Abstract
Titan has dense atmosphere that mainly consists of nitrogen and methane. Observation of Titan by Huygens prove in 2005 implied that there were water ice and liquid methane on Titan surface. Among various kinds of energy sources in Titan atmosphere, cosmic rays can penetrate deeply down to the surface, which could be most effective to produce organic compounds near the surface of Titan. We examined possible formation of bioorganic compounds in lower Titan atmosphere. A mixture of methane (1-10%) and nitrogen (balance) was irradiated with 3 MeV-protons. Solid product (tholin) was acid-hydrolyzed, and amino acids in the hydrolysate was quantified by HPLC and/or by GC/MS with a chiral column after derivatization by using chroloformate. Molecular weight of the product before hydrolysis was estimated by gel permeation chromatography (GPC) and MALDI/TOF-MS. The GPC results showed that the tholin had molecular weight of ca. 1700 and 900. Various amino acids were detected in tholin after acid hydrolysis: Indigenousness of amino acids was checked with their D/L ratio and/or by using 13CH4 as a carbon. It was suggested that cosmic rays-induced tholin could give amino acids after interaction with surface water ice and/or cometary water ice during impacts.
