Survival of prebiotic organic compounds during exogenous delivery: Implications for the origin of life on Earth and potentially on Mars

The origin of life on Earth and other planets such as Mars would have required the presence of liquid water and a continuous supply of organic carbon. Although large bodies of water were present on the early Earth and most likely on Mars, the source of organic compounds is not clear. One potential source of prebiotic organic compounds, including amino acids, would be a Miller-Urey “home-grown” type synthesis, which would require a reducing atmosphere. However, it has been argued that the atmosphere of the early Earth was non-reducing, which suggests that other sources of prebiotic organic compounds may have been required for the origin of life. Exogenous delivery of organics by asteroids, comets and interplanetary dust particles (IDPs) is one potential source.; IDPs and micrometeorites could have been a major source of the Earth's prebiotic organic inventory as these particles supply the bulk of extraterrestrial debris accreted to the Earth each year. One problem associated with micrometeorite delivery however, is that these grains can be heated and melted at temperatures up to 1200°C during atmospheric entry. Although most organic compounds are destroyed at high temperature, amino acids are volatile under reduced pressure at temperatures above ∼150°C and could therefore survive atmospheric entry by subliming from the meteorite grains. To test this hypothesis, grains from the Murchison meteorite were heated under reduced pressure inside a glass sublimation apparatus, the sublimed organic material collected on a cold finger and then analyzed for amino acids by high performance liquid chromatography (HPLC). The results suggest that glycine is the only amino acid that will sublime from micrometeorites and survive atmospheric entry at temperatures above ∼550°C. Thus, glycine may have been the most abundant amino acid in the prebiotic soup of the early Earth and possibly even on Mars.; In order to search for evidence of organic compounds derived from Mars, the Martian meteorites ALH84001 and Nakhla were analyzed for amino acids. Amino acids were detected in acid hydrolyzed hot water extracts of these meteorites by HPLC. However, the distribution and enantiomeric abundances of amino acids found in these meteorites were nearly identical to terrestrial amino acids found in the landing site environments. The terrestrial amino acid contamination of Martian meteorites appears to be a very rapid process, which makes it difficult to assess whether any organic compounds important to biology as we know it are present on Mars.