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

Abstract

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.

Mineral Radioactivity in Sands as a Mechanism for Fixation of Organic Carbon on the Early Earth

“…The early Earth received a high flux of organic molecules from extraterrestrial sources (Chyba and Sagan, 1992) that may have been photo-polymerized to a liquid organic layer at the oceanic surface (Lasaga et al, 1971;Nilson, 2002). Additionally methane from the mantle vented at the surface, concentrated in hydrothermal systems (Kelley, 1996;Holm and Charlou, 2001) and may have contributed to the formation of hydrocarbon liquids or smogs in the atmosphere (Sagan and Chyba, 1997).…”

Astrobiology and habitability studies in preparation for future Mars missions: trends from investigating minerals, organics and biota

“…Cometary and meteoritic impacts in the first 700 million years after solar system formation have delivered large amounts of organic molecules to the young planets (Chyba & Sagan 1992). Meteoritic infall to the Martian surface by small bodies and their fragmented dust is estimated to be 240 × 10 3 kg per year (Flynn 1996).…”

“…Several endogenous pathways for the production of organic matter on Mars have been proposed (e.g. Chyba & Sagan 1992;Bada & McDonald 1995). Influx of organic material from small bodies must have occurred with certainty (Flynn 1996).…”

“…Endogenous production of organic material on Mars may have proceeded through similar mechanisms as suggested for the early Earth in the presence of energy sources such as lightning, UV and particle radiation, atmospheric shocks, etc. (Chyba & Sagan 1992;Ehrenfreund et al 2006). Life on Earth originated approximately 3.5 billion years ago (Derenne et al 2008;Westall 2009) and has adapted to nearly every explored environment.…”

Organic chemistry in circumstellar envelopes: Setting the stage for prebiotic synthesis

“…Shortly thereafter, life is thought to have developed (Furnes et al 2004). It has been suggested for some time that exogenous delivery of organics to Earth during this period may have provided the necessary carbonaceous material that gave a "jump-start" to life (Anders 1989, Chyba & Sagan 1992. The organic matter brought by meteorites, comets, and dust particles probably had in part an interstellar origin, as suggested by isotopic studies (e.g., Pizzarello et al 2008, Keller et al 1994, Mumma et al 2002.…”