schaible-et-al-2024-chapter-1-the-astrobiology-primer-3-0.pdf

ASTROBIOLOGY Volume 24, Supplement 1, 2024...

ASTROBIOLOGY Volume 24, Supplement 1, 2024 ª Mary Ann Liebert, Inc. DOI: 10.1089/ast.2021.0129 Open camera or QR reader and scan code to access this article and other resources online. Chapter 1: The Astrobiology Primer 3.0 Micah J. Schaible,1,* Nadia Szeinbaum,2,* G. Ozan Bozdag,3 Luoth Chou,4,5,6 Natalie Grefenstette,7,8 Stephanie Colón-Santos,9,10 Laura E. Rodriguez,11,12 M.J. Styczinski,12,13 Jennifer L. Thweatt,14 Zoe R. Todd,15 Alberto Vázquez-Salazar,16,17 Alyssa Adams,5 M.N. Araújo,18 Thiago Altair,19,20 Schuyler Borges,21 Downloaded by 2401:4900:2354:56c7:830:ca46:c66a:1cf1 from www.liebertpub.com at 09/18/24. For personal use only. Dana Burton,22 José Alberto Campillo-Balderas,16 Eryn M. Cangi,23 Tristan Caro,24 Enrico Catalano,25 Kimberly Chen,3 Peter L. Conlin,3 Z.S. Cooper,15 Theresa M. Fisher,26 Santiago Mestre Fos,1 Amanda Garcia,27 D.M. Glaser,28 Chester E. Harman,16 Ninos Y. Hermis,12,29 M. Hooks,30 K. Johnson-Finn,31,32 Owen Lehmer,15 Ricardo Hernández-Morales,16 Kynan H.G. Hughson,2 Rodrigo Jácome,16 Tony Z. Jia,8,31 Jeffrey J. Marlow,33 Jordan McKaig,2 Veronica Mierzejewski,26 Israel Muñoz-Velasco,16,34 Ceren Nural,35 Gina C. Oliver,36 Petar I. Penev,3 Chinmayee Govinda Raj,1 Tyler P. Roche,1 Mary C. Sabuda,37,38 George A. Schaible,39 Serhat Sevgen,8,40 Pritvik Sinhadc,41,42 Luke H. Steller,43 Kamil Stelmach,44 J. Tarnas,12 Frank Tavares,45 Gareth Trubl,46 Monica Vidaurri,5,47 Lena Vincent,9 Jessica M. Weber,12 Maggie Meiqi Weng,6 Regina L. Wilpiszeki,48 and Amber Young4,21 1 School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, USA. 2 School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA. 3 School of Biological Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA. 4 NASA Goddard Space Flight Center, Greenbelt, Maryland, USA. 5 Center for Space Sciences and Technology, University of Maryland, Baltimore, Maryland, USA. 6 Georgetown University, Washington DC, USA. 7 Santa Fe Institute, Santa Fe, New Mexico, USA. 8 Blue Marble Space Institute of Science, Seattle, Washington, USA. 9 Wisconsin Institute for Discovery, University of Wisconsin–Madison, Wisconsin, USA. 10 Department of Botany, University of Wisconsin–Madison, Wisconsin, USA. 11 Lunar and Planetary Institute, Universities Space Research Association, Houston, Texas, USA. 12 NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA. 13 University of Washington, Seattle, Washington, USA. 14 Department of Biochemistry and Molecular Biology, Penn State University, University Park, Pennsylvania, USA. 15 Department of Earth and Space Sciences, University of Washington, Seattle, Washington, USA. 16 Departamento de Biologı́a Evolutiva, Facultad de Ciencias, Universidad Nacional Autónoma de México, Mexico City, Mexico. 17 Department of Chemical and Biomolecular Engineering, University of California Los Angeles, California, USA. 18 Biochemistry Department, University of São Paulo, São Carlos, Brazil. 19 Institute of Chemistry of São Carlos, Universidade de São Paulo, São Carlos, Brazil. 20 Department of Chemistry, College of the Atlantic, Bar Harbor, Maine, USA. 21 Northern Arizona University, Flagstaff, Arizona, USA. 22 Department of Anthropology, George Washington University, Washington DC, USA. 23 Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, Colorado, USA. 24 Department of Geological Sciences, University of Colorado Boulder, Boulder, Colorado, USA. 25 Sant’Anna School of Advanced Studies, The BioRobotics Institute, Pisa, Italy. 26 School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA. 27 Department of Bacteriology, University of Wisconsin–Madison, Wisconsin, USA. 28 Arizona State University, Tempe, Arizona, USA. 29 Department of Physics and Space Sciences, University of Granada, Granada, Spain. 30 NASA Johnson Space Center, Houston, Texas, USA. 31 Earth-Life Science Institute, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan. 32 Rensselaer Polytechnic Institute, Troy, New York, USA. 33 Department of Biology, Boston University, Boston, Massachusetts, USA. 34 Departamento de Biologı́a Celular, Facultad de Ciencias, Universidad Nacional Autónoma de México, Mexico City, Mexico. 35 Istanbul Technical University, Istanbul, Turkey. 36 Department of Geology, San Bernardino Valley College, San Bernardino, California, USA. 37 Department of Earth and Environmental Sciences, University of Minnesota–Twin Cities, Minneapolis, Minnesota, USA. S-4  CHAPTER 1: THE ASTROBIOLOGY PRIMER 3.0 S-5 Abstract The Astrobiology Primer 3.0 (ABP3.0) is a concise introduction to the field of astrobiology for students and others who are new to the field of astrobiology. It provides an entry into the broader materials in this sup- plementary issue of Astrobiology and an overview of the investigations and driving hypotheses that make up this interdisciplinary field. The content of this chapter was adapted from the other 10 articles in this supple- mentary issue and thus represents the contribution of all the authors who worked on these introductory articles. The content of this chapter is not exhaustive and represents the topics that the authors found to be the most important and compelling in a dynamic and changing field. Key Words: Astrobiology Primer—Origins and evolution of life—Searching for life beyond Earth. Astrobiology 24, S-4–S-39 Table of Contents 1.1. Introduction to the Astrobiology Primer 3.0 S-7 1.2. Historical, Practical, and Theoretical Definitions of Life S-7 Downloaded by 2401:4900:2354:56c7:830:ca46:c66a:1cf1 from www.liebertpub.com at 09/18/24. For personal use only. 1.2.1. Definitions of life from antiquity to modern time S-7 1.2.2. Universal features of terran life S-7 1.2.2.1. The cell as the unit of life S-8 1.2.2.2. Essential and trace elements S-8 1.2.2.3. DNA/RNA, proteins, lipids, and sugars S-8 1.2.2.4. Higher-order processes and properties S-8 1.2.3. Representing the evolutionary history of life S-8 1.2.4. Definitions of life in astrobiology S-9 1.3. The Origins of the Solar System and Earth S-9 1.3.1. The formation of stars and the elements S-9 1.3.1.1. Nucleosynthesis S-9 1.3.1.2. Stellar types and life cycles S-9 1.3.2. The formation of Earth and the Solar System S-10 1.3.3. The formation and delivery of volatiles to Earth S-10 1.3.3.1. Molecular formation in the interstellar medium and protoplanetary disk S-10 1.3.3.2. Delivery of molecules by asteroids and comets S-10 1.3.4. The atmosphere, hydrosphere, and geosphere of Earth S-11 1.4. The Origins of Life on Earth S-11 1.4.1. Geology and life on early Earth S-12 1.4.2. Prebiotic chemistry and the emergence of life on Earth S-12 1.4.2.1. Sources of energy and elements S-12 1.4.2.2. Chemical mechanisms and the first macromolecules S-12 1.4.2.3. The origins of metabolism and homochirality S-13 1.4.3. Environmental considerations for the origins of life on Earth S-13 1.4.4. The origins of the protocell S-14 1.5. The Evolution of Life on Earth S-14 1.5.1. The origins of individuality—LUCA S-14 1.5.2. The oxygenation of Earth’s atmosphere S-14 1.5.3. The origin of the eukaryotic cell S-15 1.5.4. The origin(s) and evolution of multicellularity S-15 1.6. The Diversity and Limits of Life on Earth S-15 1.6.1. The diversity of life on Earth S-16 1.6.2. Energy production mechanisms of life on Earth S-16 38 Biotechnology Institute, University of Minnesota–Twin Cities, St. Paul, Minnesota, USA. 39 Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana, USA. 40 Institute of Marine Sciences, Middle East Technical University, Erdemli, Mersin, Turkey. 41 BEYOND: Center For Fundamental Concepts in Science, Arizona State University, Arizona, USA. 42 Dubai College, Dubai, United Arab Emirates. 43 Australian Centre for Astrobiology, and School of Biological, Earth and Environmental Sciences, University of New South Wales, Kensington, Australia. 44 Department of Chemistry, University of Virginia, Charlottesville, Virginia, USA. 45 Space Enabled Research Group, MIT Media Lab, Cambridge, Massachusetts, USA. 46 Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California, USA. 47 Department of Physics and Astronomy, Howard University, Washington DC, USA. 48 Oak Ridge National Laboratory: Oak Ridge, Tennessee, USA. *Authors contributed equally to this work.  S-6 SCHAIBLE ET AL. 1.6.2.1. Sources of energy, electrons, and carbon S-16 1.6.2.2. Planetary sources of energy and nutrients S-16 1.6.3. Types of extremophiles S-17 1.6.3.1. Hot and cold temperatures S-17 1.6.3.2. High and low pressure S-17 1.6.3.3. Acidic and basic environments S-17 1.6.3.4. High salinity S-17 1.6.3.5. Dry environments S-18 1.6.3.6. High radiation S-18 1.6.3.7. Low resources S-18 1.7. Habitability of Environments Beyond Earth S-18 1.7.1. Definitions and requirements for habitability S-18 1.7.2. Habitability in the Solar System and beyond S-19 1.7.2.1. Mars S-19 1.7.2.2. Venus S-19 1.7.2.3. Europa S-19 Downloaded by 2401:4900:2354:56c7:830:ca46:c66a:1cf1 from www.liebertpub.com at 09/18/24. For personal use only. 1.7.2.4. Enceladus S-20 1.7.2.5. Titan S-20 1.7.2.6. Other icy bodies S-20 1.7.2.7. Habitability of exoplanets S-21 1.8. Searches for Life Beyond Earth S-21 1.8.1. Detection of biosignatures in the Solar System S-21 1.8.1.1. Chemical biosignatures S-21 1.8.1.2. Physical biosignatures S-22 1.8.1.3. Biosignature detection methods S-22 1.8.2. Remote detection of biosignatures S-22 1.8.2.1. Exoplanet detection S-22 1.8.2.2. Global atmospheric and pigment-based biosignatures S-22 1.8.2.3. Technosignatures S-23 1.8.3. Definitiveness of life detection S-23 1.9. How Life Could Be Different Than Life on Earth S-23 1.9.1. Exotic life and the origins of life on Earth S-24 1.9.2. The chemical and physical possibilities of exotic life S-24 1.9.2.1. Alternative elemental bases S-24 1.9.2.2. Alternative solvents S-24 1.9.2.3. Alternative compartments S-24 1.9.2.4. Alternative energy sources S-25 1.9.2.5. Alternative information mechanisms S-25 1.9.3. Agnostic biosignatures S-25 1.10. Planetary Protection and Ethical Exploration S-25 1.10.1. The motivations of planetary protection S-25 1.10.2. The history of planetary protection S-25 1.10.3. The methods of planetary protection S-26 1.10.3.1. Microbial quantification and reduction S-26 1.10.3.2. Molecular contamination S-27 1.10.3.3. Trajectory biasing S-27 1.11. Education and Involvement in Astrobiology S-27 1.11.1. Astrobiology teaching and learning S-27 1.11.2. Astrobiology education research (ABER) S-28 1.11.3. Resources, opportunities, and research institutions S-28 1.11.4. Public engagement in astrobiology S-28 A strobiology is the study of the origin, evolution, and distribution of life in the past, present, and future of the universe. Astrobiology is an interdisciplinary field seeking to planet Earth and the Solar System with known instances of life as a guide, and research topics include the origins of life, the search for life beyond Earth, and the responsible explo- answer questions ranging in scale from the microscopic (e.g., ration of space. The challenges surrounding these astrobi- the origins, evolution, and extremes of cellular life) to the ology research topics can be obscure and difficult to cosmic (e.g., the formation of planets, planetary habitability, comprehend. Thus, interpretations and guidance for future and biosignatures). Many astrobiological studies focus on astrobiology studies can derive additional benefit from  CHAPTER 1: THE ASTROBIOLOGY PRIMER 3.0 S-7 philosophical studies (e.g., how to define life). Finally, as- and the most recent literature. Numerous cross-references trobiology research requires effective tools for sharing re- are provided throughout this special issue to indicate top- search, creating community, and teaching the next generation ical links between chapters. of astrobiologists. 1.2. Historical, Practical, and Theoretical Definitions 1.1. Introduction to the Astrobiology Primer 3.0 of Life The Astrobiology Primer 3.0 (ABP3.0) provides an in- Understanding the framework of the question ‘‘what is troduction and overview of the current state of knowledge in life?’’ is fundamental to formulating other questions that are the diverse fields of study that encompass (most of) the central to astrobiology such as ‘‘where else may life be found?’’ science of astrobiology. The goal of ABP3.0 was to provide and ‘‘how do we search for life elsewhere?’’ Definitions of life students and early career scientists with the basic informa- can serve as essential guides when searching for life beyond tion needed to begin exploring the field. ABP3.0 was in- Earth. This section will provide an overview of Chapter 2: spired by and builds off of the previous versions (Mix et al., What Is Life? and the various ways in which life is scientifi- 2006; Domagal-Goldman et al., 2016). The 11 chapters of cally defined through its building blocks and behaviors. ABP3.0 were written as a collaboration of 60 unique authors Downloaded by 2401:4900:2354:56c7:830:ca46:c66a:1cf1 from www.liebertpub.com at 09/18/24. For personal use only. representing nearly 50 different institutions from around the 1.2.1. Definitions of life from antiquity to modern time world. The diversity of backgrounds and research interests The concept and definition of life largely depends on its of the co-authors represents the interdisciplinary nature of historical context, which determines our ability to test any astrobiology. given hypothesis at a particular point in time. Efforts to dis- The Astrobiology Primer 3.0 expands upon previous tinguish life from nonlife have been given increasingly more versions by (i) introducing several new topical areas to the serious consideration as human understanding has expanded. primer series, (ii) providing an updated and more com- The first formal definitions can be traced back to ancient prehensive overview of astrobiology research, and (iii) Greece and Aristotle where the concept of psyche or ‘‘soul’’ making the content available online and as a living docu- was used to describe the properties unique to living things. ment uploaded to www.astrobiologyprimer.org. The es- These definitions changed drastically starting in the Enlight- tablishment of an online version of the primer will include enment, starting with quasi-mechanical definitions by Bacon options for community input to ensure more responsive and Descartes in the 17th century, followed by Schleiden and updating of the primer series so that accurate information Schwan’s focus on the cell as the unit of life, and finally can be provided to those looking for an introduction or definitions based on evolution by Darwin, Lamarck, and reference in this ever-growing field. ABP3.0 is intended to others in the 19th century (Mix, 2018; Ribatti, 2018). In the be understandable to newcomers to astrobiology. Abundant early 20th century, models of life turned to origin-of-life citations are provided for more advanced astrobiology re- questions founded in new knowledge and discoveries. Hal- searchers looking for current research in new topical areas. dane and Oparin developed theoretical descriptions of life’s Multiple rounds of review, both formal and informal, and origins (Lazcano, 2016; Tirard, 2017), and Miller and Urey extensive feedback from astrobiology experts have helped provided experimental evidence to support the abiotic origin. refine this work. With the discovery of DNA and its composition and structure Chapter 1 was written as a summary of ABP3.0 and by Watson, Crick, and Franklin, information storage was covers the unifying concepts that appear throughout the added to perspectives and definitions of life. With the advent remaining chapters in a condensed and focused way. The of the digital era, definitions of life have become more ab- most critical features from each of the 10 subsequent stract, specifically considering functions rather than form, chapters are presented sequentially in the remainder of such as the storage and transmission of information, the Chapter 1. Each section roughly follows the general outline complexity of a system, and metabolism. of the chapter it covers. Chapter 2 introduces the history There is no current consensus on the definition of ‘‘life.’’ and current understanding of life as a scientific concept and Historical perspectives have emphasized functional features discusses how definitions of life, although difficult to agree such as chemical interactions that provide energy (i.e., upon, can be useful in the search for life beyond Earth. The metabolism) and transmission of information (i.e., DNA next eight chapters of ABP3.0 are organized to first explore replication and translation). More recent ideas expand be- the history of life in a roughly chronological sequence as a yond particular chemical or molecular interactions and use ‘‘Space-to-Earth’’ perspective followed by the exploration generalized features that are thought to be common to all of life from an ‘‘Earth-to-Space’’ perspective. Chapters 3– life. The working definition of life that NASA uses is ‘‘a 6 cover the formation of the Solar System and Earth self-sustaining chemical system capable of Darwinian evo- (Chapter 3) and the origins (Chapter 4), evolution (Chapter lution’’ (astrobiology.nasa.gov). 5), and diversity (Chapter 6) of known life. Chapters 7–10 describe potentially habitable environments beyond Earth 1.2.2. Universal features of terran life (Chapter 7), signatures and techniques used to search for life (Chapter 8), possible forms of exotic life (Chapter 9), Life on Earth is known as ‘‘terran’’ life. Although terran and ethical concerns relevant to the search for life beyond life is remarkably diverse, all organisms use the same mo- Earth (Chapter 10). Chapter 11 describes education and lecular building blocks in roughly similar proportions and research opportunities in astrobiology. Each chapter can share similar metabolic pathways. The commonality of also be used as an independent reference text and contains these elements can be explained by a shared ancestry that citations to many books, review articles, seminal papers, unites all life on Earth.  S-8 SCHAIBLE ET AL. 1.2.2.1. The cell as the unit of life. All life on Earth is packed lipids, are used for long-term energy storage. Poly- cellular—either as a single cell or multicellular unit. Cells saccharides are also commonly used as structural elements provide separation from the environment and enable life to such as cellulose, a structural component of the cell walls in maintain stable conditions inside the cell to support survival. plants, and chitin, a major component in the exoskeletons of Every cell on Earth is composed of networks of biomole- marine arthropods (e.g., crabs, lobsters, and shrimp). cules that keep the cell alive. Cells act as hosts for genetic Almost all biomolecules are chiral, meaning that their replication, bodies for unicellular organisms, and essential atoms can be arranged into two unique orientations called components for multicellular organisms. enantiomers. Like our hands, the structures of the two en- antiomers are non-superimposable. Living systems on Earth 1.2.2.2. Essential and trace elements. The elements primarily use only one of the two possible orientations for carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phos- different types of biomolecules (D-sugars and L-amino ac- phorus (P), and sulfur (S), known as CHONPS, are essential ids). Some species leverage opposite-handed molecules for components of all the cellular biomolecules. Carbon forms specific functions such as structural support, defense, and the backbone of virtually all biomolecules utilized by terran cellular communication (Aliashkevich et al., 2018). life. It can be combined to produce a diverse array of complex macroscale structures due to its ability to form Downloaded by 2401:4900:2354:56c7:830:ca46:c66a:1cf1 from www.liebertpub.com at 09/18/24. For personal use only. 1.2.2.4. Higher-order processes and proper- covalent bonds with a wide range of elements. Hydrogen ties. Common processes and properties shared by all and oxygen react easily with metals and other redox reactive known life include ordered structure, reproduction, metab- species and often drive key biological processes. They can olism, response to the environment, homeostasis, and evo- combine to form water (H2O), thus providing a solvent for lution. The collection of all genes in an organism is called a life’s processes (Westall and Brack, 2018). Most of the ni- genome. While related groups of organisms may share trogen on contemporary Earth exists as the chemically inert similar genomes, slight variations between generations can N2 that makes up the atmosphere. However, certain organ- lead to significant changes over time. All species on Earth isms can convert N2 into more reactive molecules such as share the ability to adapt to their environment. They evolve ammonium (NH4+), nitrogen dioxide (NO2-) and nitrate and change over time through mechanisms such as muta- (NO3-) that are more easily incorporated into biological tion, gene flow and exchange, and natural selection (Blount processes. Phosphorus is an important component of many et al., 2018). fundamental biomolecules (e.g., nucleic acids, lipids, and The functions and processes of life result in three major ATP), and the most common form used by life is negatively characteristics: (i) life evolves open-endedly and innovates charged orthophosphate (PO43-). Sulfur is also a funda- in relationship to its environment (Pattee and Sayama, 2019; mental part of terran life because of its ability to form redox- Davies, 2021), (ii) life is marked by major emergent tran- active disulfide bonds (S-S) and iron-sulfur clusters and to sitions (Banzhaf et al., 2016), and (iii) life is complex with exist in many oxidation states. respect to what differentiates it from a totally random or a Life selectively requires smaller quantities of many other very simple system (Chu et al., 2003). Each of these three elements as well, the most prevalent being Na, K, Cl, Ca, criteria are difficult to define with precision and have vari- Mg, and Fe. These species tend to exist on Earth as ions and ous degrees of measurability in experimental systems. Un- can generate potential differences across a membrane, help derstanding open-endedness, emergence, and complexity in stabilize charged organic molecules in solution, or even be ways that encompass all of biology and match observations used as structural components. Life also incorporates tran- of living systems can provide a helpful guide when sition metals (e.g., Fe, Mg, Mn, Cu, Ti, Co, Zn), which searching for life beyond Earth (Davies and Walker, 2016). readily gain and lose electrons, catalyze reactions, and control electron flow in metabolism. 1.2.3. Representing the evolutionary history of life 1.2.2.3. DNA/RNA, proteins, lipids, and sugars. All The commonality of building blocks across all modern known organisms on Earth rely on the same fundamental set organisms suggests that all cells, from the simplest unicel- of biomolecules to carry out the functions of life. These are lular bacteria to those that make up highly complex multi- (i) nucleic acids, (ii) proteins, (iii) lipids, and (iv) polysac- cellular species such as Homo sapiens, evolved from a charides. Cells store information about protein synthesis as single species or population of cells known as the last uni- genes in DNA. The code in DNA is mirrored by messenger versal common ancestor, or LUCA. Biological Trees of Life RNA which dictates the sequence in which amino acids are (ToLs) are often used to represent the evolutionary history combined in the ribosome to produce proteins. This typical and interconnectedness of life on Earth. On a ToL, an ex- flow of information from storage to expression (DNA / ample of which is given in Fig. 2.4, each node indicates an RNA / protein) is known in biology as the Central Dogma organism or population of organisms, with the intervening and is found across all of terran life. Proteins serve nu- branches representing times of independent evolution. A merous functions such as reaction catalysis (i.e., enzymes), node from which branches emerge represents a common structural support, and electron transport. Lipids encompass ancestor or ancestral species, and the unconnected ends of a broad range of organic molecules, including phospholipids branches (i.e., the tips or leaves) are extant organisms. (used to form cellular membranes) and triglycerides (used Moving backward from the leaves, each branch traces the for long-term energy storage). While cells use adenosine evolution of each species backward toward LUCA. Each triphosphate (ATP) and nicotinamide adenine dinucleotide branch also shows the relative order in which organisms (NAD) for short-term energy storage, polysaccharides, diverged and how evolutionarily close or distant they are in composed of glucose and similar sugars and more densely a simple diagram that is easy to understand.  CHAPTER 1: THE ASTROBIOLOGY PRIMER 3.0 S-9 In the ToL, LUCA lies just before the first branching 1.3.1. The formation of stars and the elements event. The stem beneath LUCA represents LUCA’s hypo- 1.3.1.1. Nucleosynthesis. The probability of Earth-like thetical ancestors (i.e., the first universal common ancestor, life existing elsewhere beyond Earth depends on the avail- FUCA). This stem could also have had branches of diver- ability and inventory of the fundamental elements of life gent organisms that have gone extinct and left no descen- (i.e., CHONPS). Elements are formed through nucleosyn- dants. If life were found undergoing Darwinian evolution thesis—the formation of specific types of atoms by adding elsewhere in the universe, a similar systematic classification or removing protons and neutrons. The primary nucleo- of organisms (i.e., a phylogenetic approach) could be used synthesis processes that formed the elements of the Sun and for classification. Earth are (i) the formation of light elements during the Viruses are genetically diverse microscopic agents that initial cooling of the universe after the Big Bang, (ii) the replicate only inside a cell by infection. They are an espe- formation of elements up to Ni through atomic fusion in cially important case when discussing the definition of life the cores of stars, and (iii) the formation of heavy elements and minimal requirements for inclusion in a ToL. Viruses during stellar supernovae and the merging of neutron stars. utilize DNA or RNA for information storage and can un- Big Bang nucleosynthesis began approximately 3 minutes dergo Darwinian evolution. However, because they cannot after the Big Bang and lasted approximately 17 minutes, metabolize and replicate independently (without co-opting a Downloaded by 2401:4900:2354:56c7:830:ca46:c66a:1cf1 from www.liebertpub.com at 09/18/24. For personal use only. producing about 75% hydrogen, 25% helium, and trace cell), many biologists do not consider them to be alive. amounts of lithium and beryllium. Nuclear burning in the While phylogenies can be constructed connecting viruses, cores of stars produces a range of different elements that their rapid evolution and lack of common shared elements depends on the type (e.g., mass) of the star, where heavier (e.g., ribosomal genes) make it difficult to connect viruses elements are produced at higher temperatures and pressures with universal ToLs. (Ryan and Norton, 2010). In the most massive stars, nuclear The case of viruses indicates that there are entities with burning in successive layers creates an onion-like structure some of the characteristics commonly used in working within the star. However, stellar nucleosynthesis can only definitions of life that nevertheless do not fit several aspects, generate elements up to nickel-56 (Ni56), which subse- and thus debate continues on the blurry definitions of life. quently decays to iron-56 (Fe56). The formation of elements Some biologists focus on cells, rather than replicating heavier than Ni56 requires temperatures and pressures that packets of information, and they differentiate ribocells are only present during extreme stellar events such as su- (translating the genes historically associated with the cell pernovae or neutron star mergers. membrane) and virocells (translating the genes provided by an infecting agent, the virus). Others focus on evolutionary relationships between mobile genetic elements that can be 1.3.1.2. Stellar types and life cycles. Stars are born traced by network models (Raoult and Forterre, 2008; when a gaseous nebula, typically composed predominantly Bapteste and Dupré, 2013). of hydrogen and helium, becomes unstable under its own gravity, causing collapse. Once enough material has ac- 1.2.4. Definitions of life in astrobiology creted, the gravitational pressure forces the elements to- gether, heating the core until nuclear fusion begins (Larson, One of the key goals of astrobiology is to formulate a 2003). Once a star runs out of hydrogen to burn, it begins to precise or comprehensive definition of life. Ideally, the slowly cool and eventually violently collapses. Smaller stars agreed-upon definition should be valid independent of his- (similar in mass to the Sun) initially expand to become a red torical context or technological advancement (Fry, 2000; giant and then shed their outer layers and slowly shrink to Tirard et al., 2010) rather than one describing the form and become a white dwarf. In higher-mass stars (larger than 8 mechanisms of life as we know it today. Although a widely solar masses), the core undergoes a rapid and violent col- accepted, comprehensive definition of life may always elude lapse called a supernova, which can expel several solar us, this does not diminish the value of research on the fields masses of material. Ejected material re-diffuses into the of origins of life, synthetic biology, or space exploration, interstellar medium, which is the diffuse dust and gas oc- and the search for life different from life on Earth. Modern cupying the space between the stars. attempts to define life aim to abstract certain characteristics The length of a star’s lifetime depends on how quickly its and develop operational and practical models that lead to fuel is used up: smaller stars live longer, potentially up to observable traits that enable hypothesis testing in the labo- trillions of years, while the most massive stars have life- ratory setting, computational simulations, or extraterrestrial times of only a few million years. The amount of elements environments. heavier than helium present in a star is known as its me- tallicity. Metallicity is an important measure to determine if 1.3. The Origins of the Solar System and Earth the CHONPS elements are present in distant stellar systems. The study of astrobiology relies on an understanding of Massive stars produce and spread heavier elements, thus the environments where life may be found, both on small seeding other protostellar nebulae with the building blocks and large scales. To understand how life can originate, it is for planets and, potentially, life. important to determine how solar systems and planets form As shown in Fig. 3.3, stars produce distinct photon and what features are required for the development of wavelength patterns, or spectra, that can be used to char- habitable environments. This section will provide an over- acterize the type of star. Stellar spectra vary depending on view of Chapter 3: The Origins and Evolution of Planetary the amount of heavy elements and the presence of mole- Systems and the physical and chemical processes which led cules. Astrobiological studies are typically limited to lower- to the formation of the only known habitable planet: Earth. mass stars (less than about 1.5 · the mass of the Sun), which  S-10 SCHAIBLE ET AL. have lifetimes thought to be long enough for the emergence changes before, during, and after their incorporation into of inhabited planets (i.e., F, G, K, and M type stars). The larger bodies. The study of chemistry in space is usually most compelling targets for astrobiology are stars that host divided into the fields of astrochemistry, which tends to rocky planets of roughly an Earth mass and output enough focus on the chemistry occurring in space, and cosmo- solar energy to power chemical and biological processes on chemistry, which largely deals with the chemical composi- a planetary surface. In this context, planets can be defined as tion of physical samples (e.g., meteorites and returned any rocky body, including dwarf planets and natural satel- samples). lites of planets (i.e., moons). The Sun is a yellow dwarf star with a temperature 1.3.3.1. Molecular formation in the interstellar medium of *5800 K and is designated as a G2 V star located on and protoplanetary disk. Determination of molecular the main sequence. It is included in the heaviest 5% of stars abundances in the interstellar medium (ISM) and around in the galactic neighborhood with an age currently estimated stars can give clues on the reaction mechanisms at work, at about 4.6 billion years. The Sun has a high metallicity, how certain molecules are formed, and how these can be meaning it contains a relatively high abundance of heavy incorporated into comets and asteroids that subsequently elements such as C, O, Si, and Fe. impact a planet like Earth. The ISM, asteroids, and mete- orites contain abundant complex organic molecules formed Downloaded by 2401:4900:2354:56c7:830:ca46:c66a:1cf1 from www.liebertpub.com at 09/18/24. For personal use only. 1.3.2. The formation of Earth and the Solar System through various processes, and about 300 molecular species have been identified in interstellar space. One class of Protoplanetary disks are the sites of star and planet for- complex organic molecules that has historically generated mation, two processes that are intimately linked. Once the significant interest is the polycyclic aromatic hydrocarbons collapse of a gas cloud is triggered and a star is formed, (PAHs). These molecules have been identified as an abun-