Modeling the Emplacement of Amino Acids in Impact Melt on Titan

In 2019, NASA selected the Dragonfly mission to Titan to investigate its prebiotic chemistry and geology. The mission will send a quadcopter that will navigate Titan’s equatorial region, with the goal of reaching Selk crater. Laboratory experiments have shown that the organics on Titan will produce biomolecules when oxygenated in liquid water. Impact craters like Selk present a prime opportunity for this chemistry to occur in the liquid water melt that forms during impact. Recent work has used HCN as a proxy to constrain how these biomolecules will be trapped within the melt ponds as they freeze. The study affirmed previous expectations that a high concentration of impurities will become emplaced within the middle of the melt sheet, but a significant fraction (~70%) of the total HCN will be emplaced in the upper half of the melt sheet. In this work, we seek to determine the final location of the biomolecules that are formed when HCN reacts with liquid water. We use the ice model of Buffo et al. (2020) to study the entrapment of the amino acid glycine in the melt pond. This molecule is chosen because it has been shown to form in Titan simulated environments, and the necessary chemical and thermal properties for glycine in an aqueous solution are available. The ice model tracks organic impurities in a water mixture at a freeze front (i.e. a progressing region of freezing water). The ice model is used to track only one of the two freeze fronts in Titan’s impact crater melt ponds (i.e. the top or bottom of the melt pond). The freeze front that it tracks is dependent on the impurity (or impurities) within the water; the buoyancy of the impurity determines whether the water-organic mixture within the freezing lattice is driven out or held in place. The amino acid studied here is denser than water, like salt, which will lead to more of the amino acid being concentrated in the bottom half of the melt sheet. However, the density difference is small (16%) which suggests the frozen melt sheet will retain some amount of the amino acid in the upper half, similar to that observed with HCN in the lower half (on average, 2.5 parts per thousand.). Further work will consider more complicated mixtures, with experimental work to understand whether impurities will act independently of one another or be driven by the bulk chemical and thermal properties of the melt.