Altitude Profile and Isotopic Composition for N ₂ From Pioneer Venus Mass Spectra

We report the highest resolution altitude profile and isotopic composition for N 2 at Venus, to date. These results were obtained through re‐analysis of data acquired by the Pioneer Venus Large Probe Neutral Mass Spectrometer (LNMS). The N 2 is non‐uniformly mixed across the clouds and sub‐cloud atmosphere (∼59–15 km) yet uniformly mixed across the deep lower atmosphere (DLA, <15 km). The volume percent for N 2 decreases by ∼2.1‐fold between the clouds (5.21 ± 0.18 vol%; ∼59–51 km) and DLA (2.49 ± 0.10 vol%; <15 km) with concomitant increases in the atmospheric molecular masses. Using the cloud N 2 , we extracted the 15 N/ 14 N ratio (2.93 × 10 −3 ± 0.13 × 10 −3 ) and a corresponding δ 15 N (−204 ± 35‰). Together, the LNMS results suggest that (a) N 2 is not well‐mixed >15 km, (b) the atmosphere is unstable at <15 km, and (c) the cloud δ 15 N falls between the solar wind and Earth's atmosphere.