Decomposition of amino acids in water with application to in-situ measurements of Enceladus, Europa and other hydrothermally active icy ocean worlds

Abstract

To test the potential of using amino acid abundances as a biosignature at icy ocean worlds, we investigate whether primordial amino acids (accreted or formed by early aqueous processes) could persist until the present time. By examining the decomposition kinetics of amino acids in aqueous solution based on existing laboratory rate data, we find that all fourteen proteinogenic amino acids considered in this study decompose to a very large extent (>99.9%) over relatively short lengths of time in hydrothermally active oceans. Therefore, as a rule of thumb, we suggest that if amino acids are detected at Enceladus, Europa, or other hydrothermally active ocean worlds above a concentration of 1 nM, they should have been formed recently and not be relicts of early processes. In particular, the detection of aspartic acid (Asp) and threonine (Thr) would strongly suggest active production within the ocean, as these amino acids cannot persist beyond 1 billion years even at the freezing point temperature of 273K. Identifying amino acids from the oceans of icy worlds can provide key insight into their history of organic chemistry.

Hydrothermal Experiments with Protonated Benzylamines Provide Predictions of Temperature-Dependent Deamination Rates for Geochemical Modeling

“…Examples of aqueous reactions for which rates have been extrapolated from higher-temperature experiments to model lower-temperature environments include peptide bond hydrolysis at experimental temperatures of 120−200 °C extrapolated to mesophilic prokaryote and eukaryote growth temperatures of 25−37 °C, 26 carbon monoxide oxidation to carbon dioxide via the water−gas shift reaction at experimental temperatures of 150− 300 °C extrapolated to submarine hydrothermal vents and seafloor temperatures as low as 2 °C, 16 and the decomposition of several biological amino acids at experimental temperatures ranging from 101−340 °C extrapolated to 0 °C to represent the icy ocean of Enceladus. 27 The largest extrapolations for amino acid decomposition rates span from experimental temperatures of 230 °C to natural system temperatures of 0 °C. Such extrapolations represent essential progress toward a more complete understanding of aqueous organic chemistry in natural systems but are not without limitation.…”

Returning Samples From Enceladus for Life Detection

“…It is unclear whether this points to a geologically young ocean or instead to incomplete chemical homogenization of the core due to limited pervasiveness of fluid circulation (Macdonald and Fyfe, 1985;Cable et al, 2020). A better chronometer could be provided by measuring abundances of organic compound classes that decompose in water on a variety of timescales (Truong et al, 2019). Mineral or organic chemical tracers of the ocean age can be tracked with measurements synergistic with life detection (section "Measurement Strategy and Sample Needed").…”

The subsurface habitability of small, icy exomoons

“…More generally, f m,NH 3 goes up as temperature (and thus, illumination) goes down. Suggestions of Enceladus' recent in situ formation as opposed to from Saturn's circumplanetary disk 4.5 Gyr ago (Truong et al 2019;Glein et al 2018) however makes it unlikely to possess extensive NH 3 deposits. Therefore, we adopt a no NH 3 model for Enceladus, 10% for Rhea because it is farther from Saturn and, possibly, indigenous to the Saturnian system, and 15% for Titania and Oberon since they are farther from the Sun.…”