title: Tracing the ingredients for a habitable earth from interstellar space through planet formation.
abstract: We use the C/N ratio as a monitor of the delivery of key ingredients of life to nascent terrestrial worlds. Total elemental C and N contents, and their ratio, are examined for the interstellar medium, comets, chondritic meteorites, and terrestrial planets; we include an updated estimate for the bulk silicate Earth (C/N = 49.0 ± 9.3). Using a kinetic model of disk chemistry, and the sublimation/condensation temperatures of primitive molecules, we suggest that organic ices and macromolecular (refractory or carbonaceous dust) organic material are the likely initial C and N carriers. Chemical reactions in the disk can produce nebular C/N ratios of ∼1-12, comparable to those of comets and the low end estimated for planetesimals. An increase of the C/N ratio is traced between volatile-rich pristine bodies and larger volatile-depleted objects subjected to thermal/accretional metamorphism. The C/N ratios of the dominant materials accreted to terrestrial planets should therefore be higher than those seen in carbonaceous chondrites or comets. During planetary formation, we explore scenarios leading to further volatile loss and associated C/N variations owing to core formation and atmospheric escape. Key processes include relative enrichment of nitrogen in the atmosphere and preferential sequestration of carbon by the core. The high C/N bulk silicate Earth ratio therefore is best satisfied by accretion of thermally processed objects followed by large-scale atmospheric loss. These two effects must be more profound if volatile sequestration in the core is effective. The stochastic nature of these processes hints that the surface/atmospheric abundances of biosphere-essential materials will likely be variable.
authors.affiliation: Department of Astronomy, University of Michigan, Ann Arbor, MI 48109-1107; ebergin@umich.edu.
authors.forename: Edwin A
authors.initials: EA
authors.lastname: Bergin
authors.affiliation: Division of Geological & Planetary Sciences, California Institute of Technology, Pasadena, CA 91125;
authors.forename: Geoffrey A
authors.initials: GA
authors.lastname: Blake
authors.affiliation: Department of the Geophysical Sciences and Chicago Center for Cosmochemistry The University of Chicago, Chicago, IL 60637;
authors.forename: Fred
authors.initials: F
authors.lastname: Ciesla
authors.affiliation: Department of Earth Sciences, University of Minnesota, Minneapolis, MN 55455;
authors.forename: Marc M
authors.initials: MM
authors.lastname: Hirschmann
authors.affiliation: Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109.
authors.forename: Jie
authors.initials: J
authors.lastname: Li
mesh_terms: D002244:Carbon / Q000032:analysis; D003198:Computer Simulation; D018534:Earth, Planet; D018559:Exobiology; D005118:Extraterrestrial Environment; D019015:Geologic Sediments / Q000737:chemistry; D007053:Ice; D007700:Kinetics; D008956:Models, Chemical; D009010:Monte Carlo Method; D009584:Nitrogen / Q000032:analysis; D012825:Silicon / Q000032:analysis; D055609:Stars, Celestial
chemical_list: D007053:Ice; D002244:Carbon; D009584:Nitrogen; D012825:Silicon
keywords: comets; elements; interstellar medium; meteorites; terrestrial worlds