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Salts on Europa's Surface Detected by Galileo's Near Infrared Mapping Spectrometer

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Science  22 May 1998:
Vol. 280, Issue 5367, pp. 1242-1245
DOI: 10.1126/science.280.5367.1242

Abstract

Reflectance spectra in the 1- to 2.5-micrometer wavelength region of the surface of Europa obtained by Galileo's Near Infrared Mapping Spectrometer exhibit distorted water absorption bands that indicate the presence of hydrated minerals. The laboratory spectra of hydrated salt minerals such as magnesium sulfates and sodium carbonates and mixtures of these minerals provide a close match to the Europa spectra. The distorted bands are only observed in the optically darker areas of Europa, including the lineaments, and may represent evaporite deposits formed by water, rich in dissolved salts, reaching the surface from a water-rich layer underlying an ice crust.

Europa is the second Galilean satellite outward from Jupiter. It is a lunar-sized object with a water-ice surface of relatively young geologic age (1, 2). Its density and gravity field suggest a differentiated object with an Fe-rich core and silicate mantle underlying an ice-rich crust (3). The surface features and young age (4) suggest recent resurfacing, perhaps from a liquid water ocean existing between a relatively thin ice crust and the silicate mantle. Tidal heating has been suggested as the energy source to maintain an ocean in this relatively cold part of the solar system (surface temperatures on Europa range from 100 to 120 K) (5). The existence of heat and water plus organic molecules (6) has led many to speculate whether Europa's ocean might be an environment with the potential for sustaining life (7).

Ground-based telescope observations show that Europa exhibits a high visual albedo and other spectral reflectance characteristics in the 1- to 5-μm wavelength range that were reported to indicate the dominance (>85% by weight) of surface water ice or frost (or both) and only a few weight percent impurities, with the possible presence of iron-bearing minerals that reduce the visible brightness in some regions (2, 8). In addition, forms of sulfur-rich species, including sulfur allotropes, SO, SO2, and H2S, have been suggested as impurities in Europa's surface on the basis of ultraviolet and visible spectroscopy and Voyager multispectral images (9, 10).These sulfur-rich species are thought to be derived from materials sputtered from Io and carried to Europa by Jupiter's magnetosphere, although an interior origin has also been suggested (10).

Early observations by the Near Infrared Mapping Spectrometer (NIMS) (11) indicated a surface of almost pure water ice for a north polar region of Europa (12), consistent with the earlier interpretations of the telescopic spectra. Here, we discuss subsequent NIMS observations of Europa that indicate the presence of hydrated minerals. Water-related spectral features dominate the NIMS reflectance spectra in the 1- to 3-μm range for Europa (Fig.1). These features are due to vibration modes and combinations of these modes for H–O–H (13) in ice or in association with hydrated species (14,15). The features include absorption bands centered near 1.04, 1.25, 1.5, and 2 μm; a weak temperature-sensitive ice band at 1.65 μm on the edge of the 1.5-μm feature (16); and the fundamental O–H stretching transition near 3 μm. Water is strongly absorbing longward of 2.5 μm and is mostly responsible for the low reflectance in this region for all icy Galilean satellites (6).

Figure 1

Examples of NIMS reflectance spectra for Europa and Ganymede. The spectra grade from pure water ice (top) to Europa's least ice-like (bottom) and are offset vertically from each other by 0.12 in reflectance. The water features (for example, at 1.04, 1.25, 1.5, 2.0, and 2.8 μm) are prominent. Spectra A and C are each averages of 100 NIMS pixels from the TERINC (29) observation in the E6 orbit. Spectrum A is scaled by a factor of 1.3. Spectrum B is a 130-pixel average over a dark area in the NHILAT observation in the G1 orbit. Spectrum D is a full disk average (120 pixels) of Europa's leading hemisphere taken in the E11 orbit, scaled by a factor of 1.35. Spectrum E is from a few NIMS pixels measuring a very icy region of Ganymede. Spectrum F is pure water ice from a radiative transfer model with 20-μm grain size (30). Each curve shows a number of spectrum segment mismatched overlaps associated with NIMS's spectrum sampling process (11) and calibration uncertainties.

At Europa's surface temperatures (100 to 120 K), H2O is expected to exist as water ice, and ice is responsible for some of the spectral signatures (Fig. 1) (6,8). The NIMS data indicate that H2O must also be present in other species besides ice to explain the asymmetric and broadened Europa absorption bands (6, 17). Spectra from locations within each of the NIMS Europa data sets from Galileo orbits 1 to 11 that show the most distorted H2O features were identified, and an average spectrum for each area was calculated (see example in Fig. 1). The average spectra for the most hydrate-rich areas were nearly identical, so they were averaged to produce a candidate generic hydrate spectrum.

This hydrate spectrum, a very ice-rich spectrum from portions of Ganymede, and spectra for areas on Europa with different (estimated) amounts of ice and nonice spectral characteristics were then used to estimate the proportions of ice and nonice material for each NIMS pixel and to map the distribution of the nonice material. The Ganymede nearly pure ice spectrum was used rather than the model ice spectrum in Fig. 1 because it was obtained with NIMS under similar conditions to the Europa observations and it may better represent the physically complex condition of the ice on Europa's surface. The nearly pure Ganymede ice and nearly pure Europa nonice material spectra shown in Fig. 1 and some other NIMS mixture spectra were used to calculate a series of synthetic spectra for intermediate mixtures that were compared with the spectrum for each NIMS pixel to estimate the proportions of ice and nonice materials present.

Although it is an oversimplification (18), this approach nevertheless resulted in distinctive maps of the nonice material that show that the nonice material is associated with the darker (lower visible albedo) regions, including the linea (Fig.2) (19). In some areas, such as within some lineaments, there appears to be a near-perfect match to the nonice end-member spectrum, indicating less than about 1% ice. Furthermore, the spectra for the most ice-poor areas, even at the single pixel scale, are nearly identical, suggesting that the nonice material is of about the same composition everywhere so far studied.

Figure 2

(A) A near-hemispheric region of Europa at 77 km/pixel in the 0.7-μm NIMS passband. The gray scale bar indicates the approximate albedo for each pixel. (B) The relative concentrations of the nonice and ice-rich components of the surface, with dark red corresponding to the highest nonice concentration. The color scale bar indicates the relative amount of nonice component, where 1 = 100% nonice material. The nonice material is associated with the dark regions in (A). (C) High-resolution albedo image (∼4 km/pixel) of a small region of Europa including a dark linea in the 0.7-μm passband from NIMS. (D) The distribution of the nonice component, with dark red being high nonice concentration. The nonice material is associated with the dark regions, including the linea.

The nonice end-member spectrum (Fig. 1) exhibits H2O absorptions, especially in the 1.5- and 2.0-μm regions, that are highly distorted compared with water ice features, and the continuum is increasingly depressed toward longer wavelengths. Inspection of the H2O spectral features in the nonice material indicates (14, 15) that (i) the H2O molecule has to be present (the OH functional group alone is not sufficient to explain the absorption bands because OH has no 2.0-μm absorption), (ii) the H2O molecule is active spectrally, indicating abundant water in the mineral because its features are expressed so strongly in the spectrum, and (iii) the H2O molecules are relatively closely bound to the host molecule in numerous slightly different bonding configurations to account for the strong and multiple distorting effects the host electric potential has on the H2O bonding energy. Slightly hydrated mineral species generally show narrower H2O absorption features because of fewer water molecules per formula unit, fewer sites of different potentials, and therefore less spreading of each band's energies. Thus, the candidate minerals are likely to be of the form mineral·xH2O, where x is large and the H2O is incorporated in the crystal structure as opposed to being adsorbed.

Spectral libraries (15, 20) contain only a few spectra of hydrated minerals resembling Europa's spectra. Nonhydrated and hydroxylated minerals do not exhibit similar spectral features. Hydrated clay minerals have been suggested as candidate materials for Europa's dark regions (8,21), but their spectral signatures differ from Europa's in several ways. The spectra for clays show additional absorptions in the 2.2- to 2.4-μm range, due to metal–OH (for example, Fe, Mg, and Al) absorptions (14, 20), which are not present in Europa's spectra. These features can be suppressed in mixtures containing other absorbers, which reduce the reflectance and thus the spectral contrast. In Europa's case, however, they should be evident because some locations seem to be nearly 100% hydrated mineral and the reflectance remains high (≥50% of 1-μm reflectance) in the 2.2- to 2.4-μm region. Also, clays generally have more symmetric and narrower H2O features positioned at shorter wavelengths than those observed in the Europa nonice spectrum. Clays also lack the decreasing continuum reflectance toward longer wavelengths (14,20) typical of Europa's nonice material.

The class of minerals whose spectra most closely resemble those of Europa is the hydrated salts. Of these, sulfates, carbonates, and borates have spectra (15, 20) that are most consistent with Europa's nonice spectra. Borates seem unlikely given the low solar abundance of boron and the generally complex spectra shown by many borate minerals (15, 20). From our study of available spectra, heavily hydrated carbonates and sulfates appear to be the better candidates. For example, natron (Na2CO3·10H2O) has a reflectance decrease toward longer wavelengths similar to that of Europa's nonicy regions (Fig. 3), but the 1.45- and 2.0-μm water features are a little more symmetric and less distorted at their core than Europa's. MgSO4·xH2O is also a likely candidate with hexahydrite (MgSO4·6H2O) and epsomite (MgSO4·7H2O) having the most similar spectra (Fig. 3). Other sulfate features at longer wavelengths within the NIMS spectral range in these heavily hydrated materials are masked by stronger H2O features (5). Other Mg sulfates with x < 6, such as pentahydrite (MgSO4·5H2O), do not exhibit sufficiently distorted water bands to match Europa's spectrum. Other cations, such as Na, also occur in this structure with Mg (22), but they should not substantially change H2O bonding energies and thus the spectral features analyzed here. Mixtures of carbonate and sulfate minerals provide even better matches to Europa's spectrum than the individual minerals. A mixture of salts is reasonable given that the nonice material spectrum is almost identical everywhere observed, implying a source of approximately uniform composition on a hemispherical scale.

Figure 3

Reflectance spectra from the Galileo NIMS instrument of Europa's nonicy and icy areas are averaged and scaled to qualitatively compare with reflectance spectra of possible surface minerals. The NIMS spectra are offset by 0.15 (nonicy) and 0.3 (icy) and are plotted with heavier lines. The hydrated sulfate and carbonate minerals hexahydrite (Hexa.) [MgSO4·6H2O], epsomite (Epsom.) (MgSO4·7H2O), and natron (Na2CO3·10H2O) show similar features and are the prime candidates (including in mixtures) for the material composing much of the nonice portion of the surface. No offset is used for these spectra. Hydrated clay minerals such as sepiolite [Mg4Si6O15(OH)2·H2O] and montmorillonite (Mont.) [(Na,Ca)0.33(Al,Mg)2Si4O210 (OH)2·nH20] do not have the broadened water features found for Europa. Furthermore, clays have metal–OH absorptions in the 2.2- to 2.4-μm region that do not occur for Europa. The sepiolite and montmorillonite spectra are each offset 0.6.

There are other effects that could refine our interpretations of Europa's spectra. There may be temperature-induced spectral differences between ambient laboratory and Europa observations. One effect of colder temperatures on these H2O features in hydrated minerals would be a narrowing of the bands similar to that seen in water-ice spectra (23). This phenomenon is sufficiently understood and does not alter our analysis. The particle size of materials can affect the absolute and relative strengths of absorptions in the spectrum of individual samples by changing the distance a photon travels through a grain, and thus the absorption probability, between scattering boundaries. However, the shape and symmetry of the absorption band do not change other than to become stronger or weaker, except in the case of a composite of overlapping bands with large differences in absorption strengths. This fact and our investigations over the limited available range of particle sizes for salt minerals suggest that it is unlikely that differences in grain size effects can alter our interpretations.

Hydrated salt minerals are usually formed on Earth by evaporation of brines on the surface and precipitation of the salts leaving a lag deposit of salt crystals (24). If there is a liquid ocean below the ice crust on Europa, there must be heat generated in the interior of Europa to maintain the ocean. There may be volcanic or hydrothermal activity and dissolution of ions into the ocean, as occurs on Earth's ocean floor. If an ocean brine within Europa were extruded onto the surface, it would encounter a vacuum and a temperature < 125 K. The exposed brine would be concentrated by a number of mechanisms, including flash evaporation, freezing, sublimation, and sputtering, leaving some crystallized salts as an evaporite-like deposit. Hydrated magnesium sulfate, common in evaporite deposits on Earth, is highly soluble in water and is found in aqueous-altered meteorites (25). Also, sodium carbonates, such as natron, are found in large quantities in terrestrial evaporite deposits derived primarily from carbonic acid weathering of volcanic rocks.

Models of an evolving Europa that use initial carbonaceous chondrite composition predict large amounts of hydrated MgSO4 (25), and recent experiments dissolving meteorites in water produced hydrated MgSO4-rich leachates (26). The NIMS results seem to confirm these models. Kargel (25) suggested that the state of hydration on Europa may be as high as ·12H2O, substantially more that the ·7H2O maximum easily produced at room temperatures. Greater hydration will tend to further broaden and distort the H2O absorptions, which may allow an even better match to Europa's spectrum.

The occurrence of evaporite minerals in general and carbonates in particular on Europa's surface would place important constraints on chemical processes in the interior and provide important ions that could be supportive of life in the ocean (27). For example, if Europa's ocean is enriched in dissolved carbonate species, this would require an alkaline water chemistry such as would exist with high CO2 concentrations. A similar ocean composition has been proposed as the setting for the development of primitive life on Earth (28). For Earth, the CO2 was in the early atmosphere. For Europa, the ice crust may trap CO2 released by heating of the interior.

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