Chemically distinct regions of Venus’s atmosphere revealed by measured N2 concentrations

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.