Jupiter in the Ultraviolet: Acetylene and Ethane Abundances in the Stratosphere of Jupiter from Cassini Observations between 0.15 and 0.19 μm

Henrik Melin,P G J Irwin,S G Edgington,L N Fletcher

doi:10.3847/1538-3881/ab91a6

Abstract

Abstract At wavelengths between 0.15 and 0.19 μm, the far-ultraviolet spectrum of Jupiter is dominated by the scattered solar spectrum, attenuated by molecular absorptions primarily by acetylene and ethane, and to a lesser extent ammonia and phosphine. We describe the development of our radiative transfer code that enables the retrieval of abundances of these molecular species from ultraviolet reflectance spectra. As a proof-of-concept we present an analysis of Cassini Ultraviolet Imaging Spectrograph (UVIS) observations of the disk of Jupiter during the 2000/2001 flyby. The ultraviolet-retrieved acetylene abundances in the upper stratosphere are lower than those predicted by models based solely on infrared thermal emission from the mid-stratosphere observed by the Composite Infrared Spectrometer (CIRS), requiring an adjustment to the vertical profiles above 1 mbar. We produce a vertical acetylene abundance profile that is compatible with both CIRS and UVIS, with reduced abundances at pressures <1 mbar: the 0.1 mbar abundances are 1.21 ± 0.07 ppm for acetylene and 20.8 ± 5.1 ppm for ethane. Finally, we perform a sensitivity study for the JUICE ultraviolet spectrograph, which has extended wavelength coverage out to 0.21 μm, enabling the retrieval of ammonia and phosphine abundances, in addition to acetylene and ethane.

Highlights

Ultraviolet photons are unable to penetrate the atmosphere of the Earth, and so we need instruments mounted on spacecraft to observe these photons both reflected and emitted by the giant planets
We have updated the NEMESIS radiative transfer code to model and retrieve atmospheric abundances from ultraviolet reflectance spectra, and we have identified a set of far ultraviolet (FUV) observations with which we can test these model developments against
We developed the ability of our NEMESIS radiative transfer and retrieval code to model and fit ultraviolet reflectance spectra from Jupiter between 0.15 and 0.19 μm

Summary

Introduction

Ultraviolet photons are unable to penetrate the atmosphere of the Earth, and so we need instruments mounted on spacecraft to observe these photons both reflected and emitted by the giant planets. The ultraviolet reflectance spectrum of Jupiter, observed long-ward of ∼0.15 μm, is dominated by Rayleigh-Raman scattered sunlight from the species contained within the atmosphere, mainly H2, He, CH4, and NH3. This reflectance spectrum is attenuated by wavelength-dependent absorptions by molecules with the amount of absorption being governed by the ultraviolet absorption cross-section, multiplied by the volumetric abundance at a particular altitude. Aerosols will scatter light away from and into the line-of-sight which could potentially significantly reduce the observed radiance, dependent on their properties

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