Uranus's and Neptune’s stratospheric water abundance and vertical profile from Herschel-HIFI

Nicholas A Teanby*, PGJ Irwin, Melody A S Sylvestre, CA Nixon, MA Cordiner

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

3 Citations (Scopus)
41 Downloads (Pure)

Abstract

Here we present new constraints on Uranus's and Neptune's externally sourced stratospheric water abundance using disk-averaged observations of the 557 GHz emission line from Herschel's Heterodyne Instrument for the Far-Infrared. Derived stratospheric column water abundances are ${0.54}_{-0.06}^{+0.26}$ × 1014 cm−2 for Uranus and ${1.9}_{-0.3}^{+0.2}$ ×1014 cm−2 for Neptune, consistent with previous determinations using ISO-SWS and Herschel-PACS. For Uranus, excellent observational fits are obtained by scaling photochemical model profiles or with step-type profiles with water vapor limited to ≤0.6 mbar. However, Uranus's cold stratospheric temperatures imply a ∼0.03 mbar condensation level, which further limits water vapor to pressures ≤0.03 mbar. Neptune's warmer stratosphere has a deeper ∼1 mbar condensation level, so emission-line pressure broadening can be used to further constrain the water profile. For Neptune, excellent fits are obtained using step-type profiles with cutoffs of ∼0.3–0.6 mbar or by scaling a photochemical model profile. Step-type profiles with cutoffs ≥1.0 mbar or ≤0.1 mbar can be rejected with 4σ significance. Rescaling photochemical model profiles from Moses & Poppe to match our observed column abundances implies similar external water fluxes for both planets: ${8.3}_{-0.9}^{+4.0}$ × 104 cm−2 s−1 for Uranus and ${12.7}_{-2.0}^{+1.3}$ ×104 cm−2 s−1 for Neptune. This suggests that Neptune's ∼4 times greater observed water column abundance is primarily caused by its warmer stratosphere preventing loss by condensation, rather than by a significantly more intense external source. To reconcile these water fluxes with other stratospheric oxygen species (CO and CO2) requires either a significant CO component in interplanetary dust particles (Uranus) or contributions from cometary impacts (Uranus, Neptune).
Original languageEnglish
Article number96
Pages (from-to)1-13
JournalPlanetary Science Journal
Volume3
Issue number4
Early online date29 Apr 2022
DOIs
Publication statusE-pub ahead of print - 29 Apr 2022

Bibliographical note

Funding Information:
HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada, and the United States under the leadership of SRON Netherlands Institute for Space Research, Groningen, The Netherlands, and with major contributions from Germany, France, and the United States. Consortium members are: Canada: CSA, U.Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS; Ireland, NUI Maynooth; Italy: ASI, IFSI-INAF, Osservatorio Astrofisico di Arcetri-INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: Observatorio Astronómico Nacional (IGN), Centro de Astrobiología (CSIC-INTA). Sweden: Chalmers University of Technology—MC2, RSS, & GARD; Onsala Space Observatory; Swedish National Space Board, Stockholm University—Stockholm Observatory; Switzerland: ETH Zurich, FHNW; USA: Caltech, JPL, NHSC. N.A.T., P.G.J.I., and M.S. were funded by the UK Science and Technology Facilities Council (STFC) and the UK Space Agency (UKSA). C.A.N. and M.A.C. were supported by the Fundamental Laboratory Research (FLaRe) Program at NASA GSFC for their effort in this work. Facility: Herschel (HIFI).

Funding Information:
HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada, and the United States under the leadership of SRON Netherlands Institute for Space Research, Groningen, The Netherlands, and with major contributions from Germany, France, and the United States. Consortium members are: Canada: CSA, U.Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS; Ireland, NUI Maynooth; Italy: ASI, IFSI-INAF, Osservatorio Astrofisico di Arcetri-INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: Observatorio Astronómico Nacional (IGN), Centro de Astrobiología (CSIC-INTA). Sweden: Chalmers University of Technology—MC2, RSS, & GARD; Onsala Space Observatory; Swedish National Space Board, Stockholm University—Stockholm Observatory; Switzerland: ETH Zurich, FHNW; USA: Caltech, JPL, NHSC. N.A.T., P.G.J.I., and M.S. were funded by the UK Science and Technology Facilities Council (STFC) and the UK Space Agency (UKSA). C.A.N. and M.A.C. were supported by the Fundamental Laboratory Research (FLaRe) Program at NASA GSFC for their effort in this work.

Publisher Copyright:
© 2022. The Author(s). Published by the American Astronomical Society.

Keywords

  • Neptune
  • Uranus
  • Planetary atmospheres
  • Submillimeter astronomy

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