Primitive Earth environments: organic syntheses and …
The current inventory of organic matter on Earth is dominated by biological sources, in particular the structural biopolymers of vascular plants, i.e., cellulose and lignin. While the total mass of the active biological component is estimated to be ~1013 kg, the majority of organic carbon lies preserved within sedimentary rocks. Recent estimates place the sedimentary carbon at ~1019 kg, distributed predominantly within oil shales and coal-bearing strata.- The source of such organic matter originates from the selective preservation of biomolecular compounds derived predominantly from microbiota and vascular plants.
Primitive Earth environments: organic syntheses and ..
Abiotic sources of organic carbon currently include the persistent rain of exogeneous organic carbon derived from carbonaceous chondritic meteorites, interplanetary dust particles, and the occasional comet. It has been estimated that early in Earth’s history (~4.5 billion years ago) up to ~109 kg/yr of organic carbon was delivered to Earth. Although this amount has tailed off considerably, current estimates for the influx of carbon-containing exogenous material is on the order of 2 × 108 kg/yr. Earth may also have received a portion of its volatiles from comets, potentially providing abiotic organic matter which some authors have argued is relevant to the origins and/ or evolution of life.- Impacts may also have shock-synthesized organics in the atmosphere or as a result of the impact event (i.e., impact plume syntheses). Note that impacts would also destroy or modify organic matter. As has been observed in carbonaceous chondrites, the concentration of simple organic molecules under aqueous conditions would ultimately result in the formation of some of the more complex organic compounds (e.g., amino acids, nucleic acid bases, and sugars) typically found in modern cells. However, further studies of cosmogeochemical samples, coupled with laboratory experiments, are needed to probe the degree of chemical complexity that can be attained as a result of exogenous delivery of both intact and perhaps synthesized organic compounds to early Earth, early Mars, and other habitable zones.
Despite decades of research, how life began on Earth remains one of the most challenging scientific conundrums facing modern science. It is agreed that the first step was synthesis of organic compounds essential to obtain amino acids and their polymers. Several possible scenarios that could accomplish this step, using simple inorganic molecules, have been suggested and studied over the years. The present study examines, using atomistic reactive molecular dynamics simulations, the long-standing suggestion that natural cavitation in primordial oceans was a dominant mechanism of organic molecule synthesis. The simulations allow, for the first time, direct observation of the rich and complex sonochemistry occurring inside a collapsing bubble filled with water and dissolved gases of the early atmosphere. The simulation results suggest that dissolved CH4 is the most efficient carbon source to produce amino acids, while CO and CO2 lead to amino acid synthesis with lower yields. The efficiency of amino acid synthesis also depends on the nitrogen source used (i.e., N2, NH3) and on the presence of HCN. Moreover, cavitation may have contributed to the increase in concentration of NH3 in primordial oceans and to the production and liberation of molecular O2 into the early atmosphere. Overall, the picture that emerges from the simulations indicates that collapsing bubbles may have served as natural bioreactors in primordial oceans, producing the basic chemical ingredients required for the beginning of life.
the first organic compounds synthesized by ..
Underlying these synthesis experiments, however, is the assumptionthat biomonomers (e.g., amino acids, carbohydrates) or their simpleprecursors (e.g., hydrogen cyanide, formaldehyde) were readilyproduced in the atmosphere of primitive Earth.
the synthesis of organic compounds and the ..
As with the atmospheres of the outer solar system bodies, the organic molecules in the atmospheres of the terrestrial planets, apart from Earth, listed in have been identified primarily by remote spectroscopic observations, mainly at infrared and ultraviolet wavelengths, from spacecraft missions and space- and ground-based telescopes. In situ and sounding measurements have been obtained for Venus (Mariner, Pioneer Venus, Venera), Mars (Mariner, Viking, martian meteorites), and, of course, Earth. Approximate mixing ratios for the carbon compounds are indicated in in parentheses.
Production of Some Organic Compounds under …
Early experiments to test whether there could have been a source of atmospherically derived organic carbon delivered to Earth’s surface were performed by Urey and Miller, who used an electric discharge to initiate chemical reactions in a gaseous mixture of CH4, H2O, H2, and NH3., These experiments showed that amino acids and other organic acids could be readily produced abiotically in such an atmosphere. If any of the terrestrial planets had such a reduced atmosphere, it appears likely that synthesis of organic compounds might have been a significant factor in generating a surface inventory of organic compounds. However, such a reduced composition for Earth’s early atmosphere is now considered unlikely due to the rapid photolysis of CH4 and NH3 in an atmosphere without an ultraviolet shield. Moreover, volcanic outgassing would most likely give the early Earth an atmosphere consisting of CO2 and N2 rather than CH4 and NH3. This, together with independent geochemical and cosmochemical constraints on CO2 and CH4 abundances from 2 billion to 4 billion years ago, suggests that the likely composition of Earth’s early atmosphere was predominantly N2 and CO2., Subsequent experiments performed on CO2/N2 atmospheres (with and without small amounts of CH4 and NH3) have shown that the yield of organic compounds via spark discharge is considerably less than in a highly reduced atmosphere containing mostly CH4 and NH3.
Production of Some Organic Compounds under Possible ..
Other possibilities for the generation of organic compounds via atmospheric chemistry have also been explored. An intriguing mechanism involves the synthesis of HCN via photochemical reactions between CH4 (of volcanic origin) and N2. If early in Earth’s history the mantle was much more reduced than it is currently, then the amount of CH4 emitted from volcanoes would have been greater. Rainout of substantial quantities of HCN would make the subsequent synthesis of purines, pyrimidines, and amino acids possible in the aqueous phase. Similarly, photolytic reactions initiated from CO2 and H2O at ultraviolet wavelengths have been postulated to produce rainout of formaldehyde (CH2O) with the possibility of subsequent condensation reactions yielding primitive sugars. This assumes that the concentration of formaldehyde rose high enough in aqueous solution on Earth’s surface.