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Latitudinal variation in the abundance of methane (CH4) above the clouds in Neptune's atmosphere from VLT/MUSE Narrow Field Mode Observations

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posted on 2019-06-06, 08:57 authored by PGJ Irwin, D Toledo, AS Braude, R Bacon, PM Weilbacher, NA Teanby, LN Fletcher, GS Orton
Observations of Neptune, made in 2018 using the new Narrow Field Adaptive Optics mode of the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope (VLT) from 0.48 - 0.93 micron, are analysed here to determine the latitudinal and vertical distribution of cloud opacity and methane abundance in Neptune's observable troposphere (0.1 - ~3 bar). Previous observations at these wavelengths in 2003 by HST/STIS (Karkoschka and Tomasko 2011, Icarus 205, 674-694) found that the mole fraction of methane above the cloud tops (at ~2 bar) varied from ~4% at equatorial latitudes to ~2% at southern polar latitudes, by comparing the observed reflectivity at wavelengths near 825 nm controlled primarily by either methane absorption or H2-H2/H2-He collision-induced absorption. We find a similar variation in cloud-top methane abundance in 2018, which suggests that this depletion of methane towards Neptune's pole is potentially a long-lived feature, indicative of long-term upwelling at mid-equatorial latitudes and subsidence near the poles. By analysing these MUSE observations along the central meridian with a retrieval model, we demonstrate that a broad boundary between the nominal and depleted methane abundances occurs at between 20 - 40S. We also find a small depletion of methane near the equator, perhaps indicating subsidence there, and a local enhancement near 60 - 70S, which we suggest may be associated with South Polar Features (SPFs) seen in Neptune's atmosphere at these latitudes. Finally, by the use of both a reflectivity analysis and a principal component analysis, we demonstrate that this depletion of methane towards the pole is apparent at all locations on Neptune's disc, and not just along its central meridian.

Funding

We are grateful to the United Kingdom Science and Technology Facilities Council for funding this research. We thank Larry Sromovsky for providing the code used to generate our Rayleigh-scattering opacities. Glenn Orton was supported by NASA funding to the Jet Propulsion Laboratory, California Institute of Technology. Leigh Fletcher was supported by a Royal Society Research Fellowship at the University of Leicester. PMW received support from BMBF Verbundforschung (project MUSE-NFM, grant 05A17BAA). The observations reported in this paper have the ESO ID: 60.A-9100(K). Facilities: VLT (MUSE).

History

Citation

Icarus, 2019, 331, pp. 69-82

Author affiliation

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

Version

  • VoR (Version of Record)

Published in

Icarus

Publisher

Elsevier for Academic Press

issn

0019-1035

Acceptance date

2019-05-08

Copyright date

2019

Available date

2019-06-06

Publisher version

https://www.sciencedirect.com/science/article/pii/S0019103518306870?via=ihub

Language

en

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