TY - JOUR
T1 - The quest for H3+ at Neptune
T2 - Deep burn observations with NASA IRTF iSHELL
AU - Melin, H.
AU - Fletcher, L. N.
AU - Stallard, T. S.
AU - Johnson, R. E.
AU - O'Donoghue, J.
AU - Moore, Luke
AU - Donnelly, P. T.
N1 - Funding Information:
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. We would like to express our gratitude to Steve Miller for helpful comments and suggestions.
Publisher Copyright:
© 2017 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society.
PY - 2018/3/1
Y1 - 2018/3/1
N2 - 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.
AB - 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.
KW - Planets and satellites: atmospheres
KW - Planets and satellites: composition
KW - Planets and satellites: individual: Neptune
KW - Planets and satellites: individual: Uranus
KW - Techniques: spectroscopic
KW - planets and satellites: aurorae
UR - http://www.scopus.com/inward/record.url?scp=85040225604&partnerID=8YFLogxK
U2 - 10.1093/mnras/stx3029
DO - 10.1093/mnras/stx3029
M3 - Article
SN - 0035-8711
VL - 474
SP - 3714
EP - 3719
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
IS - 3
ER -