Characteristics of a habitable planet

What is a habitable planet? There is no formal definition at present, but the term is generally understood to mean a planet that can sustain life in some form. This concept is of limited use in practice since the conditions required to support life are poorly constrained. A narrower definition of a habitable planet is one that shares some characteristics with Earth, and hence one that could support at least some of Earth's inhabitants. A commonly adopted minimum requirement is that a planet can sustain liquid water on its surface for geological periods of time. Earth is the only body in the Solar System that qualifies as habitable in this sense. One advantage of this definition is that it can be used to categorize hypothetical and observable planets in a relatively straightforward manner, and we will use it in the rest of this chapter. However, one should bear in mind that not all life-sustaining environments will be included under this definition. Tidally heated satellites of giant planets, like Europa, are likely to possess oceans of liquid water beneath a layer of ice (Cassen et al., 1979), but these objects would not be ‘habitable’ according to the conventional usage.

Planets that can support liquid water at their surface must have an atmosphere, and surface temperatures and pressures within a certain range. These planets will occupy a particular range of orbital distances from their star that is commonly referred to as the star's habitable zone (HZ).

Introduction

The scope of life models and simulations is as broad as that of biology. It encompasses studies on cell metabolism, intercellular communication, immunology, physiology, development, cognitive processes, molecular evolution, population genetics, epidemiology etc. However, for the purpose of the present book, we will focus on the use of computing and simulation approaches as tools for studying the origins of life. Under the global denomination of ‘automata’, a large number of different frameworks have been used to implement and test models accounting for the emergence of life.

Despite their diversity, these approaches call upon the same fundamental grounding: bottom-up model building. Instead of identifying state variables of the whole system and formulating their relationships by equations, the idea is to start with elementary components and then specify how they interact with one another and with the environment. The whole system behaviour is therefore not an a priori descriptor of the model, but rather emerges from within the system. This kind of reasoning is obviously appropriate for research on the origins of life, where the main question is precisely to find out how a property (life) which is valid for the system (a living being) has emerged out of multiple elements (chemical components), which are not individually endowed with this property.

Multiwavelength images of normal, interacting, merging, starburst and active galaxies are presented. Galaxies are ordered by right ascension in each category sample as listed in Table 1 (page 18). The images come from a variety of sources – some single-wavelength images are presented in false color, whilst multiwavelength images are color coded.

A brief description of each galaxy including its major multiwavelength properties (literature references start on page 243) is given. The description is ordered via wavelength, usually starting with X-ray properties and ending with radio properties. However, papers that present multiwavelength properties have their findings summarized concurrently. Each galaxy description is provided to highlight some of the more recent and interesting scientific findings. The description is, however, not intended to be a comprehensive description of the galaxy. Readers interested in comprehensive literature searches on individual galaxies should use a nearby mirror site of the NASA Astrophysics Data System listed in, e.g.

http://ads.nao.ac.jp/mirrors.html

or use the NASA/IPAC Extragalactic Database (NED)

http://nedwww.ipac.caltech.edu/

Technical information about the galaxy images – telescope/instrument, observer(s), λ/v/energy/filter, exposure, resolution and image source – is listed in Appendix C (page 224). For each image north is up and east is to the left unless otherwise indicated. Inclinations of galaxies are referred to in the sense that 0° is face-on and 90° is edge-on to our line of sight. The spatial scale of each galaxy image is not always the same although the field of view is listed when it is known.

Introduction

At one end, the Earth, we have complex molecular structures giving rise to life; at the other end, the diffuse interstellar medium (hereinafter ISM), we have atoms floating in an almost empty space. How, when and where did the transition from unbound atoms to complex molecular structures occur? Had it occurred already in the dense molecular-cloud phase of the ISM and/or during the formation and evolution of the protosolar nebula? These are the questions whose answers will help in understanding whether, as the Nobel prizewinner C. De Duve wrote, ‘The building blocks of life form naturally in our galaxy and, most likely, also elsewhere in the cosmos. The chemical seeds of life are universal’ (De Duve, 2005). What we know for sure is that a long process, of a few billions years, brought matter from the diffuse state of the ISM to the condensed state of planets (Earth), comets and meteorites (see Chapter 7 of this volume). We also know that primitive meteorites, the oldest fossils we have from the Solar-System formation aeons, contain the ‘seeds of life’ that De Duve alluded to: amino acids.

In this chapter we will show that the formation process of solar-type stars, while bringing matter from a diffuse to a condensed state, also leads to increasing molecular complexity. Although solar-type star-forming regions are not the only places in the ISM where organic molecules are found, two reasons lead us to focus here on them: (1) they are among the places with the richest harvest of organic molecules; and (2) they are regions similar to our Solar-System progenitor, so that the organic chemistry observed there is directly linked to the possible inheritance of terrestrial life from the ISM.

Naturally occurring physical and chemical constraints of life and the biosphere

Deep-sea and deep-subsurface environments have been recognized to be among the most extreme biotopes potentially placed very close to an interface between the habitable and the uninhabitable terrains for life on Earth. The concept of habitability appears difficult to define, particularly in terms of an astrobiological perspective. Nevertheless, it is widely accepted that the harshest habitats for life, such as deep-sea and deep-subsurface environments in this ‘highly habitable’ planet, the Earth, may be approximated to the most plausible environments for extraterrestrial life in some ‘hardly habitable’ planets and moons of our Solar System. Thus, to understand the limits of life and the biosphere in the deep-sea and deep-subsurface environments of the Earth could be a key for elucidating the potential habitability of extraterrestrial life in the Universe. In this chapter, the possible factors that limit life and the biosphere on the Earth are overviewed and discussed from insights gained from the recent biogeochemical and geomicrobiological explorations in the deep-sea and deep-subsurface biosphere.

In the deep-sea and deep-subsurface environments many physical and chemical parameters limiting the activities of microbial life have been elucidated. The best example is temperature. In the terrestrial and oceanic surface environments, liquid water boils at around 100°C, while with an increasing pressure (hydrostatic), liquid water can be present at up to 373°C for pure water and 407°C for seawater (critical points) (Bischoff and Rosenbauer,1988).

Introduction

Biomineralization is the process by which organisms form minerals; this is a widespread phenomenon and more than 60 minerals of biological origin have been identified up to now (e.g. Lowenstam, 1981; Baeuerlein, 2000; Weiner and Dove, 2003). Particular attention has been paid so far to eukaryotic biominerals, including the siliceous frustules of diatoms (e.g. Poulsen et al., 2003; Sumper and Brunner, 2008), the calcitic tests of foraminifers (e.g. Erez, 2003) and the aragonitic skeleton of modern scleractinian corals (e.g. Cuif and Dauphin, 2005; Meibom et al., 2008; Stolarski, 2003). However, prokaryotes can form minerals as well (Figure 27.1; Boquet et al., 1973; Krumbein, 1979). For instance, stromatolites are carbonate deposits that are usually interpreted as the result of bacterial biomineralization. Interestingly too, some bacteria, called ‘magnetotactic’, can produce intracellular magnetite crystals seemingly aimed at directing their displacements using the local magnetic field (Blakemore, 1982). While eukaryotes obviously synthesize minerals exhibiting very specific structures (although ascertaining quantitatively why it is obvious might be an issue), the biogenicity of prokaryotic biominerals is more difficult to infer. The morphology, the structure (e.g. crystallinity, presence/absence of defects) and the chemistry (including the isotopic composition) of these prokaryote biominerals have, however, frequently been proposed as potential biosignatures (e.g. Konhauser, 1998; Little et al., 2004). Such biosignatures have been used to infer the presence of traces of life not only in ancient terrestrial rocks but also in extraterrestrial rocks such as the Martian meteorite ALH 84001 (McKay et al., 1996).

Origin and diversity of eukaryotes

Origin

Life on Earth is classed into three phylogenetic domains: the Archaea, the Bacteria and the Eucarya. Eukaryotic cells are less diverse than prokaryotes metabolically, but have a complex cellular architecture comprising a nucleus, a cytoskeleton (a proteinic network structuring the cytoplasm to facilitate intracellular traffic, endo- and exo-cytosis and amoeboid locomotion; Cavalier-Smith, 2002), an endomembrane system (a system of internal membranes subdivided into several organelles, and used for synthesis, processing, packaging and transport of macromolecules such as lipids and proteins) and organelles such as mitochondria (or derived organelles) and chloroplasts in photosynthetic eukaryotes. Archaea and Bacteria are called prokaryotes because their cells do not contain a nucleus or organelles, and because transcription and translation are coupled, i.e. they do not occur in different cellular compartments. The prokaryotes have diverse and complex metabolisms, but simpler cellular architecture. They possess proteins playing the role of cytoskeleton, but not the motor proteins involved in intracellular transport as in eukaryotes (Moller-Jensen and Lowe, 2005; Cabeen and Jacobs-Wagner, 2005) and some (the planctomycetales) may possess endomembrane systems (e.g. Fuerst, 2005).

The eukaryotic genome is seen as a mosaic of bacterial genes (involved in energy and carbon metabolism) and archaeal genes (related to DNA replication, transcription and translation), but it also contains a core set of genes and proteins unique to eukaryotes (e.g. Kurland et al., 2006), while the membrane lipids are closer to those of the bacteria (Rivera et al., 1998).

The young Sun: activity and radiation

Magnetic activity in the young Sun

The Sun's magnetic activity has steadily declined throughout its main-sequence lifetime. This is an immediate consequence of the declining dynamo as a star spins down by losing angular momentum through its magnetized wind. Along with the decline in magnetic activity, solar radiation ultimately induced by the magnetic fields declined as well, and hence the short-wavelength radiative input into planetary atmospheres diminished with time. (By contrast, solar radiation at visible wavelengths increased with time, as discussed below.) Similarly, the magnetically guided solar wind and high-energy particle fluxes were very likely to be different in the young solar environment compared to present-day conditions. A closer understanding of the magnetic behaviour of the young Sun is therefore pivotal for further modelling of young planetary atmospheres, their chemistry, heating and erosion.

Magnetic activity expresses itself in a variety of features, including dark photospheric, magnetic spots, photospheric faculae and chromospheric plage producing optical and ultraviolet excess radiation, and – most dramatically – magnetically confined coronae containing million-degree plasma that emits extreme-ultraviolet and X-ray emission. Occasional magnetic instabilities (flares) and shocks both in the corona and in interplanetary space accelerate particles to energies much beyond 1 MeV; related electromagnetic radiation (e.g. from collisions) is emitted in the hard X-ray and gamma-ray range (Lin et al., 2002).

The origin of life, as with any other process of structure formation, should have been accompanied by a loss of entropy. Since the second law of thermodynamics states that the entropy of an isolated system tends to increase, any self-organizing system must exchange free energy (closed system) and/or matter (open system) with its environment in order that the overall entropy increases (Kondepudi and Prigogine, 1998). This simple observation emphasizes the importance of energy transfers in the origin and development of early life. As far as biochemical systems are concerned, energy exchanges mostly involve chemical energy that is brought about by ‘high-energy’ carriers so that energy flows through metabolic pathways from free energy-rich compounds towards low-energy molecules, the difference being released in the environment as heat. When the occurrence of a thermodynamically unfavourable reaction makes it necessary, fresh energy is provided to the system through coupled reactions involving a free-energy carrier such as ATP. The principle that energy is brought about by ‘high-energy’ carriers applies to most metabolic pathways, though some of them do not simply follow this rule. An example is the process of energy collection leading to ATP synthesis, in which ‘chemical’ energy is generated from a ‘physico–chemical’ source: a gradient of concentration between two compartments separated by the plasma–cell membrane (Mitchell, 1961).

Introduction

The first two thirds of the history of life on Earth are dominated by single-celled microorganisms with prokaryotes characterizing the time period up to at least the Palaeoproterozoic Period (from 2.5 to about 1.8 billion years (Ga) ago). The oldest recognizable eukaryotes appear in the Mesoproterozoic Era (and are dated at between 1.6 to 1.8 Ga (Javaux et al., 2001, 2004; see also review in Knoll et al., 2006). This chapter on early life will concentrate on the traces of life contained in the oldest crustal rocks potentially capable of hosting well-preserved biosignatures, i.e. Early to Mid-Archaean, 3.5 to 3.0 Ga-old sediments and volcanic rocks from greenstone belts in both the Pilbara (NW Australia) and the Barberton (East South Africa) Greenstone Belts. The fossil traces of early microorganisms in these rocks resemble prokaryotes in terms of their morphology, metabolic processes and interactions with the environment. Life is directly influenced by its environment and, reciprocally, it can also influence its immediate environment. On the microbial scale, this influence is in proportion to the size of the microbial colonies, biofilms or mats, which can range from tens of microns to several metres or more (sometimes up to kilometres) for well-developed mats. For instance, if one takes into consideration the probable microbial control on the rise of oxygen in the atmosphere (between 2.4 and 2.0 Ga; Bekker et al., 2004; Canfield, 2005), this influence also reaches the planetary scale.

Introduction

In a paper entitled ‘The prospect of alien life in exotic forms on other worlds’ published in 2006, the authors write:

Similar comments can be found in several papers (Bains, 2004).

We are not planning to address the possibility of an alien life, but wish to focus on the issue of the solvent in order to try to demonstrate that water is an essential component of all living systems. Living systems are complex both at the molecular and supra-molecular levels (Schulze-Makuch and Irwin, 2006). Water plays, at both these levels, a role which is crucial for the structure, the stability and the biological function of all molecules that are essential for life, a role that cannot be played by any other solvent or any other molecule.

A solvent is never an inert medium and always interacts with the solute molecules. These interactions affect not only the solute but also the solvent. Water is a unique solvent because solute-induced modifications are very important in this medium.

As mentioned in the first chapter, astrophysical applications played a crucial role in the development of atomic physics. In their 1925 paper, Russell and Saunders [2] derived the rules for spectroscopic designations of various atomic states based on the coupling of orbital angular momenta of all electrons into a total L, and the coupling of all spin momenta into a total S, called the LS coupling scheme. Each atomic state is thus labelled according to the total L and S.

Atomic structure refers to the organization of electrons in various shells and subshells. Theoretically it means the determinations of electron energies and wavefunctions of bound (and quasi-bound) states of all electrons in the atom, ion or atomic system (such as electron–ion). As fermions, unlike bosons, electrons form structured arrangements bound by the attractive potential of the nucleus. Different atomic states arise from quantization of motion, orbital and spin angular momenta of all electrons. Transitions among those states involve photons, and are seen as lines in observed spectra.

This chapter first describes the quantization of individual electron orbital and spin angular momenta as quantum numbers l and s, and the principal quantum number n, related to the total energy E of the hydrogen atom. The dynamic state of an atom or ion is described by the Schrödinger equation. For hydrogen, the total energy is the sum of electron kinetic energy and the potential energy in the electric field of the proton.