Inputs and outputs of the analytical model in forward mode (left) and inverse mode (right) for He⁺ and He²⁺ ion species. The “forward” mode uses the recommended water outgassing rate Q_S13 of Snodgrass et al. (2013) to calculate theoretical flux ratios R_F. The “inverse” mode uses RPC-ICA observations of He⁺/He²⁺ particle flux ratios to derive the corresponding outgassing rates Q_obs. n_H2O is the number density of neutral species H₂O, so here species A = H₂O.