Cosmic Carbon Chemistry: from the Interstellar Medium to the Early Earth

Downloaded from cshperspectives.cshlp.org on October 22, 2010 - Published by Cold Spring Harbor Laboratory Press Cosmic Carbon Chemistry: From the Interstellar Medium to the Early Earth Pascale Ehrenfreund1,2 and Jan Cami3,4 1Leiden Institute of Chemistry, 2300 RA Leiden, The Netherlands 2Space Policy Institute, Washington DC 3Department of Physics and Astronomy, UWO, London, ON, Canada 4SETI Institute, Mountain View, California 94043 Correspondence: [email protected] Astronomical observations have shown that carbonaceous compounds in the gas and solid state, refractory and icy are ubiquitous in our and distant galaxies. Interstellar molecular clouds and circumstellar envelopes are factories of complex molecular synthesis. A surpris- ingly large number of molecules that are used in contemporary biochemistry on Earth are found in the interstellar medium, planetary atmospheres and surfaces, comets, asteroids and meteorites, and interplanetary dust particles. In this article we review the current knowl- edge of abundant organic material in different space environments and investigate the con- nection between presolar and solar system material, based on observations of interstellar dust and gas, cometary volatiles, simulation experiments, and the analysis of extraterrestrial matter. Current challenges in astrochemistry are discussed and future research directions are proposed. arbon is a key element in the evolution of variety of organic molecules in circumstellar Cprebiotic material (Henning and Salama and interstellar environments. During the for- 1998), and becomes biologically interesting mation of the solar system, this interstellar in compounds with nitrogen, oxygen and organic material was chemically processed and hydrogen. Our understanding of the evolution later integrated in the presolar nebula from of organic molecules—including such com- which planets and small solar system bodies pounds—and their voyage from molecular formed. The remnant planetesimals in the clouds to the early solar system and Earth pro- form of comets and asteroids impacted the vides important constraints on the emergence young planets in the early history of the solar of life on Earth and possibly elsewhere (Ehren- system (Gomes et al. 2005). The large quantities freund and Charnley 2000). Figure 1 shows of extraterrestrial material delivered to young the cycle of organic molecules in the universe. planetary surfaces during the heavy bombard- Gas and solid-state chemical reactions form a ment phase may have played a key role in life’s Editors: David Deamer and Jack W. Szostak Additional Perspectives on The Origins of Life available at www.cshperspectives.org Copyright # 2010 Cold Spring Harbor Laboratory Press; all rights reserved. Advanced Online Article. Cite this article as Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a002097 1 Downloaded from cshperspectives.cshlp.org on October 22, 2010 - Published by Cold Spring Harbor Laboratory Press P. Ehrenfreund and J. Cami Complex carbon chemistry in Organic chemistry circumstellar envelopes of in interstellar clouds evolved stars. Processing of organics in presolar nebula; condensation into asteroids, comets and planets. Meteoritic infall of organic matter Figure 1. Carbon pathways between interstellar and circumstellar regions and the forming solar system. origin (Chyba and Sagan 1992, Ehrenfreund during stellar collapse and supernova explo- et al. 2002). How elements are formed, how sions. In the denser regions of interstellar space, complex carbonaceous molecules in space are, the so-called interstellar clouds, active chemical what their abundance is and on what timescales pathways form simple and complex carbon mol- they form are crucial questions within cosmo- ecules from carbon atoms (van Dishoeck and chemistry. Blake1998).Circumstellarenvelopesareregarded as the largest factories of carbon chemistry in space (Kwok 2004, 2009). Inventory of Cosmic Carbon Organic molecules in the solar system are Carbon is found in space in all its allotropic found in planetary atmospheres and on the sur- forms: diamond, graphite, and fullerene (Ca- face of many outer solar system moons (e.g., taldo et al. 2004). Astronomical observations Cruikshanket al. 2005; Raulin 2008; Lorenz et al. in the last decade have shown that carbonaceous 2008). More than 50 molecules have been iden- compounds (gaseous molecules and solids) are tified in cometary comae (Crovisier et al. 2009). ubiquitous in our own as well as in distant gal- Many small organic molecules observed in axies (Ehrenfreund et al. 2006a). The first chem- cometary comae probably originate wholly or ical enrichment of the universe may likely be partially from the decomposition of larger mol- connected to the first generation of stars ecules or particles, indicating that large poly- (Spaans 2004). Large carbon abundances are mers such as polyoxymethylene and HCN- already extrapolated from observations of the polymers are present in comets (Ehrenfreund strong C[II] and CO lines in the hosts of the et al. 2004; Cottin and Fray 2008). Carbona- most distant quasars (Bertoldi et al. 2003). ceous chondrites (meteorites) and micromete- Carbon in space was first produced in stellar orites do contain a variety of organics (e.g., see interiors in fusion reactions and was later Alexander et al. 2007; Sephton 2002; Septhon ejected into interstellar and intergalactic space and Botta 2005 for reviews). They are fragments 2 Advanced Online Article. Cite this article as Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a002097 Downloaded from cshperspectives.cshlp.org on October 22, 2010 - Published by Cold Spring Harbor Laboratory Press Cosmic Carbon Chemistry of cometary and asteroidal bodies. Investigating followed by CO, the most abundant carbon- 24 their organic composition often indicates the containing species, with CO/H2 10 . nature of the parental body (Hiroi et al. 1993; In cold dark clouds with a temperature of Ehrenfreund et al. 2001; Nesvorny et al. 2009). 3–10 K the sticking coefficient of most atoms and molecules is close to unity and all species (except H and He) freeze out (Ehrenfreund Cosmic Cycling of Organics 2 and Charnley 2000). At 10 K only H, D, C, O, The interstellar medium, the space between the and N atoms have sufficient mobility to interact stars, is composed primarily of H and He and on the surfaces of grains. Dark clouds offer a constitutes a few percent of the galactic mass. protected environment for the formation of Interstellar material is dominated by interstellar larger molecules. Those regions have a rather gas (99%). The remaining 1% is composed of high density (106/ cm3) and experience a low solid silicate and carbon-based mm-sized dust radiation field of 103 photons cm2/s induced particles present throughout interstellar clouds by cosmic rays (Prasad and Tarafdar 1983). that provide surfaces for accretion of gas phase The diffuse interstellar medium is charac- species and subsequent grain surface chemistry terized by a low density (103 atoms/cm3) and (Ehrenfreund and Charnley 2000; Ehrenfreund temperatures 100 K. Diffuse clouds are fila- and Fraser 2003). Fundamental physical param- mentary structures that surround cold dense eters such as temperature and density vary interstellar regions. Ices are not present in those strongly across the spectrum of interstellar regions and a strong radiation field of 108 regions. The diversity of interstellar clouds photons/cm2/s (Mathis et al. 1983) dominates results from energy injected by supernova the formation and evolution of molecules and shockwaves and stellar outflows and radiative larger structures. Small carbonaceous mole- losses (Wolfire et al. 2003). The most recent cules in the gas phase are easily destroyed by classification by Wooden et al. (2004) describes radiation. Atoms with ionization potentials less in detail very low-density hot gas, environments than 13.6 eV are photoionized. The identifica- with warm intercloud gas, and regions with tion of many small molecules in dense clouds denser and colder material, see Table 1. Cosmic implies that their destruction is well balanced abundances in the interstellar medium are by active formation routes. Ion-molecule reac- derived by measuring elemental abundances tions,dissociative recombinationwithelectrons, in stellar atmospheres. These cosmic elemental radiative association reactions and neutral- abundances determine the amount of elements neutral reactions contribute to gas phase proc- available for the formation of molecules and esses and influence interstellar chemistry (Snow particles. Gas phase and solid-state reactions and McCall 2006). and gas-grain interactions lead to the formation “Stardust,” in the form of dust and mole- of complex molecules. H2 is by far the most cules, is injected by stellar sources in their abundant molecule in cold interstellar regions, late stage of evolution into interstellar clouds. Table 1. Phases of the interstellar medium (adapted from Wooden et al. 2004). ISM component Designation Temperature (K) Density [cm23] Hot ionized medium coronal gas 106 0.003 Warm ionized medium diffuse ionized gas 104 .10 Warm neutral medium intercloud HI 104 0.1 Atomic cold neutral medium diffuse clouds 100 10–100 Molecular cold neutral medium dark clouds, molecular ,50 103 –105 clouds, dense clouds Molecular hot cores protostellar cores 100–300 .106 Adapted from Wooden et al. 2004. Advanced Online Article. Cite this article as Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a002097 3 Downloaded

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