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The Evolution of Prebiotic Chemical Complexity and the Organic Inventory of Protoplanetary Disks and Primordial Planets

Life on Earth is tied to the abiotic formation and chemical history of compounds based mainly on the elements C, H, N, O, S, and P. Apart from H, the evolution of these biogenic elements starts with their nucleosynthesis in stars and ejection into the interstellar medium (ISM). Once in the ISM, the ejecta are modified by physical and chemical processes such as UV and cosmic ray irradiation, gas-phase reactions, accretion and reaction on grain surfaces, and shock waves. Within molecular clouds, the birthplace of stars and planets, most of these elements are frozen onto small, cold dust grains where they are processed into more complex molecules by UV photons, cosmic rays, and catalytic surface reactions. For the past thirty years, laboratories around the world have simulated UV photochemistry and cosmic-ray bombardment processes on interstellar and Solar System ice analogs that reveal the central role these ices play in the astrobiology and astrochemistry in star and planet-forming regions. From this work it is clear that ices play important roles not only as repositories of reactants, but also as structures that mediate the reactions that occur on and in them. Studies of the role of surface chemistry on astrophysically relevant solids have just begun, with laboratory work investigating surface reactions of atoms and simple molecules. However, processes involving a wider range of astrophysically-relevant molecules on relevant minerals and ices have been largely unexplored.

The NAI Ames team seeks a greater understanding of chemical processes at every stage in the evolution of organic chemical complexity, from quiescent regions of dense molecular clouds, through all stages of cloud collapse, protostellar disk, and planet formation, and ultimately to the materials that rain down on planets, and understanding how these depend on environmental parameters like the ambient radiation field and the abundance of H₂O. This team is structured as an integrated, coherent program of astrochemical experiments, quantum chemical computations, disk modeling, and observations of astronomical sources. The ultimate goal is to move beyond identifying particular molecules, ions, or radicals present in astronomical objects to understanding their place in the dynamics of the chemical evolution of different astrophysical environments, with particular attention to the formation sites of new planetary systems and our own Solar System.

The research of the NASA Ames team consists of three Modules, each of which has well defined tasks but that integrates into the other two. The Modeling and Observations of Disks and Exoplanets Module leverages the abundance of available observational data, including line and continuum spectra from protoplanetary disks and data from exoplanet surveys, to understand the formation and evolution of organics during planet formation. Members of their team are conducting a thorough investigation of the evolution of organic molecules using theoretical disk models that include a new, specific, thermo-chemical network constructed with inputs from laboratory experiments and quantum chemical theory, and chemo-dynamical models that follow the transport and irradiation of these species as disks evolve, form planets, and eventually disperse.

Exoplanet surveys suggest that planet formation may occur in situ and thus may not depend on large-scale Type I migration. The mode of planet formation has strong consequences for the compositional mixes and catalytic processes that are most astrobiologically relevant. The Ames team, therefore, combines statistical modeling of the observed planetary census with the results of structural models to determine the distribution of planetary core-envelope fractions, which encode key information regarding the disk environment in which the planets formed. They also carry out dynamical simulations that test migratory and in situ planet formation theories, and determine disk parameters that are consistent with exoplanetary architectures. Chemical evolution of these prototypical extrasolar nebulae are followed to track the incorporation of materials from different parts of the full condensation sequence, and radiative transfer modeling are conducted to predict line emission signatures that stem from molecular processes in planet-forming disks.

The Laboratory Module utilizes conditions gleaned from the literature and obtained from the theoretical modeling described above, to conduct laboratory studies with mixtures of relevant minerals, organic molecules, and ices to study radiation- and surface-mediated (i.e., catalysis) chemistry. Experimental series are designed to investigate specific regions of protoplanetary disks. The analyses used to study the organic products are carried out with several goals in mind: (i) characterize the products and compare them to organics found in relevant regions of protoplanetary disks, exoplanets, as well as meteorites and samples from sample return missions; (ii) search for products of specific astrobiological interest, including amino acids, amphiphiles, quinones, sugar-like compounds, and nucleobases; (iii) search for specific compounds that are under consideration as potential biomarkers to test if they are robust bio-indicators or can be formed abiotically; and (iv) determine whether the organic residues produced have interesting astrobiological behaviors (for example, determine whether they form membranes).

The third Module of the Ames team makes use of computational quantum chemistry techniques, which have become vital tools to elucidate the detailed mechanisms of chemical reactions both in the gas and condensed phases, as well as diverse catalytic processes. Using ab initio and density functional theory approaches, including quantum mechanical/molecular mechanics methods, it is possible to determine which reaction products are more feasible or even possible under various physical conditions.

The Ames team also investigates the reactions of organic molecules that are present in the different environments of protoplanetary disks, including gas-phase, gas surface and condensed-phase chemical reactions both in H₂O-rich (‘hydrous’) and H₂O-free (‘anhydrous’) environments. The formation mechanisms of small biogenic molecules, such as amino acids and nucleic acids, are being examined first, starting from simple C,N,O-bearing precursors. Mechanisms that lead to the formation of these molecules reveal key information about the environments in which these reactions can or cannot occur. Similar to their prior studies, the catalytic role played by ices (mainly H₂O) and by anhydrous grains is also being investigated.

The three modules of the Ames team are not carried out in isolation, but are part of an interacting whole. The results of disk and exoplanet modeling are used to establish the environmental conditions (temperature, composition, radiation environment, etc.) that are examined in the laboratory studies and quantum computational chemistry modules. Results from the laboratory studies (reaction rates, chemical products, etc.) are fed back into the disk models to improve their prediction of organic synthesis and are used to test the results of the quantum chemistry approach. Quantum chemistry results (reaction mechanisms, reaction rates, product ratios, etc.) are used to help interpret the laboratory results, and feed into improved parameters for the disk modeling studies. In addition, validation of the quantum chemistry results will allow the team to address environments and conditions that may be predicted by disk models, but that are outside the range of experimental conditions that the laboratory effort can address.

Figure 1. Overview for the interactions among the three modules. Module1-Modeling & Observational Studies, Module 2-Laboratory Studies, and Module 3-Quantum Chemistry Studies.