic chemistry, and organic chemistry, are essential. If research efforts continue on many of the issues that have been mentioned, we can anticipate significant progress for the future.

 

ADDITIONAL READING

 

- Andrew, B. H., ed.: interstellar Molecules. Reidel, Dordrecht, 1980.

- Bar-Nun, A.: Acetylene Formation of Jupiter: Photolysis or Thunderstorms? Icarus, vol. 38, May 1979, pp.180-191.

- Bunch, T.; and Chang, S.: Carbonaceous Chondrites II: Carbonaceous Chondrite Phyllosilicates and Light Element Geochemistry as Indicators of Parent Body Processes and Surface Conditions. Geochim. Cosmochim. Acta, vol.44,1980, pp.1543-1577.

- Chang, S.: Comets: Cosmic Connections with Carbonaceous Meteorites, Interstellar Molecules and the Origin of Life. In: Space Missions to Comets, NASA SP-2089,1979, pp.59-111.

- Dalgarno, A.; and Black, J. H.: Molecule Formation in the Interstellar Gas. Reports on Prog. Phys., vol. 39, June 1976, pp.573-612.

- Delsemme, A. H., ed.: Comets-Asteroids-Meteorites. Univ. of Toledo Press, 1977

- Flinn, E. A., ed.: Scientific Results of the Viking Project. J. Geophys. Res., vol.82, Sept.30, 1977, pp.3959-4680.

- Gehrels, T., ed.: Jupiter. Univ. of Arizona Press, Tucson,1976.

- Goldsmith, D.; and Owen, T. C.: The Search for Life in the Universe. Benjamin-Cummings Publishing Co., Menlo Park, Calif.,1980.

- Gordon, M. A.; and Snyder, L. E., eds.: Molecules in the Galactic Environment. John Wiley & Sons, New York, 1973.

[46] - Miller, S. L.; and Orgel, L. E.: The Origins of Life on the Earth. Prentice-Hall, Englewood Cliffs, New Jersey, 1974.

- Mission to Jupiter and Its Satellites, American Association for the Advancement of Science, 1979 (a special reprint collection); see also Science vol. 204, no. 4396, June 1, 1979, pp. 913-921 and 945-1008.

- Nagy, B.: Carbonaceous Meteorites. Elsevier Publishing Co., New York, 1975.

- Windley, B. F., ed.: The Early History of the Earth. John Wiley & Sons, New York, 1976.


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