Laboratory simulations of organic synthesis in Enceladus: Implications for the origin of organic matter in the plume

Abstract

In-situ observations of Enceladus' plume and Saturn's E ring by the Cassini spacecraft have revealed that some ice particles erupted from Enceladus contain a large inventory of organic materials. These include both high- and low-molecular-weight hydrocarbon chains, aromatic-, nitrogen-, and oxygen-bearing compounds. Here we report experimental results on organic synthesis through hydrothermal (up to 150 °C) and freezing (down to –40 °C) processes using starting solutions simulating Enceladus' ocean. We find that, owing to HCN and NH3 in the starting solutions, amino acids, together with aldehydes, carboxylic acids, amines, and nitriles, are the primary products of hydrothermal synthesis. Freezing of the starting solutions can also form simple amino acids, such as glycine. Comparing with Cassini's observations, most of our hydrothermal products are in good agreement with observations arguing for a deep plume source, but amino acid-relevant molecular signals in the experiments appear to be absent in Enceladus' organic-rich particles. One possibility for this discrepancy is that partitioning of amino acids into salt-rich plume particles may obscure detection. Macromolecules with aromatic constituents and long hydrocarbon chains in Enceladus cannot be replicated in our experiments. Primordial organic matter or catalytic reactions at elevated temperatures (>150 °C) might contribute to the formation of macromolecules in Enceladus.