  We investigate the enrichment patterns of several delivery scenarios of the
volatiles to the atmospheres of ice giants, having in mind that the only well
constrained determination made remotely, i.e. the carbon abundance measurement,
suggests that their envelopes possess highly supersolar metallicities, i.e.
close to two orders of magnitude above that of the protosolar nebula. In the
framework of the core accretion model, only the delivery of volatiles in solid
forms (amorphous ice, clathrates, pure condensates) to these planets can
account for the apparent supersolar metallicity of their envelopes. In
contrast, because of the inward drift of icy particles through various
snowlines, all mechanisms invoking the delivery of volatiles in vapor forms
predict subsolar abundances in the envelopes of Uranus and Neptune.
Alternatively, even if the disk instability mechanism remains questionable in
our solar system, it might be consistent with the supersolar metallicities
observed in Uranus and Neptune, assuming the two planets suffered subsequent
erosion of their H-He envelopes. The enrichment patterns derived for each
delivery scenario considered should be useful to interpret future in situ
measurements by atmospheric entry probes.
