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

H. Melin, L. N. Fletcher, T. S. Stallard, R. E. Johnson, J. O'Donoghue, Luke Moore, P. T. Donnelly

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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 H3+ 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 × 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 H3+, thus greatly reducing the column integrated H3+ densities. This upper limit also provide constraints on future attempts at detecting H3+ using the James Webb Space Telescope.
Original languageEnglish
Pages (from-to)3714-3719
Number of pages6
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Early online date23 Nov 2017
Publication statusPublished - 01 Mar 2018
Externally publishedYes


  • Planets and satellites: atmospheres
  • Planets and satellites: composition
  • Planets and satellites: individual: Neptune
  • Planets and satellites: individual: Uranus
  • Techniques: spectroscopic
  • planets and satellites: aurorae


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