First N2 Profile for Venus’ Deep Lower Atmosphere

We present the first N2 profile for Venus’ deep lower atmosphere (<15 km) and a constrained isotopic composition for cloud N2 [1]. This work directly addresses unresolved questions for Venus using legacy observations. Prior to this work, there were no reported measurements for N2 abundances at <15 km and the isotopic composition remained unconstrained [2]. The N2 parameters are critical to understanding the evolution and thermal properties of the atmosphere [3-7]. Our N2 results were obtained by re-analysis of data acquired in 1978 by the Pioneer Venus Large Probe Neutral Mass Spectrometer (LNMS) [8]. The archived mass spectra from 64.2 to 0.2 km were treated using the analytical procedures specifically developed for the LNMS [9-11]. Judicious peak fitting permitted disambiguation of (A) N2+ and CO+ at 28 u and (B) 14N15N+, 13CO+, and C2H5+ at 29 u. Quality controls included comparing the (A) LNMS CO+/CO2+ ratios to the NIST database and literature and (B) fitted counts for CO+ to the expected counts of CO+ calculated from C18O+ and 13CO+ using the LNMS 16O/18O and 12C/13C ratios (obtained from CO2). Our results show that N2 is uniformly mixed across the deep lower atmosphere between ~0.2 and 15 km (2.49 ± 0.10 v%). In contrast, N2 is non-uniformly mixed across the sub-cloud atmosphere and clouds (~15–59 km), where N2 abundances increase by ~ 2-fold between ~15 km (2.45 ± 0.32 v%) and ~59-51 km (5.21 ± 0.18 v%). Using the cloud data, we also obtained a constrained 15N/14N ratio (2.93×10-3 ± 0.13×10-3) and δ15N value (-204 ± 35‰). Thus, the LNMS results [1] suggest that (A) the atmosphere is unstable at <15 km, (B) N2 is not well-mixed >15 km, and (C) the cloud δ15N falls between Earth and the solar wind [12, 13]. Comparisons of the N2 abundances and isotopic compositions for nitrogen, carbon, and oxygen to other Venus measurements will be discussed.