Saturn's Formation at the Carbon Dioxide Iceline

Overview & Synopsis

Saturn's bulk metallicity and atmospheric volatile enrichments exceed those of Jupiter. Combined with their similar heavy-element masses, this suggests that Saturn formed farther out in a protoplanetary disk whose metallicity increased with heliocentric distance. Numerous models argue that Jupiter formed near the H$_2$O iceline; Saturn's composition is therefore expected to reflect accretion beyond this region. We test whether Saturn's elemental enrichments can be reproduced by formation near the CO$_2$ or CO icelines using a one-dimensional viscous $α$-disk model that tracks the transport, sublimation, and recondensation of 13 volatile carriers. We compute time-dependent radial distributions of solids and vapors for $α= 5\times10^{-4}$--$5\times10^{-3}$ and varied CO/CO$_2$ abundance ratios, and compare the resulting metallicities and enrichments with Saturn's observed composition. Strong carbon and oxygen enrichment peaks result from the cold-finger effect, most prominently at the CO$_2$ iceline, where supersolar metallicities comparable to Saturn's envelope are achieved only for $α\le 10^{-3}$. Smaller C and O enrichment peaks also occur at the CO iceline, but the local protosolar nebula metallicity there never reaches Saturn's inferred value. At $\sim$0.2--0.3 Myr, carbon and phosphorus enrichments in this region match Saturn's atmospheric values, while noble-gas abundances remain near protosolar owing to their long diffusion times. These results identify material originating near the CO$_2$ iceline as the most likely source of Saturn's heavy-element inventory. For the conditions adopted in our model, our results imply that Saturn accreted this material early ($\lesssim0.3$ Myr), in a weakly turbulent disk ($α\lesssim10^{-3}$) with a CO$_2$-rich volatile inventory.