10.3 Astrochemical constraints on the emergence of life

Astrochemistry plays a crucial role in understanding life's origins. By examining the cosmic abundance of key elements and the formation of complex organic molecules in space, we gain insights into the building blocks of life.

The RNA world hypothesis suggests that self-replicating RNA molecules were the first genetic material. Astrochemical processes may have contributed to forming RNA precursors, potentially kickstarting life on early Earth through cosmic delivery.

Elements and Molecules for Life

Cosmic Abundance of CHNOPS

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• The cosmic abundance of elements, particularly CHNOPS (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur), plays a crucial role in the potential for life's emergence
• These elements are the building blocks of organic molecules essential for life as we know it (amino acids, nucleic acids, lipids, carbohydrates)
• The relative abundances of these elements in the universe influence the likelihood of complex organic molecules forming and the potential for life to emerge in various environments

Formation and Distribution of Complex Organic Molecules

• The formation and distribution of complex organic molecules, such as amino acids, nucleobases, and sugars, in interstellar space and on celestial bodies can provide the necessary ingredients for the emergence of life
• Organic molecules have been detected in interstellar clouds, comets (Hale-Bopp), and meteorites (Murchison meteorite), indicating their widespread presence in the universe
• The delivery of these organic molecules to planetary surfaces through cometary impacts, asteroid collisions, and interplanetary dust particles may have contributed to the prebiotic inventory necessary for life's origin

Energy Sources for Astrochemical Reactions

• The availability of energy sources, such as ultraviolet radiation and cosmic rays, can drive astrochemical reactions and promote the synthesis of complex organic compounds in space
• Ultraviolet photons from stars can induce photochemical reactions in interstellar clouds, leading to the formation of complex organic molecules (polycyclic aromatic hydrocarbons)
• Cosmic rays, high-energy particles originating from supernovae and other energetic events, can ionize and dissociate molecules, facilitating chemical reactions and the synthesis of organic compounds in space

Liquid Water and Habitability

• The existence of liquid water, a key solvent for biochemical reactions, on a celestial body is considered a critical factor in determining its potential habitability and the likelihood of life's emergence
• Liquid water provides a medium for the dissolution, transport, and interaction of chemical compounds, enabling the formation of complex molecular structures and the occurrence of metabolic processes
• The presence of liquid water is used as a primary criterion in the search for potentially habitable environments beyond Earth (Mars, Europa, Enceladus)

Water's Role in Life's Origin

Water as a Solvent and Medium

• Water is essential for life as we know it, serving as a solvent for biochemical reactions, a medium for nutrient transport, and a regulator of temperature
• The unique properties of water, such as its high heat capacity, cohesive and adhesive properties, and ability to form hydrogen bonds, make it an ideal solvent for life's processes
• Water facilitates the dissolution and interaction of chemical compounds, enabling the formation of complex molecular structures and the occurrence of metabolic reactions

Liquid Water and Habitability

• The presence of liquid water is considered a key requirement for habitability and is used as a primary criterion in the search for potentially habitable environments beyond Earth
• Liquid water provides a stable environment for the assembly and function of biomolecules, as well as the maintenance of cellular structures and processes
• The discovery of subsurface oceans on icy moons like Europa and Enceladus suggests the possibility of habitable environments beyond Earth where life could potentially emerge

Water Abundance in the Universe

• Water is abundant in the universe, found in various forms such as gas, ice, and liquid on planets, moons, comets, and asteroids
• Water vapor has been detected in the atmospheres of exoplanets (HD 189733b) and in interstellar clouds, indicating its widespread presence in the universe
• The presence of water ice on comets and the delivery of water to Earth through cometary impacts may have contributed to the origin of Earth's oceans and the emergence of life

Water Delivery to Earth

• The delivery of water to Earth through cometary impacts and asteroid collisions during the Late Heavy Bombardment may have been crucial for the origin and sustenance of life
• Comets, often described as "dirty snowballs," contain a significant amount of water ice and could have delivered a substantial portion of Earth's water inventory
• The isotopic composition of water in some comets (Hartley 2) closely matches that of Earth's oceans, supporting the idea of cometary water delivery

Chirality in Biological Systems

Chirality and Enantiomers

• Chirality refers to the property of molecules that exist in two non-superimposable mirror-image forms, called enantiomers, which are designated as left-handed (L) or right-handed (D)
• Chiral molecules have the same chemical composition but differ in their spatial arrangement, leading to distinct properties and interactions
• Examples of chiral molecules include amino acids, sugars, and some organic compounds (bromochlorofluoromethane)

Homochirality in Life

• Biological systems exhibit homochirality, meaning that life on Earth almost exclusively uses L-amino acids and D-sugars, a preference that is critical for the proper functioning of enzymes, DNA, and other biomolecules
• Homochirality ensures the correct folding and function of proteins, the formation of stable double-helical DNA, and the efficient recognition and binding of molecules in biochemical processes
• The origin of homochirality in life is a major question in the study of life's emergence, and astrochemical processes have been proposed as potential sources of enantiomeric excess

Astrochemical Origins of Chirality

• Observations of circularly polarized light in star-forming regions suggest that asymmetric photochemical reactions induced by this light could lead to enantiomeric excesses in organic molecules formed in space
• Circularly polarized light can preferentially destroy one enantiomer over the other, resulting in an enantiomeric excess that could be preserved in organic molecules delivered to planetary surfaces
• The discovery of enantiomeric excesses in amino acids found in meteorites, such as the Murchison meteorite, supports the idea that astrochemical processes could have contributed to the origin of homochirality on Earth

Implications for the Origin of Life

• The emergence of homochirality is considered a key step in the origin of life, as it enables the formation of functional biomolecules and the development of complex biological systems
• Astrochemical processes that produce enantiomeric excesses in organic molecules could have provided a prebiotic source of homochirality, which was then amplified and maintained by biological systems
• Investigating the astrochemical origins of chirality can provide insights into the conditions and processes that led to the emergence of life on Earth and the potential for life's emergence elsewhere in the universe

The RNA World Hypothesis

RNA as an Early Genetic Material

• The RNA world hypothesis proposes that self-replicating RNA molecules served as the earliest genetic material and catalysts before the evolution of DNA and proteins, playing a central role in the origin of life
• RNA can store genetic information, catalyze chemical reactions (ribozymes), and replicate itself, making it a plausible candidate for the first biopolymer in the early stages of life's emergence
• The ability of RNA to perform both genetic and catalytic functions suggests that it could have served as a precursor to the more stable and specialized DNA and protein-based systems in modern life

Astrochemical Contributions to RNA Building Blocks

• Astrochemical processes could have contributed to the formation of RNA building blocks, such as nucleobases and sugars, in space or on early Earth
• Nucleobases, such as adenine and guanine, have been detected in meteorites (Murchison, Lonewolf Nunataks 94102) and simulated interstellar ice experiments, indicating their potential formation in space
• The delivery of these RNA precursors to Earth via comets, asteroids, or interplanetary dust particles could have provided the necessary ingredients for the emergence of an RNA world

Experimental Support for the RNA World

• Experimental studies have demonstrated the possible synthesis of RNA nucleotides and the self-assembly of RNA-like polymers under conditions that mimic those of the early Earth or in interstellar ice analogs, supporting the plausibility of the RNA world hypothesis in an astrochemical context
• The synthesis of ribose, a key component of RNA, has been achieved under prebiotic conditions using formaldehyde and glycolaldehyde, which are known to be present in interstellar space
• The self-assembly of RNA-like polymers from nucleotides has been demonstrated in laboratory experiments, suggesting that the spontaneous formation of RNA molecules could have occurred in the prebiotic environment

Implications for the Origin of Life

• The RNA world hypothesis provides a plausible scenario for the origin of life, bridging the gap between the abiotic synthesis of organic molecules and the emergence of more complex biological systems
• The astrochemical formation and delivery of RNA building blocks could have kickstarted the RNA world on early Earth, leading to the development of self-replicating and catalytic RNA molecules
• The transition from an RNA world to the current DNA-RNA-protein system could have occurred through the gradual evolution of more stable and efficient genetic and catalytic molecules, ultimately giving rise to the diversity of life we observe today