Carbonaceous Chondrites, Comets, Cosmic Dust and Life

Dear Colleagues,

The origin of life is an enigma that is one of the greatest unsolved mysteries of Science. Did life originate on Earth or is it widely distributed throughout the Universe? The widely-accepted hypothesis of the endogenous origin of life on Earth has become a virtual paradigm. Life is found on Earth wherever water, energy and a small group of biogenic elements (C, H, O, N, P, S, etc.) co-exist. All known forms of life are carbon-based even though carbon is extremely depleted in the inner Solar System and approaches photospheric abundances only beyond the main asteroid belt. Comets, asteroids, cosmic dust and carbonaceous chondrites deliver ~3 to 300 metric tons of carbon-rich debris to Earth every day. Aqueously altered minerals in CI and CM carbonaceous chondrites establish that low temperature liquid water existed on their parent bodies. Carbonaceous chondrites contain a vast array of organic/prebiotic chemicals and life-critical biomolecules. These include 8 of the 20 protein amino acids; 3 of the 5 nucleobases as well as fatty acids, sugar alcohols, carboxylic acids, long chain nonpolar hydrocarbons, etc. Biomolecules in carbonaceous meteorites were originally dismissed as terrestrial contaminants. Recent measurements of chirality and the fractionation of isotopes of carbon (d ¹³C) and nitrogen (d ¹⁵N) have proven they are indigenous and extraterrestrial. The absence of unstable protein amino acids, nucleobases, DNA and other biomolecules show that the meteorites are not contaminated by modern terrestrial life. Carbon isotope studies of d¹³C show that the valine, alanine, glycine and glutamic acid in the Murchison meteorite are both indigenous and extraterrestrial and that they exhibit a relationship analogous to the biological fractionation of these amino acids in terrestrial microorganisms. Glycine was found in Stardust samples returned from comet Wild 2 and detected in the coma of 67P/Churymov-Gerasimenko by Rosetta along with water, methane and very abundant molecular oxygen by the Alice Spectrograph (O₂/H₂O 11-68%). Molecular oxygen is highly reactive and replenished into Earth's atmosphere by oxygenic photosynthesis of living organisms. The co-existence of molecular oxygen with water and methane in the atmosphere of an exoplanet has long been considered a valid signature of life. Carbon in dust particles emitted by the comet is bound in large kerogen-like macromolecular compounds analogous to the insoluble organic matter (approximate elemental composition C₁₀₀H₄₆N₁₀O₁₅S_4.5) that comprises the bulk of the carbon present in carbonaceous chondrites.

Biomolecules and microfossils from ancient crustal rocks and vents indicate life appeared on Earth earlier than 4.1-3.8 gya. The study of the living and fossil life forms found on planet Earth, the most informative body of our Solar System, is of profound importance to understanding how to recognize the remains of life in ancient terrestrial and extraterrestrial materials. Bacterial Paleontology discoveries have shown that life on Earth had a much greater spatial, temporal and environmental distribution than previously thought possible. Recognizable fossils of life forms found in carbonaceous chondrites merits serious consideration. Recent space missions have shown that prokaryotic and eukaryotic extremophiles can survive exposure to the vacuum and radiation environment of space and it has been shown that many species of microorganisms can remain viable after cryopreservation for millions of years in glacial ice, permafrost, and salt crystals. The view that comets, cosmic dust and carbonaceous meteorites may have played a major role in the distribution and delivery of pre-biotic molecules to early Earth has now become more acceptable by the scientific community. It has recently been discovered that irradiation of formamide (NH₂CHO) by 170 MeV protons using powdered meteorites as a catalyst can yield all 5 nucleobases as well as nucleosides, ribose, deoxyribose, carboxylic acids and a host of other organic/pre-biotic chemicals and life-critical biomolecules. Formamide has been observed on several comets and is a common constituent of star forming regions that foster planetary systems within the galactic habitable zone. The synthesis, concentration and amplification of life-critical biomolecules on icy moons or cometary nuclei could dramatically alter the temporal, spatial and environmental parameters for the origin of life. By addressing these recent discoveries, and new developments in biochemistry, paleontology, molecular biology and meteoritic research this Special Issue will consider the role that carbonaceous meteorites, comets and cosmic dust may have played in the origin, distribution, and evolution of pre-biotic organic chemicals, biomolecules and life.

Prof. Richard B. Hoover
Academician Alexei Yu. Rozanov
Academician Eric M. Galimov
Guest Editors

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