Biosphere Origin and Evolution

pp 55-68

Comets, Carbonaceous Meteorites, and the Origin of the Biosphere

  • Hoover R.B.Affiliated withAstrobiology Laboratory, NASA/NSSTC

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Evidence for indigenous microfossils in carbonaceous meteorites suggests that the paradigm of the endogenous origin of life on Earth should be reconsidered. It is now widely accepted that comets and carbonaceous meteorites played an important role in the delivery of water, organics and life-critical biogenic elements to the early Earth and facilitated the origin and evolution of the Earth’s biosphere. However, the detection of embedded microfossils and mats in carbonaceous meteorites implies that comets and meteorites may have played a direct role in the delivery of intact microorganisms and that the biosphere may extend far into the cosmos. Recent space observations have found the nuclei of comets to have very low albedos (˜0.03) and these jet-black surfaces can become very hot (T ˜ 400 K) near perihelion. This chapter reviews recent observational data on comets and suggests that liquid water pools could exist in cavities and fissures between the internal ices and rocks and the exterior carbonaceous crust. The presence of light and liquid water near the surface of the nucleus enhances the possibility that comets could harbor prokaryotic extremophiles (e.g., cyanobacteria, sulfur bacteria and archaea) capable of growth over a wide range of temperatures. The hypothesis that comets are the parent bodies of the CI1 and the CM2 carbonaceous meteorites is advanced. Electron microscopy images will be presented showing forms interpreted as indigenous microfossils embedded in freshly fractured interior surfaces of the Orgueil (CI1) and Murchison (CM2) meteorites. The size range and morphological characteristics of these forms are consistent with known representatives of morphotypes of all five subsections (orders) of Phylum Cyanobacteria. Energy dispersive X-ray spectroscopy (EDS) elemental data show that the forms in the meteorites have anomalous C/N and C/S as compared with modern extremophiles and cyanobacteria. These images and spectral data indicate that the clearly biogenic and embedded remains cannot be interpreted as recent biological contaminants and therefore are indigenous microfossils in the meteorites.