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      Comet impacts and chemical evolution on the bombarded Earth

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      Origins of Life and Evolution of the Biosphere
      Springer Science and Business Media LLC

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          Endogenous production, exogenous delivery and impact-shock synthesis of organic molecules: an inventory for the origins of life.

          Sources of organic molecules on the early Earth divide into three categories: delivery by extraterrestrial objects; organic synthesis driven by impact shocks; and organic synthesis by other energy sources (such as ultraviolet light or electrical discharges). Estimates of these sources for plausible end-member oxidation states of the early terrestrial atmosphere suggest that the heavy bombardment before 3.5 Gyr ago either produced or delivered quantities of organics comparable to those produced by other energy sources. Which sources of prebiotic organics were quantitatively dominant depends strongly on the composition of the early terrestrial atmosphere. In the event of an early strongly reducing atmosphere, production by atmospheric shocks seems to have dominated that due to electrical discharges. Organic synthesis by ultraviolet light may, in turn, have dominated shock production, but only if a long-wavelength absorber such as H2S were supplied to the atmosphere at a rate sufficient for synthesis to have been limited by ultraviolet flux, rather than by reactant abundance. In the apparently more likely case of an early terrestrial atmosphere of intermediate oxidation state, atmospheric shocks were probably of little importance for direct organic production. For [H2]/[CO2] ratios of approximately 0.1, net organic production was some three orders of magnitude lower than for reducing atmospheres, with delivery of intact exogenous organics in interplanetary dust particles (IDPs) and ultraviolet production being the most important sources. At still lower [H2]/[CO2] ratios, IDPs may have been the dominant source of prebiotic organics on the early Earth. Endogenous, exogenous and impact-shock sources of organics could each have made a significant contribution to the origins of life.
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            Chemistry of hot springs on the East Pacific Rise and their effluent dispersal

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              Annihilation of ecosystems by large asteroid impacts on the early Earth.

              Large asteroid impacts produced globally lethal conditions by evaporating large volumes of ocean water on the early Earth. The Earth may have been continuously habitable by ecosystems that did not depend on photosynthesis as early as 4.44 Gyr BP (before present). Only a brief interval after 3.8 Gyr exists between the time when obligate photosynthetic organisms could continuously evolve and the time when the palaeontological record indicates highly evolved photosynthetic ecosystems.
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                Author and article information

                Journal
                Origins of Life and Evolution of the Biosphere
                Origins Life Evol Biosphere
                Springer Science and Business Media LLC
                0169-6149
                1573-0875
                September 1991
                September 1991
                : 21
                : 5-6
                : 317-338
                Article
                10.1007/BF01808305
                dcec6c97-8b75-4782-b6c1-4d7e5c013ad6
                © 1991

                http://www.springer.com/tdm

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