Chapter 3: The Origins and Evolution of Planetary Systems
Abstract
The materials that form the diverse chemicals and structures on Earth—from mountains to oceans and biological organisms—all originated in a universe dominated by hydrogen and helium. Over billions of years, the composition and structure of the galaxies and stars evolved, and the elements of life, CHONPS, were formed through nucleosynthesis in stellar cores. Climactic events such as supernovae and stellar collisions produced heavier elements and spread them throughout the cosmos, often to be incorporated into new, more metal-rich stars. Stars typically form in molecular clouds containing small amounts of dust through the collapse of a high-density core. The surrounding nebular material is then pulled into a protoplanetary disk, from which planets, moons, asteroids, and comets eventually accrete. During the accretion of planetary systems, turbulent mixing can expose matter to a variety of different thermal and radiative environments. Chemical and physical changes in planetary system materials occur before and throughout the process of accretion, though many factors such as distance from the star, impact history, and level of heating experienced combine to ultimately determine the final geophysical characteristics. In Earth's planetary system, called the Solar System, after the orbits of the planets had settled into their current configuration, large impacts became rare, and the composition of and relative positions of objects became largely fixed. Further evolution of the respective chemical and physical environments of the planets—geosphere, hydrosphere, and atmosphere—then became dependent on their local geochemistry, their atmospheric interactions with solar radiation, and smaller asteroid impacts. On Earth, the presence of land, air, and water, along with an abundance of important geophysical and geochemical phenomena, led to a habitable planet where conditions were right for life to thrive.
Some of the questions asked in astrobiology and planetology are: Where and how are planets formed? What makes terrestrial planets habitable? What kinds of planetary environments can develop a biosphere? As described in Chapter 2, many different elements are required to build the molecules and structures of complex life-forms. Because life as we know it requires liquid water, an Earth-like planet must form at or migrate to a distance from the star where surface temperatures are amenable to its presence. However, the materials of Earth and terran (meaning Earth-like) life were exposed to a wide variety of physical environments and chemical transformations long before the planet was even formed.
Many planetary features, such as surface composition, plate tectonics, a planetary dynamo (magnetic field), and tidal forces can contribute to the origin and evolution of life. Additionally, the radiative output of and distance from the star, impact history of comets and asteroids, and presence or loss of an atmosphere can also drastically affect planetary geochemistry and thus the availability of energy and building blocks for life (see Chapter 7 for a more detailed discussion of planetary habitability). It is not fully understood how life originated on Earth (the current understanding of the origin of life is discussed in Chapter 4), nor are the mechanisms used by life to survive in extreme conditions (see Chapter 6 for more details about the diversity and extremes of known life). Regardless, searches for life are ongoing (biosignatures and detection methods are described in Chapter 8), and the possibilities for non-Earth-like life remain open (see Chapter 9 for discussions of exotic life).
The goal of this chapter is to introduce the basics of planetary systems and their relationship to the field of astrobiology. The focus is primarily on the Earth/Sun planetary system, the Solar System, for which the most detailed historical record is available and which contains the only known instance of life. Chapter 3.1 reviews the formation of elements through the various nucleosynthesis mechanisms and the life cycles of stars. Chapter 3.2 and Chapter 3.3 describe the physical and chemical aspects, respectively, of planetary system formation. Chapter 3.4 addresses major planetary geophysical features—atmospheres, hydrospheres, and geospheres—and their diversity throughout the Solar System.
3.1. Beginnings of Matter and the Basics of Stars
3.1.1. The beginnings of matter: Energy and nucleosynthesis
The probability of life existing elsewhere in the universe besides Earth depends on how the fundamental components of life (i.e., CHONPS; see Chapter 2.2.2) are formed and combined to produce environments hospitable to life. Nucleosynthesis is the addition or removal of protons and neutrons to a preexisting atomic nucleus. The relative importance of various nucleosynthesis mechanisms in the formation of the naturally occurring elements and the relative abundance of the elements in the Solar System are shown in Fig. 3.1 (Johnson, 2019).

3.1.1.1. The basics of energy and matter in the universe
The universe, as it is currently understood, is composed of ordinary matter (5%), dark matter (∼27%), and dark energy (∼68%). Ordinary matter is a combination of positively charged protons, neutrally charged neutrons (together called nucleons), and negatively charged electrons. Ordinary matter is the material we can see and interact with on an everyday basis (e.g., Earth and stars). Charge is neutral throughout most of the universe (i.e., the number of protons and electrons are balanced). In high-temperature or radiation-dominated regions, nucleons and electrons exist as an unbound plasma, but when temperatures cool, the opposite charges of nuclei and electrons can bring them together to form an atom.
Additional details about the different forms of matter and energy are given in the supplementary material.
3.1.1.2. The Big Bang and the formation of the first atoms
The universe formed in the Big Bang about 13.8 billion years ago, an event that generated all energy and matter in the universe. Big Bang nucleosynthesis—the process of forming simple nuclei—began approximately 3 minutes after the Big Bang. The high temperature and density of the universe during this time resulted in energies and collision frequencies that were high enough to bring nucleons close enough for the attractive strong force, the force holding atomic nuclei together, to achieve nuclear fusion from combinations of fundamental particles (Weinberg, 1993). This period lasted approximately 17 minutes and produced about 75% hydrogen, 25% helium (by mass), and trace amounts of lithium and beryllium (the four lightest elements on the periodic table).
3.1.1.3. Growing the elements: stellar nucleosynthesis
Stellar nucleosynthesis is the process by which stars create progressively heavier elements through the fusion of atomic nuclei. The nucleosynthetic pathways that are possible in a star (i.e., the range of different elements produced during their lifetimes) depend on the type (e.g., mass) of the star (Ryan and Norton, 2010). Nuclear fusion processes at the stellar core, commonly referred to as nuclear burning (not to be confused with combustion), form progressively heavier elements as the density and temperature of the core increase. Nuclear fusion reactions occur sequentially within a single star and in a generational series of stars. The successive stages of nuclear burning create an onion-like structure within the star, with the heaviest elements and highest temperatures at the center (see Fig. 3.S1). Heavy elements formed in an earlier generation can be incorporated into a newly formed star. The amount of heavy metals present in a star is known as the metallicity. In astronomy, it is common to call all elements other than H and He “metals,” although other fields (e.g., chemistry and geology) tend to reserve the word only for the elements that form electrically conducting solids.
Additional details on star formation in the early universe and nuclear burning are given in the supplementary material.
As indicated in Fig. 3.2, elements with even atomic numbers (number of protons in the nucleus) tend to be the most abundant. Nuclear burning of elements heavier than carbon occurs by the alpha ladder process, which is the fusion of a helium nucleus (also known as an alpha particle with an atomic number of two) with a carbon (or oxygen, neon, etc.) nucleus to form increasingly heavy elements. The alpha ladder process can occur in the cores of massive stars or during supernovae. Stellar core nucleosynthesis creates isotopes as heavy as nickel (56Ni) through exothermic (energy-yielding) reactions that release the energy that sustains stars. Isotopes with masses larger than 56Ni can only be generated with endothermic (energy-consuming) reactions (e.g., the elevated temperatures and pressures that occur during a supernova). While 56Ni is the last fusion product produced in the cores of high-mass stars, it rapidly undergoes radioactive decay (half-life ∼6 days) into 56Fe through successive gamma ray emissions. The high binding energy per nucleon for elements near iron results in their efficient formation during supernovae due to oxygen and silicon fusion, and this significantly increases their abundance relative to other first-row transition elements in the periodic table (Fig. 3.2). The balance of the various nucleosynthesis and radioactive decay channels ultimately determines the final relative abundances of the isotopes of the elements (Lodders, 2010).

3.1.1.4. Supernova nucleosynthesis and merging neutron stars
Fusion acts to counter the force of gravity to keep the star from collapsing under its own weight. Once fusion can no longer support a star, gravity will overcome pressure generated through heat, and the core will collapse. There are two pathways a dying star can take. For low- to intermediate-mass stars (less than ∼8 M☉, where the mass of the Sun = 1 M☉), outer layers are ejected, and the core collapses to become a white dwarf—a hot, compact ball of helium, carbon, and oxygen in which nuclear fusion can no longer occur (see Fig. 3.S1). The ejected layers consist of mostly hydrogen and are known as planetary nebulae (although they have almost nothing to do with planets). For more massive stars, stellar death is accompanied by core-collapse supernovae (Arnett, 1996), during which the outer layers of the star are ejected, and the remnant core forms a neutron star or black hole (depending on the initial mass of the star). The explosive energy release during a supernova can drive the endothermic nucleosynthesis reactions required to form elements heavier than nickel (Chapter 3.1.1.3).
Additional details on supernova nucleosynthesis are given in the supplementary material.
3.1.1.5. Spallation and radiogenic decay
Additional details on nucleosynthesis through spallation, radiogenic decay, and in high-energy particle accelerators are given in the supplementary material.
3.1.2. The formation and life cycle of stars
While planets are typical sites of astrobiological interest, stars must first be considered due to their intimate connections with the planets they host. The process of stellar birth, evolution, death, and rebirth is illustrated schematically in Fig. 3.S2. In brief, a diffuse cloud leads to the formation of a translucent cloud, which in turn eventually becomes a dense cloud, collapses into a dense core, then a hot core, eventually forms a star and planetary system, and the process is restarted once the star goes supernova and forms a planetary nebula or re-diffuses the matter into the interstellar medium (ISM). Of particular interest to astrobiology are stars that host rocky planets of roughly an Earth mass and output enough solar energy to power chemical and biological processes on the planetary surface. Stellar age, life stage, mass, radius, temperature, and metallicity all play important roles in determining the potential habitability of orbiting planets (see Chapter 7.1.4).
3.1.2.1. Core collapse and stellar formation
Stars are formed when a gaseous nebula, typically composed of predominantly hydrogen and helium with small amounts of dust, becomes unstable under its own gravity, causing collapse. Gravitational collapse is a complicated process involving the balance of gravitational and hydrodynamic forces, and where magnetic fields, angular momentum, and molecular chemistry may play a role. The specific condition necessary for collapse is known as the Jeans instability: collapse occurs when the internal gas pressure is insufficient to counteract gravity. The mass at which collapse occurs, known as the Jeans mass, depends on the temperature and density of a gaseous cloud. Systems with larger mass, smaller size, and colder temperatures are more prone to gravitational collapse. Once enough material has accreted, the gravitational pressure forces the elements together, heating the core until nuclear fusion begins (see Shu, 1977; Larson, 2003 for more details).
3.1.2.2. Brief introduction to spectral type, luminosity, radius
The most obvious measurable feature of a star is its luminosity (i.e., the total amount of radiant energy emitted per unit time), which depends mainly on its temperature and radius. More massive stars tend to be larger, brighter, and hotter. Stars produce a distinct photon wavelength pattern, or spectrum, that can be used to characterize the type of the star, as illustrated in Fig. 3.3, and the spectra can vary depending on amount of heavy elements present (the metallicity) of the star and various molecules in the ISM and protoplanetary disks (see Chapter 3.3.1). The most commonly studied stellar types for astrobiology are F (6000–7500 K, 1.04–1.4 M☉), G (5200–6000 K, 0.8–1.04 M☉), K (3700–5200 K, 0.45–0.8 M☉), and M (2400–3700 K, 0.08–0.45 M☉).

The Hertzsprung–Russell diagram (HRD) is typically used to relate the effective temperature of a star to its overall luminosity (Montmerle and Ekström, 2015). As a star ages, both the luminosity and the radius can change, and typical tracks are traced on the HRD over a stellar lifetime. Thus, knowing a star's overall luminosity and temperature allows both the mass and age to be estimated. An example HRD is shown in Fig. 3.4, where several well-known stars are indicated, and representative stellar tracks are given for one and five solar-mass stars.

3.1.2.3. Main sequence versus evolved/pre-main sequence stars
The length of a star's lifetime depends on how quickly its fuel is used up: smaller stars live longer, potentially up to trillions of years, while the most massive stars have lifetimes of only a few million years (Phillips, 1999). Thus, astrobiological studies are typically limited to lower-mass stars (less than about 1.5 × the mass of the Sun) which have lifetimes thought to be long enough for the emergence of inhabited planets. However, the most massive stars also play a key role by forming and spreading the heavy elements. As discussed further in Chapter 7.1.4 in the supplementary material for Chapter 7, the mass of a star along with its radiant intensity and stellar activity can all have direct effects on the habitability of orbiting planets.
3.1.3. Context of the Sun compared to other stars
The Sun is a yellow dwarf star with a temperature of ∼5800 K and is designated as a G2 V star located on the main sequence (see Fig. 3.4). It is included in the heaviest 5% of stars in the galactic neighborhood (Robles et al., 2008), but not so massive that it burned out before life had a chance to develop. The age of the Sun is currently estimated at about 4.6 Ga, and it is expected to enter its red giant phase in ∼8 Ga, at the age of 12 Ga. The Sun is not a binary star, which are common (Duchêne and Kraus, 2013); thus the likelihood of forming stable planetary orbits is greater. Further, the Sun has a high metallicity, meaning it contains a relatively large abundance of heavy elements such C, O, Si, and Fe.
3.2. How Planets Form: Physics of Planet Formation
Protoplanetary disks are the sites of star and planet formation; these processes are intimately linked together. Once the collapse of a gas cloud is triggered and a star is formed (see Chapter 3.1.2), protoplanetary disk formation begins. The protoplanetary disk contains particles of gas and dust which can extend up to thousands of astronomical units (AU) from the central star (typical sizes ∼100–200 AU around A-type stars). Although Earth and the Solar System have evolved far beyond these stages, similar planetary systems can be observed in various stages of evolution by looking out into the stars. Figure 3.5 shows three protoplanetary disks imaged by the Atacama Large Millimeter Array (ALMA) telescope, each of which has unique structure and morphology. The ALMA images reveal the presence of disk substructures that could be signs of planet formation, including rings, gaps, and spiral structures (Huang et al., 2018).

This section will briefly review the processes controlling the growth of planets in the protoplanetary disk and various gravitational interactions that can influence the evolution of a planetary system through time.
3.2.1. Core accretion versus gas instability (merits and drawbacks)
There are two fundamentally different hypotheses for seeding planets in protoplanetary disks: core accretion and gravitational instability. The core accretion hypothesis states that dust collisions over time will build up larger and larger particles, making first small grains, then pebbles and boulders, ultimately generating planetesimals and protoplanets. In this scenario, collisions within the protoplanetary disk are invoked to increase the size of particles and enable the growth of large bodies. A potential caveat is the possibility that collisions may not be fully accretional; for example, two colliding bodies could result in fracture or loss of material.
An alternative to core accretion is gravitational instability (Zel'Dovich, 1970; Boss, 1997). In this scenario, protoplanetary disks can undergo further fragmentation and collapse. Although this model presents difficulties in producing the observed properties of the Solar System and many exoplanetary systems, it may operate very early in the life of massive protoplanetary disks where instabilities can lead to fragmentation of the disk and the formation of massive objects at large separation distances, such as gas giant planets and brown dwarf stars (Kratter and Lodato, 2016).
3.2.2. Growth of planets
Protoplanetary disks are initially composed of diffuse gas and dust particles, and gravity causes grains to settle into the midplane of the disk. According to the core accretion model, collisions between particles during this settling process are thought to result in their growth from small grains to centimeter-sized pebbles to kilometer-sized bodies and finally planetesimals and protoplanets. Once larger bodies (e.g., planetesimals or planetary embryos; see Johansen and Lambrechts, 2017) are formed, a process called pebble accretion, where smaller “pebbles” accumulate on larger bodies, could occur, leading to continued growth. The precise details and constraints on these processes of planet formation are still open areas of investigation.
Once significantly large planetary bodies accumulate in protoplanetary disks, the processes of runaway growth and oligarchic growth occur. Under runaway growth, planetary bodies in a disk grow proportionally to their mass therefore, the larger bodies grow faster than smaller bodies (primarily due to enhanced gravity). Runaway growth is terminated when the random velocities of planetesimals increase enough to largely prevent collisions. The next phase, oligarchic growth, occurs once a population of planetesimals has formed. Here, the largest planetary embryos grow quickly at the expense of the smaller bodies due to increased gravitational forces. As a result of oligarchic growth, the mass distribution bifurcates to contain (i) a population of moon-to-Mars-mass embryos and (ii) a population of a larger number of smaller planetesimals. The last stage of planet formation after oligarchic growth is embryo-embryo collisions (Mandell, 2015).
Additional details of the various phases of planetary accumulation are given in the supplementary material.
3.2.3. Models of the Solar System's formation
Although it is impossible to directly observe the formation of the Solar System, many questions can be explored to help explain how planetary system formation works in general. For example, what sets the relative orbital distances of the planets? How can the presence of the asteroid belt, Kuiper Belt, and Oort Cloud be explained? Why is there a compositional difference in the planets in the inner Solar System (rocky, terrestrial) and the outer Solar System (gas/ice giants)? Why is Mars so much less massive than Earth? Ideally, a formation theory for the Solar System would explain most, if not all, of these observations (Morbidelli and Raymond, 2016).
3.2.3.1. The Grand Tack theory
The Grand Tack theory of Solar System formation, first put forth by Walsh et al. (2011), describes the broad-scale migration of the planets to explain their current positions and mass differences. In the Grand Tack theory, Jupiter initially began accreting around 3.5 AU but migrated inward and toward the Sun as it continued to grow (Lin and Papaloizou, 1986). Saturn, since it is smaller, would have migrated inward faster than Jupiter, thus allowing the planets to enter a 2:3 mean motion resonance (i.e., where Jupiter orbits the Sun three times in the same amount of time as Saturn orbits twice). This resonance could have caused the two planets to migrate outwards under certain disk conditions (Masset and Snellgrove, 2001; D'Angelo and Marzari, 2012). The location and timing of the potential planetary migrations are not known, so the Grand Tack theory seeks to constrain these parameters by understanding the effects of such an inward-then-outward migration on characteristics of the Solar System (Raymond and Morbidelli, 2014).
Additional details about the Grand Tack theory are given in the supplementary material.
The planetary migration invoked in the Grand Tack theory is also intriguing from the point of view of comparing the Solar System to observed exoplanetary systems (see Chapter 8.4.2). Planetary migration appears to be common in planetary systems, with gas giant planets found very close to their host stars. So-called “hot Jupiters” (gas giants roughly the size of Jupiter that orbit very close to their host stars, often far interior to Mercury's orbit of our own Sun) are not likely to have formed where they are found given that there would not have been enough access to solid material to grow to masses large enough to accrete gases. Consequently, the currently accepted view of hot Jupiter formation is that these planets formed outside the snow line in their respective systems then migrated inward. The Grand Tack theory postulates that the difference between the Solar System and these exoplanetary systems was the presence of Saturn, which caused the reversal of Jupiter's migration. While the Grand Tack theory has been shown to successfully reproduce some aspects of the Solar System architecture, many unknowns still remain, and planetary migration studies are an area of active research.
3.2.3.2. The Nice model
The Nice model, named for a series of ideas and papers developed by researchers in Nice, France, focuses on the long-term dynamical evolution of the Solar System (Gomes et al., 2005; Morbidelli et al., 2005; Tsiganis et al., 2005). The Nice model can explain the eccentricities and inclinations of the giant planets, the presence of Jupiter's Trojan asteroids, and the postulated Late Heavy Bombardment (LHB). However, some disagreement remains whether all of these features can be explained by outward migration of Jupiter and Saturn due to interactions with planetesimals in the outer Solar System.
Giant planets are thought to form on essentially circular and coplanar orbits, yet the Solar System gas and ice giants presently have nonzero eccentricities and inclinations. If Jupiter and Saturn passed through a 1:2 orbital resonance, the orbital motions of the planets could have become chaotic. Subsequent close encounters between planets could then lead to increasing inclinations. The current orbital architecture of the outer planets is consistent with expectations that the 1:2 orbital resonance was indeed crossed (Morbidelli et al., 2005; Tsiganis et al., 2005). The Jupiter/Saturn mean motion resonance crossing stipulated by the Nice model can also help explain the LHB, a hypothetical brief period early in the history of the Solar System when the inner planets experienced a uniquely heavy flux of impactors. The crossing of the orbital resonance would have scattered small bodies, thus leading to a brief period of intense impact activity throughout the Solar System. However, the LHB is somewhat contested in recent literature which suggests bombardment may have been more drawn out (see Chapter 3.3.4).
3.2.4. Comparing the Solar System to exoplanetary systems
For most exoplanets, the characteristics that can be measured using current instrumentation include a lower limit on the mass of the planet, the radius of the planet, orbital eccentricity, orbital period, and the average distance between the planet and the star. As illustrated in Fig. 3.6, NASA's Kepler and TESS missions have discovered thousands of planet candidates (5312 according to the Exoplanet Archive [https://exoplanetarchive.ipac.caltech.edu], as of March 16, 2023). Exoplanet searches have found several planets similar in size to Earth (Fressin et al., 2012) and planets potentially in the habitable zone (Hill et al., 2023), as well as a growing number of multiplanet systems (Steffen and Lissauer, 2018) and planets in systems with multiple stars (e.g., Doyle et al., 2011; Schwamb et al., 2013).

While many of the first exoplanet detections were gas giants orbiting close to their star, as exoplanet detection techniques have improved (see Chapter 8.4.1), astronomers have found progressively smaller extrasolar planets—in particular, “super-Earths,” which are planets with masses up to 10 times the mass of Earth and radii less than ∼1.5 times larger than Earth's, and “mini-Neptunes,” which are larger than super-Earths but smaller than Neptune (Howard et al., 2012). The increasing number of exoplanet detections has yielded attempts at creating empirical mass-radius relationships for extrasolar planets. (e.g., Wolfgang and Laughlin, 2012; Lopez and Fortney, 2013; Weiss and Marcy, 2014). The analyses of these relationships suggest a “breakpoint” around 1.5 Earth radii (∼5 Earth masses); above that size, increasingly larger planets have a composition that is dominated by H2/He gas (e.g., Rogers, 2015; Fulton and Petigura, 2018). This boundary is not sharp, however, and there are planets above this threshold that may be potentially rocky, such as 55 Cancri e (Bourrier et al., 2018).
Following the end of Kepler's primary mission in 2012, the telescope continued to find new planets until 2018. Other efforts are continuing to make new discoveries, such as the TRAPPIST survey, which detected a seven-planet system orbiting a small host star designated as TRAPPIST-1 (see Chapter 7.2.10.2). All the planets in the TRAPPIST-1 system are small and likely rocky (Dorn et al., 2018) and arranged compactly around their host star. This is a sharp contrast to the widely spaced and disparately sized planets of the Solar System. Another search is the CARMENES program, which recently discovered two Earth-sized planets orbiting a close-by, dim red star that had only been discovered itself relatively recently, Teegarden's Star (Teegarden et al., 2003; Zechmeister et al., 2019). Planetary systems orbiting small stars like Teegarden's Star and TRAPPIST-1 provide a unique perspective on planet formation and evolution that is distinct from the Solar System.
3.3. How Planets Form: Chemistry of Planet Formation
As described in Chapter 3.2, young planetary systems are filled with rocky and icy bodies condensed from the diffuse pre-stellar nebula. The constituent molecules and minerals undergo a wide variety of chemical changes before, during, and after their incorporation into larger bodies (Leroux, 2009; Williams and Cieza, 2011; Henning and Semenov, 2013; Öberg, 2016). The study of chemistry in space is usually divided into the fields of astrochemistry and cosmochemistry. Astrochemistry tends to focus on the chemistry occurring in space, both in the ISM and on planetary objects and their satellites. Cosmochemistry largely deals with the chemical composition of physical samples, namely meteorites, and how they can be used to understand the history of the Solar System. This section will give a brief overview of chemistry occurring in the Solar System and beyond.
3.3.1. Astrochemistry in the ISM and protoplanetary disks
Astrochemistry has several important links with the larger field of astrobiology, in particular the search for prebiotic molecules in systems that are thought to be precursors of habitable planetary systems. Determination of molecular abundances in the ISM and around stars can give clues on the reaction mechanisms at work, how certain molecules are formed, and how these can be incorporated into comets and asteroids. In general, one can distinguish the formation of molecules through gas phase collisions or through surface chemistry occurring on dust grains or icy mantles. More details about the different regions of the ISM are given in the supplementary material.
The ISM, asteroids, and meteorites contain abundant complex organic molecules (COMs) formed in the diverse chemistry and radiation environments of the ISM and protoplanetary disks (Glavin et al., 2006; Herbst and van Dishoeck, 2009; Ehrenfreund and Cami, 2010; Pizzarello and Shock, 2010). Almost 200 COMs have been identified in space, and those with at least seven atoms (excluding large fullerene molecules) are shown in Fig. 3.7. COMs are of particular interest to astrobiology as these can provide a feedstock for more complex prebiotic reactions and, potentially, serve as the starting point for biomolecule formation (see Chapter 3.3.4 and Chapter 4.2). One class of molecules that has historically generated significant interest are the polycyclic aromatic hydrocarbons (PAHs). These molecules have been identified as an abundant component of the ISM (Allamandola et al., 1985; Tielens, 2005; Yamamoto, 2017) and protoplanetary disks (e.g., Seok and Li, 2017), and have been hypothesized to play important roles in prebiotic chemistry (Groen et al., 2012).

FIG. 3.7. Complex organic molecules (COMs) consisting of at least seven atoms and detections in other cosmic bodies. Image credit: K. Stelmach (2021).
References: 1McGuire, 2021; 2Grady et al., 2018; 3Bandurski and Nagy 1976; Becker et al., 1997; Briscoe and Moore, 1993; Cooper and Cronin 1995; Cooper et al., 2001; Jungclaus et al., 1976; Simmonds et al., 1969.
Notes: ^No amino acids have been confirmed to exist outside the Solar System. *Methane and methanethiol consist of less than seven atoms.
3.3.1.1. Astrochemistry methods
Research in astrochemistry can be roughly divided into observational, theoretical, and laboratory investigations. The telescopes used for observational astrochemistry, described in Fig. 3.S3, make use of primarily the millimeter/submillimeter (radio and microwave) and infrared wavelength regimes. The former can be detected using ground-based instruments, whereas the latter, due to atmospheric absorptions, mostly requires instruments to be placed in the stratosphere or in space. Computational modeling can be roughly divided into three areas: astrochemical modeling, chemical modeling, and radiative-transfer modeling. Experimental work can be divided into gas phase and solid phase experiments. More details on various astrochemical methods are given in the supplementary material.
3.3.1.2. Gas phase astrochemistry
Gas phase reactions can be induced by radiation or through collisions. In the case of single atoms or molecules, interactions with UV photons, electrons, and ions can lead to ionization or dissociation. Ion-neutral reaction mechanisms are used to explain how complex molecules form. Interstellar photons typically only provide energies up to 13.6 eV, which is insufficient to ionize H2 (which requires 15.4 eV). Cosmic rays provide more energy than interstellar photons and are thus an important component in the ionization of H2. Proton transfer reactions are a special type of ion-molecule reaction where the ion is H+. For example, the trihydrogen cation, H3+, plays a crucial role in the conversion of atomic hydrogen into H2 by acting as a proton donor, and proton transfer reactions are also key in the formation of water and organic molecules (Tielens, 2005; Yamamoto, 2017).
Recombination reactions are commonly used to explain molecule formation in space (Tielens, 2005). Molecules and atoms can interact with UV photons through photodissociation reactions (e.g., AB + hν → A+ + B-) (Tielens, 2005; Yamamoto, 2017). These reactions are simply the reverse of the radiative recombination reaction and are limited to molecules. Reactions involving photons are rather limited in relatively dense environments because of molecular self-shielding. For example, an H2 cloud strongly absorbs the UV in the outer regions such that the interior of the cloud is left unaffected.
One example of molecular formation in space that is highly relevant to astrobiology is water. A common interstellar molecule, water forms in three stages (Yamamoto, 2017): (i) the formation of OH+, (ii) reaction with H2, and (iii) the final formation of water through recombination. In the (i) stage, atomic O is ionized and protonated:
The next step (ii) is a series of proton abstraction reactions:
And the final recombination (iii) mechanism is:
Recent modeling of expected deuterium-to-hydrogen enrichments suggests that the Solar System's water was created in interstellar ices (Cleeves et al., 2014). This may end up being true for typical planetary systems. Additional details on various gas phase astrochemistry reactions are given in the supplementary material.
3.3.1.3. Surface astrochemistry
Dust grain surfaces are important in the formation of many astrochemical species because they can increase the potential for atoms/molecules to react and additionally act as a sink for excess reaction energy (Vasyunin and Herbst, 2013). Important processes in surface astrochemistry include (i) accretion of atoms/molecules from the gas phase, (ii) desorption of a molecule or atom from the solid surface, (iii) diffusion on the solid surface, and (iv) chemical reaction with the underlying substrate and/or other adsorbed species. There are two general types of dust in space: amorphous carbon and silicates. Silicate grains can be further subdivided into amorphous silicates and crystalline silicates. The latter are more common around protoplanetary disks where the hotter temperatures from the developing star can melt the silicate and allow it to crystallize as its temperature cools.
The importance of dust grains is evident in the formation of H2 (Yamamoto, 2017). Gas phase mechanisms are too slow to explain observed abundances, and the dust grain surfaces provide an outlet for the excess energy of formation from the reaction H+ + H+ → H2 + energy, thereby increasing the formation rates. Excess energy is either adsorbed by the dust grain or lost through sublimation into the gas phase. Surface chemistry is important for the formation of hydrides like water, which is formed on grains by simple atomic addition (H + O → OH, H + OH → H2O or H2 + OH → H2O + H).
3.3.2. Composition of forming planets
Planet formation is far from straightforward and uniform. The bulk composition of planets is predominantly determined by the protoplanetary disk materials present in the region where the body accumulates. Heat from the central star generates a temperature gradient, with temperatures >10,000 K in close orbits and dropping to <10 K in the outermost regions. This temperature gradient creates a natural concentration gradient where only materials that can withstand high heat are present close to the star, and more volatile species condense on surfaces in the cold outer regions of the planetary system.
More details on the compositional variations in the Solar System during and after the epoch of planet formation are discussed in the supplementary material.
3.3.3. Chemical inventory in asteroids and comets
Asteroids and comets in the Solar System are thought to be “leftovers” from planet formation. Asteroids mostly reside between the orbits of Mars and Jupiter, while comets originate mainly in the outer Solar System Kuiper Belt and Oort Cloud. Some asteroids and comets may contain pristine records of their formation conditions, giving us a window into the composition and molecules present in the early Solar System (Mumma and Charnley, 2011). However, radiation, chemical processing, and differentiation can further modify the original materials. Many questions remain as to how much of the chemistry of comets/asteroids is native (or unaltered from its initial formation state) versus transformed during the history of the Solar System. Sample return (discussed in the context of biosignatures in Chapter 8.2.4) from these bodies provides some of the best evidence about the composition of molecules derived relatively without alteration from the protoplanetary disc (Matzel et al., 2010; Nittler, 2010).
Additional details on the different comet and asteroid types and recent spacecraft missions to several comets and asteroids are given in the supplementary material.
3.3.4. Delivery of volatiles and water to early Earth
The emergence of life on Earth required the presence of a variety of volatile elements and other species such as small organic molecules and water. However, the region of the Solar System in which Earth formed is believed to have been dry and mostly depleted in volatiles (based on the volatile-poor composition of the innermost asteroids), thus suggesting that the delivery of these species (or their precursors) to Earth occurred through impact delivery (Dreibus and Wänke, 1989). Since Earth is differentiated (Chapter 3.4.3.1), denser substances, known as highly siderophilic elements (HSEs; e.g., Pt and Au), preferentially accumulated at the center of the planet, while lighter elements (i.e., silicate rocks and carbon-rich species) remained at the top. The presence of heavy elements in the mantle at abundances above what is expected suggests that a “late veneer” of material was delivered to Earth from an extraterrestrial source after the process of core formation and surface solidification.
Craters on the Moon, Mars, and Mercury point to a period of intense bombardment, commonly known as the Late Heavy Bombardment (LHB), about ∼4.0 to ∼3.5 Ga after planet formation by residual materials present in the protoplanetary disk. Initially it was argued that dating of the major impact basins on the Moon (centered around ∼3.9 Ga) was evidence for a spike in impact events at that time (referred to as the late/lunar “cataclysm” or late heavy bombardment; e.g., Ryder, 1990). However, recent evidence suggests these dates are incorrect and that the bombardment was drawn out from 4.25–3.87 Ga (Zellner, 2017; Michael et al., 2018). Additionally, measurements of mantle-derived volatiles match isotopic abundances of primitive chondrites, indicating that Earth may have retained many of the volatiles delivered early in its accumulation (Broadley et al., 2020).
Given their ice-rich composition, comets were originally suggested as the primary source of Earth's water (Owen and Bar-Nun, 1995; Delsemme, 2000). However, this idea has since been largely discredited due to the low probability of cometary collisions with Earth (Morbidelli et al., 2000) and a different hydrogen isotopic composition (deuterium/hydrogen ratio) of water in many comets compared to Earth (Marty and Yokochi, 2006). Although relatively water-poor in comparison with comets, asteroidal material has also been suggested as the primary water source given their higher probability of colliding with Earth (Morbidelli et al., 2000; Raymond et al., 2004). Most asteroids are small, predominantly rocky bodies, but it has long been known that some asteroids likely contained a greater volume of water early in Solar System history (Alexander et al., 2018). Additionally, impacts would have delivered an abundance of diverse organics and other species necessary for life to early Earth (Chyba et al., 1990; Morbidelli et al., 2000; Pasek and Lauretta, 2005; Altwegg et al., 2020). It has been estimated that early Earth could have experienced as much as 109 kg/year of organic carbon delivery (Chyba and Sagan, 1992), thus providing important precursors for more complex prebiotic molecules thought to be important for origins of life (see Chapter 4.2.3).
3.3.5. Astrophysical origins of biomolecular chirality
Chiral molecules are found in abundance in space. Recent astronomical observations have detected chiral propylene oxide in star-forming regions (McGuire et al., 2016), and many of the molecules found in asteroids and comets have chiral centers (Cronin and Chang, 1993; Goesmann et al., 2015). Multiple analyses of organic molecules extracted from primitive meteorites contain amino acids and sugars with enantiomeric excesses of, respectively, the L- and D-enantiomers (Cronin and Pizzarello, 1997; Glavin et al., 2012; Cooper and Rios, 2016; Burton and Berger, 2018; Glavin et al., 2020), the same chirality as found in biologic molecules (see Chapter 2.2.3.5). However, abiotic mechanisms that form these molecules tend to produce both enantiomers in an equal abundance (Bernstein et al., 2002).
Additional details of various chiral selective processes that could be operational in space are given in the supplementary material.
3.4. Fundamentals of Planetary Geophysics
The geophysical processes which shape and form planetary lithospheres, atmospheres, and hydrospheres played a direct role in the formation and evolution of life on Earth (see Chapter 4.1 and Chapter 5.2). Additionally, geochemistry is expected to play an important role in the search for life elsewhere (see Chapter 7.1 and Chapter 8), including in the case for exotic life (see Chapter 9). This section will discuss these geologic features in the context of what is known from planetary bodies in the Solar System and briefly discuss how they contribute to planetary evolution and the development of habitable conditions.
3.4.1. Atmospheres: Gas phase geochemistry
Atmospheres exist on most of the familiar terrestrial planets (Venus, Earth, and Mars), as well as gas giants and even around some icy bodies (see Chapter 7.2). The composition of a planet's atmosphere is affected by its planet, host star, and environment, resulting in atmospheres that can be thicker (e.g., Venus) or thinner (e.g., Mars) than Earth's present-day atmosphere.
Planetary atmospheres are critical components in the origin and evolution of life (e.g., Sasselov et al., 2020). They facilitate the existence of liquid water (see Chapter 2.2.2.1), provide a medium for accessing the biogenic elements through respiration (see Chapter 6.2.1), and shape surface environments via weather and climate. Atmospheric gases absorb and emit radiation at many different wavelengths so that it is transparent at some wavelengths and opaque at others. Because Earth's atmosphere is opaque in much of the harmful UV, X-ray, and gamma ray portions of the spectrum (see Fig. 3.8), the atmosphere serves as a sort of “shield” against high-energy radiation (e.g., Rugheimer et al., 2015). The wavelength-dependent transparency of atmospheres allows them to be studied using spectroscopic techniques at both ground- and space-based observatories (see Chapter 8.4).

3.4.1.1. Formation and general structure of atmospheres
There are two types of planetary atmospheres: primary and secondary. Primary atmospheres are composed of light or volatile gasses that accrete with the planet and have a composition similar to the region where the planet is formed (e.g., Jupiter). By contrast, secondary atmospheres form later and represent a delicate balance between supply from the planetary interior and small impactors, and loss due to large impacts and thermal escape (Zahnle et al., 2010). Earth's primary atmosphere was likely lost soon after formation, largely due to the Moon formation event (see Chapter 4.1.2). The dominance of N2, CO, and CO2 in the atmospheres of the terrestrial planets of the inner Solar System is consistent with these bodies having secondary atmospheres. In general, most atmospheres share similar thermal structures to Earth. Atmospheric chemistry can complicate this structure on worlds like Titan (Chapter 7.2.5).
3.4.1.2. Reducing and oxidizing atmospheres
Depending on its gas composition, an atmosphere can be either reducing or oxidizing, and this has major implications for surface geochemistry and life (see Chapter 6.2). Reducing atmospheres contain many reducing agents (e.g., H2, CO, and CH4), and oxidizing atmospheres contain many oxidizing agents (e.g., CO2 and H2O). The mantle redox state is critical to understanding the composition of the atmosphere (Ortenzi et al., 2020; Van Hoolst et al., 2019), and degassing occurs from both melt and solid phases as a planet cools (Bower et al., 2019). Newly formed planets tend to have strongly reducing atmospheres with a large amount of H since this is the most abundant species in the solar nebula and reduced species are the least soluble in most melts (Gaillard and Scaillet, 2014). Over geological time, planets with reducing primary atmospheres may, if conditions are right, transform to an oxidized secondary atmosphere via volcanic activity and outgassing, redox reactions, and biotic interactions (see Chapter 5.2).
Although complex life appears to depend on oxygen, it is possible that life could have arisen in an early anoxic (or oxygen-poor) atmosphere (see Chapter 4.1.2). Biological processes tend to catalyze redox reactions and can produce far more oxidants compared with geological processes. Because life can catalyze oxygen production orders of magnitude faster than abiotic production, high oxygen concentration in an atmosphere is a potential biosignature, although oxygen detection alone is not sufficient for life detection since it can also be produced abiotically (see Chapter 8.3.1).
3.4.1.3. Orbital parameters and climate
Variations in obliquity are key in determining how an atmosphere evolves in the short term (millions of years) and over geological time. Nonzero obliquity causes changes in stellar radiation at a given latitude, resulting in seasons and changes in the length of day. Beyond the local weather effects, the poles can influence global climate; on Earth, polar vortices, or regions of low pressure and cold temperatures, have been linked to distant weather patterns (Tang et al., 2014). On Mars, seasonal effects drive sublimation and deposition of CO2 and water vapor from the poles, with up to 30% of the total atmospheric CO2 being part of this cycle (James et al., 1992).
Planetary rotation rates also affect atmospheres. Tidally locked planets rotate on their axis at the same rate as they orbit their star, so the same hemisphere is constantly illuminated. This can lead to large temperature differences between the two hemispheres and, for planets with sufficiently thick atmospheres, potentially strong winds. Although no planets in the Solar System synchronously rotate, Venus has a day that is approximately half the length of its year.
A third orbital parameter that affects atmospheric evolution is the eccentricity, /e/ or how circular an orbit is. Planets with an eccentricity, /e/ of zero have perfectly circular orbits; all real planets have nonzero eccentricity, though many are nearly circular (e.g., Earth has /e/ = 0.017). High eccentricities may cause significant atmospheric changes from aphelion (the point farthest from the star) to perihelion (the point closest). Ongoing research suggests that, on Mars (e = 0.0934), dust storm activity and loss of water to space may be enhanced at perihelion (Cangi et al., 2024).
3.4.1.4. Energy balance and the greenhouse effect
The temperature of a planet's surface and the surface layer of its atmosphere are important to the development and continued existence of life (see Chapter 7.1.3.1 in the supplementary material for Chapter 7). Terran life can only survive over a limited temperature range (see Chapter 6.3.1), and it is typically assumed that life in general requires a relatively stable, equilibrated temperature state. For life to be detectable on exoplanets, it must be a globally distributed habitable environment in contact with the planet's atmosphere (see Chapter 8.3).
Generally, the surface temperature of a planet is set by the balance between the incoming stellar radiation that heats the planet and the outgoing thermal radiation from the planet that cools it (Trenberth et al., 2009). Planets may experience net warming, net cooling, or a careful balance that can vary as a function of the orbital parameters, the atmospheric composition, and albedo. Net warming is known as the greenhouse effect. On Earth, this effect can lead to positive effects, such as keeping the surface warm enough for liquid water to exist, although in some cases (e.g., Venus; see Chapter 7.2.2) planets can enter what is known as a runaway greenhouse. Additional details about the runaway greenhouse phenomena are given in the supplementary material.
3.4.1.5. Atmospheric loss through escape to space
All planetary atmospheres undergo loss through time. Although atmospheric loss may be relatively minor on large planets (e.g., Earth and Venus), smaller bodies can have significantly greater loss rates and thinner atmospheres. If an atmosphere is too thin (<0.01 atm), liquid water cannot exist on the surface. Escape processes that depend on mass or charge tend to cause isotope fractionation (e.g., enrichment of the more massive isotope compared to the lighter one). One of the most common examples of isotopic fractionation is the D/H ratio, or the ratio of deuterium (2H or D) to hydrogen (1H, or just H). Depending on the atmospheric chemistry and surface conditions on a planet, an elevated D/H ratio is often used as an indicator of water loss (predominantly H2O) throughout the planet's history. In the Solar System, both Venus and Mars have D/H ratios higher than Earth for reasons which seem to indicate a history of water loss (Kasting and Pollack, 1983; Owen et al., 1988). Additional details on the different types of atmospheric loss are given in the supplementary material.
3.4.1.6. Surface interactions
A planet's atmosphere and surface evolve together. Species enter the atmosphere through evaporation, sublimation, and volcanic eruptions. Conversely, particles are removed from the atmosphere through dissolution or condensation. Gas phase species can also dissolve in liquids (e.g., CO2 dissolves in Earth's oceans) or catalyze surface oxidation reactions. Surfaces can also be shaped by wind (i.e., aeolian processes). Material transport by wind, while less efficient than water, contributes heavily to the transport of nutrients on Earth. The most common landforms derived from aeolian transport are dunes, and these features have been identified on all Solar System bodies with atmospheres whose surface pressure is on the order of 1 kPa or greater (i.e., Venus, Earth, Mars, and Titan). Besides freezing into ice, water can be incorporated into the solid phase through hydration of crystalline structures. Phyllosilicates, also known as clay, are an example of this process.
3.4.2. Hydrospheres: The influences of surface liquids
Hydrospheres are defined as the total water content on the surface of a planet. Some planetary bodies, like Europa and Enceladus, may contain an ocean beneath a layer of water ice, or even alternative liquids such as the “methanosphere” on Titan. This subsection will focus on water and discuss the importance of hydrospheres in the Solar System. The importance of liquid water to life and planetary habitability of a planet are further discussed in Chapter 2.2.2.1, Chapter 4.2, and Chapter 7.1.
3.4.2.1. Hydrologic cycle on Earth
Due to the temperature ranges on Earth's surface, water is continuously cycled among all three phases of matter: gas, liquid, and solid. The physical transformation and movement of water is called the hydrologic cycle. The hydrologic cycle generally includes evaporation, condensation, precipitation, and runoff. Further, ices can melt and subsequently evaporate, or directly sublimate to return water to the atmosphere. An additional contribution to Earth's modern hydrologic cycle is transpiration, or the exhalation of water vapor by plants.
3.4.2.2. Erosion and sediment transport
Water is the dominant erosive and transport agent on Earth and plays an important role in facilitating the carbonate–silicate cycle. The carbonate–silicate cycle helps control the long-term climate stability of Earth by acting as a feedback loop for CO2 in the atmosphere (Berner et al., 1983; Dasgupta and Hirschmann, 2010). In the carbonate–silicate cycle, atmospheric CO2 produces carbonic acid in rainwater, which can effectively weather both carbonate and silicate rocks on Earth's surface. Runoff ultimately carries the resulting dissolved materials to oceans or lakes. In the absence of a biological presence, the presence of enough calcium and bicarbonate/carbonate ions in a body of water will cause precipitation of calcium carbonate, often called the Urey reaction (Urey, 1952). Precipitates sink to the bottom of the ocean floor and are buried, and eventual melting in the mantle can release CO2 which is returned to the atmosphere through volcanism (see Chapter 3.4.3.4). When there is excess CO2 in the atmosphere, weathering of sediments will increase, causing more calcium carbonate precipitation, which acts as a sink for the excess CO2. Conversely, if CO2 abundance is reduced, weathering will be less significant, leading to less precipitation of CaCO3 and therefore less pull-down of CO2 from the atmosphere.
3.4.2.3. Surface hydrologic cycles on other planets
Other bodies in the Solar System may have, or have previously had, enough water to drive hydrologic cycles of their own. Mars shows numerous indications of having been substantially wetter in the past and indeed sculpted by liquid water (see Chapter 7.2.1). While Mars currently does not have significant amounts of surface liquid water, the polar caps maintain a large ice reservoir at or near the surface, and a small amount of water vapor is present in the atmosphere. Additionally, Saturn's moon Titan has a methane-based “methanologic” cycle, where methane replaces water as the species found in lakes, clouds, and ice (see Chapter 7.2.5).
3.4.2.4. Geology of subsurface ocean worlds
Beyond Earth, numerous objects in the Solar System have been found, or are presumed, to contain large subsurface oceans of liquid water (see Chapter 7.2.3–7.2.7). These oceans are attractive targets for astrobiological investigation due to the ubiquitous association of terrestrial life with liquid H2O. The liquid oceans of Enceladus and Europa may be in direct contact with their rocky mantles, allowing for water/rock interactions and potentially habitable conditions to form (see Chapter 7.1.3 in the supplementary material for Chapter 7). Due to the thickness of the water layer on Ganymede, interior pressure modeling suggests that the subsurface ocean is not exposed to the rocky mantle but instead a tetragonal ice phase that is denser than the subsurface liquid water (Vance et al., 2014; Journaux et al., 2020).
3.4.3. Geospheres: Mass transport and energetics
The physical media of astrobiology is planets, and the bulk of a planet's mass is contained in its geosphere. The geosphere is the portion of the planet that includes the rocks and minerals. This subsection introduces the various physical processes which shape and modify the geospheres of planets over time.
3.4.3.1. Planetary differentiation
Planetary bodies of sufficient mass and/or temperatures (e.g., driven by planetary accretion or continued radioactive decay) inevitably undergo physical planetary differentiation: the separation of materials through melting and gravitational separation. During planetary differentiation, high-density materials tend to sink while lighter materials float toward the surface. On Earth, this resulted in differentiation into four distinct regions (from surface to center): the crust, mantle, outer core, and inner core. In addition to Earth, Mercury, Venus, Mars, the Moon, the asteroid Vesta, and Ganymede all appear to have undergone significant physical differentiation resulting in the formation of an iron core. Europa, Titan, and Ceres display some physical characteristics of physical differentiation, but the extent to which this process proceeded on these bodies remains largely unknown. Additional details of Earth's differentiation are given in the supplementary material.
3.4.3.2. Planetary magnetospheres and dynamos
There are currently eight known Solar System objects thought to have actively maintained magnetic fields: Mercury, Earth, Jupiter, Ganymede, Saturn, Uranus, Neptune, and the Sun. Magnetic fields can play an important protective role by deflecting incoming charged particles away from the surface, although deeper penetration is possible near magnetic poles and during solar outburst events. Planets without a magnetic field are directly exposed to incoming solar winds, and the likelihood of radiation reaching the surface increases as the atmospheric thickness decreases. Magnetospheres may help protect planetary atmospheres from erosion by the solar wind (see Chapter 3.4.1.5), and the reduction in incoming energetic charged particles is certainly helpful for the development of surface-dwelling biology. More details on the current theories of planetary dynamo formation are given in the supplementary material.
3.4.3.3. Plate tectonics
In the theory of plate tectonics, the uppermost portion of Earth is mechanically divided into the rigid lithosphere (the crust plus some brittle mantle) and the plastic asthenosphere (the uppermost plastic portion of the mantle). The lithosphere is further broken up into cohesive fragments called tectonic plates, and these ride over the fluidlike asthenosphere driven primarily by gravitational and convective forces. Plate tectonics describes the large-scale motion of the discrete plates which make up Earth's lithosphere and relates the internal dynamics of Earth to the motion and generation of the crust and its geologic, geomorphic, and compositional diversity. New crust is created at divergent boundaries such as mid-ocean ridges, derived from the upper mantle. Divergent boundaries occur where two plates move apart from each other, also known as rifting. Old crust is removed from the lithosphere via subduction into the mantle at convergent boundaries, where it is incorporated into the mantle. Plate boundaries represent zones of high thermal and energetic gradients and are therefore of particular interest to astrobiology (see Chapter 4.3.2, and Chapter 7.1.3 in the supplementary material for Chapter 7). Additional details on Earth's plate tectonics are given in the supplementary material.
Currently, plate tectonics have not been observed on any other object in the Solar System, although certain tectonic features found on Earth are also found elsewhere. The possible existence of some current or past form of plate tectonics on Mars is a subject of continuous debate, with some researchers suggesting that Valles Marineris may be a plate boundary (Yin, 2012). Additionally, there is evidence of divergent-style plate boundaries on some of the icy moons, particularly Europa, Ganymede, Enceladus, and Triton. For further reading, see Frisch et al. (2011) and references therein.
3.4.3.4. Volcanism
Volcanism is the process of erupting molten rock (magma) onto the surface of a planetary body. Much like plate boundaries, volcanic regions represent attractive targets for astrobiological exploration due to their high energy gradients and propensity to form hydrothermal systems. On Earth, volcanism is intimately linked to plate tectonics, with the majority of volcanic activity resulting from seafloor spreading (divergent boundaries) and arc volcanism (convergent boundaries). Magmas are typically formed through decompression melting of crustal or mantle material through tectonic uplift, and/or by the addition of volatile species such as water and CO2 during slab devolatilization at subduction zones (as discussed in Chapter 3.4.3.3). Additional details about volcanism on Earth are given in the supplementary material.
Despite the apparent absence of plate tectonics on other worlds, volcanism is abundant in the Solar System. Putative silicate volcanoes and volcanic fields have been noted on Mercury, Venus, the Moon, and Mars. The martian volcano Olympus Mons is the largest planetary mountain in the Solar System. Io, the innermost satellite of Jupiter, is the most volcanically active object in the Solar System. Spacecraft- and Earth-based observations have identified >150 active volcanoes on the ionian surface with hundreds more predicted to exist. Unlike the terrestrial planets, where most of the internal planetary heat comes from primordial heat and radioactive decay, Io's volcanism is driven by friction from tides (tidal heating) caused by its elliptical orbit in Jupiter's massive gravitational field. On colder, volatile-rich objects, analogous melts may form in so-called “cryovolcanic” systems. Water and nitrogen eruptions have only been detected on Enceladus and Triton, respectively, though putative cryovolcanoes and cryovolcanic features have been interpreted on Ceres, Europa, Enceladus, Titan, Miranda, Triton, Pluto, and Charon. For further reading on volcanism in the Solar System, see Sigurdsson (2015) and references therein.
3.4.3.5. Gravitational tides
Tides on planets and moons in the Solar System are predominantly caused by interactions with the Sun or by planet-satellite interactions within planetary systems. The most obvious and ubiquitously known form of tidal deformation is Earth's marine tides. Marine tides on Earth form due to gravitational interactions with the Sun and Moon. In addition to marine tides, atmospheric and solid tides also exist but are much less perceptible without instrumental aids.
On other worlds, tidal interactions create large stresses and supply substantial amounts of energy to their geosphere. This is particularly true of Jupiter's moons Europa and Io which are locked in a mean-motion resonance with each other and Ganymede. The resonance maintains the orbital eccentricities of these two moons, thus creating large tidal variations as they move toward and away from Jupiter. Io, the innermost satellite, is the most tidally disrupted body in the Solar System (Lainey and Tobie, 2005; Van Hoolst et al., 2020). In combination with nonsynchronous rotation and the existence of a subsurface ocean, tidal stresses likely contribute to the many tectonic landforms seen on the surfaces of Europa and Enceladus (Greenberg et al., 1998; Porco et al., 2006). Understanding the effects of brittle fracturing and strain in ice shells induced by tidal stresses is of astrobiological interest as these processes likely influence material transport through the ice shell as well as affect the morphology and oceanography at the potentially habitable ice-ocean interfaces.
3.4.3.6. Impact cratering
Impact cratering is the dominant geologic process that disrupts and modifies the surfaces of most solid planets and satellites in the Solar System, and likely the universe as a whole. The relative influence of impact cratering as a geologic process is reduced on worlds with active, or recently active, tectonic processes, or atmospheric and hydrospheric weathering and erosional processes. From an astrobiological perspective, after the initial impact, craters represent potential environments for endemic biology and prebiotic chemistry. Analysis of the impact cratering record on Earth, Mars, and Ceres suggests that hydrothermal activity is commonplace on planetary surfaces rich in H2O in the aftermath of an impact crater–generating event and can be long lived (up to millions of years; Osinski et al., 2013). Although impacts with protoplanetary-sized objects likely sterilize a planetary surface, smaller impact sites may produce conditions favorable to habitability (see Chapter 7.1.3.1 in the supplementary material for Chapter 7). Additional details on impact cratering are given in the supplementary material.
3.4.4. Planetary geophysics and habitability
As described further in Chapter 4 and Chapter 5, life is ultimately a planetary process defined by the energy fluxes and molecular components available. The exchange of energy and chemical species between the atmosphere, hydrosphere, and geosphere ultimately defines the paths that are available to drive molecular complexification and biologic diversification into the great variety found on Earth today. As described above, numerous interactions between the various constituent materials (rocks, water, carbon dioxide, etc.), the dynamics of the planetary geospheres (atmosphere, hydrosphere, and lithosphere), and the origins and evolution of life have combined to irreversibly change the surface of Earth. Understanding planetary geophysics and geochemistry is thus crucial to understanding the origins and evolution of life on Earth.
A wide diversity of geophysical processes are important to the evolution of Earth, and such processes must also play a major role in developing and maintaining planetary habitability and the evolution of life (see Chapter 7.1). Other bodies in the Solar System and in exoplanetary systems also contain atmospheres, hydrospheres, and geospheres; some of these share many characteristics with Earth, while some are vastly different. Depending on the size and location of a planetary body, a multitude of physical factors can vary widely, such as temperature, pressure, chemical composition, and geological activity. The combination of these factors results in a specific set of characteristics and environments that may culminate in a habitable body.
3.5. Conclusion
The combination of elements and circumstances from which a habitable world can develop reaches all the way back to the beginning of the universe. As described in Chapter 3.1.1, the various forms of nucleosynthesis—the Big Bang, stellar cores, supernovae, and other exotic events—have gradually increased the relative abundance of heavy elements critical for life (Chapter 2.2.2). The size, composition, and orbit of planetary bodies is also important to their habitability, and recent decades have shown a vast diversity of planetary systems that differ from our own. Studies of exoplanetary systems are a new frontier in astrobiology; Chapter 7.2.10 discusses the potential habitability of several known exoplanets, while Chapter 8.4 discusses searches for exoplanets and remote detection of biosignatures. It is important to keep in mind that, since Earth is our only current example of a planet with life, our perceptions of habitability could be skewed toward favoring Earth-like circumstances.
Abbreviations Used
- ALMA
- Atacama Large Millimeter Array
- AU
- astronomical units
- COMs
- complex organic molecules
- HRD
- Hertzsprung–Russell diagram
- ISM
- interstellar medium
- LHB
- Late Heavy Bombardment
Acknowledgments
The authors would like to thank two anonymous reviewers and Linda Billings for their helpful and constructive feedback that helped to improve this manuscript. We also acknowledge the authors of the Astrobiology Primer 2.0 for several of the structural elements of Chapter 3.1, as well as E.H. Mitchell, J. Huang, and J. Berger for feedback on earlier versions of this manuscript. Work by M.J.S. was supported by an appointment to the NASA Postdoctoral Program at Georgia Institute of Technology, administered by Universities Space Research Association through a contract with NASA, and from the Solar System Research Virtual Institute (SSERVI) under cooperative agreement NNA17BF68A. Work by Z.R.T was supported by the NASA Hubble Fellow Program (HST-HF2-51471). E.M.C. would like to thank her advisors for their support and acknowledges the NSF GRFP for funding (grant #DGE 1650115). Work by K.B.S was supported by the NSF Astronomy Program (2009365).
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Published online: 18 March 2024
Published in print: March 2024
Accepted: 14 November 2023
Received: 8 August 2021
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M.J.S. was the primary author of Chapter 3.1, Chapter 3.3.4–5, Chapter 3.4.4, and Chapter 3.5, and also led the editing process, organized the chapter for submission, and was the primary point of contact throughout the revision process.
Z.R.T. was the primary author of Chapter 3.2.1–3 and Chapter 3.3.2–3, contributed to the authorship of Chapter 3.4.2, Chapter 3.4.4, and Chapter 3.5, and also aided in editing and revising the chapter for submission.
E.M.C. was the primary author of Chapter 3.4.1.
C.E.H. was the primary author of Chapter 3.2.4 and contributed to the authorship of Chapter 3.1.2.
K.H.G.H. was the primary author of Chapter 3.4.3 and contributed to the authorship of Chapter 3.4.2.
K.B.S. was the primary author for Chapter 3.3.1 and contributed to Chapter 3.3.5, and also aided in editing and revising the chapter for submission.
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