Decomposition of Amino Acids in Water with Application to Enceladus and other Ocean Worlds.

Amino acids are often considered to be one of the most important biosignatures in searching for life on other worlds. However, before a biological origin can be deduced for the future potential finding of amino acids in Enceladus, other non-biological sources must be considered, including 1) Accretion of primordial materials or 2) Geochemical synthesis in the ocean. In the first scenario, Enceladus and other ocean worlds that would be accreted from icy planetesimals might have retained an abundance of primordial amino acids.

Therefore, the first step in using amino acids as biosignatures is to determine if amino acids were to be detected at Enceladus, could those species have been maintained from primordial synthesis process? Amino acids which are sensitive to destruction, unless protected or recently emplaced by another process, must have been recently formed and can be identified based on their relative stabilities. Following the previous work from Monroe et al. 2017, we use chemical kinetics data to estimate rates and destruction timescales of amino acids in scenarios applicable to Enceladus and hydrothermally active ocean worlds.

Amino acids decomposition timescales are calculated by solving analytically kinetics equations with the corresponding kinetic rate constant. In order to predict rate constants at environmentally relevant temperatures, data are extrapolated using an Arrhenius relation between ln[k] vs. 1/T. This approach also was used to estimate racemization timescales for meteoritic amino acids based on asteroid parent body temperature. As a large range extrapolation may involve substantial error due to changes in reaction mechanisms, we employed a student-t distribution approach to calculating the confidence interval of the simulated rate constants.

The destruction timescale of amino acids depends strongly on the environment's temperature and the residence timescale of amino acids in elevated hydrothermal temperature. On the basis of existing laboratory data, among 16 amino acids considered in this study, aspartic acid, threonine, isoleucine, serine, arginine are relatively sensitive to decomposition. Any of those species detected at Enceladus cannot be primordial, such that in situ detection would indicate recent (< 1 Myr) production of the amino acid.