Detection of a hydrogen corona at Callisto

Abstract

In December 2001, the Space Telescope Imaging Spectrograph of the Hubble Space Telescope obtained far‐ultraviolet spectral images of Jupiter's moon Callisto. The leading and trailing hemispheres were observed in the spectral range 1190 Å to 1720 Å when the moon was at eastern and western elongations, respectively. We analyzed the hydrogen Lyman α (1216 Å) signal in the two observations and found that faint atmospheric emissions extending up to several moon radii away are present in addition to the solar flux reflected off the surface. We show that the detected atmospheric Lyman α emissions are consistent with an escaping hydrogen corona with a vertical column density in the range of (6–12) ×1011 cm−2. The derived hydrogen abundance is about 2 times higher when the moon is at eastern elongation, possibly related to increased water sublimation when the visibly darker leading hemisphere is illuminated by the Sun. The detected hemispheric difference is larger than the measurement uncertainties providing first evidence for variations in Callisto's neutral atmosphere.

Callisto's Atmosphere: First Evidence for H2 and Constraints on H2O

“…We estimate an uncertainty of up to 30%, factoring in, e.g., the variability of the solar intensity and simplifications about the resonant scattering properties (neglecting all optical thickness effects). Comparing the g-factor from Roth et al (2017a) to the scaled Lyman-alpha g-factors from Killen et al (2009), the latter are about 25% lower (when scaled with distance from the Sun and daily solar flux) confirming our estimated uncertainty. This ≤30% uncertainty translates linearly to the derived abundances everywhere and therefore does not affect our conclusions on the roles of H 2 O and H 2 for producing the observed corona profiles on and off the disk.…”

“…As originally suggested in Roth et al (2017a), a fit to the HST/STIS observation can be generated by a scenario with a nearly uniformly distributed source of H, as is the case in our simulations with H 2 . Therefore, we considered other possible sources for the observed H. Using the plasma parameters from Vorburger et al ( 2019) (see Tables D3 and D4 in Appendix D), proton charge-exchange with all atmospheric species considered in the models as well as with the observed CO 2 and O components produces a negligible source rate of H. In addition, Vorburger et al (2015) assumed H was produced via sputtering from hydrated nonice surface materials; however, the peak number densities they obtained were ∼10° cm −3 (Figure 3 therein).…”

“…Since O 2 does not freeze out on the surface, even on the night-side, it permeates the porous regolith and enriches the atmosphere limited primarily by reactions in the regolith and gas-phase ionizing and dissociative processes. An H corona was detected by the Hubble Space Telescope (HST; Roth et al, 2017a), which was suggested to be produced via photolytic or electron-impact dissociation of sublimated H 2 O or, possibly, radiolytically produced H 2 , with a very small contribution due to direct sputtering from its icy surface. Carberry Mogan et al (2021b), hereafter referred to as "CM21," showed that even though radiolytically produced H 2 can have a peak-density orders of magnitude less than that of the sublimated H 2 O component, it can be the primary producer of H near and beyond the terminator.…”

“…The importance of the interrelated surface and atmospheric processes at Callisto is exemplified by its O 2 atmosphere (Cunningham et al., 2015) and its H corona (Roth et al., 2017a). The former is likely produced via radiolysis in Callisto's icy surface (e.g., Johnson, 1990) as well as, to a much less extent, via a series of photochemical reactions of sublimated H 2 O (e.g., Yung & McElroy, 1977).…”

Callisto's Atmosphere and Its Space Environment: Prospects for the Particle Environment Package on Board JUICE

“…However, even this reduced estimate for the density of Callisto's atmosphere (column density ≥10 15 cm −2 ) is exceeded among other solar system satellites only by the atmospheres on Io, Triton, and Titan (Cunningham et al, 2015). Faint atmospheric emissions above Callisto's limb were recently detected from HST-Space Telescope Imaging Spectrograph observations (Roth et al, 2017), likely originating from resonant scattering by an H corona. This corona was suggested to be produced via dissociation of sublimated H 2 O and sputtered or radiolytically produced H 2 (e.g., Carberry Mogan et al, 2021) from Callisto's icy surface.…”