Journal ArticleDOI
The Early Faint Sun Paradox: Organic Shielding of Ultraviolet-Labile Greenhouse Gases
Carl Sagan,Christopher F. Chyba +1 more
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It is shown that ultraviolet absorption by steady-state amounts of high-altitude organic solids produced from methane photolysis may have shielded ammonia sufficiently that ammonia resupply rates were able to maintain surface temperatures above freezing.Abstract:
Atmospheric mixing ratios of approximately 10(-5 +/- 1) for ammonia on the early Earth would have been sufficient, through the resulting greenhouse warming, to counteract the temperature effects of the faint early sun. One argument against such model atmospheres has been the short time scale for ammonia photodissociation by solar ultraviolet light. Here it is shown that ultraviolet absorption by steady-state amounts of high-altitude organic solids produced from methane photolysis may have shielded ammonia sufficiently that ammonia resupply rates were able to maintain surface temperatures above freezing.read more
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Atmospheric Influence of Earth's Earliest Sulfur Cycle
TL;DR: Mass-independent isotopic signatures in Precambrian rocks indicate that a change occurred in the sulfur cycle between 2090 and 2450 million years ago, implying that atmospheric oxygen partial pressures were low and that the roles of oxidative weathering and of microbial oxidation and reduction of sulfur were minimal.
Journal ArticleDOI
Evolution of the Solar Activity over Time and Effects on Planetary Atmospheres. I. High-Energy Irradiances (1-1700 Å)
TL;DR: In this paper, the results of the Sun in Time multiwavelength program (X-rays to UV) of solar analogs with ages covering 0.1-7 Gyr were reported.
Journal ArticleDOI
The habitat and nature of early life
Euan G. Nisbet,Norman H. Sleep +1 more
TL;DR: It is possible that early life diversified near hydrothermal vents, but hypotheses that life first occupied other pre-bottleneck habitats are tenable (including transfer from Mars on ejecta from impacts there).
Journal ArticleDOI
On the origins of cells: a hypothesis for the evolutionary transitions from abiotic geochemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells
TL;DR: The universal ancestor the authors infer was not a free-living cell, but rather was confined to the naturally chemiosmotic, FeS compartments within which the synthesis of its constituents occurred, leading to the emergence of prokaryotic lineages from inorganic confines.
Reference EntryDOI
Radiative Forcing of Climate Change
TL;DR: This article reviewed the current understanding of mechanisms that are, or may be, acting to cause climate change over the past century, with an emphasis on those due to human activity, and discussed the general level of confidence in these estimates and areas of remaining uncertainty.
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The Carbon cycle and atmospheric CO[2] : natural variations Archean to present
TL;DR: The paper as discussed by the authors presents the papers given at a conference on the atmospheric chemistry of carbon dioxide, including the carbon cycle, tropospheric methane cycle, the last deglaciation, reef growth, climatic change, carbon deposition rates in the Atlantic Ocean, low-latitude biomass, carbon isotopes, geochemistry, charcoal fluxes, volcanism, geologic ages, tectonics, carbonate rocks, marine surveys, and biogeochemistry.
Journal ArticleDOI
Evidence for life on Earth before 3,800 million years ago.
Stephen J. Mojzsis,Gustaf Arrhenius,Kevin D. McKeegan,T. M. Harrison,Allen P. Nutman,Clark R.L. Friend +5 more
TL;DR: In this article, ion-microprobe measurements of the carbon-isotope composition of carbonaceous inclusions within grains of apatite (basic calcium phosphate) from the oldest known sediment sequences a approx. 3,800 Myr-old banded iron formation from the Isua supracrustal belt, West Greenland and a similar formation from Akilia island that is possibly older than 3,850 Myr.
Journal ArticleDOI
Endogenous production, exogenous delivery and impact-shock synthesis of organic molecules: an inventory for the origins of life
TL;DR: 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.
Journal ArticleDOI
Organic Compound Synthesis on the Primitive Earth
Stanley L. Miller,Harold C. Urey +1 more
Journal ArticleDOI
Earth and Mars: evolution of atmospheres and surface temperatures.
Carl Sagan,George Mullen +1 more
TL;DR: Solar evolution implies, for contemporary albedos and atmospheric composition, global mean temperatures below the freezing point of seawater less than 2.3 aeons ago, contrary to geologic and paleontological evidence, but ammonia mixing ratios of the order of a few parts per million in the middle Precambrian atmosphere resolve this and other problems.