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

Abstract

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.

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

“…The vapor density fractions of 71% and 65% of the values for pure ice found for the two hemispheres is suggestive of a relatively high ice fraction; the various measurements and model uncertainties (in particular the uncertainty about the electron properties and the sensitivity of the vapor pressure to small temperature changes) prevent further conclusions. Additionally, recent analysis of Ganymede's FUV reflectance suggests that the surface ice contains a small fraction (<1% by volume) of UV-absorbing impurities [39], which may modify the expected sublimation rate.…”

“…During these periods the JUICE UV spectrograph (UVS) instrument plans to map the location, brightness, and altitude distribution of OCFB emissions of OI 1356 Å and OI 1304 Å over a wide range of longitudes. For the planned surface reflectance mapping measurements at far-UV wavelengths using the 1650 Å absorption edge feature [39,49], it will be useful to search for latitudinal trends associated with the transport of water vapor away from the sub-solar point and to determine if the transport extends to persistently shaded regions near Ganymede's poles.…”

Laboratory Architecture as an Infrastructural Capability to Increase the Science Returned by Ocean World Missions

“…Laboratory data on the spectral signatures of trace organics in ice, salts and acids is greatly lacking, and none that we know of covering the deep UV to 5 micron range of the Europa clipper mission. Also, puzzling is the lack of the ~165nm water absorption edge in the far-ultraviolet spectra as revealed by Hubble observations of Europa and Ganymede (Becker et al, 2018, Molyneux et al, 2020. What unique endogenic/exogenic processes are at play at Europa's top surface veneer that mutes this distinct water ice signature observed for many Saturnian satellites?…”

On Charon's Far-Ultraviolet Surface Reflectance

“…The tholins modeled by Hendrix et al (2010) have wavelength-independent reflectances of ∼ 0.05 in the FUV and act as spectrally neutral darkening agents at these wavelengths. Further, other surface contaminants such as silicates, sulfur compounds, and organics that are not tholins can also be spectrally neutral in the FUV (see, e.g., Molyneux et al 2020). Thus, the lower overall FUV reflectance of Charon compared to Enceladus could plausibly be explained by Charon having more and/or less-reflective surface contaminants.…”

“…Except for Pluto, all of these bodies show the characteristic upturn in reflectance near 1650 Å that is diagnostic of H 2 O ice. Impurities in the H 2 O ice matrix can alter the shape of the upturn (e.g., Ganymede; Molyneux et al 2020) and different grain sizes can shift the wavelength where the upturn begins (Hendrix & Hansen 2008a), but neither can produce a constant FUV reflectance between 1400 and 1850 Å. The brightest regions on Pluto are dominated by CH 4 , N 2 , and CO ices (Stern et al 2015a;Grundy et al 2016), none of which show an FUV absorption feature.…”