Probing Ganymede's atmosphere with HST Lyα\alpha images in transit of Jupiter

We report results from far-ultraviolet observations by the Hubble Space Telescope of Jupiter's largest moon Ganymede transiting across the planet's dayside hemisphere. {Within} a targeted campaign on 9 September 2021 two exposures were taken during one transit passage to probe for attenuation of Jupiter's hydrogen Lyman-$\alpha$ dayglow above the moon limb. The background dayglow is slightly attenuated over an extended region around Ganymede, with stronger attenuation in the second exposure when Ganymede was near the planet's center. In the first exposure when the moon was closer to Jupiter's limb, the effects from the Ganymede corona are hardly detectable, likely because the Jovian Lyman-$\alpha$ dayglow is spectrally broader and less intense at this viewing geometry. The obtained vertical H column densities of around $(1-2)\times 10^{12}$~cm$^{-2}$ are consistent with previous results. Constraining angular variability around Ganymede's disk, we derive an upper limit on a local H$_2$O column density of $(2-3)\times 10^{16}$~cm$^{-2}$, such as could arise from outgassing plumes in regions near the observed moon limb

Mid-ultraviolet Hubble Observations of Europa and the Global Surface Distribution of SO₂

We present spatially resolved reflectance spectra of Europa’s surface in the wavelength range of 210–315 nm obtained by the Hubble Space Telescope Imaging Spectrograph in 2018 and 2019. These data provide the first high-quality, near-global spectral observations of Europa from 210 to 240 nm. They show that the reflectance of Europa’s leading, trailing, anti-Jovian, and sub-Jovian hemispheres is ∼5% near 210 nm, with varying spectral slopes across the mid-UV. This low albedo, even on the more “pristine” leading hemisphere, indicates a lack of the signature far-UV spectral edge characteristic of water ice. We detected and mapped a strong absorption feature at 280 nm that is consistent with an S–O bond that has previously been attributed to SO2 on the surface, hypothesized to be formed through radiolytic processing of Iogenic sulfur ions that have been preferentially emplaced on Europa’s trailing hemisphere by Jupiter’s magnetic field. Our models show that small inclusions of SO2 (0.1%) within the water ice are sufficient to produce the 280 nm feature without producing a feature at 4.07 μm, which has not been observed in ground-based spectral observations of Europa. This data set is the first to produce a spatially resolved, near-global map of the assumed SO2 feature, which is primarily concentrated near the apex of the trailing hemisphere and correlated with large-scale darker regions in both the visible and the ultraviolet. This distribution is consistent with “cold” exogenic sulfur ion bombardment on Europa

Isolating the 130.4 nm and 135.6 nm emissions in Ganymede’s aurora using broadband optics

We discuss a technique for isolating the two main Far Ultraviolet emission lines in Ganymede’s aurora by adding flight proven transmission filters to a broad- band, wide-field imager design. We find that the ra- tio of OI emissions at 135.6 nm and 130.4 nm can be recovered if the transmission of the filters and other optical elements are well known. This ratio allows constraints to be placed on the relative abundances of O atoms and O2 molecules within Ganymede’s at- mosphere, leading to more accurate models of atmo- spheric composition

New constraints on Ganymede's hydrogen corona: Analysis of Lyman-alpha emissions observed by HST/STIS between 1998 and 2014

Far-ultraviolet observations of Ganymede's atmospheric emissions were obtained with the Space Telescope Imaging Spectrograph (STIS) onboard of the Hubble Space Telescope (HST) on several occasions between 1998 and 2014. We analyze the Lyman-alpha emission from four HST campaigns in order to constrain the abundance and variation of atomic hydrogen in Ganymede's atmosphere. We apply a forward model that estimates surface reflection and resonant scattering in an escaping corona of the solar Lyman-alpha flux, taking into account the effects of the hydrogen in the interplanetary medium. The atmospheric emissions around Ganymede's disk derived for the observations taken between 1998 and 2011 are consistent with a hydrogen corona in the density range of (5-8) x 10(3) cm(-3) at the surface. The hydrogen density appears to be generally stable in that period. In 2014, Ganymede's corona brightness is approximately 3 times lower during two observations of Ganymede's trailing hemisphere and hardly detectable at all during two observations of the leading hemisphere. We also investigate extinction of Ganymede's coronal emissions in the Earth's upper atmosphere or geocorona. For small Doppler shifts, resonant scattering in the geocorona of the moon corona emissions can effectively reduce the brightness observed by HST. In the case of the 2014 leading hemisphere observations, an estimated extinction of 80% might explain the non-detection of Ganymede's hydrogen corona. Geocoronal extinction might also explain a previously detected hemispheric difference from Callisto's hydrogen corona

Autofluorescence Signatures of Seven Pathogens: Preliminary in Vitro Investigations of a Potential Diagnostic for Acanthamoeba Keratitis

PURPOSE: Acanthamoeba keratitis can cause devastating damage to the human cornea and is often difficult to diagnose by routine clinical methods. In this preliminary study, we investigated whether Acanthamoeba may be distinguished from other common corneal pathogens through its autofluorescence response. Although only a small number of pathogens were studied, the identification of a unique Acanthamoeba signature would indicate that autofluorescence spectroscopy as a diagnostic method merits further investigation. METHODS: Samples of 7 common pathogens (Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, Elizabethkingia miricola, Achromobacter ruhlandii, Candida albicans, and Acanthamoeba castellanii) in solution were excited with ultraviolet light at a number of successive, narrow wavebands between 260 and 400 nm, and their fluorescence response recorded. Principal Component Analysis was used to allow better visualization of the differences in response to UV light for different species. RESULTS: Acanthamoeba was found to possess a characteristic autofluorescence response and was easily distinguished from E. coli, S. aureus, P. aeruginosa, E. miricola, A. ruhlandii, and C. albicans over a wide range of excitation wavelengths. We also found a clear discrimination between E. coli, C. albicans, and P. aeruginosa at an excitation wavelength of 274 nm, whereas E. miricola, S. aureus, and A. ruhlandii could be separated using an excitation wavelength of 308 nm. CONCLUSIONS: Our results, although preliminary, indicate that autofluorescence spectroscopy shows promise as a diagnostic technique for keratitis. We intend to expand the set of pathogens studied before assessing the feasibility of the technique in vivo by introducing cultures onto pig corneas

A sublimated water atmosphere on Ganymede detected from Hubble Space Telescope observations

Ganymede’s atmosphere is produced by charged particle sputtering and sublimation of its icy surface. Previous far-ultraviolet observations of the O i 1,356 Å and O i 1,304 Å oxygen emissions were used to infer sputtered molecular oxygen (O2) as an atmospheric constituent, but an expected sublimated water (H2O) component remained undetected. Here we present an analysis of high-sensitivity spectra and spectral images acquired by the Hubble Space Telescope revealing H2O in Ganymede’s atmosphere. The relative intensity of the oxygen emissions requires contributions from the dissociative excitation of water vapour, indicating that H2O is more abundant than O2 around the subsolar point. Away from the subsolar region, the emissions are consistent with a pure O2 atmosphere. Eclipse observations constrain atomic oxygen to be at least two orders of magnitude less abundant than these other species. The higher H2O/O2 ratio above the warmer trailing hemisphere compared with the colder leading hemisphere, the spatial concentration in the subsolar region and the estimated abundance of ~1015 molecules of H2O per cm2 are consistent with sublimation of the icy surface as source

HST UV Observations of Asteroid (16) Psyche

The Main Belt Asteroid (16) Psyche is the target object of the NASA Discovery Mission Psyche. We observed the asteroid at ultraviolet (UV) wavelengths (170-310 nm) using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope during two separate observations. We report that the spectrum is very red in the UV, with a blue upturn shortward of similar to 200 nm. We find an absorption feature at 250 nm and a weaker absorption feature at 275 nm that may be attributed to a metal-oxide charge transfer band. We find that the red-sloped, relatively featureless spectrum of (16) Psyche is best matched with the reflectance spectrum of pure iron; however, our intimate mixture models show that small grains of iron may dominate the reflectance spectrum even if iron only comprises up to 10% of the material on the surface. We also stress that there is a limited database of reflectances for planetary surface analogs at UV wavelengths for comparison with the spectrum of (16) Psyche. The mid- and far-UV spectra (<240 nm) are markedly different for each of the four asteroids observed at these wavelengths so far, including ones in the same spectral class, indicating that UV observations of asteroids could be used to better understand differences in the composition and processing of the surfaces of these small bodies

Ganymede’s tenuous atmosphere

Ganymede possesses a tenuous water-based atmosphere, thought to be produced by the solar and Jovian plasma irradiation of its icy surface. Observational evidence for Ganymede's atmosphere is provided by far-ultraviolet (FUV) emission from the atomic oxygen (O) and hydrogen (H) constituents. The relative strengths of the two observed FUV atomic oxygen emission multiplets, which are produced by electron-impact on the atmosphere, allow inference of molecular O2 and H2O components and their relative abundances. Minor species besides O and H have not been detected and the observational constraints can be considered loose overall, partly due to the inherent ambiguity and difficulty in relating the electron-excited emissions to atmospheric properties. Much of our understanding of the configuration of Ganymede’s atmosphere is based on modelling efforts. The modelling studies suggest that O2 is produced primarily through radiolysis and sputtering by the plasma precipitation in the icy polar regions. Since it does not condense, O2 is yet abundant everywhere and likely the dominant species over most of the surface. Ice sublimation increases the abundance of H2O near the sub-solar point, locally exceeding the O2 abundance. The high altitudes, in contrast, are expected to be primarily populated by the lighter H2 molecules everywhere. Inferred column densities for the primary molecules, O2 and H2O, are on the order of 1014 cm-2 to 1015 cm-2. In addition, models suggest that the atmosphere is collisional only up to a few tens of kilometers above the surface, particular in the dense sub-solar region.QC 20220125</p

Ganymede’s tenuous atmosphere

Ganymede possesses a tenuous water-based atmosphere, thought to be produced by the solar and Jovian plasma irradiation of its icy surface. Observational evidence for Ganymede's atmosphere is provided by far-ultraviolet (FUV) emission from the atomic oxygen (O) and hydrogen (H) constituents. The relative strengths of the two observed FUV atomic oxygen emission multiplets, which are produced by electron-impact on the atmosphere, allow inference of molecular O2 and H2O components and their relative abundances. Minor species besides O and H have not been detected and the observational constraints can be considered loose overall, partly due to the inherent ambiguity and difficulty in relating the electron-excited emissions to atmospheric properties. Much of our understanding of the configuration of Ganymede’s atmosphere is based on modelling efforts. The modelling studies suggest that O2 is produced primarily through radiolysis and sputtering by the plasma precipitation in the icy polar regions. Since it does not condense, O2 is yet abundant everywhere and likely the dominant species over most of the surface. Ice sublimation increases the abundance of H2O near the sub-solar point, locally exceeding the O2 abundance. The high altitudes, in contrast, are expected to be primarily populated by the lighter H2 molecules everywhere. Inferred column densities for the primary molecules, O2 and H2O, are on the order of 1014 cm-2 to 1015 cm-2. In addition, models suggest that the atmosphere is collisional only up to a few tens of kilometers above the surface, particular in the dense sub-solar region.QC 20220125</p