Modeling the Distribution of Organic Carbon and Nitrogen in Impact Crater Melt on Titan

Abstract

Titan is a chemically rich world that provides a natural laboratory for the study of the origin of life. Titan’s atmospherically derived CxHyNz molecules have been shown to form amino acids when mixed with liquid water, but the transition from prebiotic chemistry to the origin of life is not well understood. Investigating this prebiotic environment on Titan is one of the primary motivations behind NASA’s Dragonfly mission. One of its objectives is to visit the 80 km diameter Selk crater, where a melt sheet of liquid water would have formed during the impact cratering process. Organic molecules on Titan’s surface could have mixed with this water, forming molecules of prebiotic interest. Constraining how this material becomes trapped in the refreezing ice is necessary for Dragonfly to effectively target and interpret the samples it aims to acquire. In this work, we adapt the planetary ice model of Buffo et al. to Titan conditions to track how organic molecules will become trapped within the ice of the freezing melt sheet. We use HCN as a model impurity because of its abundance on Titan and its propensity to form amino acids in aqueous solutions. We show that without hydrolysis, HCN will be concentrated in the upper and middle portions of the resolidified melt sheet. In a closed system like Selk crater, the highest concentration of HCN appears 75% of the way into the frozen melt pond (relative to the surface), but HCN should be accessible at high concentrations nearer the surface as well.