Jupiter in the ultraviolet: acetylene and ethane abundances in the stratosphere of Jupiter from Cassini observations between 0.15 and 0.19 $\mu$m

TL;DR

This study develops a radiative transfer model to analyze ultraviolet spectra of Jupiter, retrieving molecular abundances of acetylene and ethane, and explores the potential for future measurements of ammonia and phosphine with extended wavelength coverage.

Contribution

The paper introduces a new radiative transfer code for UV spectra analysis and demonstrates its application to Cassini data, refining molecular abundance profiles in Jupiter's stratosphere.

Findings

UV-retrieved acetylene abundances are lower than infrared model predictions.

A compatible vertical abundance profile for acetylene and ethane is produced.

Sensitivity analysis indicates JUICE UVS can measure additional molecules like ammonia and phosphine.

Abstract

At wavelengths between 0.15 and 0.19 $\mu$m, 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 $1.21 \pm 0.07$ ppm for acetylene and $20.8 \pm 5.1$ ppm for ethane. Finally, we perform a sensitivity study for the JUICE UVS instrument, which has extended wavelength coverage out to 0.21 $\mu$m, enabling the retrieval of ammonia and phosphine abundances, in addition to acetylene and ethane.