Carl Edward Sagan

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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(More)
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. Ammonia mixing ratios of the order of a few parts per million in the middle Precambrian atmosphere resolve this and other problems. Possible(More)
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(More)
It has long been speculated that Earth accreted prebiotic organic molecules important for the origins of life from impacts of carbonaceous asteroids and comets during the period of heavy bombardment 4.5 x 10(9) to 3.8 x 10(9) years ago. A comprehensive treatment of comet-asteroid interaction with the atmosphere, surface impact, and resulting organic(More)
As Voyager 1 flew through the Saturn system it returned photographs revealing many new and surprising characteristics of this complicated community of bodies. Saturn's atmosphere has numerous, low-contrast, discrete cloud features and a pattern of circulation significantly different from that of Jupiter. Titan is shrouded in a haze layer that varies in(More)
The cameras aboard Voyager 1 have provided a closeup view of the Jupiter system, revealing heretofore unknown characteristics and phenomena associated with the planet's atmosphere and the surfaces of its five major satellites. On Jupiter itself, atmospheric motions-the interaction of cloud systems-display complex vorticity. On its dark side, lightning and(More)
Matthews (1992) has proposed that HCN "polymer" is ubiquitous in the solar system. We apply vacuum deposition and spectroscopic techniques previously used on synthetic organic heteropolymers (tholins), kerogens, and meteoritic organic residues to the measurement of the optical constants of poly-HCN in the wavelength range 0.05-40 micrometers. These(More)
In its December 1990 fly-by of Earth, the Galileo spacecraft found evidence of abundant gaseous oxygen, a widely distributed surface pigment with a sharp absorption edge in the red part of the visible spectrum, and atmospheric methane in extreme thermodynamic disequilibrium; together, these are strongly suggestive of life on Earth. Moreover, the presence(More)