The Enigmatic Abundance of Atomic Hydrogen in Saturn’s Upper Atmosphere

Abstract

A planet’s Lyα emission is sensitive to its thermospheric structure. Here we report joint Hubble Space Telescope and Cassini cross-calibration observations of the Saturn Lyα emission made 2 weeks before the Cassini grand finale. To investigate the long-term Saturn Lyα airglow observed by different ultraviolet instruments, we cross-correlate their calibration, finding that while the official Cassini/UVIS sensitivity should be lowered by ∼75%, the Voyager 1/UVS sensitivities should be enhanced by ∼20% at the Lyα channels. This comparison also allowed us to discover a permanent feature of the Saturn disk Lyα brightness that appears at all longitudes as a brightness excess (Lyα bulge) of ∼30% (∼12σ) extending over the latitude range ∼5°–35° N compared to the regions at equator and ∼60° N. This feature is confirmed by three distinct instruments between 1980 and 2017 in the Saturn north hemisphere. To analyze the Lyα observations, we use a radiation transfer model of resonant scattering of solar and interplanetary Lyα photons and a latitude-dependent photochemistry model of the upper atmosphere constrained by occultation and remote-sensing observations. For each latitude, we show that the Lyα observations are sensitive to the temperature profile in the upper stratosphere and lower thermosphere, thus providing useful information in a region of the atmosphere that is difficult to probe by other means. In the Saturn Lyα bulge region, at latitudes between ∼5° and ∼35°, the observed brightening and line broadening support seasonal effects, variation of the temperature vertical profile, and potential superthermal atoms that require confirmation.

Seasonal Variation of Saturn's Lyα Brightness

“…The revised Voyager Lyα brightnesses are roughly consistent with those observed by Cassini/UVIS (Koskinen et al 2020). Using HST observations from an Earth orbit as a calibration standard, Ben-Jaffel et al (2023) challenged the downward revision of the Voyager brightnesses and proposed instead a recalibration of the Cassini/ UVIS instrument at Lyα that would increase the observed brightnesses by 80%. After scaling with the solar Lyα flux at different times, they compared several observations by Cassini/ UVIS (in 2007, 2013, and 2014), HST/STIS (in 2017), and HST's Goddard High Resolution Spectrograph (in 1996) to arrive at this conclusion.…”

“…In addition to scattered solar flux, a galactic contribution to the background of 40 R has been identified at large heliocentric distances (Gladstone et al 2021;Pryor et al 2022). While there remains uncertainty on the density of the LISM, we compare Cassini/UVIS Lyα observations to the model of Quémerais et al (2013a) and find good agreement between Cassini/UVIS observed and modeled brightnesses, without the proposed recalibration by a factor of 1.7 (Ben-Jaffel et al 2023;Pryor et al 2024). Limb observations and solar occultations, in conjunction with photochemical models, can provide further constraints on the atomic hydrogen columns, Lyα emissions.…”

“…We note that this bulge shows no longitudinal variation and therefore differs significantly from the Lyα bulge that has been observed on Jupiter, which is fixed in system III longitude (Dessler et al 1981;Skinner et al 1988). Ben-Jaffel et al (2023) proposed two primary mechanisms to drive the emission bulge: a variation of the temperature profile in the lower thermosphere and upper stratosphere or a previously unidentified suprathermal atomic hydrogen population at high altitudes, both of which could vary seasonally. They proposed that the suprathermal population could be created, for example, by a significant influx of material from the rings or Enceladus into the upper atmosphere.…”

“…Recently, Ben-Jaffel et al (2023) identified a bulge in Lyα emissions from Saturn's thermosphere in the northern hemisphere between latitudes of 5°and 35°N, with observations from the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) and Cassini/UVIS. They also identified the same bulge in Voyager/UVS observations that probed Saturn's atmosphere close to the northern spring equinox 35 yr earlier (Yelle et al 1986).…”

Modeling Cassini UVIS Interplanetary Hydrogen Lyα Observations from 1999 to 2017

“…There are still differing opinions on the interstellar hydrogen density; for example, models presented in Katushkina et al (2017) used an interstellar value of 0.14 cm −3 , as did Model 3 of Izmodenov & Alexashov (2015), lower than the estimate of 0.21 cm −3 just found here. Now that we have an estimate for the decline in Cassini UVIS sensitivity at Lyα, it will be interesting to see how this affects studies of the Saturn Lyα dayglow from UVIS observations (e.g., Ben-Jaffel et al 2023).…”