Noble gas fractionation predictions for high speed sampling in the upper atmosphere of Venus

Venus, our neighboring planet, is an open-air laboratory that can be used to study why Earth and Venus evolved in such different ways and even to better understand exoplanets. Noble gases in planetary atmospheres are tracers of their geophysical evolution, and measuring the elemental and isotopic composition of noble gases in the Venus atmosphere informs us about the origin and evolution of the entire planet. In this work we describe a new mission concept, Venus ATMOSpheric - Sample Return (VATMOS-SR), that would return gas samples from the upper atmosphere of Venus to Earth for scientific analysis. This could be the first sample return mission for an extraterrestrial atmosphere. To ensure it is possible to relate the composition of the sampled gases (acquired when the spacecraft is traveling $>10$ km/s) to the freestream atmospheric composition, large-scale numerical simulations are employed to model the flow into and through the sampling system. In particular, an emphasis is placed on quantifying noble gas elemental and isotopic fractionation that occurs during the sample acquisition and transfer process, to determine how measured isotopic ratios of noble gases in the sample would compare to the actual isotopic ratios in the Venusian atmosphere. We find that lighter noble gases are depleted after they are sampled compared to the freestream conditions, and heavier ones are enriched, due to the high pressure gradients present in the flowfield. We also observe that lighter noble gases are more affected than heavier ones by changes in the freestream conditions. Finally, we observe that, in general, the numerical parameters do not have a major impact on the observed fractionation. We do, however, note that the freestream velocity and density have a major impact on fractionation, and do need to be precisely known to properly reconstruct the fractionation in the sampling system. We demonstrate that the sample fractionation can be predicted with numerical simulations, and believe that VATMOS-SR, which could be the first mission to bring back samples from another planet, could answer key scientific questions related to understanding the evolution of Venus.