New chemical scheme for giant planet thermochemistry

Abstract

Context. Several chemical networks have been developed to study warm (exo)planetary atmospheres. The kinetics of the reactions related to the methanol chemistry included in these schemes have been questioned. Aims. The goal of this paper is to update the methanol chemistry for such chemical networks based on recent publications in the combustion literature. We also aim to study the consequences of this update on the atmospheric compositions of (exo)planetary atmospheres and brown dwarfs. Methods. We performed an extensive review of combustion experimental studies and revisited the sub-mechanism describing methanol combustion in a scheme published in 2012. The updated scheme involves 108 species linked by a total of 1906 reactions. We then applied our 1D kinetic model with this new scheme to the case studies HD 209458b, HD 189733b, GJ 436b, GJ 1214b, ULAS J1335+11, Uranus, and Neptune; we compared these results with those obtained with the former scheme. Results. The update of the scheme has a negligible impact on the atmospheres of hot Jupiters. However, the atmospheric composition of warm Neptunes and brown dwarfs is modified sufficiently to impact observational spectra in the wavelength range in which James Webb Space Telescope will operate. Concerning Uranus and Neptune, the update of the chemical scheme modifies the abundance of CO and thus impacts the deep oxygen abundance required to reproduce the observational data. For future 3D kinetics models, we also derived a reduced scheme containing 44 species and 582 reactions. Conclusions. Chemical schemes should be regularly updated to maintain a high level of reliability on the results of kinetic models and be able to improve our knowledge of planetary formation.

Atmospheric chemistry on Uranus and Neptune

“…The uncertainty arises from observational difficulties and model dependencies in the fitting of the pressure-broadened wings of the CO absorption features. By contrast, the most stringent upper limit for tropospheric CO on Uranus is as low as 2.1 × 10 −9 [78]-a low value that presumably results from Uranus' expected less efficient tropospheric mixing (as supported by its low internal heat flux [103]) and potentially smaller intrinsic deep oxygen abundance [95,96]. Observational constraints on the tropospheric CO abundances on Uranus and Neptune have been used to constrain the deep O/H abundance on these planets, assuming that the CO derives from quenching from the deep interior [17,52,53,95,96,98].…”

“…By contrast, the most stringent upper limit for tropospheric CO on Uranus is as low as 2.1 × 10 −9 [78]-a low value that presumably results from Uranus' expected less efficient tropospheric mixing (as supported by its low internal heat flux [103]) and potentially smaller intrinsic deep oxygen abundance [95,96]. Observational constraints on the tropospheric CO abundances on Uranus and Neptune have been used to constrain the deep O/H abundance on these planets, assuming that the CO derives from quenching from the deep interior [17,52,53,95,96,98]. Early estimates via time-scale arguments [17,36,53,98] have given way to more complete thermochemical kinetics and transport models [52,95,96], based on those developed for Jupiter and extrasolar planets [99][100][101][102].…”

“…Observational constraints on the tropospheric CO abundances on Uranus and Neptune have been used to constrain the deep O/H abundance on these planets, assuming that the CO derives from quenching from the deep interior [17,52,53,95,96,98]. Early estimates via time-scale arguments [17,36,53,98] have given way to more complete thermochemical kinetics and transport models [52,95,96], based on those developed for Jupiter and extrasolar planets [99][100][101][102]. The full suite of these estimates suggests a deep oxygen enrichment relative to hydrogen on Neptune of 250-650 times solar, while the upper limit for Uranus is less than 45-260 times solar.…”

“…The full suite of these estimates suggests a deep oxygen enrichment relative to hydrogen on Neptune of 250-650 times solar, while the upper limit for Uranus is less than 45-260 times solar. Figure 2 shows the predicted vertical profiles of a few species in the deep troposphere of Uranus and Neptune from the nominal thermochemical kinetics and diffusion model of Venot et al [95], updated from Cavalié et al [96]. Both planets are assumed to have a deep-tropospheric eddy diffusion coefficient K zz = 10 8 cm 2 s −1 in this model, as estimated from mixing-length theory (for which K zz varies with the internal heat flux to the one-third power, so the factor of approximately 10 difference in the internal heat flux of the two planets translates to only a factor of approximately 2 difference in K zz , although it is possible that convection is inhibited 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 Figure 2.…”

Neptune and Uranus: ice or rock giants?

“…For this to happen, the mixing timescale must be much less than the loss timescale [44]. The required magnitude of O/H enrichment relative to solar composition depends on details of the chemical scheme, but ranges from 250 to 650 [44,85,86,88], with an O/H enrichment of approximately 250 being inferred in the most recent study [88].…”