  Neptune remains a mysterious world that deserves further exploration and is a
high-priority objective for a future planetary mission in order to better
understand the formation and evolution of ice giant planets. We have developed
a coupled ion-neutral 1D photochemical model of Neptune's atmosphere to study
the origin and evolution of the hydrocarbons and the oxygen species. The
up-to-date chemical scheme is derived from one used for Titan's atmosphere,
which led to good agreements with the Cassini-CIRS observations for oxygen
species and the main hydrocarbons. The main results we obtain are the
following: The ion-neutral chemistry coupling produces aromatics (and benzene
in particular) in the atmosphere of Neptune with relatively high abundances.
Our model results are in good agreement with observations (taking model
uncertainties into account). Two ionospheric peaks are present in the
atmosphere located above the pressure level of 10$^{-5}$ mbar and around
10$^{-3}$ mbar. The influx of oxygen species in the upper atmosphere of Neptune
has an effect on the concentration of many ions. We show that in situ
exploration of Neptune's atmosphere would provide very interesting constraints
for photochemical models concerning in particular the origin of oxygen species
and the contribution of ion chemistry. A precise description of upper
atmospheric chemistry is crucial for a better understanding of the internal
composition and the formation processes of this planet.
