Title: The quest for H$_3^+$ at Neptune: deep burn observations with NASA IRTF iSHELL,
Abstract: 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 $1.0^{+1.2}_{-0.8}\times10^{13}$ 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.