Meridional Variations of C2H2 in Jupiter's Stratosphere From Juno UVS Observations

Rohini S Giles,Leigh N Fletcher,Patrick G J Irwin,Henrik Melin,Thomas K Greathouse,Vincent Hue,Joshua A Kammer,Steven M Levin,Scott J Bolton,G Randall Gladstone,Bertrand Bonfond,Maarten H Versteeg,Denis C Grodent

doi:10.1029/2021je006928

Abstract

AbstractThe Ultraviolet Spectrograph (UVS) instrument on the Juno mission records far‐ultraviolet reflected sunlight from Jupiter. These spectra are sensitive to the abundances of chemical species in the upper atmosphere and to the distribution of the stratospheric haze layer. We combine observations from the first 30 perijoves of the mission in order to study the meridional distribution of acetylene (C2H2) in Jupiter's stratosphere. We find that the abundance of C2H2 decreases toward the poles by a factor of 2–4, in agreement with previous analyses of mid‐infrared spectra. This result is expected from insolation rates: near the equator, the UV solar flux is higher, allowing more C2H2 to be generated from the UV photolysis of CH4. The decrease in abundance toward the poles suggests that horizontal mixing rates are not rapid enough to homogenize the latitudinal distribution.

Highlights

Hydrocarbons play a significant role in Jupiter’s stratosphere
Methane (CH4 ) is the most abundant spectroscopically absorbing gas in the stratosphere and its photolysis by UV photons leads to a rich hydrocarbon photochemistry (Moses et al, 2004)
The most abundant products of CH4 photolysis are ethane (C2 H6 ) and acetylene (C2 H2 ), the latter of which is the focus of this paper

Summary

Introduction

Hydrocarbons play a significant role in Jupiter’s stratosphere. Methane (CH4 ) is the most abundant spectroscopically absorbing gas in the stratosphere and its photolysis by UV photons leads to a rich hydrocarbon photochemistry (Moses et al, 2004). As discussed in Nixon et al (2007), a decrease in C2 H2 abundance towards the poles can be understood by considering the lifetime of C2 H2 in Jupiter’s stratosphere. We present the first full-planet meridional retrievals of C2 H2 from ultraviolet reflected sunlight observations of Jupiter, using data from the UVS instrument on the Juno spacecraft. This builds on previous work from Melin et al (2020) who retrieved low-latitude abundances from Cassini UVIS data, which has a lower spatial resolution but higher spectral resolution than the Juno UVS observations.

Juno UVS

Reflected sunlight spectra

Radiative transfer and retrieval code

Latitudinal retrievals

Discussion and conclusions

Data Availability Statement