6.2 Astrochemistry of comets and asteroids

Comets and asteroids are cosmic time capsules, preserving the chemical makeup of our early solar system. They're like nature's delivery trucks, potentially bringing water and organic compounds to planets. This topic dives into their composition and the chemical processes that shape them.

Understanding comets and asteroids is crucial for unraveling the origins of life on Earth. We'll explore how these celestial bodies might have seeded our planet with the building blocks of life, and what that means for the possibility of life elsewhere in the universe.

Chemical Composition and Structure of Comets and Asteroids

Composition of Comets

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• Comets are composed of a mixture of volatile ices (water, carbon dioxide, methane, and ammonia), dust particles, and organic compounds
• The nucleus of a comet is a solid, irregularly shaped object typically a few kilometers in diameter, consisting of a mixture of ice and dust
• As a comet approaches the Sun, solar radiation causes the volatile ices to sublimate, creating a coma (an extended atmosphere) and tail(s) of gas and dust (Halley's Comet, Comet Hale-Bopp)

Composition of Asteroids

• Asteroids are primarily composed of rock and metals, with a smaller proportion of volatile materials compared to comets
• The three main types of asteroids are C-type (carbonaceous), S-type (silicaceous), and M-type (metallic), each with distinct chemical compositions
• C-type asteroids are the most common and contain a significant amount of organic compounds and water-bearing minerals (Ceres, Bennu)

Organic Molecules in Comets and Asteroids

• Both comets and asteroids can contain complex organic molecules, such as amino acids, nucleobases, and polycyclic aromatic hydrocarbons (PAHs)
• The chemical composition of comets and asteroids provides insight into the early solar system's composition and the materials available for planet formation
• Organic molecules found in comets and asteroids contribute to the inventory of prebiotic compounds that could have been delivered to early Earth (Murchison meteorite, Comet Wild 2 samples)

Chemical Processes in Comets vs Asteroids

Dynamic Chemical Processes in Comets

• Comets undergo more dynamic chemical processes due to their higher content of volatile ices compared to asteroids
• Sublimation of volatile ices in comets leads to the formation of a coma and tail(s) as they approach the Sun, exposing the interior to space and facilitating chemical reactions
• Photodissociation and photoionization of molecules in the coma can lead to the formation of reactive radicals and ions (CN-, C2, C3, OH)

Chemical Processes in Asteroids

• Asteroids experience less intense chemical processing due to their lower content of volatile materials and more stable environment
• Aqueous alteration is a key chemical process in some asteroids, particularly in C-type asteroids, where water-rock interactions can lead to the formation of hydrated minerals and organic compounds (phyllosilicates, carbonates)
• Thermal metamorphism can occur in asteroids, causing chemical and mineralogical changes due to heat from radioactive decay or solar radiation (ordinary chondrites, enstatite chondrites)

Space Weathering and Impact Events

• Both comets and asteroids can experience space weathering, which involves the alteration of surface materials due to exposure to solar radiation, cosmic rays, and micrometeorite impacts
• Impacts between comets, asteroids, and other bodies can result in the mixing and redistribution of materials, as well as the creation of new surfaces exposed to space (Shoemaker-Levy 9 comet impact on Jupiter, Chicxulub impact event)

Role of Comets and Asteroids in Planetary Delivery

Delivery of Water to Planets

• Comets and asteroids are thought to have played a significant role in the delivery of water and organic compounds to the early Earth and other terrestrial planets
• The high water content of comets and some asteroids (e.g., C-type) makes them potential sources for the Earth's oceans and other planetary water reservoirs (Comet 67P/Churyumov-Gerasimenko, Ceres)
• The deuterium-to-hydrogen (D/H) ratio in Earth's oceans is similar to that found in some comets, supporting the idea of cometary water delivery

Delivery of Organic Molecules

• Organic molecules found in comets and asteroids, such as amino acids and nucleobases, could have contributed to the inventory of prebiotic compounds on early Earth
• The delivery of these molecules by comets and asteroids may have been crucial for the emergence of life on Earth and potentially other planets
• The Murchison meteorite, a carbonaceous chondrite, contains a wide range of organic compounds, demonstrating the potential of asteroids as sources of prebiotic molecules (amino acids, nucleobases, carboxylic acids)

Timing and Extent of Delivery

• The timing and extent of cometary and asteroidal delivery of water and organics to planets are still debated, with evidence suggesting a more significant role in the early stages of planetary formation
• The Late Heavy Bombardment, a period of increased impact events approximately 4.1 to 3.8 billion years ago, may have been a key time for the delivery of water and organic compounds to the inner solar system
• The ongoing study of comets and asteroids helps constrain the timing and extent of their contributions to the chemical inventory of planets

Comets and Asteroids as Sources of Prebiotic Molecules

Diverse Range of Organic Compounds

• Comets and asteroids contain a diverse range of organic compounds that are relevant to prebiotic chemistry and the origin of life
• These include amino acids, carboxylic acids, alcohols, aldehydes, ketones, sugars, and nucleobases, among others
• The presence of these molecules suggests that comets and asteroids could have been important sources of the building blocks of life on Earth and potentially other planets (glycine in Comet 67P/Churyumov-Gerasimenko, uracil in Murchison meteorite)

Variability in Organic Content

• The organic content of comets and asteroids varies depending on their formation conditions and subsequent processing
• Some comets, such as 67P/Churyumov-Gerasimenko, have been found to contain a wide range of organic molecules, including glycine, the simplest amino acid
• Carbonaceous chondrites, a type of primitive meteorite originating from C-type asteroids, are particularly rich in organic compounds and are often studied as analogs for the prebiotic chemistry of the early Earth (Murchison meteorite, Tagish Lake meteorite)

Experimental Studies on Prebiotic Chemistry

• Experimental studies simulating the conditions on comets and asteroids have demonstrated the formation of complex organic molecules from simpler precursors
• These experiments suggest that the chemical conditions on comets and asteroids are conducive to the synthesis of prebiotic molecules
• For example, the formation of amino acids from simple precursors has been observed in experiments simulating the conditions in cometary ices (Miller-Urey experiment, Greenberg's interstellar ice analog experiments)

Implications for the Origin of Life

• The delivery of prebiotic molecules by comets and asteroids to planetary surfaces could have provided the necessary ingredients for the emergence of life, although the specific mechanisms and pathways for prebiotic chemistry and the origin of life remain an active area of research
• The study of comets and asteroids as sources of prebiotic molecules helps constrain the potential pathways for the emergence of life on Earth and guides the search for habitable environments and signs of life beyond Earth
• The ongoing exploration of comets and asteroids, such as the OSIRIS-REx mission to Bennu and the Rosetta mission to 67P/Churyumov-Gerasimenko, provides new insights into the role of these bodies in the delivery of prebiotic molecules and the origin of life