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

Alday, Juan; Roth, Lorenz; Ivchenko, Nickolay; Retherford, Kurt D.; Becker, Tracy M.; Molyneux, Philippa M.; Saur, Joachim

doi:10.1016/j.pss.2017.10.006

Cited by 24 publications

(43 citation statements)

References 42 publications

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“…We have presented FUV reflectance spectra of Ganymede's orbital leading and trailing hemispheres, derived from HST/COS observations in 2014. We found that the leading hemisphere becomes brighter than the trailing hemisphere at wavelengths >170 nm, confirming and constraining a hemispheric reflectance dichotomy that has previously been noted to begin somewhere in the 155–200 nm region (Alday et al, 2017; Musacchio et al, 2017). The observed shape of Ganymede's FUV reflectance spectrum is notably different to those of Saturn's icy moons, which exhibit a sharp increase in reflectivity at ~160 nm.…”

Section: Discussionsupporting

confidence: 88%

“…This is consistent with previous observations of Ganymede in the FUV. The higher sensitivity of COS, however, places a much tighter constraint on the wavelength at which the leading hemisphere becomes brighter than the trailing, which has previously been noted to fall somewhere in the 155–200 nm region (Alday et al, 2017; Musacchio et al, 2017).…”

Section: Ganymede's Fuv Reflectancementioning

confidence: 98%

“…Alday et al (2017) measured Ganymede's leading and trailing hemisphere reflectance at 121.6 nm (Lyman‐α) using HST/STIS data from previous observation campaigns (Feldman et al, 2000; McGrath et al, 2013; Molyneux et al, 2018; Saur et al, 2015), deriving average albedos of 3.0 ± 0.2% and 2.0 ± 0.2% and for the trailing and leading hemispheres, respectively. Musacchio et al (2017) similarly used data from previous HST/STIS and HST/Goddard High Resolution Spectrograph (Hall et al, 1998) observations to derive Ganymede's reflectance in the 141–155 nm region.…”

Section: Ganymede's Fuv Reflectancementioning

confidence: 99%

“…Previous observations of Ganymede in the far ultraviolet (FUV) indicate that the hemispheric reflectance asymmetry reverses somewhere in the 155–200 nm region, with the trailing hemisphere becoming more reflective than the leading hemisphere at shorter wavelengths (Alday et al, 2017; Musacchio et al, 2017). These previous campaigns observed Ganymede with the HST Space Telescope Imaging Spectrograph (HST/STIS), using the G140L grating to obtain spectra in the 115–170 nm region.…”

Section: Introductionmentioning

confidence: 99%

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Ganymede's Far‐Ultraviolet Reflectance: Constraining Impurities in the Surface Ice

et al. 2020

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We present reflectance spectra of Ganymede's leading and trailing hemispheres in the wavelength range 138–215 nm, obtained by the Hubble Space Telescope Cosmic Origins Spectrograph (HST/COS) in 2014. The most notable feature of both spectra is the absence of a sharp water absorption edge at ~165 nm, seen in laboratory measurements of ice reflectivity and in previous observations of Saturn's icy moons and rings. Rather than displaying a sharp change in the reflectivity at the wavelength of the water ice absorption edge, Ganymede's reflectance gradually increases with wavelength at λ > 165 nm. We show that the observed shape of Ganymede's UV reflectance is inconsistent with intimate mixture models of pure ice with UV‐dark materials including tholins, amorphous carbon, graphite, and silicates. However, we find that intraparticle models, in which a small proportion of a UV‐absorbing contaminant is trapped as inclusions within the ice matrix, are able to suppress the 165 nm feature at contaminant concentrations of <1%. We show that models of ice with inclusions of silicates, Triton‐type tholin, or H2O2 are able to produce the observed gradual increase in reflectivity at λ > 165 nm, but additional absorbing surface materials are required to produce a good fit to Ganymede's previously observed near‐UV and visible reflectance.

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Section: Discussionsupporting

confidence: 88%

Section: Ganymede's Fuv Reflectancementioning

confidence: 98%

Section: Ganymede's Fuv Reflectancementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Ganymede's Far‐Ultraviolet Reflectance: Constraining Impurities in the Surface Ice

et al. 2020

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“…On other Jovian satellites, the spectral inversion, where the visible dark regions are bright in the UV, begins at longer wavelengths as well. On Ganymede, Alday et al () show a clear inversion between 140 and 150 nm and detect a possible, though muted inversion between 160 and 170 nm. Whether changes to the compositional or microphysical structure of the surface provide the best explanation for the weathering the Galilean satellites remains unclear.…”

Section: Discussionmentioning

confidence: 99%

The Far‐UV Albedo of Europa From HST Observations

et al. 2018

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We present an analysis of Europa's far‐UV spectral albedo using observations during the 1999–2015 time period made by the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. Disk‐integrated observations show that the far‐UV spectrum in the ~130 to 170‐nm range is relatively flat or slightly blue (increasing albedo with decreasing wavelength) for the studied hemispheres: the leading, trailing, and anti‐Jovian hemispheres. At Lyman‐α (121.6 nm), the albedo of the trailing hemisphere continues the blue trend, but it reddens for the leading hemisphere. Also at this wavelength, the albedo of the leading hemisphere, which is higher than the trailing hemisphere at near‐UV and visible wavelengths, is lower than the trailing hemisphere, exhibiting spectral inversion. We find no evidence of a sharp water‐ice absorption edge at 165 nm on any hemisphere of Europa, which is intriguing since such an absorption feature has been observed on the icy Saturnian satellites.

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Moons and Jupiter Imaging Spectrometer (MAJIS) on Jupiter Icy Moons Explorer (JUICE)

Poulet,

Piccioni,

Langevin

et al. 2024

Space Sci Rev

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The MAJIS (Moons And Jupiter Imaging Spectrometer) instrument on board the ESA JUICE (JUpiter ICy moon Explorer) mission is an imaging spectrometer operating in the visible and near-infrared spectral range from 0.50 to 5.55 μm in two spectral channels with a boundary at 2.3 μm and spectral samplings for the VISNIR and IR channels better than 4 nm/band and 7 nm/band, respectively. The IFOV is 150 μrad over a total of 400 pixels. As already amply demonstrated by the past and present operative planetary space missions, an imaging spectrometer of this type can span a wide range of scientific objectives, from the surface through the atmosphere and exosphere. MAJIS is then perfectly suitable for a comprehensive study of the icy satellites, with particular emphasis on Ganymede, the Jupiter atmosphere, including its aurorae and the spectral characterization of the whole Jupiter system, including the ring system, small inner moons, and targets of opportunity whenever feasible. The accurate measurement of radiance from the different targets, in some case particularly faint due to strong absorption features, requires a very sensitive cryogenic instrument operating in a severe radiation environment. In this respect MAJIS is the state-of-the-art imaging spectrometer devoted to these objectives in the outer Solar System and its passive cooling system without cryocoolers makes it potentially robust for a long-life mission as JUICE is. In this paper we report the scientific objectives, discuss the design of the instrument including its complex on-board pipeline, highlight the achieved performance, and address the observation plan with the relevant instrument modes.

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

Cited by 24 publications

References 42 publications

Ganymede's Far‐Ultraviolet Reflectance: Constraining Impurities in the Surface Ice

Ganymede's Far‐Ultraviolet Reflectance: Constraining Impurities in the Surface Ice

The Far‐UV Albedo of Europa From HST Observations

Moons and Jupiter Imaging Spectrometer (MAJIS) on Jupiter Icy Moons Explorer (JUICE)

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