Ion-neutral chemistry at Ganymede

The Galileo spacecraft flew by Ganymede, down to 0.1 R_G from the surface for the closest, six times, giving us insight into its plasma environment. Its ionosphere, made of ions born from the ionisation of neutrals present in Ganymede's exosphere, represents the bulk of the plasma near the moon around closest approach. Galileo and Juno revealed that near the closest approach, the ion population is dominated by low-energy ions from the water ion group (O⁺, HO⁺, H₂O⁺) and O₂⁺. As we showed in [1] using a collisionless test particle model, the ion composition during most flybys was a priori dominated by H₂⁺and O₂⁺. However, during Juno's flyby of Ganymede, plasma data revealed the additional presence of H₃⁺ that may only stem from ion-neutral reactions between H₂ and H₂⁺. 

 

We have updated our test particle model to account for these ion-neutral collisions of which H₂ + H₂⁺. We show how it modifies the ion composition compared with [1] and assess the role of these collisions in the production of new ion species within Ganymede's exo-ionosphere. We highlight that the ion composition exhibits asymmetries mainly dayside/nightside and jovian/anti-jovian.  This will help to interpret plasma observations made by Juno and in the future by JUICE around Ganymede.

 

[1] Beth et al., EGU24, https://doi.org/10.5194/egusphere-egu24-11772, 2024