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 $1.0_{-0.8}^{+1.2}\times 10^{13}$ 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.

arXiv comment8 pages, 6 figures, published in Monthly Notices of the Royal Astronomical Society

References in corpus (4)

• Dust Ablation on the Giant Planets: Consequences for Stratospheric Photochemistry
• Neptune at Summer Solstice: Zonal Mean Temperatures from Ground-Based Observations 2003-2007
• Giant Planet Observations with the James Webb Space Telescope
• Conjugate observations of Saturn's northern and southern H3+ aurorae

Cited by in corpus (5)

• Atmospheric chemistry on Uranus and Neptune
• Modelling H${}_3^{+}$ in planetary atmospheres: effects of vertical gradients on observed quantities
• The upper atmospheres of Uranus and Neptune
• ExoMol line lists -- L: High-resolution line lists of H${}_3^{+}$, H${}_2$D${}^{+}$, D${}_2$H${}^{+}$ and D${}_3^{+}$
• Detection of the infrared aurora at Uranus with Keck-NIRSPEC