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.

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 pyrolysis demonstrates that organics will not survive impacts at velocities greater than about 10 kilometers per second and that even comets and asteroids as small as 100 meters in radius cannot be aerobraked to below this velocity in 1-bar atmospheres. However, for plausible dense (10-bar carbon dioxide) early atmospheres, we find that 4.5 x 10(9) years ago Earth was accreting intact cometary organics at a rate of at least approximately 10(6) to 10(7) kilograms per year, a flux that thereafter declined with a half-life of approximately 10(8) years. These results may be put in context by comparison with terrestrial oceanic and total biomasses, approximately 3 x 10(12) kilograms and approximately 6 x 10(14) kilograms, respectively.

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Voyager 2 images of Neptune reveal a windy planet characterized by bright clouds of methane ice suspended in an exceptionally clear atmosphere above a lower deck of hydrogen sulfide or ammonia ices. Neptune's atmosphere is dominated by a large anticyclonic storm system that has been named the Great Dark Spot (GDS). About the same size as Earth in extent, the GDS bears both many similarities and some differences to the Great Red Spot of Jupiter. Neptune's zonal wind profile is remarkably similar to that of Uranus. Neptune has three major rings at radii of 42,000, 53,000, and 63,000 kilometers. The outer ring contains three higher density arc-like segments that were apparently responsible for most of the ground-based occultation events observed during the current decade. Like the rings of Uranus, the Neptune rings are composed of very dark material; unlike that of Uranus, the Neptune system is very dusty. Six new regular satellites were found, with dark surfaces and radii ranging from 200 to 25 kilometers. All lie inside the orbit of Triton and the inner four are located within the ring system. Triton is seen to be a differentiated body, with a radius of 1350 kilometers and a density of 2.1 grams per cubic centimeter; it exhibits clear evidence of early episodes of surface melting. A now rigid crust of what is probably water ice is overlain with a brilliant coating of nitrogen frost, slightly darkened and reddened with organic polymer material. Streaks of organic polymer suggest seasonal winds strong enough to move particles of micrometer size or larger, once they become airborne. At least two active plumes were seen, carrying dark material 8 kilometers above the surface before being transported downstream by high level winds. The plumes may be driven by solar heating and the subsequent violent vaporization of subsurface nitrogen.