Future Venus missions and flybys: A collection of possible measurements with mass spectrometers and plasma instruments

This study contains predictions for mass spectrometry and plasma instrument measurements during upcoming Venus flybys of BepiColombo and Solar Orbiter and discusses the possibility of a phosphine detection with mass spectrometry in Venus’ upper atmosphere. The results extend the ones published previously in Gruchola et al. (2019), where predictions for the proposed ESA mission EnVision and the Venus flyby of JUICE were included. Both the Venus flyby of BepiColombo and the one of Solar Orbiter will take place in August 2021, only 1 day apart. BepiColombo, carrying the neutral mass gas spectrometer STROFIO, could probe the atmosphere around closest approach and obtain data on the thermal and hot neutral particle populations in the upper atmosphere. According to this study, the thermal hydrogen population as well as the hot species H, C, N and O, including isotopes, should be visible to STROFIO. Especially data on the abundance of the hot species would yield important insight into the planetary escape processes.

The Solar Orbiter on the other hand carries the plasma instrument SWA-HIS, designed to measure the energetic solar wind ions. During its second Venus gravity assist it will traverse the magnetosheath, the region between bow shock and ion composition boundary, where in addition to the solar wind ions energetic planetary ions are present. The planetary pickup ions can be measured by SWA-HIS, providing important information on the outflow of planetary ions and the ionization processes in the upper atmosphere itself.

The recent reports of phosphine in Venus’ cloud decks with an abundance of 20 ppb at 80 km probably overestimate the actual PH3 abundance, as the data are currently being reanalyzed (Greaves et al., 2020). However, even with this upper limit of 20 ppb a phosphine detection with a mass spectrometer e.g. on-board ESA’s proposed EnVision mission, seems unlikely. To resolve the PH3 peak and the 16O18O peak a mass resolution of almost 10’000 is required, and the PH2D peak is masked by the Cl fragment peak of HCl. Furthermore, NGMS on-board Pioneer Venus with a mass resolution of around 440 did most likely not detect phosphine, as it scanned only a few masses in the mass range of interest where more abundant species than phosphine are present.