The Far‐UV Albedo of Europa From HST Observations

Abstract

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.

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

“…Figure 3b shows the 300 μm H 2 O reflectance spectrum scaled to Ganymede's leading hemisphere to indicate where Ganymede's FUV reflectance diverges from that of pure water ice. A potentially similar modification of the FUV water ice feature has been noted in UV HST/STIS observations of Europa (Becker et al, 2018), although Europa's reflectance in the 170–210 nm region has not yet been published and the exact FUV spectral shape therefore remains unclear.…”

“…There are many materials we could add to darken the intraparticle models and fit Ganymede's near‐UV and visible reflectance, compared with only one method we have identified that may brighten the intimate model, and we cannot currently confirm that this method would work due to a lack of optical constants. Additionally, the water absorption edge is absent from the reflectance spectrum of Europa (Becker et al, 2018), and the Europan surface is thought to be considerably icier than Ganymede's. An improved silicate‐H 2 O intimate model may prove to be bright enough for Ganymede, but we anticipate that it will be difficult to identify silicates bright enough to reproduce Europa's visible reflectance, which is >0.7 at 500 nm (Spencer et al, 2004).…”

“…The Wagner et al (1987) spectral reflectance atlas, for example, includes silicate spectra with higher visible reflectance values, but we are unable to model these materials as optical constants are not currently available. Becker et al (2018) had similar difficulty in fitting Europa's UV albedo using intimate mixture models, finding that mixtures which are dark enough in the~160-200 nm region remain too dark at longer UV and visible wavelengths. Becker et al (2018) suggest that a thin veneer of UV-absorbing lag deposit, transparent at longer wavelengths, may explain the apparent lack of the UV H 2 O absorption edge at Europa.…”

“…Becker et al (2018) had similar difficulty in fitting Europa's UV albedo using intimate mixture models, finding that mixtures which are dark enough in the~160-200 nm region remain too dark at longer UV and visible wavelengths. Becker et al (2018) suggest that a thin veneer of UV-absorbing lag deposit, transparent at longer wavelengths, may explain the apparent lack of the UV H 2 O absorption edge at Europa. We calculated the mean optical path length (MOPL; the average distance traveled by photons within a particular material) of 100-700 nm photons in particulate graphite and silicate surfaces to determine whether the inability of our intimate mixture models to match Ganymede's >200 nm reflectance may be due to the visible photons probing a deeper, icier part of the surface.…”

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?…”