Hydrogen Dimers in Giant-planet Infrared Spectra

Abstract

Despite being one of the weakest dimers in nature, low-spectral-resolution Voyager/IRIS observations revealed the presence of (H 2 ) 2 dimers on Jupiter and Saturn in the 1980s. However, the collision-induced H 2 -H 2 opacity databases widely used in planetary science (Borysow et al. 1985;Orton et al. 2007;Richard et al. 2012) have thus far only included free-to-free transitions and have neglected the contributions of dimers. Dimer spectra have both fine-scale structure near the S(0) and S(1) quadrupole lines (354 and 587 cm −1 , respectively), and broad continuum absorption contributions up to ±50 cm −1 from the line centres. We develop a new ab initio model for the free-to-bound, bound-to-free and bound-to-bound transitions of the hydrogen dimer for a range of temperatures (40-400 K) and para-hydrogen fractions (0.25-1.0). The model is validated against low-temperature laboratory experiments, and used to simulate the spectra of the giant planets. The new collision-induced opacity database permits high-resolution (0.5-1.0 cm −1 ) spectral modelling of dimer spectra near S(0) and S (1) in both Cassini Composite Infrared Spectrometer (CIRS) observations of Jupiter and Saturn, and in Spitzer Infrared Spectrometer (IRS) observations of Uranus and Neptune for the first time. Furthermore, the model reproduces the dimer signatures observed in Voyager/IRIS data near S(0) (McKellar 1984) on Jupiter and Saturn, and generally lowers the amount of para-H 2 (and the extent of disequilibrium) required to reproduce IRIS observations.

ARES. II. Characterizing the Hot Jupiters WASP-127 b, WASP-79 b, and WASP-62b with the Hubble Space Telescope*

“…In this study we use the plane-parallel approximation to model the atmospheres, with pressures ranging from 10 −2 to 10 6 Pa, uniformly sampled in log-space with 100 atmospheric layers. We included the Rayleigh scattering and the collisioninduced absorption (CIA) of H 2 -H 2 and H 2 -He (Abel et al 2011(Abel et al , 2012Fletcher et al 2018). A summary of the fitted retrieval parameters is given in Table 1.…”

WASP-117 b: An Eccentric Hot Saturn as a Future Complex Chemistry Laboratory

“…We included in our simulations the contribution of trace gases whose opacities were taken from the ExoMol (Tennyson et al 2016) (Rothman et al 2010). Additionally, we included the collision-induced absorption (CIA) from H 2 -H 2 (Abel et al 2011;Fletcher et al 2018) and H 2 -He (Abel et al 2012), as well as Rayleigh scattering for all molecules. In our retrieval analysis, we used uniform priors for all parameters as described in Table 2.…”

ARES. III. Unveiling the Two Faces of KELT-7 b with HST WFC3*

“…In this study we use the plane-parallel approximation to model the atmospheres, with pressures ranging from 10 −2 to 10 6 Pa, uniformly sampled in log-space with 100 atmospheric layers. We included the Rayleigh scattering and the collision induced absorption (CIA) of H 2 -H 2 and H 2 -He (Abel et al 2011(Abel et al , 2012Fletcher et al 2018). Constant molecular abundance profiles were used, and allowed to vary freely between 10 −12 and 10 −1 in the volume mixing ratio.…”