The Search for Life's Origins Progress and Future Directions in Planetary Biology and Chemical Evolution (1990) / Chapter Skim
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2. The Cosmic History of the Biogenic Elements and Compounds
2. The Cosmic History of the Biogenic Elements and Compounds
Pages 21-55
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From page 21... ...
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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From page 22... ...
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.
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From page 23... ...
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 evolution on Earth?
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From page 24... ...
The most massive objects, "giant" molecular 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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From page 25... ...
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 circumstellar 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.
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From page 26... ...
More systematic observations of selected fractionation ratios as functions of cloud temperature and density are required to refine current theories further. Once these theories have become more quantitative, theoretical treatments of how these isotopic ratios can be preserved as the interstellar cloud becomes a protosolar nebula will be most useful.
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From page 27... ...
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 hydrocarbons; see discussion following objective 2)
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From page 28... ...
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 molecules are in good agreement among themselves and with observation, they differ significantly in their predictions of complex molecule abundances.
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From page 29... ...
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)
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From page 30... ...
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.
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From page 31... ...
Inside dense molecular clouds the much lower ultraviolet flux from embedded newly formed stars or from cosmic-ray excitation of molecular hydrogen may be sufficiently high to transform the simple icy molecules into more complex molecules, which are more refractory. This process may also be the source of the more refractory grain mantles possibly observed in the diffuse interstellar medium.
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From page 32... ...
32 CO an o o V Cat sat en Cat D ._ Cat U: o ~¢ sit Ct ._ V' C)
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From page 33... ...
Presumably, these PAHs are the extension of the size distribution of interstellar carbon grains into the molecular domain, and these large molecules are formed as the condensation nuclei of carbon grains in the circumstellar outflow of carbon-rich planetary nebulae and red giants. The observed interstellar infrared spectrum is compared in Figure 2.2 to the laboratory-measured Raman spectra of carbonaceous grains in interplanetary dust particles, carbonaceous chondrites, and auto exhaust.
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From page 34... ...
Because other interstellar dust components may also have survived incorporation into the presolar nebula, it is important that searches be conducted for interstellar or circumstellar signatures for the biogenic elements in other primitive interplanetary, meteoritic, or cometary dust components such as silicates, carbides, and ices. INFRARED SPECTROSCOPY AND ASTRONOMY As already demonstrated, infrared spectroscopy is a useful tool for studying the composition of cosmic dust.
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From page 35... ...
of about 3000 is necessary to study the grains, whereas even higher resolution is required for observations of the gas phase. The composition of interstellar, circumstellar, and protostellar dust is very complex and probably varies from object to object.
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From page 36... ...
The possibility of studying actual interstellar grains in the laboratory rather than by remote sensing is an exciting prospect and would undoubtedly revolutionize our knowledge of interstellar grains and their connection with primitive interplanetary particles. OBJECTIVE 3: To assess the efficacy of chemical and physical processes in the solar nebula for altering preexisting materials and producing new compounds and phases containing the biogenic elements.
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From page 37... ...
in the Allende carbonaceous chondrite imply that the CAIs formed by a complex sequence involving condensation, partial evaporation, and recondensation. Such observations and theoretical models strongly suggest that evaporation and recondensation leading to thermal and chemical equilibration were very probable in the inner regions of the solar nebula.
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From page 38... ...
Understanding the effects of these latter processes on the chemistry of the biogenic elements and compounds is important, and much more effort should be devoted to their investigation. Nonequilibrium thermal effects in a cooling parcel of gas and dust in the solar nebula will be favored when the characteristic cooling time (or the characteristic radial mixing time)
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From page 39... ...
Other inherently nonequilibrium processes, such as photochemistry, solar flares, lightning, coronal discharges, and planetesimal impacts, must also be considered for their influences on biogenic element chemistry in the solar nebula. In general, the net effect of such processes will be to increase molecular complexity and diversity over that expected if the chemistry of
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From page 40... ...
The ultraviolet flux from nearby stars is also a potential nonequilibrating mechanism in the solar nebula. Although the corresponding flux from the early Sun is orders of magnitude greater and may lead to a larger number of molecular dissociations, subsequent pyrolysis of the product molecules in the hot inner regions of the solar nebula may lead to a very small overall net yield of nonequilibrium species (e.g., both simple and complex organic compounds)
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From page 41... ...
It is also important to use realistic laboratory simulation experiments and quantitative theoretical modeling to study the effects of photochemically pumped nonequilibrium chemistry on the biogenic elements and their compounds under pressure, temperature, composition, and photon flux conditions consistent with currently accepted models of the solar nebula. In this regard it would be particularly valuable to try to simulate the effects of the ultraviolet flux from nearby stars on the gas-solid distribution of the biogenic elements carbon, nitrogen, sulfur, and phosphorus between a cold solar composition gas and the grains embedded in it.
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From page 42... ...
During particle growth, adsorption and eventual trapping of volatiles (including noble gases) within the aggregates could have taken place, thus preserving a record of the gas composition of the solar nebula.
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From page 43... ...
The possible fates of compounds and phases containing the biogenic elements in this stage of planetoid formation are many and diverse. To varying degrees of intensity, impacts could have caused pyrolytic decomposition of heat-sensitive organic matter to form both gaseous and refractory products, thermal and aqueous alteration of minerals and carbonaceous grains, melting, near-surface volatile transport, and loss of volatiles to transient atmospheres from either the target or the projectile.
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From page 44... ...
A comparable situation holds for the high molecular weight insoluble organic matter that contains the bulk of the carbon and nitrogen in primitive meteorites. This chemically heterogeneous material contains small amounts of isotonically anomalous hydrogen, carbon, nitrogen, and noble gases attributable to presolar origins, but the amount and nature of the material that may have been affected by alteration or synthesized on the parent bodies are poorly understood.
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From page 45... ...
For the biogenic compounds and phases used as starting materials in these experiments, modifications of physical, chemical, and isotopic properties as a function of environmental conditions must be determined. Deeper understanding of the conditions of aqueous alteration, the identities of the precursor phases, and the nature of the resulting hydrous phases should be sought in petrographic and mineral-chemical studies of carbonaceous chondrites and IDPs.
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From page 46... ...
The study of primitive material in meteorites has provided valuable information about the chemical composition of the solar system and the chemical and physical processes operating in the solar nebula and early solar system. However, the enormous advantage in studying primitive asteroidal
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From page 47... ...
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)
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From page 48... ...
Both X and gamma rays can provide qualitative and semiquantitative analyses for a large number of elements. Both nondestructive mapping techniques and destructive analytical techniques may be required to measure the abundances and chemical forms of the major biogenic elements.
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From page 49... ...
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 welldefined sequence of mineral phases, which became progressively more volatile-rich, formed from this homogeneous cloud with decreasing temperature.
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From page 50... ...
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 interstellar clouds.
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From page 51... ...
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 volatile-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 accreted both refractory and volatile material formed in cold regions of the protosolar nebula.
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From page 52... ...
Although resembling primitive carbonaceous chondrites in exhibiting 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.
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From page 53... ...
Interplanetary Dust Particles Interplanetary dust particles (IDPs) are extraterrestrial particles, typically less than 1 mm in diameter, that survive entry into the upper atmosphere and are currently collected by high-flying aircraft.
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From page 54... ...
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.
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From page 55... ...
the abundances of the biogenic elements in the dust, and (4) the isotopic compositions of the biogenic elements in the gas phase.
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