Organic Aerosols in the Early Earth Atmosphere: Reaction Network an Its Isotope Signature
Experimental simulations of Titan's atmospheric and surface chemistries in order to understand the formation of complex organic molecules as well as the abiotic formation of prebiotic molecules. The thick 1.5 bar atmosphere of Titan, mostly composed of nitrogen and some methane, leads to the formation of various hydrocarbons and nitriles via organic chemistry, which also generates a complex organic haze material called tholin. However, the formation mechanism of tholin and its chemical structures are still not understood well. Over geologic time, both tholin and condensed ice mixtures of organic gases accumulate on the surface of Titan. There, further organic chemical reactions might occur in transient liquid H2O with NH3, which might be induced episodically by geologic processes, such as meteoritic impacts and cryovolcanic activity. Those interactions of organic materials with liquid H2O-NH3 may produce a wide variety of prebiotic molecules, such as amino acids and nucleic acid bases. Understanding of such abiotic formation of prebiotic molecules in an aqueous environment may provide us with important clues to the origin of life on the early Earth.
We are investigating the formation of tholin by vacuum UV light irradiation of a CH4/N2 gas mixture. In order to elucidate the dominant reaction pathways leading to tholin formation, the gaseous products are investigated quantitatively by mass spectrometry. Dependence of gaseous products on the irradiation wavelength, and the inclusion of nitrogen atoms into the photochemistry is one of the areas of interest. The chemical and optical properties of UV tholin is compared to the tholin formed by plasma irradiations. Comparing the gaseous products and tholin properties with Titan’s observations, we constrain the formation of haze particles in Titan’s atmosphere and their chemical properties. Then, we investigate the following interactions of tholin with liquid H2O-NH3. We determine the formation rates of prebiotic molecules such as amino acids, nucleic acid bases, and amphiphilic membrane vesicles. A series of measurements are conducted as a function of pH and temperature close to the conditions at the surface of Titan. With these experimental simulations, the distribution of prebiotic molecules and their stability may be estimated. These results could contribute to a search for extraterrestrial organisms on a future Titan mission.