Sampling Venus' atmosphere to measure noble gases and their isotope ratios

Measuring noble gases and their isotope ratios would inform on Venus' geological evolution and help understand why Earth and Venus have evolved so differently. Noble gases and their isotope ratios inform on (1) the origin of the volatiles, (2) the atmospheric loss mechanisms, and (3) the history of volcanic outgassing. A small probe equipped with a miniaturized mass spectrometer that would skim Venus' atmosphere and collect samples below the homopause could make such measurements [1]. The high velocity of the probe has raised the question of fractionation during sampling. Numerical simulations using the open source code SPARTA [2,3] have been performed to test such an approach. We report on the results of the first numerical simulations that have investigated the effect of geometry and number densities on the fractionation pattern. Preliminary results show that isotope ratios are weakly affected by the sampling but that the relative amount of noble gases is more affected by the sampling. In addition, the numerical simulations include chemical reactions and provide all physical (pressure, temperature, velocities, heat flux) and chemical (number densities of neutral and ionic species) parameters outside and inside the probe. These numerical simulations help define the optimal geometry to conduct laboratory tests at the ArcJet facilities at Ames Research Center. These tests cannot exactly match the Venus environment where the samples would be acquired. However, these tests would validate the numerical simulations run with the laboratory parameters. The numerical simulations could then be performed with the parameters of the Venus environment and would provide the necessary information for the performance model of the sampling system. The uncertainties related to the sampling system would be added to the uncertainties of the measurements by the mass spectrometer [4] in order to assess the potential of such measurements to answer the science questions on the geological evolution of Venus.

This work has been performed at the Jet Propulsion Laboratory (JPL), California Institute of Technology (Caltech), under contract to NASA. The information in this paper is predecisional and is presented for planning and discussion purposes only.

References: [1] Sotin et al., LPSC 2018 [2] Gallis et al. RGD29 2014 [3] Plimpton S. and M. Gallis, SPARTA Users Manual, Sandia National Laboratories, 2018. https://sparta.sandia.gov/ [4] Avice G. et al., submitted to JAAS, 2018.