Introduction

The modern study of the ‘habitability’ of circumstellar environments started almost half a century ago (Huang, 1959). The concept of a circumstellar habitable zone (CHZ) is relatively well defined, being tightly related to the requirement of the presence of liquid water as a necessary condition for life-as-we-know-it; the corresponding temperature range is a function of the luminosity of the star and of the distance of the planet from it. An important amount of recent work, drawing on various disciplines (planetary dynamics, atmospheric physics, geology, biology etc.) has refined considerably our understanding of various factors that may affect the CHZ; despite that progress, we should still consider the subject to be in its infancy (e.g. Chyba and Hand, 2005; Gaidos and Selsis, 2007; and references therein).

Habitability on a larger scale was considered a few years ago by Gonzalez et al. (2001), who introduced the concept of the galactic habitable zone (GHZ). The underlying idea is that various physical processes, which may favour the development or the destruction of complex life, may depend strongly on the temporal and spatial position in the Milky Way (MW). For instance, the risk of a supernova (SN) explosion sufficiently close to represent a threat to life is, in general, larger in the inner Galaxy than in the outer one, and has been larger in the past than at present.

Introduction

Meteorites have so far provided our best analytical window into the cosmochemical evolution of the elements that make up Earth's complex chemical systems. They are for the most part fragments of asteroids, i.e. of the small-sized and odd-shaped planetesimals that orbit the Sun in great numbers between Mars and Jupiter. According to the regular spacing of inner planets from the Sun (the Titius–Bode law), this orbit should be occupied by a planet, but it appears that the smaller lumps of early Solar-System material on their way to form a planet in this region fell under the strong gravity of the gas-giant Jupiter, which slung many away and left the rest unable to fully coalesce. Asteroids, therefore, are the remnants of a planet that never was and, just like comets and other smaller bodies in the Solar System that avoided the geological reprocessing of planet formation, may offer a pristine record of early Solar-System material. Yet, they have the important distinction of being concentrated in a crowded orbit and, subjected still to the gravitational pull of planets nearby and many collisions, regularly send their fragments to the Earth as meteorites, whose direct laboratory analyses secure unequivocal data of their extraterrestrial material.

In the case of carbonaceous meteorites, the delivery has taken on an astrobiological significance because this subgroup of meteorites contains abundant and diverse organic material, including compounds having identical counterparts in the biosphere, such as amino acids (e.g. Pizzarello et al., 2006).

Introduction

The word ‘cell’ was first used by the scientist Robert Hooke in the seventeenth century in his book Micrographia (1665), where he described observations made with his own handmade microscope (Hooke, 1665). In particular, he noticed small cavities in a piece of cork delimitated by cell walls of cellulose and suber that he called cells. However, neither Hooke nor his contemporaries realized the importance of this discovery and it was only during the nineteenth century that the cell stood out as a central dogma in biology: the cellular theory. This theory is based on the work of Matthias Schleiden and his friend Theodor Schwann, who showed that all living organisms (plants, animals, moulds) are made of microscopic building units: cells (Schwann and Schleyden, 1847). The cellular theory is based on two central ideas. First, the cell is the unit of structure, physiology and organization in all living beings. Thus, the cell retains a dual existence – as a distinct entity and as a building block in the construction of organisms: unicellular organisms correspond to a single and autonomous cell, whereas multicellular organisms are made of two up to several billions of often highly differentiated cells. The second concept of the cellular theory is that ‘every cell stems from another cell’, and was formulated by Rudolf Virchow in 1855 (Virchow, 1855).

Stars exist in great variety. They are among the most stable, as well as occasionally the most unstable, objects in the Universe. While extremely massive stars have short but very active lifetimes of only millions of years after birth, the oldest stars have estimated ages of up to 14 billion years at the present epoch, not much shorter (though it must be) than the estimated age of the Universe obtained by other means, such as the cosmological Hubble expansion. In fact, the estimates of the age of the Universe are thereby constrained, since the Universe cannot be younger than the derived age of the oldest stars – an obvious impossibility. Stellar ages are estimated using well-understood stellar astrophysics. On the other hand, variations in the rate of Hubble expansion may depend on the observed matter density in the Universe, the gravitational ‘deceleration parameter’, the ‘cosmological constant’, ‘dark’ (unobserved) matter and energy, and other exotic and poorly understood entities. Needless to say, this is an interesting and rather controversial area of research, and is further discussed in Chapter 14.

But stars are the most basic astronomical objects, and astronomers are confident that stellar physics is well-understood. This confidence is grounded in over a century of detailed study of stars, with the Sun as the obvious prototype. Most of this knowledge is derived from spectroscopy which, in turn, yields a wealth of information on nearly every aspect of stellar astrophysics; stellar luminosities, colours, temperatures, sizes, ages, composition, etc.

Conceptual and historical framework

Botanists of the late nineteenth century were already familiar with microbial symbioses. In fact, the term ‘symbiosis’, meaning literally ‘living together’ was introduced by Anton de Bary and Albert Bernard Frank discussing lichens and mycorrhizae, respectively, at the end of the 1870s. However, until recent times, the idea that microbial associations are central in evolution remained almost marginal. The historian Jan Sapp (1994) proposed several reasons for that situation, including the traditional accent on microorganisms as causative agents of diseases and the prominent concepts of conflict and competition as major evolutionary driving forces. Darwin's metaphoric use of the term ‘struggle for existence’ included the ‘dependence of one being on another’, and he actually recognized the existence of species taking advantage of another species, but also emphasized the difficulty for his natural-selection theory to explain the emergence of structures in one species to benefit another (Darwin, 1859, p. 200). Lynn Margulis' contributions since the late 1960s on a symbiotic theory for cell evolution bridged the previous intellectual gap, and proposals made by forgotten biologists that had been dismissed were reopened – especially those of the Russian botany school (Khakhina, 1992; Margulis, 1993). Today there is a wide consensus on the essential role played by symbiosis during the origin and evolution of eukaryotic cells, although very passionate and fundamental debates still persist (de Duve, 2007).

Atomic astrophysics and spectroscopy

Spectroscopy is the science of light–matter interaction. It is one of the most powerful scientific tools for studying nature. Spectroscopy is dependent on, and therefore reveals, the inherent as well as the extrinsic properties of matter. Confining ourselves to the present context, it forms the link that connects astronomy with fundamental physics at atomic and molecular levels. In the broadest sense, spectroscopy explains all that we see. It underlies vision itself, such as the distinction between colours. It enables the study of matter and light through the wavelengths of radiation (‘colours’) emitted or absorbed uniquely by each element. Atomic astrophysics is atomic physics and plasma physics applied to astronomy, and it underpins astrophysical spectroscopy. Historically, astrophysical spectroscopy is older than modern astrophysics itself. One may recall Newton's experiments in the seventeenth century on the dispersion of sunlight by a prism into the natural rainbow colours as an identification of the visible band of radiation. More specifically, we may trace the beginning of astrophysical spectroscopy in the early nineteenth century to the discovery of dark lines in the solar spectrum by Wollaston in 1802 and Fraunhofer in 1815. The dark lines at discrete wavelengths arise from removal or absorption of energy by atoms or ions in the solar atmosphere. Fraunhofer observed hundreds of such features that we now associate with several constituent elements in the Sun, such as the sodium D lines.

Introduction

Water is known to be ubiquitous in the Universe, from the dark spots of the Sun up to the most distant galaxies. It is also a major component of Solar-System objects, especially in the outer parts beyond heliocentric distances of a few astronomical units (one astronomical unit = AU = average Earth–Sun distance = 149.6 × 106 km). It should be mentioned, however, that the Earth is the only place in the Solar System where water can be present in its three states: solid, liquid and vapour. So far, outside the Earth, water has always been found in the form of vapour or ice, although there are some indications that liquid water might be – or have been – present elsewhere in the Solar System. Liquid water was probably present in the past on the surface of Mars and also possibly Venus; it might currently exist in the interiors of some satellites of giant planets.

The presence of large amounts of water, vapour or ice in the Universe is a natural outcome of the large cosmic abundances of the hydrogen and oxygen elements which form the water molecule. In addition, the large abundance of water in the outer Solar System is also a natural consequence of the formation scenario of the Solar System, which led to the accretion of two classes of planets, the terrestrial and the giant ones, separated by the ‘snow line’, which basically corresponds to the heliocentric distance of water condensation in the primordial solar nebula.

Introduction

The awareness that genes and genomes are extraordinarily rich historical documents from which a wealth of evolutionary information can be retrieved has widened the range of phylogenetic studies to previously unsuspected heights. The development of efficient sequencing techniques, which now allows the rapid sequencing of complete cellular genomes, combined with the simultaneous and independent blossoming of computer science, has led not only to an explosive growth of databases and new sophisticated tools for their exploitation, but also to the recognition that, in spite of many lateral gene-transfer (LGT) events, different macromolecules are uniquely suited as molecular chronometers in the construction of nearly universal phylogenies.

Cladistic analysis of rRNA sequences is acknowledged as a prime force in systematics and from its very inception had a major impact on our understanding of early cellular evolution. The comparison of small-subunit ribosomal-RNA (16/18S rRNA) sequences led to the construction of a trifurcated unrooted tree in which all known organisms can be grouped in one of three major monophyletic cell lineages, i.e. the domains Bacteria (Eubacteria), Archaea (Archaeabacteria) and Eukarya (Eukaryotes) (Woese et al., 1990), which are all derived from an ancestral form, known as the last common ancestor (LCA).

In ionized plasmas spectral formation is due to particle collisions or radiative excitations. In astrophysical situations there is usually a primary energy source, such as nuclear reactions in a stellar core, illumination of a molecular cloud by a hot star or accretion processes around a black hole. The ambient energy is transferred to the kinetic energy of the particles, which may interact in myriad ways, not all of which are related to spectroscopy.

Electron collisions with ions may result in excitation or ionization. The former process is excitation of an electron into discrete levels of an ion, while the latter is excitation into the continuum, or ionization, as shown in Fig. 3.1 and discussed in Chapter 3. A practically complete description of the (e + ion) excitation process requires collisional information on the ions present from an observed astrophysical source, and for all levels participating in spectral transitions. As the excitation energy from the ground state to the higher levels increases, the ionization energy EI is approached. The negative binding energy of the excited states increases roughly as E ~ –z2/n2, where z is the ion charge. As n → ∞, E → 0, i.e., the electron becomes free.

At first sight, therefore, it might seem like a very large number of levels need to be considered for a given atomic system in order to interpret its spectrum completely.

The origin of spectral lines depends on the matter and radiation fields that characterize the physical conditions in the source. However, the lines actually observed also depend on the intervening medium towards the observer. The wide variety of astrophysical sources span all possible conditions, and their study requires both appropriate modelling and necessary atomic parameters. The models must describe the extremes of temperature, density and radiation encountered in various sources, from very low densities and temperatures in interstellar and intergalactic media, to the opposite extremes in stellar interiors and other environments. As such, no single approximation can deal with the necessary physics under all conditions. Different methods have therefore been developed to describe spectral formation according to the particular object, and the range of physical conditions under consideration.

This is the first chapter devoted mainly to astrophysical applications. The theoretical formulation of atomic spectroscopy described hitherto is now applied to the analysis of emission-line observations in three widely disparate regions of the electromagnetic spectrum: the visible, X-ray and far-IR. Examples include some of the most well-known and widely used lines and line ratios. Emission line analysis depends on accurate calculations of emissivities, which, in turn, are derived from fundamental parameters such as collision strengths for (e + ion) excitation and recombination, and radiative transition probabilities. However, spectral models in complicated situations, such as line formation in transient plasmas and in the presence of external radiation fields, assume a level of complexity that requires consideration of a variety of processes and parameters.

This book aims at exploring several crucial issues related to the origin(s) and evolution of life in the Universe, starting from the only example of life known so far: terrestrial life. It is clear, though, that many of the circumstances that surrounded the emergence of life on Earth may have occurred, are occurring or will occur in other regions of our Galaxy or in other galaxies of our Universe. Therefore, the critical exploration of those conditions and the elaboration of models explaining the transition from the organic chemistry of the Universe to the biochemistry of terrestrial living forms are relevant at a much more global scale.

Just as with this volume, the field of astrobiology is by nature multidisciplinary. Astrophysicists, geologists, chemists, biologists, computer scientists and philosophers, as well as scientists working at the different interfaces between those disciplines, can all contribute to a better understanding of the processes and conditions that led to the emergence of life. The points of view and approaches of those different disciplines should not only superimpose, but also converge towards a unified explanation of the phenomenon of life in our Universe.

This book is an attempt to contribute to such an ambitious objective. It summarizes a series of lectures presented by selected speakers during two successive summer courses sponsored by the French Research Council (CNRS, Centre National de la Recherche Scientifique): Exobio'05 and Exobio'07, Ecole d'exobiologie du CNRS, which were respectively held in September 2005 and September 2007 in Propriano, Corsica (http://www.u-bordeaux1.fr/exobio07/).

Which atoms were formed first, in what proportion and when? The relationship between atomic spectroscopy and cosmology rests on the answer to these questions. According to big bang nucleosynthesis (BBN), before the creation of the first atoms, the Universe would have been filled with a highly dense ensemble of nuclei, free electrons, and radiation. The standard model from high-energy particle physics implies that most observable matter is made of baryons, such as protons and neutrons; electrons are leptons and much less massive. The baryons are themselves made of more exotic fundamental particles, such as quarks, gluons and so forth. According to the BBN theory, given a fixed baryon-to-photon ratio in the first three minutes of origin, a few primordial nuclear species made of baryons appeared. The atomic nuclei created during the BBN were predominantly protons and helium nuclei (2He3, 2He4), with very small trace amounts of deuterium (heavy hydrogen 2H1) and lithium (3Li6, 3Li7). Atomic physics then determines that singly ionized helium He II (not hydrogen!) would have been the first atoms(ions) formed.

The process of formation is (e + ion) recombination: He III + e → He II + hν. This temporal marker in the history of the Universe is referred to as the recombination epoch. The reason that He II was the first atomic species is not difficult to see, given the extremely hot plasma that preceded the recombination epoch when nuclei and electrons were free in the fully ionized state.

Astrobiology, also known as bioastronomy or exobiology, is the study of the origin, evolution and distribution of life in the Universe. These are subjects which have been of interest to mankind throughout recorded history. Although questions of origins have most frequently invoked divine beings, non-supernatural speculation on these fundamental issues dates back at least to the Ionian school of pre-Socratic Greek philosophers. Anaximander, the successor to Thales, is reported as saying that all living creatures arose from the moist element (water) through the action of the Sun (Freeman, 1966), a prescient insight given current ideas that life as we know it requires water, that radiation acting on inorganic matter can produce the molecular components of life (amino acids, nucleic acids, etc.) and that the Sun is the ultimate energy source for almost all life on Earth. In fact, Anaximander seems to have gone further and suggested that human beings arose from fish-like creatures (presumably a natural result of life having originated in water).

Speculation about life beyond the Earth has also had a long tradition. Although Pythagoras himself is not known to have recorded his teachings, his school (in particular, Philolaus, ca. 400 BCE) is said to have written that the Moon appears Earth-like because it is inhabited with animals and plants (Dreyer, 1953). At roughly the same time the atomist school of Leucippus and Democritus taught that the Universe is infinite and contains innumerable worlds.

The origin of life and its existence elsewhere than on planet Earth

Studies of the origins of life are closely interwoven with exobiology (Raulin-Cerceau et al., 1998). It is highly probable that the full range of conditions present on Earth since its formation are present elsewhere. On a virtual trip through the Universe, we would travel not only in space, but also back in time into the Earth's biological history. The search for past, dormant or currently existing extraterrestrial life is one of the most thought-provoking challenges for biology. It is based on the certainty that liquid water and other key chemical and physical environmental conditions for the development of living organisms, as we know them, were, or are, present elsewhere in the Universe than on our planet. Any evidence of extraterrestrial life, from Mars sample analysis for example, would be of major interest for all biology. It would contribute to an understanding not only of the definition and origin of life, but also of the evolution and adaptation of molecular mechanisms in living cells, or of how organisms adapt and develop within ecosystems.

Why study life in extreme environments?

Life on Earth is almost everywhere! And because it is almost everwhere around us, we can hope to define the extreme limits for its existence by studying it here on Earth.