HST Detection of H 2 Raman Scattering in the Jovian Atmosphere

Abstract

Raman scattered features by molecular hydrogen have been detected in Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) observations of Jupiter. The measurements were obtained with the G190H grating and red detector combination spanning 158.0–232.0 nm at about 0.3 nm resolution. The data were corrected for scattered light, and careful modeling of the line spread function (LSF) of the instrument was performed to accurately degrade the solar spectrum obtained by SOLSTICE (solar-stellar irradiance comparison experiment) to the spectral resolution of the FOS. A cross-correlation method was used to align features in the planetary spectra to those in the SOLSTICE solar spectrum. At all latitudes longward of 210.0 nm, the resulting I/F displayed discrete features up to 20% of the continuum level that anticorrelate with the solar spectrum. A radiative transfer code was developed to include the effect of rotational and vibrational multiple Raman scattering for the first few lowest energy rotational states of molecular hydrogen under the approximation that the Raman component of the scattering phase function is isotropic. Simulations show not only that the detected features are indeed due to Raman scattering by H 2, but are sensitive to its ortho–para ratio as well. An analysis of the equatorial spectrum reveals that the features are consistent with an equilibrium or normal population of H 2 at 130 K.