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«INSTITUTE OF PHYSICS PUBLISHING REPORTS ON PROGRESS IN PHYSICS Rep. Prog. Phys. 65 (2002) 1427–1487 PII: S0034-4885(02)04039-3 Astrophysical and ...»

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Rep. Prog. Phys. 65 (2002) 1427–1487 PII: S0034-4885(02)04039-3

Astrophysical and astrochemical insights into the

origin of life

P Ehrenfreund1,2, W Irvine3, L Becker4, J Blank5, J R Brucato6, L Colangeli6,

S Derenne7, D Despois8, A Dutrey9, H Fraaije2, A Lazcano10, T Owen11, F Robert12, an International Space Science Institute ISSI-Team13 Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands Soft Matter/Astrobiology Laboratory, Leiden Institute of Chemistry, PO Box 9502, 2300 RA Leiden, The Netherlands Astronomy Department, 619 Lederle Graduate Research Center, University of Massachusetts, Amherst, MA 01003, USA University of California, Institute of Crystal Studies, Department of Geological Sciences, 1148 Girvetz Hall, Santa Barbara, CA 93106, USA Lawrence Livermore National Laboratory, H-Division/Shock Physics Group, PO Box 808, L-415 Livermore, CA 94551, USA INAF-Osservatorio Astronomico di Capodimonte via Moiariello 16, I-80131, Napoli, Italy Laboratoire de Chimie Bioorganique et Organique Physique, UMR CNRS 7573, Ecole Nationale Superieure de Chimie de Paris, 11, rue Pierre et Marie Curie, 75231 Paris Cedex 05, France Observatoire Aquitain des Sciences de l’Univers (OASO), BP 89, F-33270 Floirac, France Laboratoire d’Astrophysique de l’Observatoire de Grenoble (LAOG), BP 53, F-38041 Grenoble Cedex 9, France Facultad de Ciencias, UNAM, Apdo. Postal 70-407, Cd. Universitaria, 04510 Mexico DF, Mexico Institute for Astronomy, 2680 Woodlawn Ave, Honolulu HI 96822, USA Laboratoire de Mineralogie, Museum National d’Histoire Naturelle, 61 rue Buffon 75005 Paris, France ISSI Team: ‘Prebiotic matter in space’ (all the authors belong to that team) Received 27 March 2002, in final form 16 July 2002 Published 23 August 2002 Online at stacks.iop.org/RoPP/65/1427 Abstract Stellar nucleosynthesis of heavy elements such as carbon allowed the formation of organic molecules in space, which appear to be widespread in our Galaxy. The physical and chemical conditions—including density, temperature, ultraviolet (UV) radiation and energetic particles—determine reaction pathways and the complexity of organic molecules in different space environments. Dense interstellar clouds are the birth sites of stars of all masses and their planetary systems. During the protostellar collapse, interstellar organic molecules in gaseous and solid phases are integrated into protostellar disks from which planets and smaller solar 0034-4885/02/101427+61$90.00 © 2002 IOP Publishing Ltd Printed in the UK 1427 1428 P Ehrenfreund et al system bodies form. After the formation of the planets 4.6 billion years ago, our solar system, including the Earth, was subjected to frequent impacts for several hundred million years. Life on Earth may have emerged during or shortly after this heavy bombardment phase, perhaps as early as 3.90–3.85 billion years ago, but the exact timing remains uncertain. A prebiotic reducing atmosphere, if present, predicts that building blocks of biopolymers—such as amino acids, sugars, purines and pyrimidines—would be formed in abundance. Recent modelling of the Earth’s early atmosphere suggests, in contrast, more neutral conditions (e.g. H2 O, N2, CO2 ), thus, precluding the formation of significant concentrations of prebiotic organic compounds.

Moreover, even if the Earth’s atmosphere were reducing, the presence of UV photons would readily destroy organic compounds unless they were quickly sequestered away in rocks or in the prebiotic ocean. Other possible sources of organic compounds would be high temperature vent chemistry, although the stability of such compounds (bases, amino acids) in these environments remains problematic. Finally, organic compounds may have been delivered to the Earth by asteroids, comets and smaller fragments, such as meteorites and interplanetary dust particles.

It is likely that a combination of these sources contributed to the building blocks of life on the early Earth. It may even have taken several starts before life surpassed the less than ideal conditions at the surface. What is certain is that once life emerged, it learned to adapt quickly taking advantage of every available refuge and energy source (e.g. photosynthesis and chemosynthesis), an attribute that eventually led to complex metabolic life and even our own existence.

Current experimental research investigating the origin of life is focused on the spontaneous formation of stable polymers out of monomers. However, understanding the spontaneous formation of structure is not enough to understand the formation of life. The introduction and evolution of information and complexity is essential to our definition of life. The formation of complexity and the means to distribute and store information are currently being investigated in a number of theoretical frameworks, such as evolving algorithms, chaos theory and modern evolution theory.

In this paper we review the physical and chemical processes that form and process organic matter in space. In particular we discuss the chemical pathways of organic matter in the interstellar medium, its evolution in protoplanetary disks and its integration into solar system material. Furthermore, we investigate the role of impacts and the delivery of organic matter to the prebiotic Earth. Processes that may have assembled prebiotic molecules to produce the first genetic material and ideas about the formation of complexity in chemical networks are also discussed.

Astrophysical and astrochemical insights into the origin of life 1429

1. Introduction

The story of astrobiology and life in the Universe begins with the synthesis of the elements that play key roles in life as we know it: hydrogen, carbon, oxygen, nitrogen, sulfur and phosphorus. Other elements are required by specific terrestrial life forms, or perhaps by the Earth’s biosphere as an interacting whole, but might be replaceable by different chemical elements under somewhat different initial conditions.

Hydrogen is effectively primordial, having been formed from the soup of quarks that filled the Universe in the earliest stages of the Big Bang. All the other chemical elements that are featured in terrestrial biochemistry were formed by nucleosynthesis during the course of stellar evolution. Carbon, the basis of organic chemistry and the lightest of the ‘biogenic’ elements, is produced by the so-called triple-α process (3×4 He →12 C) in the cores of stars more massive than half a solar mass. Such stars form carbon after they have fused a significant fraction of their core hydrogen into helium and evolved off the Main Sequence (see review by Trimble (1997) and figure 1). In the same cores, some carbon is further processed to the principal isotope of oxygen by 12 C + 4 He →16 O. The fraction of the C and O produced that is delivered to the interstellar medium (ISM), from where it can be incorporated into a new generation of stars (such as the Sun), depends on the mass of the synthesizing star and its resulting final fate (e.g. Matteucci (1991)).

In the cores of evolving, massive (M 10 solar masses) stars, heavy element nuclear ‘burning’ produces the most abundant isotopes of sulfur (32 S) and phosphorus (31 P). Obviously S is built from eight 4 He units, although the detailed process is more complicated than simple successive addition of α-particles to C or O, because there are bottle necks along the route (Trimble 1997). A primary source of phosphorus in these stars is the reaction O + 16 O →31 P + 1 H (e.g. Hansen and Kawaler (1994)).

In contrast to the ‘primary’ production of C and O, nitrogen is a ‘secondary’ product, in the sense that its synthesis requires the presence of pre-existing C or O from earlier generations of stars. In particular, nitrogen is a by-product of hydrogen fusion to helium through the CN or the CNO cycles in the cores of massive stars and the shells of evolved lower mass stars, where C or O act as catalyst for the hydrogen ‘burning’ (e.g. Shu (1982)).

The step from atomic nuclei to molecules begins with the expulsion of nucleosynthetic products into the ISM by stellar winds, planetary nebula ejection and supernova explosions.

–  –  –

These events include the local formation of gas phase molecules and dust grains, the latter being predominantly organic or silicate when the progenitor star is either carbon- or oxygenrich. The molecules, which can include a rich array of organic species (e.g. Guelin et al (2000), Cernicharo (2000)), are probably mostly dissociated by ultraviolet (UV) radiation before reaching the shelter of a dense interstellar cloud. In contrast, both silicate and organic dust grains can survive for extended periods of time in the diffuse ISM and may then be incorporated into dense clouds, where they may acquire icy mantles, be processed in various ways, and serve as sites for molecular synthesis.

Interstellar clouds are the birthsites of stars of all masses (Mannings et al 2000). The gravitational collapse of interstellar dust and gas triggered by cloud instabilities leads to the formation of stars, which may be accompanied by planetary systems. Interstellar species thus provide the raw material from which planets and small solar system bodies formed.

The physical and chemical processes that have modified the original material and shaped the structure of our planetary system are partly unknown. However, our knowledge of the composition of interstellar clouds and star-forming processes has strongly increased in recent years due to more sensitive instruments installed on ground based or space borne observatories, as has our understanding of solar system bodies from the successful operation of interplanetary probes. Interplanetary dust and remnant planetesimals delivered material to the early planets and still do in the form of interplanetary dust and meteorites, which represent fragments of small solar system bodies. Part of this material contains organic matter, which may have been crucial for chemical evolution that ultimately led to the origin of life. Spectroscopic analysis of cosmic materials—be they gaseous or solid—remains a vital tool for monitoring their evolution in space and tracing their path from star-forming regions to planetesimals and solar system bodies.

On the other hand, it may very well be that life emerged on the early Earth simply due to a combination of the right local conditions, without any ‘help’ from space. The steps from the presence of organic species, such as formaldehyde (H2 CO), amino acids or polycyclic aromatic hydrocarbons (PAHs) to a self-replicating protected cell structure are immense and far beyond the field of astrophysics.

This review provides insights into the evolution and distribution of organic material in space, the exogenous delivery of organics to Earth, as well as their possible relevance within the basic hypotheses about the emergence of life on Earth. We have compiled an inventory of organics in interstellar clouds, comets and meteorites, as well as the limited data available for protostellar disks. We also discuss laboratory experiments to aid the identification of compounds in different space environments including isotopic data from the ISM and throughout the solar system. We investigate impact properties and delivery processes as well as the necessary conditions for the origin of life. We conclude with a discussion of future perspectives in astronomy and biochemistry that might help to reveal some of the steps that led to life on Earth and possibly other planets as well.

2. The chemistry of dense interstellar clouds

The ISM in our Milky Way and in other galaxies includes regions of differing density, temperature and radiation intensity. The mass of the ISM comprises a few per cent of the baryonic mass of the Galaxy. For the last two decades, debate has continued about the volume fraction of the ISM occupied by (a) hot (∼106 K), very low density cavities created by supernova remnants, (b) ‘warm’ (∼104 K) ‘intercloud’ gas that is either mostly ionized or mostly neutral, Astrophysical and astrochemical insights into the origin of life 1431

–  –  –

the simplest mantle molecules (water (H2 O)), ammonia (NH3 ), methane (CH4 ), etc) can be explained by simple exothermic hydrogen addition reactions. However, the presence of more complex molecules (e.g. methanol, CH3 OH) and various organics requires reaction processes that are acting on and in the icy mantles (Brown and Charnley 1990).

Water ice is the most abundant ice component formed in icy grain mantles and abundances of other species are, therefore, scaled relative to water ice. Highly volatile species such as pure CO, O2 and N2 sublimate around or below 20 K. This implies that successive layers of ice are formed with different ice compositions according to the prevailing temperatures and gas pressures (or absolute densities) in protostellar regions. Hydrogen-rich ices (polar ices), dominated by H2 O ice, are formed when H is abundant in the interstellar gas. They contain besides water ice: CO, CO2, CH4, NH3, CH3 OH, and possibly traces of formic acid (HCOOH) and formaldehyde (H2 CO). Such polar ices generally evaporate around 100 K under astrophysical conditions and can therefore survive in higher temperature regions closer to the star (Tielens and Whittet 1997). Trace species such as carbonyl sulfide (OCS), H2 CO, HCOOH, methane (CH4 ) and isocyanate (OCN− ) have been observed with the Infrared Space Observatory (ISO) near some protostars and are characterized by abundances between less than a per cent to a few per cent relative to water ice (Ehrenfreund and Charnley 2000, Gibb et al 2000a, Keane et al 2001). Apolar or hydrogen-poor ices are formed far away from the protostar and are composed of molecules with high volatility (evaporation temperatures of 20 K) such as CO, O2 and N2 (Ehrenfreund et al 1997).

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