Coupled 1D Chemical Kinetic-Transport and 2D Hydrodynamic Modeling Supports a modest 1-1.5x Supersolar Oxygen Abundance in Jupiter's Atmosphere

TL;DR

This work addresses Jupiter's deep atmospheric oxygen abundance by coupling a 1D thermochemical kinetic–transport model with a 2D hydrodynamic framework. It uses a rate-based chemical network from the Reaction Mechanism Generator (RMG) and pays special attention to the Hidaka reaction CH3OH + H → CH3 + H2O, deriving a quasi-steady $K_{ m zz}$ consistent with observed CO. The 1D results favor a subsolar O/H around $0.5$–$0.6\times Z_{\odot}$ under standard mixing, while the 2D SNAP simulations support a modest supersolar O/H of $1$–$1.5\times Z_{\odot}$ with an emergent $K_{ m zz}$ of $3\times10^{6}$–$5\times10^{7}$ cm$^2$ s$^{-1}$; a C/O ratio of about $2.9$ is implied. The integrated approach provides a physically grounded framework for constraining chemical and dynamical processes in giant-planet atmospheres and is applicable to exoplanets as well.

Abstract

Understanding the deep atmospheric composition of Jupiter provides critical constraints on its formation and the chemical evolution of the solar nebula. In this study, we combine one-dimensional thermochemical kinetic-transport modeling with two-dimensional hydrodynamic simulations to constrain Jupiter's deep oxygen abundance using carbon monoxide (CO) as a proxy tracer. Leveraging a comprehensive chemical network generated by Reaction Mechanism Generator (RMG), we assess the impact of updated reaction rates, including the often-neglected but thermochemically significant Hidaka reaction (CH3OH + H -> CH3 + H2O). Our 1D-2D coupled approach supports a modest supersolar oxygen enrichment of 1.0-1.5x the solar value. We also present a method for deriving Jupiter's eddy diffusion coefficient Kzz = 3e6 to 5e7 cm2/s) from 2D hydrodynamic simulations using the quasi steady-state approach. This method is applicable to exoplanet atmospheres, where Kzz remains highly uncertain despite its strong influence on atmospheric chemistry. Finally, our results imply a significantly elevated planetary carbon-to-oxygen (C/O) ratio of ~2.9, highlighting the importance of clarifying the mechanisms behind the preferential accretion of carbon-rich material during Jupiter's formation. By integrating thermochemical and hydrodynamic processes, our study offers a more complete framework for constraining chemical and dynamical processes in (exo)planetary atmospheres.

Figures (11)

• Figure 1: The temperature dependence of the rate coefficients for the reaction CH3OH + H $\rightarrow$CH3 + H2O from various references. The solid blue line represents the original rate coefficient reported by hidaka1989thermal; the blue dashed line corresponds to an incorrect version of this rate listed in the NIST database NIST_2020; the red solid line shows the rate coefficient calculated by moses2011disequilibrium; the lime solid line represents the rate calculated by sanches2017novel using the d-TST method; and the grey dotted line shows the collision limit calculated using the equation (1) from chen2017_collision_limit for reference.
• Figure 1: The temperature-pressure ($T$–$P$) and eddy diffusion coefficient ($K_{\text{zz}}$) profiles adopted in the current study. The red solid line represents the $T$–$P$ profile from seiff1998thermaljupitersimon2006jupiter, while the black solid line shows the $K_{\text{zz}}$ profile from moses2005photochemistry. In this work, the deep atmospheric $K_{\text{zz}}$ value (uniform at pressures $P\gtrsim8$ bar) has been varied from $5\times10^6$ to $10^9$ cm$^2$/s, with a nominal value of $10^8$ cm$^2$/s shown in this figure.
• Figure 2: Vertical mixing‐ratio profiles of carbon monoxide ([CO]/[H2]) in Jupiter’s atmosphere for various oxygen abundances O/H (Section \ref{['subsec:elemental']}): (a) $2.3\times Z_{\odot}$, (b) $1.5\times Z_{\odot}$, (c) $0.6\times Z_{\odot}$, and (d) $0.3\times Z_{\odot}$. In each panel, we vary the eddy diffusion coefficient $K_{\rm zz}$ [cm$^2$/s] and adopt the Hidaka reaction rate coefficient from either moses2011disequilibrium (nominal) or sanches2017novel (indicated as d-TST) Panel (d) additionally shows the CO profile simulated with the Hidaka reaction omitted from the chemical network described in Section \ref{['subsec:chemical_network']}. The red square with error bars indicates the observed upper‐tropospheric CO mixing ratio from bezard2002carbon, with uncertainties from bjoraker2018gas. The light blue shaded region indicates the water cloud decks between 4 and 10 bars.
• Figure 2: Vertical CO mole fraction profiles ($X_{\rm CO}$) in Jupiter’s troposphere were computed under identical conditions ($K_{\rm zz}=10^9$ [cm$^2$/s]; O/H = 7$\times Z_\odot$; He, C, N, S as in Section \ref{['subsec:elemental']}), using various chemical networks. The RMG network from this study (green), V20 network venot2020new (blue), and V12 network venot2012chemical (red) show close agreement down to $\sim$1200 K, below which vertical mixing dominates. The green dotted line shows the RMG network with the Hidaka reaction rate updated from moses2011disequilibrium to the dTST value from sanches2017novel. The gray shaded region reflects the range of network predictions in Wang_2016_Jupiter. While overall differences are modest, the RMG network predicts slightly more efficient CO-to-CH4 conversion than V20 and earlier networks under these oxygen-rich conditions.
• Figure 3: Major reaction pathways at the CO quenching point ($P \sim 505$ bar, $T \sim 1050$ K) for CO–CH4 conversion under O/H=$1.5\times Z_{\odot}$ and $K_{\rm zz}=10^{7}$ [cm$^2$/s]. Red numbers indicate branching ratios for species destruction. For example, 0.06 shows that 6% of CO destruction occurs via the CO+H2$\rightarrow$H2CO reaction, with the remainder (94%) occurring via CO+H$\rightarrow$HCO. Therefore, branching ratios sum to 1 (100%). The black numbers on the far right indicate the logarithm of the absolute total rate [molecules/cm$^3$/s] of reaction pathways included in the light beige-shaded bars. For example, a value of 6.783 indicates a total HCO destruction rate of 10$^{6.783}$ [molecules/cm$^3$/s], representing the combined rates of the reactions HCO+H2$\rightarrow$H2CO+H and HCO+H2S$\rightarrow$H2CO+SH. The green highlighted region identifies the Hidaka reaction, discussed in detail in Section \ref{['subsec:hidaka_reaction']}.