Stability of organic species in ocean world interiors with geochemical models

The icy moons in the Solar System, such as Europa, Enceladus and Titan, are  of great interest to the astrobiological and biogeochemical communities, owing to their subsurface liquid oceans. These oceans are potential targets for deep explorations of biochemical processes in our cosmic neighbourhood.

In our study, we use the Deep Earth Water (DEW) model to calculate the thermodynamic properties of water and aqueous compounds. We employ DEWPython, a modular software interface, enabling access to both DEW and SUPCRT models, with the purpose of investigating the stability of organic molecules in ocean worlds. In particular, we model the equilibrium constant and Gibbs free energy (GFE) as a function of pressure and temperature for reactions involving metabolically essential amino acids, basic molecules possibly involved in the emergence of life (H2, NH3, CH4, etc.), as well as products of water-rock interaction such as serpentinization and abiotic methanogenesis. Considering the results from space missions such as Europa Clipper and Dragonfly, such molecular stability analyses can contribute to the search for conditions ideal for habitability and extraterrestrial life.

We map out reaction networks, by first calculating the individual equilibrium constants and GFE changes of the hydrothermal reactions that (i) form a given organic species from inorganics, and then (ii) decompose into other (in)organic molecules. In the final step, we obtain the net equilibrium constant as a measure of the stability of the species, by proportioning the total equilibrium constant of the reactions responsible for the formation of a particular species to those responsible for its destruction. At a given pressure and temperature, a value greater or less than unity indicates that the formation of a species is favourable or unfavourable, respectively.

Our current results suggest that alanine and glycine, two common and simple amino acids, can remain stable throughout the ocean columns, and just beneath the seafloors, of Titan and Europa, and thus could potentially be sampled by plumes or cryovolcanism.

In the next development phase of our model, we will further constrain the depths at which the species of interest can be found, by identifying and ruling out regions of high pressure ice stability, using the SeaFreeze model.

The research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). © 2022. Jet Propulsion Laboratory, California Institute of Technology. Government sponsorship acknowledged.

Fig 1 : Alanine stability (contours) through Europa’s ocean and ocean floor (colored lines). Positive (negative) regions indicate stable (unstable) conditions.