Cometary Material and the Origins of Life on Earth

Abstract

Comets are the most pristine minor bodies in the Solar System. With the exception of phosphorus, all the organogenic elements (H, C. N, O, S, P) which are necessary for life have been found in comets. By comparing their carbon abundances with those of carbonaceous chondrites it has been estimated that 10% or more of the cometary matter is made of organic compounds. The nature of the chemical species detected in comets suggests that the complexity of their parent molecules is at least comparable to that of interstellar molecules.

The possible sources and origins of cometary organic molecules are: (1) the primordial solar nebula, which must have been the primary source of all the organic and inorganic compounds in comets; (2) the organic molecules formed by the ionized radiation resulting from the decay of ²⁶Al and other short-lived presolar isotopes during the early stages of differentiation of the solar nebula; (3) organic compounds produced by the cosmic-ray heating of the matter in the Opik-Oort cloud during its lifetime, and (4) the more complex organic compounds synthesized in the icy surface layers of the cometary nuclei by the action of the solar wind, UV radiation and induced exothermic reactions as comets approach perihelion.

The reactivity of some of the chemical species detected in comets (H, OH, NH₂, CO, CS, C₂, C₃, HCN, CH₃CN, H₂O) indicate that a number of biochemical compounds, such as amino acids, purines, pyrimidines, etc., may be present in cometary nuclei. However, due to the small mass (~10¹⁸g), low temperatures (<200°K) and other environmental conditions, such as the absence of a proper atmosphere, hydrosphere and lithosphere as on the Earth, it is not likely that the processes of organic synthesis in comets have proceeded beyond the stage of chemical evolution of the organic matter in carbonaceous chondrites.

Although it is highly improbable that life could have appeared in comets, the antiquity of terrestrial life, as is indicated by the presence of a complex fossil microbiota of 3.5 × 10⁹ years old suggests that life emerged on Earth during the time in which the influx of cometary material was considerably higher than the present values. Different estimates have shown that the Earth acquired about 10²² grams of carbon by accretion of cometary nuclei during its early history. It is likely that most of the cometary carbon compounds were decomposed into highly reactive chemical species by the heat and shock waves of cometary collisions, thus creating transient, relatively reducing atmospheric environments in which abiotic organic synthesis occurred by quenching and recombination of the reactive species. Furthermore, upon settling to the Earth’s surface, some of these compounds must have given rise through different condensation reactions to the formation of the more complex biochemical monomers and polymers essential to living systems. Therefore, we can conclude that comets contributed significantly to the processes of chemical evolution necessary for the emergence of life on the Earth.