Acyclic hydrocarbon environments ⩾ n-C 18 on the early terrestrial planets

Abstract

The possible occurrence on the surface of the early Earth, Mars and Venus of hydrocarbon environments mainly composed by acyclic alkane molecules ⩾ n-C 18 has been revised. These hydrocarbons could be accumulated from the contribution of endogenous Fischer–Tropsh-type reactions and post-impact recombination reactions, as well as from exogenous sources such as comets, meteorites and dust particles. Such heavy alkane environments could offer protection for the synthesis and survival of biomolecules on the early terrestrial planets. Amounts of heavy n-alkanes delivered by large impactors, dust particles or produced by post-impact recombination on Venus would have been higher than those delivered or produced by the same sources on Earth and Mars before 3600 Myr ago. However, the high values of the total frequency of impacts by bolides >14-km in diameter estimated in this time period (viz. 3.9×10 3, Mars; 2.2×10 4, Earth, and 3.8×10 4 Venus) and the high surface temperatures generated by those impactors suggest the existence of very unstable conditions on the early terrestrial planets for the survival and long-term accumulation of acyclic hydrocarbons. Therefore, the most significant accumulation of n-alkanes could have occurred only during the longer intervals (10 5– 10 7 yr ) between each impact through the contribution mainly of IDPs, and thereby a high decomposition rate would be expected for the accumulated n-alkanes by successive impacts. Amounts of n-alkanes accumulated from IDPs in these intervals have been estimated between 2.3×10 9 and 2.2×10 10 kg 3600– 3800 Myr ago. These processes are expected to occur on other planetary bodies or satellites belonging to our solar system and probably in analogs of the early solar system.