Linear mixture modeling of Europa's non‐ice material based on cryogenic laboratory spectroscopy

Abstract

[1] Linear mixture modeling has been applied to Galileo Near Infrared Mapping Spectrometer (NIMS) spectra of Europa's dark, non-water-ice terrain using cryogenic laboratory spectra of hydrated compounds. Water ice alone could not account for the distorted and asymmetric spectral features attributed to water of hydration in the Europa spectrum. The modeling was conducted using cryogenic reference spectra of hexahydrite, epsomite, bloedite, sodium sulfide nonahydrate, mirabilite, magnesium sulfate dodecahydrate, sulfuric acid hydrate, and saturated brines of magnesium sulfate, sodium carbonate, and sodium sulfate. All were measured between 77 and 130K. The best model fit was attained using abundances of 14% hexahydrite, 11% bloedite, 12% mirabilite, and 62% sulfuric acid hydrate. Results indicate that sulfuric acid hydrate and hydrated sulfate salts, taken together, produce a better spectral match than either class of compound considered alone.Citation: Dalton, J. B., III (2007), Linear mixture modeling of Europa's non-ice material based on cryogenic laboratory spectroscopy, Geophys. Res. Lett., 34, L21205,

Chemical Composition of Icy Satellite Surfaces

“…At the surface temperatures of icy Galilean satellites (typically within 80-150 K) the infrared spectra of hydrated materials are markedly different than they are at room temperature or even at 273 K. Recent work using cryogenic laboratory spectra of hydrates indicates that the best spectral matches to Europa observations are achieved using mixtures of sulfuric acid hydrate, sodium and magnesium sulfate hydrates, and water ice of grain sizes varying from 50 to 250 microns (Dalton 2007b;Dalton et al 2009;Shirley et al 2010).…”

Spectroscopy of Icy Moon Surface Materials

“…By the time Kuiper (1957) and Moroz (1965) confirmed the presence of water ice on Europa and Ganymede, water ice and a few other volatile ices had been characterized sufficiently to enable their detection. Derivation of abundances for most other surface species awaits cryogenic laboratory measurements of these species (Carlson et al 1999a(Carlson et al , 1999b(Carlson et al , 2005Dalton 2007). …”

“…Water ice, as predicted by Lewis (1972), is nearly Table 1 Some of the major compounds identified on the outer solar system satellites, and others predicted by models or experiments. Sources: Hanel et al (1979); Orton et al (1996); Carlson et al (1996Carlson et al ( , 2005Carlson et al ( , 2009); Davies et al (2000); ; Nash and Betts (1998); Spencer et al (1999); Hibbitts et al (2000); Roush (2001); Clark et al (2005); Emery et al (2005); ; Waite et al (2006Waite et al ( , 2009; Verbiscer et al (2006Verbiscer et al ( , 2008; Grundy et al (2006); Cook et al (2007); ; Cruikshank et al (2007); Dalton (2007); Moore et al (2007);Newman et al (2007); Hodyss et al (2009) Schenk et al (2001) attribute this caldera on Ganymede (center) to cryovolcanic flows. Sublimation degradation (Moore et al 2004) enlarges pits such as this one on Callisto (right), providing volatiles that are re-deposited as bright ice limning the ridges ubiquitous among the large outer solar system satellites, failing to dominate only Io, Titan and Triton.…”

“…Absorption coefficients of solid methane (Grundy et al 2002) enable models of CH 4 ice on Titan and Pluto. measured cryogenic reflectance spectra of hydrated salts predicted to be on the surface of Europa, and demonstrated their utility with linear mixture modeling of Europa's surface composition (Dalton 2007;Shirley et al 2010). However, these salt hydrate measurements do not extend above 2.5 µm, while published spectra of hydrogen peroxide ice do not extend below 2.5 µm, so a composite model with all of these materials cannot be constructed.…”

Seeking Europa's Ocean

“…Wherever Europa's surface is somehow breached by geological activity to expose subsurface material, that material is relatively dark and a ruddy ochre in color, in contrast to the bright icy older surface. Multispectral characterization suggests that the colored materials contain hydrated sulfate salts and hydrated sulfuric acid (Dalton et al, 2007;. Salts may originate in the ocean (McCord et al, 1999;Zolotov and Kargel, 2009), while sulfuric acid hydrate may be a product of radiation-driven chemistry, radiolytically generated as high-energy charged particles of Jupiter's magnetosphere slam into Europa's surface.…”