Noble Gas Fractionation Predictions for High Speed Sampling in the Upper Atmosphere of Venus

TL;DR

The paper evaluates noble gas fractionation during high-speed sampling from Venus’ upper atmosphere and proposes VATMOS-SR to return samples for Earth-based analysis. Using SPARTA-based DSMC simulations with a flight-relevant, 3D sampling system, it quantifies elemental and isotopic fractionation as gases traverse shock, valve, and piping, establishing a transfer function that maps freestream Venus composition to tank composition. The key finding is that fractionation is mass-dependent, with lighter gases depleted and heavier gases enriched, primarily driven by pressure diffusion in the compression layer; isotopic differences are typically small (well below 1%/amu) and predictable via the simulations. The work demonstrates that VATMOS-SR could yield representative noble gas measurements, enabling insights into Venus’ volatile origin and atmospheric evolution, and highlights the need for precise knowledge of freestream conditions and continued validation for mission planning.

Abstract

Venus, our neighboring planet, is an open-air laboratory that can be used to study why Earth and Venus evolved in such different ways. 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 SmallSat mission concept, Venus ATMOSpheric - Sample Return (VATMOS-SR), that would return gas samples from the upper atmosphere of Venus to Earth for scientific analysis. To ensure it is possible to relate the composition of the sampled gases (acquired when the spacecraft is traveling >10 km/s) to the free stream atmospheric composition, large-scale numerical simulations are employed to model the flow into and through the sampling system. 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. Finally, we observe that, in general, the numerical parameters do not have a major impact on the observed fractionation. However, the freestream velocity and density have a major impact on fractionation and 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.

Figures (10)

• Figure 1: Possible design for the VATMOS-SR Vehicle, composed of a cruise stage, separation rings, and a probe (total diameter less than 1 m). The four sampling tanks are located in the probe. The predicted dry mass is less than 75 kg (100 kg with 30 % contingency). A 60$^\circ$ sphere-cone aeroshell is shown, with the sampling system inlet located at the stagnation point of the vehicle. However, the design later evolved to a 45$^\circ$ sphere-cone, which is the geometry used in the simulations presented in this work, for aerodynamic stability.
• Figure 2: Schematic of the sampling system for VATMOS-SR, including the nominally closed Mindrum valves.
• Figure 3: Zoomed-in schematic view of the (top) "open valve" geometry, that includes the sampling tank, and (bottom) "closed valve" geometry. Only a quarter of the actual geometry is simulated.
• Figure 4: Computational grid once the simulations have reached steady state.
• Figure 5: Translational temperature and pressure for the baseline conditions. The view is zoomed-in on the nose of VATMOS-SR vehicle, where the sampling occurs, and the bow shock and compression layer region, as well as the sampling inlet.