The quest for H${}_3^{+}$ 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 $\mu$m taken with the newly commissioned iSHELL instrument on the NASA Infrared Telescope Facility in Hawaii, obtained 17-20 August 2017. We spent 15.4 h integrating across the disk 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 $1.0_{-0.8}^{+1.2}\times {10}^{13}$ m${}^{-2}$, which is an improvement of 30% on the best previous observational constraint. This result means that models are over-estimating 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.