The atmosphere of Neptune: Results of radio occultation measurements with the Voyager 2 spacecraft

Radio tracking data acquired during Voyager 2's occultation by Neptune have been used to study the thermal structure and composition of Neptune's troposphere and stratosphere. The occultation began near 62° North planetographic latitude and aided near 45°South latitude. The data cover an altitude interval of about 250 km. A comparison with infrared observations indicates that the gas at the tropopause, which was observed near the 100 mbar level, consists of 77 to 85 % hydrogen by number density with the remainder being mostly helium. A layer with a small refractivity scale height that was detected approximately 50 km below the tropopause, may be a region where the CH₄ mixing ratio is decreasing with increasing altitude due to condensation effects. This interpretation leads to a CH₄ mixing ratio of 1 to 2 % by number density at the base of the layer where the pressure is approximately 1.7 bars. The depth of the occultation measurements was limited by microwave absorption. By assuming that the absorption was caused by saturated NH₃ vapor, one obtains a pressure, temperature, and NH₃ mixing ratio of 6 bars, 130 K, and 600 parts per billion, respectively, at the lowest level where the radio link was detected. For internal consistency, this solution requires an atmospheric rotation period of 14.2 ± 0.5 hours near 60° North latitude which corresponds to a prograde zonal wind velocity of 170 ± 50 m/sec relative to the magnetic field. The 1 bar isobaric surface has an equatorial radius of 24,764 ± 15 km and a polar radius of 24,341 ± 30 km. The corresponding oblateness, (R_eq‐R_p)/R_eq, is 0.0171 ± 0.0014.