The Search for Life's Origins Progress and Future Directions in Planetary Biology and Chemical Evolution (1990) / Chapter Skim
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3. Early Planetary Environments: Implications for Chemical Evolution and the Origin of Life
3. Early Planetary Environments: Implications for Chemical Evolution and the Origin of Life
Pages 56-77
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From page 56... ...
In a more general context, the organic chemistry of planetary environments is an extension of the cosmic evolution of the biogenic elements (see Chapter 2) into the planetary epoch.
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From page 57... ...
The occurrence of selective pathways for the synthesis of organic compounds related to abiotic processes on the primitive Earth can also be investigated by studying the processes and products found in these environments. Furthermore, it may be possible to determine the relationships among the materials in asteroids, the satellites and planetary rings of the outer solar system, and the components of primitive meteorites and comets.
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From page 58... ...
THE OUTER PLANETS OBJECTIVE 1: To determine the origin and distribution of organic matter and disequilibrium products containing the biogenic elements in the hydrogen-rich atmospheres of the outer planets. The giant planets Jupiter, Saturn, Uranus, and Neptune are composed of large amounts of gas.
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From page 59... ...
, reactions driven by lightning discharges, and the effects of convection in the atmospheres themselves, which can bring species formed in one thermal regime into another where they will be out of equilibrium. Indeed evidence of these processes is available in the form of ethane and acetylene, formed from methane in the upper atmospheres of these planets; as yet unidentified colored material in atmospheric clouds and hazes; and constituents such as phosphine, germane, and possibly CO that have been formed at low elevations and brought up to visible levels by vertical currents.
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From page 60... ...
This chemistry is driven primarily by precipitating electrons from Saturn's magnetosphere, but there are also contributions from solar ultraviolet photons TABLE 3.1 Composition of Titan's Atmosphere Species Name Abundance Major Components N2 Nitrogen 73-99% Ar Argon 10-15% CH4 Methane 1-6% H2 Hydrogen 0.1-0.4% Hydrocarbons C2H6 Ethane 20 ppm C3H~ Propane 20-50 ppm C2H2 Acetylene 2 ppm C2H4 Ethylene 400 ppb C4H2 Diacetylene 30 ppb CH3C2H Methylacetylene 30 ppb Nitriles HCN Hydrogen cyanide 20 ppb HC2CN Cyanoacetylene 10-1000 ppb C2N2 Cyanogen 10-100 ppb Oxygen Compounds CO2 Carbon dioxide 10 ppb CO Carbon monoxide 60 ppm
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From page 61... ...
OBJECTIVE 3: To characterize the organic matter on the dark surfaces of the asteroids, satellites, and planetary rings of the outer solar system. Atmospheres are not the only locales for organic chemistry in the outer solar system.
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From page 62... ...
THE TERRESTRIAL PLANETS Like all the terrestrial planets, the two discussed here Earth and Marsare composed mainly of silicates and iron and contain only trace to minor amounts of the volatiles and the biogenic elements that are necessary for life. Historically, two scenarios have been proposed to explain the extreme depletion of the terrestrial planets in gases and volatile compounds.
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From page 63... ...
supplied to the terrestrial planets were prerequisites for the origin of life, yet their origin and delivery were the results of astrophysical processes operating elsewhere in the solar system. Earth Accretion by Impacts According to the scenario described earlier, the terrestrial planets grew as a result of larger planetesimals capturing smaller ones by gravitational attraction and collision.
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From page 64... ...
Fluxes larger than the interior heat flux would have afforded significant geological effects. For example, accretion of the Earth in 10 million years would correspond to a heat flux of about 4000 W/m2, which may have been enough to form a massive water greenhouse with molten rock at the surface.
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From page 65... ...
The origin of an atmosphere and an ocean following the last major impact was probably rapid, but quantitative observations exist only for radiogenic rare gases. The massive water atmosphere would have condensed into oceans more or less the present size, leaving behind a thick atmosphere containing the noble gases as well as hydrogen-, carbon-, nitrogen-, and sulfur-bearing species.
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From page 66... ...
Implicit in the idea of early core formation and mantle melting is that preexisting metallic iron carried in by accreting bodies would have been removed from near-surface regions. If segregation of iron into the core was inefficient and metal remained in the upper mantle to buffer the redox state of magmas, or if the metallic core, mantle, and outgassed volatiles were all in thermodynamic equilibrium during the period of rapid heat loss, CO and CH4 would have been the predominant thermodynamically stable forms of carbon injected into the atmosphere from the interior.
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From page 67... ...
If a substantial fraction of the carbon presently tied up in carbonate minerals was originally in the atmosphere as CO2 after accretion, a massive CO2 atmosphere of the order of several tens of bars could have existed. The greenhouse effect associated with this atmosphere would have countered the glacial temperatures inferred to have been the consequence of the lower luminosity of the early Sun; CO2 dissolved in the water would have precipitated CaCO3 at the moderate temperatures of the marine (shallow)
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From page 68... ...
The aspects of plate tectonics that do not involve continents, including midoceanic ridges with hydrothermal vents, oceanic islands, and island arcs, probably existed as early as did liquid water and tectonism, but there is little hard evidence for this. The origin of continents is more poorly understood, and the best estimates of continental growth and recycling come from geological data.
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From page 69... ...
Impacts and Their Influence on Environmental Conditions for the Origin and Maintenance of Life In theory, life could have arisen at any time after Earth was fully accreted and liquid water appeared on its surface (i.e., by about 4.5 billion years ago)
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From page 70... ...
For large impacts a transient water greenhouse traps much of the energy. A 400-km-diameter object would evaporate the ocean and evaporate life.
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From page 71... ...
As is true for Earth, the key to understanding the occurrence or absence of chemical evolution, the origin of life, and extant life on Mars lies in deciphering the planet's history of water, its geochemical cycles, and its atmosphere. OBJECTIVE 3: To assess the isotopic, molecular, morphological, and environmental evidence for chemical evolution and the origin of life on Mars.
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From page 72... ...
The bulk of the CO2 is in sedimentary rocks. A decrease, for example, in metamorphism would lead to less gas in the atmosphere.
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From page 73... ...
Liquid water reacts readily with basalt (or basaltic sediments)
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From page 74... ...
A high-velocity impact generates rock vapor, but the vapor escapes into space rather than surrounding the planet. The effects of ejecta and tidal waves on the ocean on Earth are also not likely to be applicable to Mars.
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From page 75... ...
Primordial organic matter on Mars may be preserved at depth in the regolith or in ancient sedimentary deposits protected against destruction by oxygen, ozone, and other oxidants in the atmosphere and surface dust. Mounting evidence, some alluded to in the preceding section, indicates that Mars' surface environment 4.5 to 3.5 billion years ago was quite different from the Mars of today.
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From page 76... ...
Despite their differences today, the young Mars may have been similar to the young Earth in terms of several environmental features critical to chemical evolution: moderate temperature active tectonism, the presence of liquid water, and the occurrence of biogenic elements in the atmosphere and surface rocks. It would have been within this environmental context that the process of chemical evolution might have occurred and left some relict evidence (see Chapters 4 and 5)
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From page 77... ...
The committee wishes to emphasize that exobiological interest in Mars is not limited to a simple search for microfossils although such an investigation would certainly be part of any sample analysis strategy. Exobiologists are interested in a comprehensive examination of Martian sediments and other samples, including isotopic geochemical analysis of hydrogen-, carbon-, nitrogen-, oxygen-, sulfur-, and phosphorous-bearing materials in igneous rocks and the atmosphere; organic geochemical studies of any preserved organic materials; and inorganic geochemical and mineralogical studies of clays, carbonates, sulfates, phosphates, and other phases that are composed of the biogenic elements and are associated, at least on Earth, with water or biological activity.
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