A Caltech Library Service

Tracing the ingredients for a habitable earth from interstellar space through planet formation

Bergin, Edwin A. and Blake, Geoffrey A. and Ciesla, Fred and Hirschmann, Marc M. and Li, Jie (2015) Tracing the ingredients for a habitable earth from interstellar space through planet formation. Proceedings of the National Academy of Sciences of the United States of America, 112 (29). pp. 8965-8970. ISSN 0027-8424. PMCID PMC4517224.

[img] PDF - Published Version
See Usage Policy.

[img] PDF - Supplemental Material
See Usage Policy.


Use this Persistent URL to link to this item:


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.

Item Type:Article
Related URLs:
URLURL TypeDescription Information CentralArticle
Bergin, Edwin A.0000-0003-4179-6394
Blake, Geoffrey A.0000-0003-0787-1610
Li, Jie0000-0002-3733-4587
Additional Information:© 2015 National Academy of Sciences. Edited by Mark H. Thiemens, University of California at San Diego, La Jolla, CA, and approved June 5, 2015 (received for review January 15, 2015). Published ahead of print July 6, 2015. We are grateful to I. Cleeves for providing the kinetic chemical calculations from published work and to both anonymous referees for providing useful feedback that improved this manuscript. This work was supported by funding from the National Science Foundation Grant AST-1344133 (INSPIRE) (to E.A.B., G.A.B., J.L., and M.M.H.); F.C. was supported by NASA Grant NNX12AD59G. Author contributions: E.A.B., G.A.B., F.C., M.M.H., and J.L. designed research; E.A.B., G.A.B., F.C., M.M.H., and J.L. performed research; and E.A.B., G.A.B., F.C., M.M.H., and J.L. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. This article contains supporting information online at
Funding AgencyGrant Number
Subject Keywords:terrestrial worlds; elements; interstellar medium; comets; meteorites
Issue or Number:29
PubMed Central ID:PMC4517224
Record Number:CaltechAUTHORS:20150710-133724040
Persistent URL:
Official Citation:Edwin A. Bergin, Geoffrey A. Blake, Fred Ciesla, Marc M. Hirschmann, and Jie Li Tracing the ingredients for a habitable earth from interstellar space through planet formation PNAS 2015 112 (29) 8965-8970; published ahead of print July 6, 2015, doi:10.1073/pnas.1500954112
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:58851
Deposited By: Tony Diaz
Deposited On:15 Jul 2015 19:42
Last Modified:09 Mar 2020 13:19

Repository Staff Only: item control page