  The subsurface oceans of icy satellites are among the most compelling among
the potentially habitable environments in our Solar System. The question of
whether a liquid subsurface layer can be maintained over geological timescales
depends on its chemical composition. The composition of icy satellites is
linked to that of the circumplanetary disk (CPD) in which they form. The CPD
accretes material from the surrounding circumstellar disk in the vicinity of
the planet, however, the degree of chemical inheritance is unclear. We aim to
investigate the composition of ices in chemically reset or inherited
circumplanetary disks to inform interior modeling and the interpretation of in
situ measurements of icy solar system satellites, with an emphasis on the
Galilean moon system. We used a radiation-thermochemical code to produce
circumplanetary disk models and extract the ice composition from time-dependent
chemistry, incorporating gas-phase and grain-surface reactions. The initial
sublimation of ices during accretion may result in a CO2-rich ice composition.
Sublimated ammonia ice is destroyed by background radiation while drifting
towards the CPD midplane. Liberated nitrogen becomes locked in N2 due to
efficient self-shielding, leaving ices depleted of ammonia. A significant
ammonia ice component remains only when ices are inherited from the
circumstellar disk. The observed composition of the Galilean moons is
consistent with the sublimation of ices during accretion onto the CPD. In this
scenario, the Galilean moon ices are nitrogen-poor and CO2 on Callisto is
endogenous and primordial. The ice composition is significantly altered after
an initial reset of accreted circumstellar ice. The chemical history of the
Galilean moons stands in contrast to the Saturnian system, where the
composition of the moons corresponds more closely with the directly inherited
circumstellar disk material.
