The chemical nature of Europa surface material and the relation to a subsurface ocean

Abstract

The surface composition of Europa is of special interest due to the information it might provide regarding the presence of a subsurface ocean. One source of this information is the infrared reflectance spectrum. Certain surface regions of Europa exhibit distorted H₂O vibrational overtone bands in the 1.5 and 2.0 μm region, as measured by the Galileo mission Near Infrared Mapping Spectrometer (NIMS). These bands are clearly the result of highly concentrated solvated contaminants. However, two interpretations of their identity have been presented. One emphasizes hydrated salt minerals and the other sulfuric acid, although each does not specifically rule out some of the other. It has been pointed out that accurate chemical identification of the surface composition must depend on integrating spectral data with geochemical models, and information on the tenuous atmosphere sputtered from the surface. It is also extremely important to apply detailed chemistry when interpreting the spectral data, including knowledge of mineral dissolution chemistry and the subsequent optical signatures of ion solvation in low-temperature ice. We present studies of flash frozen acid and salt mixtures as Europa surface analogs and demonstrate that solvated protons, metal cations and inorganic anions all influence the spectra and must all, collectively, be considered when assigning Europa spectral features. These laboratory data show best correlation with NIMS Europa spectra for multi-component mixtures of sodium and magnesium bearing sulfate salts mixed with sulfuric acid. The data provide a concentration upper bound of 50-mol% for MgSO₄ and 40-mol% for Na₂SO₄. This newly reported higher sodium and proton content is consistent with low-temperature aqueous differentiation and hydrothermal processing of carbonaceous chondrite-forming materials during the formation and early evolution of Europa.

Introduction

Europa is the subject of intense scrutiny because of the possibility that its icy shell may conceal a liquid ocean capable of harboring life Carr et al., 1998, Chyba, 2000, Chyba and Phillips, 2001. Evidence regarding crustal composition is limited, but includes sputtered atmospheric constituents Brown, 2001, Brown and Hill, 1996, Hall et al., 1995 and near-infrared reflectance spectra of surface regions from the Galileo NIMS investigation (Carlson et al., 1996). Reflectance spectra of certain Europa surface regions exhibit highly distorted H₂O vibrational overtone bands. One interpretation is that these suggest endogenic frozen salt mineral mixtures with some Na₂SO₄ converted to H₂SO₄ under irradiation at the surface McCord et al., 1998a, McCord et al., 1999, McCord et al., 2002. The other proposes that H₂SO₄ in ice gives the best single-component match to the NIMS spectra (Carlson et al., 1999). In the former, the salts come from the ocean below. In the latter, H₂SO₄ is from radiation processing and sulfur ion implantation in water ice from the jovian plasma torus. Hopes of accurate chemical identification of the surface material, and extrapolation to a subsurface ocean, will depend on integrating UV, visual and IR spectral data (Fanale et al., 1999), geochemical models Kargel et al., 2000, Spaun and Head, 2001, Zolotov and Shock, 2001, and information on the tenuous and likely sputtered atmosphere Johnson, 2000, Johnson et al., 2002, Leblanc et al., 2002 in a holistic view. However, interpretation of the reflectance spectra requires knowledge of mineral dissolution chemistry and the subsequent optical signatures of ion-solvation in low-temperature ice. All plausible models of Europa's formation and thermo-chemical evolution indicate that the crustal ice is not pure, but rather must contain some mineral content remnant of its formation from solar nebula solids.

Europa is a differentiated Moon-sized object, with a silicate core, water-rich mantle and ice crust that is subjected to strong energy input from Jupiter-induced tidal flexing Carr et al., 1998, Geissler et al., 1998, Greeley et al., 2004, Khurana et al., 1998, Pappalardo et al., 1998. Thus, the layers beneath the icy crust could melt forming a liquid ocean which could foster the dissolution of core minerals. Though the thickness of the ice remains controversial, it is clear that it has been disrupted extensively from below. The materials present on the surface of Europa are the products of radiation transformations, cryovolcanism, impact and gardening events that have occurred over time. In fact, chemical alteration of the ice has been shown to produce condensed hydrogen peroxide within at least some icy surface regions (Carlson et al., 1999). Also, an atmosphere composed mostly of atomic and molecular hydrogen with some atomic and molecular oxygen from radiation processing of surface ice is seen Hall et al., 1995, Orlando and Sieger, 2003, Sieger et al., 1998, Wu et al., 1978.

The chemical identity of the surface, when analyzed in conjunction with other mission data, can be considered as fingerprints of the past geochemical and geophysical activity. The association of the material with the disrupted regions on the surface strongly suggests an endogenic origin McCord et al., 1998a, McCord et al., 1998b. Geochemical models suggest that salts such as MgSO₄ with Na₂SO₄ can be produced by low-temperature aqueous alteration of solar nebula material, as seen in carbonaceous chondrites (Kargel et al., 2000). This is expected to lead to the formation of MgNaCa-sulfate rich material, which, due to the presence of sulfidic base material, could also form some sulfuric acid (Kargel et al., 2000). Typically, salts in natural environments occur as mixtures, controlled by their source material, solubilities and formation temperatures. The relative abundances of these compounds depend upon the planetary condensation and internal Europa evolution temperatures, with lower temperatures favoring a relatively high sodium concentration. Some modeling has also been carried out which addresses the role of basic (Marion, 2001) and acidic conditions (Marion, 2002) on the freezing of aqueous solutions containing sodium and magnesium bearing sulfate salts.

The Galileo mission's Near Infrared Mapping Spectrometer (NIMS) returned infrared (IR) reflectance spectra (Carlson et al., 1996) exhibiting asymmetric absorption bands in the 1–3 μm spectral region McCord et al., 1998a, McCord et al., 1998b. The primary features in this region are attributed to water ice overtones that shift in frequency and become asymmetric when ice contains certain types of impurities or when water molecules are in confined geometries. The asymmetric spectral signatures in the 1–3 μm region are concentrated on Europa in lineaments and chaotic terrain McCord et al., 1998a, McCord et al., 1998b. We previously demonstrated that rapidly frozen brines give better spectral matches to the NIMS non-ice endmember spectrum than hydrous crystalline minerals (McCord et al., 2002). This rapid quenching/glass forming process preserves the water structures associated with the solvated ions in brine solutions. The complex cation, anion and proton solvation structures formed during flash freezing of aqueous solutions are possible sources of the perturbed optical signatures observed in the NIMS spectra. Thus, we undertook studies of flash frozen acid and salt mixtures as Europa surface analogs to examine the relative contribution of endogenic vs exogenic surface contamination.