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.

References:

[1] Mogul R. et al. (2025) GRL 52.

[2] Hoffman J. H. et al. (1979) Science 205.

[3] Moroz V. et al. (1997) Adv. Space Res. 19.

[4] Avice G. et al. (2022) Space Sci. Rev. 218.

[5] Vandaele A. C. et al. (2016) Adv. Space Res. 57.

[6] Morellina S. et al. (2020) Icarus 350.

[7] Limaye S. S. et al. (2017) Icarus 294.

[8] Hoffman J. H. et al. (1980) JGR Space Sci. 85.

[9] Mogul R. et al. (2023) Icarus 392.

[10] Mogul R. et al. (2023) MethodsX 11.

[11] Mogul R. et al. (2025) JGR Planets 130.

[12] Marty B. et al. (2011) Science 332.

[13] Füri E. et al. (2015) Nature Geosci. 8.