Astrochemistry and Astrobiology

Some of those topics in physical chemistry that are especially relevant to astrochemistry and astrobiology are introduced in this chapter. I start with some discussion of the chemical elements: their relative abundances, their electronic structure, and how chemical bonds are formed in simple molecules. This leads to a discussion of how changes between energy levels lead to molecular spectra that can be used to identify molecules at a distance – even the vast distances from Earth to astronomical objects. Having considered forces within molecules, I then discuss the weaker forces between molecules, including hydrogen bonding. The next section focuses on chemical reactions from both the standpoint of thermodynamics and that of chemical kinetics. Finally, some consideration is given to surface processes, which can occur on the dust particles found in the interstellar medium, and enzyme kinetics, which is of great importance in biology.

This chapter presents a description of the interstellar medium. It starts with a summary of the interstellar medium structure and how the various phases are related to each other. The emphasis is put on molecular clouds, and on their densest regions, the dense cores, which are the birth place of stars. The evolution of matter during the star formation process and its observable consequences, especially in term of chemical composition is presented. The next section is dedicated to the constituents of the interstellar medium, with separate presentations of the gas species and the dust grains. Methods used by astronomers to derive useful information on the structure, temperature, ionization rate of interstellar environments as well as magnetic fields are briefly described. The last part of the chapter presents the telescopes and their instruments used for studying the interstellar medium across the electromagnetic spectrum.

Models of the chemical composition of the interstellar medium incorporate networks of chemical reactions. The rate coefficients and the products of these reactions are important components of the model. In this chapter I review the determinants of these components and the methods used to measure them experimentally and calculate them using theory. The bulk of the chapter is devoted to ion + neutral molecule and neutral molecule + neutral molecule reactions. I also briefly discuss radiative association, dissociative recombination and reactions occurring on surfaces. The conditions of low pressure and low temperature in the interstellar medium place considerable demands on experiment and theory, which are particularly severe for reactions between neutral species. Many reactions can be estimated with tolerable accuracy. Others require a combination of high level electronic structure calculations, coupled with detailed theory and low temperature experimental measurements.

We discuss models that astrochemists have developed to study the chemical composition of the interstellar medium. These models aim at computing the evolution of the chemical composition of a mixture of gas and dust under astrophysical conditions. These conditions, as well as the geometry and the physical dynamics, have to be adapted to the objects being studied because different classes of objects have very different characteristics (temperatures, densities, UV radiation fields, geometry, history etc); e.g., proto-planetary disks do not have the same characteristics as proto-stellar envelopes. Chemical models are being improved continually thanks to comparisons with observations but also thanks to laboratory and theoretical work in which the individual processes are studied.

A decade of exoplanet research has led to surprising discoveries, from giant planets close to their star, to planets orbiting two stars, all the way to the first hot, confirmed rocky worlds with potentially permanent lava on their surfaces due to the star’s proximity. Observation techniques have reached the sensitivity to explore the chemical composition of the atmospheres as well as physical structure of some detected exoplanets and to detect planets of less than 10 Earth masses (M_Earth) and 2 Earth radii, so called Super-Earths, among them some that may be habitable. To characterize a planet’s atmosphere and its potential habitability, we explore absorption features in the emergent and transmission spectra of the planet that indicate the presence of biology. This Chapter discusses our strategy to characterize rocky exoplanets remotely, the basics underlying the concept of the Habitable Zone as well as chemical markers that indicate life through geological time.

All life on Earth needs water to survive, and special strategies are needed to cope with water scarcity, for instance because of extremes of either heat or cold. This situation has promoted the common view that water is a prerequisite for life in the universe as a whole, with important consequences for predictions about the likelihood of habitable environments. But we cannot assess that claim until we have a thorough understanding of the part that water does play in sustaining terrestrial life. In this chapter I will review the case for considering water to be a versatile, adaptive component of the cell that engages in a wide range of biomolecular interactions: for example, mediating protein-protein and receptor-substrate interactions, facilitating proton transport, driving hydrophobic interactions and their sensitivity to small solutes, acting as a reagent in biochemical reactions, and modulating electronic excitation energies. The chapter will aim to provide some basis for assessing water’s often-alleged uniqueness as life’s solvent. I conclude that, while we cannot with any confidence assert that all life must be aqueous, it is hard to identify any other solvent that could match the versatility and in particular the responsiveness of water in mediating the kind of molecular interactions likely to be required in any living system.

The boundaries of life are set by the physical and chemical limits beyond which functions associated with life, including growth and reproduction, cannot occur. Although these limits might appear to be specific to terrestrial life, thermodynamics and the basic biophysical properties of carbon-based molecules mean that the boundary of life using carbon as a molecular backbone and water as a solvent (the ‘biospace’) is likely to be universal, although exhibiting small variations depending on the particular molecular architecture adopted by life. Entirely novel biospaces using different chemistries (e.g. ammonia as a solvent) might be possible, although there is currently no empirical evidence for these alternative life chemistries.

The energy processes that support life are analysed with respect to their thermodynamic and kinetic requirements: (1) a flow of energy in order that self-organisation does not violate the 2nd Law of thermodynamics and (2) the fact that life must be regarded as a kinetic state of matter. Aside from anabolism consisting in the synthesis of metabolites, including the activated precursors of biopolymers, the need for energy flow coming from catabolism or physical sources of energy is emphasised. Quantitative conclusions are reached by considering the lifetime of side-reactions and the absolute temperature. This relationship is consistent with the fact that self-organisation involves covalent bonds and implies the contribution of energy sources with a high thermodynamic potential. These constraints lead to a definition of the conditions under which self-organisation is possible, contribute to determine the nature of the system, and bring about a new concept with regard to the habitability of exoplanets: the compatibility with the origin of life.

Replication is a fundamental process that is critical to life as we know it. While replication today is carried out by complex biochemical machineries that have been evolving for billions of years, it must have originated with relatively small molecules in simple systems. Here we explore this concept, focusing on the physicochemical characteristics and prebiotic potential of two classes of biological macromolecules: nucleic acids and lipids. We discuss the informational and catalytic capabilities of DNA and RNA, the thermodynamic limits of information transfer, the structure and function of lipid membranes, and the formation and maintenance of primitive ‘protocells’.

This chapter is conceived as a brief exposition of the content of the previous nine chapters, a commentary on them and added material, with the intent to enlarge reflection on the general theme, Physical Chemistry in Action. It can be considered as a guide to the book and, in its attempt to be syncretic, perhaps as a guide to the perplexed, confronted with the separate domains of physical chemistry, astrochemistry and astrobiology.