The quest for H₃⁺ at Neptune: deep burn observations with NASA IRTF iSHELL

Emission from the molecular ion H|₃⁺| is a powerful diagnostic of the upper atmosphere of Jupiter, Saturn, and Uranus, but it remains undetected at Neptune. In search of this emission, we present near-infrared spectral observations of Neptune between 3.93 and 4.00 μm taken with the newly commissioned iSHELL instrument on the NASA Infrared Telescope Facility in Hawaii, obtained 2017 August 17–20. We spent 15.4 h integrating across the disc of the planet, yet were unable to unambiguously identify any H|₃⁺| line emissions. Assuming a temperature of 550 K, we derive an upper limit on the column integrated density of |1.0+1.2-0.8× 10¹³| m−2, which is an improvement of 30 per cent on the best previous observational constraint. This result means that models are overestimating the density by at least a factor of 5, highlighting the need for renewed modelling efforts. A potential solution is strong vertical mixing of polyatomic neutral species from Neptune’s upper stratosphere to the thermosphere, reacting with H|₃⁺|⁠, thus greatly reducing the column integrated H|₃⁺| densities. This upper limit also provide constraints on future attempts at detecting H|₃⁺| using the James Webb Space Telescope.