A defining characteristic of the planet Venus is its thick, CO2-dominated atmosphere. Despite over 50 years of robotic exploration of Venus, including thirteen successful atmospheric probes and landers, our knowledge of N2, the second most abundant compound in the atmosphere, is highly uncertain1. We report the first measurement of the nitrogen content of Venus’s atmosphere at altitudes between 60 and 100 km. Our result, 5.0 ± 0.4 v% N2, is 40% higher than the value of 3.5 v% N2 reported for the lower atmosphere (<50 km altitude). Our discovery of altitude-dependent variations in the N2 content of the atmosphere defies early expectations of homogeneous composition below 100 km (for example, see ref. 1), and necessitates complete chemical modelling to investigate chemical versus physical explanations for the enhanced N2 in the upper atmosphere. The existence of chemically distinct lower and upper atmosphere regions complicates the use of remote sensing measurements of the upper atmosphere to infer the properties of the lower atmosphere and surface, an important lesson that also extends to the growing field of exoplanet astronomy.
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The MESSENGER Neutron Spectrometer data used in this study are publicly available via NASA’s Planetary Data System (https://pds-geosciences.wustl.edu/missions/messenger/index.htm). Those data include the NS LG energy spectra used to produce the measured neutron count rates shown in Fig. 1 (middle and bottom panels), as well as the spacecraft ephemeris data in Fig. 1 (top panel).
The export-controlled MCNPX radiation transport code, which formed the basis of our modeling effort, is available from https://mcnp.lanl.gov. Data processing was handled using the Interactive Data Language (IDL), with spectral processing codes following a routine detailed in Lawrence et al.13 and standard statistical analysis (for example, reduced-χ2 analyses) as detailed in the Methods section.
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This analysis has benefited from many funding sources over a number of years. The MESSENGER mission was supported by the NASA Discovery Program under contract NAS5-97271 to the Johns Hopkins University Applied Physics Laboratory and NASW-00002 to the Carnegie Institution of Washington. Data analysis was originally supported, in part, by a MESSENGER Participating Scientist grant NNX08AN30G to D.J.L. Improvements to the spectral analysis and modelling codes were funded by the US Department of Energy, grant DE-SC0019343. Finally, manuscript preparation was funded by NASA’s Discovery Data Analysis Program, grant NNX16AJ949, to the Johns Hopkins University Applied Physics Laboratory.
The authors declare no competing interests.
Peer review information Nature Astronomy thanks Paul Byrne and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Peplowski, P.N., Lawrence, D.J. & Wilson, J.T. Chemically distinct regions of Venus’s atmosphere revealed by measured N2 concentrations. Nat Astron 4, 947–950 (2020). https://doi.org/10.1038/s41550-020-1079-2