Field studies at terrestrial analogue sites represent an important contribution to the science of ocean worlds. The value of the science and technology investigations conducted at field analogue sites depends on the relevance of the analogue environment to the target ocean world. We accept that there are no perfect analogues for many of the unique environments represented by ocean worlds but suggest that a one-to-one matching of environmental characteristics and conditions is not crucial to the success or impact of the work. Instead, we must instead determine which processes and parameters are required to map directly to the target ocean world environment with high fidelity to address the science question or engineering challenge. Where there are discrepancies between the model and target environment, we must fully understand how those limitations impact the applicability of the study, and mitigate these where possible using alternative approaches. Here we present a two-step approach to 1) identify the most crucial processes and parameters associated with a given science question and 2) assess the fidelity of these processes and parameters at a proposed field site to those expected for the target ocean world. We demonstrate this approach in a test case evaluating three types of ocean world analogue environments with respect to a science question. Our proposed framework will not only enhance the scientific rigor of field research but also provide access to a broader range of field sites relevant to ocean worlds processes, enabling a greater diversity of ocean and geological science researchers.

The Oman Drilling Project established an “Active Alteration” multi-borehole observatory in dunite and harzburgite undergoing low-temperature serpentinization in the Samail ophiolite. The highly serpentinized rocks are in contact with strongly reducing fluids. Distinct hydrological regimes, governed by differences in rock porosity and fracture density, give rise to steep redox (Eh +200 to -750 mV) and pH (pH range 8.5 to 11.2) gradients within the 300 to 400 meter deep boreholes. The serpentinites and fluids host an active subsurface ecosystem. Microbial cell abundances vary at least 6 orders of magnitude, from ≤3.5*101 cells/g to 2.9*107 cells/gram. Low levels of biological sulfate reduction (2-1000 fmol/cm3/day) can be detected in rock cores, particularly in rocks in contact with reduced groundwaters with pH <10.5. Thermodesulfovibrio is the predominant sulfate reducer identified via metagenomic sequencing of adjacent groundwater communities. We infer that transport and reaction of microbially generated sulfide with the serpentine and brucite assemblages gives rise to optical darkening and sulfide overprinting, including the formation of tochilinite-vallerite group minerals, potentially serving as an indicator that this system is inhabited by microbial life. Olivine mesh-cores replaced with ferroan brucite and minor awaruite, abundant veins containing hydroandradite garnet and polyhedral serpentine, and late-stage carbonate veins are suggested as targets for future spatially-resolved life-detection investigations. The high-quality whole-round core samples that have been preserved can be further probed to define how life distributes itself and functions within a system where chemical disequilibria are sustained by low-temperature water/rock interaction, and how biosignatures of in-situ microbial activity are generated.