1D photochemical model of the ionosphere and the stratosphere of Neptune

TL;DR

This study develops a 1D photochemical model of Neptune's atmosphere, revealing ion-neutral chemistry's role in producing hydrocarbons and oxygen species, aligning well with observations and aiding understanding of Neptune's composition.

Contribution

The paper introduces a coupled ion-neutral 1D photochemical model for Neptune, incorporating updated chemistry from Titan's atmosphere, to explain observed atmospheric constituents and ionospheric features.

Findings

Ion-neutral chemistry produces aromatics like benzene in Neptune's atmosphere.

Model results agree well with Cassini observations.

Two ionospheric peaks are identified above 10^{-5} mbar and 10^{-3} mbar.

Abstract

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.