Jupiter's Para-H$_2$ Distribution from SOFIA/FORCAST and Voyager/IRIS 17-37 $\mu$m Spectroscopy

TL;DR

This study maps Jupiter's para-H$_2$ distribution using SOFIA and Voyager spectra, revealing latitudinal variations and asymmetries, and discusses implications for hydrogen equilibration and atmospheric composition.

Contribution

It provides the first spatially resolved maps of Jupiter's para-H$_2$ distribution from far-infrared spectroscopy, comparing new SOFIA data with Voyager IRIS spectra to analyze latitudinal variations.

Findings

Para-H$_2$ increases from equator to pole.

High-$f_p$ air at high latitudes, especially in the north.

Para-H$_2$ distribution correlates with aerosols, not clouds or gases.

Abstract

Spatially resolved maps of Jupiter's far-infrared 17-37 $\mu$m hydrogen-helium collision-induced spectrum were acquired by the FORCAST instrument on the Stratospheric Observatory for Infrared Astronomy (SOFIA) in May 2014. Spectral scans in two grisms covered the broad S(0) and S(1) absorption lines, in addition to contextual imaging in eight broad-band filters (5-37 $\mu$m) with spatial resolutions of 2-4". The spectra were inverted to map the zonal-mean temperature and para-H$_2$ distribution ($f_p$, the fraction of the para spin isomer with respect to the ortho spin isomer) in Jupiter's upper troposphere (the 100-700 mbar range). We compared these to a reanalysis of Voyager-1 and -2 IRIS spectra covering the same spectral range. Para-H$_2$ increases from equator to pole, with low-$f_p$ air at the equator representing sub-equilibrium conditions (i.e., less para-H$_2$ than expected from thermal equilibration), and high-$f_p$ air and possible super-equilibrium at higher latitudes. In particular, we confirm the continued presence of a region of high-$f_p$ air at high northern latitudes discovered by Voyager/IRIS, and an asymmetry with generally higher $f_p$ in the north than in the south. We note that existing collision-induced absorption databases lack opacity from (H$_2$)$_2$ dimers, leading to under-prediction of the absorption near the S(0) and S(1) peaks. There appears to be no spatial correlation between para-H$_2$ and tropospheric ammonia, phosphine and cloud opacity derived from Voyager/IRIS at mid-infrared wavelengths (7-15 $\mu$m). We note, however, that para-H$_2$ tracks the similar latitudinal distribution of aerosols within Jupiter's upper tropospheric and stratospheric hazes observed in reflected sunlight, suggesting that catalysis of hydrogen equilibration within the hazes (and not the main clouds) may govern the equator-to-pole gradient. [Abridged]