Spectroscopic Study of Ice Analogs of Trans-Neptunian Objects Exposed to Ionizing Radiation. II. Infrared Spectra and Functional Groups

Abstract The surfaces of airless trans-Neptunian objects (TNOs) are subject to continuous ionizing radiation from solar winds and Galactic cosmic rays accompanied by alteration of surface compositions. The crust resulting from chemical reactions and products at various radiation levels can influence their spectral gradients and surface colors, which are essentially controlled by the chromophores of complex organic molecules. This study presents comprehensive infrared spectra of TNO-analog ices processed by ionizing radiation, which include water, methane, ammonia, carbon monoxide, carbon dioxide, and methanol along with their binary mixtures. The spectral data suggest that complex organic and inorganic products are synthesized when carbon-bearing ice mixtures are exposed to proxies of Galactic cosmic-ray and solar wind irradiation. A series of functional groups of hydrocarbons, hydroxyl, amine, carbonyl, imine, cyanate, and nitrile are identified, which can be incorporated in complex compounds including alcohols, aldehydes, ketones, carboxylic acids, esters, amines, amides, cyanates, and nitriles along with critical prebiotic molecules such as sugar and amino acids. By combining them with corresponding visible reflectance spectra, these species play a crucial role in revealing the color diversity of TNOs and are also highly relevant to the emergence of life in our solar system. The findings in this study serve as an important starting point to fully unravel the chemical complexity, compositions, and evolutionary processes of objects in the outer solar system and will also provide fundamental support for the interpretation of astronomical observations such as those from the James Webb Space Telescope.