Water ice sublimation and O-H isotopic fractionation in terrestrial and extraterrestrial environments: new insights gained from numerical modelling and laboratory experiments

Stable isotopes of oxygen and hydrogen are a powerful multipurpose tool widely used across multiple disciplines in Earth and Planetary sciences. In hydrology, δ¹⁸O and δ²H in the water molecule are commonly used in stream water source apportionment and transit time analyses. In paleoclimate research, ice core water isotope records are used as a temperature proxy, documenting past climate variability over hundreds of thousands of years. Oxygen and hydrogen isotopes are also versatile fingerprints for retracing the formation of planets and other celestial bodies.

These examples should not obscure the fact that many unknowns and uncertainties remain inherent to the use of stable isotopes of O and H as tracers and fingerprints of processes in terrestrial and extra-terrestrial environments. To this date, only a few experimental studies have investigated water ice sublimation rates and the effect of isotopic fractionation processes – notably on water ice under lunar environmental conditions.

Here we present results from a combined experimental and modelling approach. With an instrumental set-up developed at LIST, we simulate the sublimation of water ice under extreme environmental conditions (very high vacuum and/or very low temperatures) with the goal of exploring O-H isotopic fractionation processes in both (extreme) terrestrial and extraterrestrial environments. An understanding of these processes is necessary for interpreting the isotope signatures of water in planetary exploration missions, such as ESA’s PROSPECT project for lunar exploration, and in terrestrial hydrology of cold regions.

The current experimental setup consists of a sublimation chamber capable of operating at pressures down to 10⁻⁶ Pa and temperatures as low as 110 K, with high stability and control over sublimation conditions. The system can simulate controlled environments for the phase transition of water (ice-vapor), isotopic fractionation, and the movement of water vapor across different phases of the experimental run. This includes transferring gas to a series of parallel cold traps, analyzing isotopic content using laser spectroscopy.

We have developed a stochastic lagrangian numerical model to verify the existing theories of phase transition, diffusion, and O-H isotopic fractionation based on the Langevin equation. The model allows for sublimation, diffusive transport, and condensation of water and its isotopes through an isothermal domain representing the volume of the experimental prototype. Lagrangian models are highly adaptive for handling complex boundary conditions and well-suited for solving fluid mechanics problems with various types of particles.

A sensitivity analysis of the model using different sublimation temperatures shows consistent results with our experimental data. Results obtained from the dual isotope analysis (δ¹⁸O and δ²H) of ice samples obtained from Greenland Summit Precipitation (GRESP) and Antarctica snow show trends consistent with theoretical predictions and meteoric water line, suggesting that the setup is operating reliably. Observed deviations in the isotopic compositions indicate influences from environmental variables such as humidity, pointing towards the need for tighter control and validation. Our experimental set-up lays a foundation for further investigations into the problems of fast diffusion, non-equilibrium thermodynamics, and the isotopic signature of water.