Distribution of matched cases in the entire case range is shown. Matched cases (i.e. red circles) represent the escape scenarios that can produce present Venusian atmospheric ²⁰Ne/²²Ne, ³⁶Ar/³⁸Ar and ²⁰Ne/³⁶Ar − 11.8 ± 0.7, 5.5 ± 0.6, and 0.25 ± 0.1, respectively (von Zahn et al. 1983; Donahue & Pollack 1983). Conversely, ‘unmatched cases’ do not satisfy all three isotopic ratios simultaneously. The X-axis represents the initial nebular hydrogen content in the Venusian atmosphere and the Y -axis represents the initial H₂O content in the atmosphere. The horizontal dashed line represents an initial H₂O inventory of 1.05 × 104 moles/cm² and CO₂ inventory of 2.3 × 10³ moles/cm² (about 90 bar of CO₂ on present Venus). Cases above the line assume higher initial CO₂ , while those below assume lower initial CO₂ . The vertical dashed line represents an initial H₂ content of 210 moles cm⁻² (equal to 0.00004 wt%). For cases with initial nebular-source H₂ below 210 moles cm⁻², we coupled the radiactive-convective atmosphere model to the escape model and updated the escape height (where escape flux works) at each time step, assuming an initial ²⁰Ne/³⁶Ar ratio of 0.33 (i.e. a strongly fractionated value compared to the solar value). For cases with initial nebular-source H₂ above 210 moles cm⁻², our calculation applied only the escape model, keeping r₀ fixed over time and assuming an initial ²⁰Ne/³⁶Ar ratio of 44.7 (i.e. the solar value).