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

Abstract

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.

Thermal Reactions between H2S and O3: Implications for Europa Surface Chemistry

“…However, the lack of detection of O 3 on Europa is a bit more puzzling, as laboratory studies have shown that O 3 is easily produced from ion and electron irradiation of O 2 ice (Famá et al 2002;Bennett & Kaiser 2005) and heavy ion irradiation of H 2 O ice that is deposited while being actively irradiated (Teolis et al 2006). While it is unclear why O 3 is absent on the leading hemisphere of Europa, we have noted previously that the lack of O 3 on the trailing hemisphere of Europa could be due to thermal reactions with SO 2 (Loeffler & Hudson 2016), which has been detected previously (Lane et al 1981;Hendrix et al 2011;Becker et al 2022). Here we show that H 2 S will also react with O 3 on the order of hours or days at the temperatures found on Europa (∼90-130 K).…”

Sulfur Ion Implantations Into Condensed CO2: Implications for Europa

“…Therefore, it is reasonable to suggest that the implantation of sulfur ions into the Europan H 2 O surface ices yields H 2 SO 4 hydrates (Ding et al, 2013;Strazzulla et al, 2007Strazzulla et al, , 2009, which are then dissociated to yield SO 2 upon further irradiation. It is also possible that alternative formation mechanisms, such as the radiolytic decomposition of sulfate minerals (McCord et al, 2001) and the implantation of cold (sub-keV) magnetospheric sulfur ions into the Europan surface ice (Becker et al, 2022), may further contribute to the presence of SO 2 .…”

“…The detection of SO2 ice on the trailing hemisphere of Europa by the International Ultraviolet Explorer spacecraft (Lane et al 1981) led many researchers to suggest that an influx of magnetospheric sulphur ions could contribute to its formation there as part of a wider radiolytic sulphur cycle (Carlson et al 1999), in which sulphur is chemically transformed to various molecular forms as a result of its interaction with ionising radiation. Indeed, the distribution of SO2 on the Europan trailing hemisphere is such that it adopts a so-called bulls-eye pattern (Carlson et al 2005, Hendrix et al 2011, Becker et al 2022. This is suggestive of an exogenic sulphur source in which magnetospheric sulphur ions implanting into the surface ices yield SO2 as a primary product.…”

“…Indeed, the distribution of SO 2 on the Europan trailing hemisphere is such that it adopts a so‐called bulls‐eye pattern (Becker et al., 2022; Carlson et al., 2005; Hendrix et al., 2011). This is suggestive of an exogenic sulfur source in which magnetospheric sulfur ions implanting into the surface ices yield SO 2 as a primary product.…”

Hydrogen peroxide at the poles of Ganymede

“…On Europa, it is possible that underlying ice abundance (relative to non-ice materials) is responsible for this dichotomy (21,66), as we suggest may be the case for Ganymede. Another possibility is that SO 2 (72)(73)(74) formed as a result of the preferential sulfur bombardment of Europa's trailing hemisphere (67) acts to deplete H 2 O 2 via thermal reactions that produce sulfate (75). However, while similar reactions warrant consideration for Ganymede, they may present a less viable mechanism for its trailing hemisphere, as models suggest that Ganymede's intrinsic magnetic field should direct most of the impinging sulfur ions to the high latitudes of both hemispheres (13,61).…”