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Seasonal stratospheric photochemistry on Uranus and Neptune

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journal contribution
posted on 23.07.2018, 15:14 by Julianne I. Moses, Leigh N. Fletcher, Thomas K. Greathouse, Glenn S. Orton, Vincent Hue
A time-variable 1D photochemical model is used to study the distribution of stratospheric hydrocarbons as a function of altitude, latitude, and season on Uranus and Neptune. The results for Neptune indicate that in the absence of stratospheric circulation or other meridional transport processes, the hydrocarbon abundances exhibit strong seasonal and meridional variations in the upper stratosphere, but that these variations become increasingly damped with depth due to increasing dynamical and chemical time scales. At high altitudes, hydrocarbon mixing ratios are typically largest where the solar insolation is the greatest, leading to strong hemispheric dichotomies between the summer-to-fall hemisphere and winter-to-spring hemisphere. At mbar pressures and deeper, slower chemistry and diffusion lead to latitude variations that become more symmetric about the equator. On Uranus, the stagnant, poorly mixed stratosphere confines methane and its photochemical products to higher pressures, where chemistry and diffusion time scales remain large. Seasonal variations in hydrocarbons are therefore predicted to be more muted on Uranus, despite the planet's very large obliquity. Radiative-transfer simulations demonstrate that latitude variations in hydrocarbons on both planets are potentially observable with future JWST mid-infrared spectral imaging. Our seasonal model predictions for Neptune compare well with retrieved C 2 H 2 and C 2 H 6 abundances from spatially resolved ground-based observations (no such observations currently exist for Uranus), suggesting that stratospheric circulation — which was not included in these models — may have little influence on the large-scale meridional hydrocarbon distributions on Neptune, unlike the situation on Jupiter and Saturn.

Funding

This material is based on research supported by the National Aeronautics and Space Administration (NASA) Science Mission Directorate under grant NNX13AH81G from the Planetary Atmospheres Research Program. The oxygen chemistry portion was supported by NASA grant NNX13AG55G. Fletcher was supported by a Royal Society Research Fellowship and European Research Council Consolidator Grant (under the European Union’s Horizon 2020 research and innovation programme, grant agreement No. 723890) at the University of Leicester. Orton acknowledges support from NASA to the Jet Propulsion Laboratory, California Institute of Technology.

History

Citation

Icarus, 2018, 307, pp. 124-145

Author affiliation

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

Version

AM (Accepted Manuscript)

Published in

Icarus

Publisher

Elsevier for Academic Press

issn

0019-1035

eissn

1090-2643

Acceptance date

02/02/2018

Copyright date

2018

Available date

10/02/2019

Publisher version

https://www.sciencedirect.com/science/article/pii/S0019103517307935?via=ihub#!

Notes

The file associated with this record is under embargo until 12 months after publication, in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above.

Language

en