  The flybys of Jupiter by the Voyager spacecraft in 1979, and over two decades
later by Cassini in 2000, have provided us with unique datasets from two
different epochs, allowing the investigation of seasonal change in the
atmosphere. In this paper we model zonal averages of thermal infrared spectra
from the two instruments, Voyager 1 IRIS and Cassini CIRS, to retrieve the
vertical and meridional profiles of temperature, and the abundances of the two
minor hydrocarbons, acetylene (C2H2) and ethane (C2H6). The spatial variation
of these gases is controlled by both chemistry and dynamics, and therefore
their observed distribution gives us an insight into both processes. We find
that the two gases paint quite different pictures of seasonal change. Whilst
the 2-D cross-section of C2H6 abundance is slightly increased and more
symmetric in 2000 (northern summer solstice) compared to 1979 (northern fall
equinox), the major trend of equator to pole increase remains. For C2H2 on the
other hand, the Voyager epoch exhibits almost no latitudinal variation, whilst
the Cassini era shows a marked decrease polewards in both hemispheres. At the
present time, these experimental findings are in advance of interpretation, as
there are no published models of 2-D Jovian seasonal chemical variation
available for comparison.
