  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.
