Composition of Early Planetary Atmospheres I: Connecting Disk Astrochemistry to the Formation of Planetary Atmospheres

Abstract

We present a model of the early chemical composition and elemental abundances of planetary atmospheres based on the cumulative gaseous chemical species that are accreted onto planets forming by core accretion from evolving protoplanetary disks. The astrochemistry of the host disk is computed using an ionization driven, non-equilibrium chemistry network within viscously evolving disk models. We accrete gas giant planets whose orbital evolution is controlled by planet traps using the standard core accretion model and track the chemical composition of the material that is accreted onto the protoplanet. We choose a fiducial disk model and evolve planets in 3 traps - water ice line, dead zone and heat transition. For a disk with a lifetime of 4.1 Myr we produce two Hot Jupiters (M = 1.43, 2.67 M$_{\rm Jupiter}$, r = 0.15, 0.11 AU) in the heat transition and ice line trap and one failed core (M = 0.003 M$_{\rm Jupiter}$, r =3.7 AU) in the dead zone. These planets are found with mixing ratios for CO and H$_2$O of $1.99\times 10^{-4}$, $5.0\times 10^{-4}$ respectively for both Hot Jupiters. Additionally for these planets we find CO$_2$ and CH$_4$, with mixing ratios of $1.8\times 10^{-6}\rightarrow 9.8\times 10^{-10}$ and $1.1\times 10^{-8}\rightarrow 2.3\times 10^{-10}$ respectively. These ranges correspond well with the mixing ratio ranges that have been inferred through the detection of emission spectra from Hot Jupiters by multiple authors. We compute a carbon-to-oxygen ratio of 0.227 for the ice line planet and 0.279 for the heat transition planet. These planets accreted their gas inside the ice line, hence the sub-solar C/O.