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The quest for H-3(+) at Neptune: deep burn observations with NASA IRTF iSHELL

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journal contribution
posted on 26.04.2018, 14:18 by Henrik Melin, Leigh N. Fletcher, Thomas S. Stallard, Rosie E. Johnson, J. O'Donoghue, L. Moore, Padraig T. Donnelly
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.


This work was supported by the UK Science and Technology Facilities Council (STFC) Grant ST/N000749/1 for HM and TSS. LNF was supported by a Royal Society Research Fellowship at the University of Leicester. REJ and PTD were supported by STFC studentships. Support for JO’D. comes from an appointment to the NASA Postdoctoral Program at Goddard Space Flight Center, administered by Universities Space Research Association under contract with NASA. LM was supported by NASA under Grant NNX17AF14G issued through the SSO Planetary Astronomy Program. HM, REJ, and TSS are Visiting Astronomers at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract NNH14CK55B with the National Aeronautics and Space Administration.



Monthly Notices of the Royal Astronomical Society, 2018, 474 (3), pp. 3714-3719 (6)

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/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy


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Monthly Notices of the Royal Astronomical Society


Oxford University Press (OUP), Royal Astronomical Society





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