Spectral Behavior of Hydrated Sulfate Salts: Implicationsfor Europa Mission Spectrometer Design
JAMES BRADLEY DALTON, III
ABSTRACT
Remote sensing of the surface of Europa with near-infrared instruments has suggested thepresence of hydrated materials, including sulfate salts. Attention has been focused on thesesalts for the information they might yield regarding the evolution of a putative interior ocean,and the evaluation of its astrobiological potential. These materials exhibit distinct infraredabsorption features due to bound water. The interactions of this water with the host mole-cules lead to fine structure that can be used to discriminate among these materials on the ba-sis of their spectral behavior. This fine structure is even more pronounced at the low tem-peratures prevalent on icy satellites. Examination of hydrated sulfate salt spectra measuredunder cryogenic temperature conditions provides realistic constraints for future remote-sens-ing missions to Europa. In particular, it suggests that a spectrometer system capable of 2–5nm spectral resolution or better, with a spatial resolution approaching 100 m, would be ableto differentiate among proposed hydrated surface materials, if present, and constrain theirdistributions across the surface. Such information would provide valuable insights into theevolutionary history of Europa. Key Words: Hydrated salts—Europa—Infrared spectroscopy—Remote sensing. Astrobiology 3, 771-784.
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INTRODUCTION
EUROPA HAS BEEN IDENTIFIED as a primary can-didate for Outer Solar System exploration in
the next decade. Mounting evidence for a sub-surface ocean (Cassen et al., 1979; Pappalardo etal., 1999; Kivelson et al., 2000; Stevenson, 2000) hasfostered discussion regarding the astrobiologicalpotential of this putative ocean (cf. Chyba, 2000;Borucki et al., 2002). Knowledge of the surfacecomposition will be critical for modeling of sur-face and interior processes and especially forevaluating astrobiological potential (Dalton,
2002; Dalton et al., 2003). If the surface composi-tion is indicative of interior composition, surfacestudies by either orbiting or landed instrumentswill provide an important window into the inte-rior and its chemical evolution. While much in-formation on surface composition has been de-rived from the Galileo Near-Infrared MappingSpectrometer (NIMS) and Solid-State Imager(SSI) investigations, some discoveries have cre-ated more questions than answers. Any futureEuropa mission will attempt to capitalize onlessons learned from Galileo and try to answerthese questions. The primary aim of this paper is
SETI Institute, NASA Ames Research Center, Moffett Field, California.
to provide bounds on the spatial and spectral res-olution for a reflectance spectrometer necessaryto identify and discriminate among the proposedsurface constituents. In addition to and in sup-port of this goal, new laboratory measurementsare presented that illustrate spectral characteris-tics of these materials that may be exploited forthis purpose.
Infrared remote sensing of Europa has indi-cated the presence of ordinary crystalline andamorphous water ice (Pilcher et al., 1972; Cassenet al., 1979; Clark and McCord, 1980; McCord etal., 1999a,b; Hansen and McCord, 2004), as wellas water with an unusual spectral signature.Galileo NIMS spectra of certain regions on Eu-ropa (Fig. 1) exhibit several absorption featuresindicative of water, yet with unusual characteris-tics as compared with a spectrum of ordinary wa-ter ice at similar temperature. The water in theseregions appears to be bound in a hydrated state(Dalton and Clark, 1998; McCord et al., 1998,1999a,b, 2002) as in hydrated sulfate salts and/orother sulfur compounds such as sulfuric acid oc-tahydrate (Carlson et al., 1999b, 2002). Althoughit is not yet clear what the host molecule(s) is(are), it is apparent that this material is heavilyhydrated (McCord et al., 1998, 1999a,b; Dalton,2000). The hydrate abundance appears to be mostconcentrated in disrupted landscapes such as thelinea and chaos terrains (McCord et al., 1998,1999a,b; Greenberg et al., 1999; Dalton, 2000;Fanale et al., 2000). This hydrated material is con-sidered by many researchers to be indicative of a
putative interior ocean composition (McCord etal., 1998, 1999a,b; Pappalardo et al., 1999; Kargelet al., 2000). It has been postulated that this oceancould have at one time harbored prebiotic com-pounds or even extraterrestrial life (Sagan et al.,1993; Fanale et al., 1998; McCord et al., 1998; Chybaand Phillips, 2001). Terrestrial extremophiles areactually known to thrive in environments rich insulfur compounds, including sulfuric acid andmagnesium sulfates (Rothschild, 1990; Rothschildand Mancinelli, 2001). Identifying and mappingthe distributions of these compounds on Europawould be an important step in evaluating its as-trobiological potential.
At present, the surface appears to be composedof a mixture of water ice, hydrated salts, radiol-ysis products (such as hydrogen peroxide, othersulfur compounds, and possibly simple organicslike formaldehyde), and some as yet unidentifiedcomponent (McCord et al., 1998; Carlson et al.,1999a,b, 2002; Fanale et al., 1999; Dalton, 2000).While the global distribution of hydrogen perox-ide detected on Europa appears linked to themagnetospheric plasma flux (Carlson et al.,1999a), it shows no obvious correlation to the dis-rupted terrains.
Questions remain concerning the surface com-position for three primary reasons. First, radi-olytic processing can obscure the signature of andchemically alter the original surface materials,particularly on the trailing hemisphere (Johnsonand Quickenden, 1997; Cooper et al., 2001). Ob-servations of disrupted terrains on the leading
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Europa FIG. 1. Galileo NIMS spectrumof hydrated material on Europafrom observation G1ENNHI-LAT01, shown with syntheticspectrum of water ice havingnominal grain size of 100 mm, cal-culated with optical constants ofwater ice measured at 100 K (fromDalton, 2000). The NIMS spectrumhas been vertically offset by 10.3for viewing purposes. Water ice ex-hibits known absorption featuresnear 1.0, 1.25, 1.5, and 2.0 mm,which appear symmetric and “U”-shaped. The distorted and asym-metric absorptions at the 1.5- and2.0-mm positions in the Europaspectrum are interpreted to indi-cate the presence of hydrated ma-terials at the surface.
hemisphere, which encounters far lower radi-olytic fluxes, would help to address this issue.Second, the surface displays a great deal of het-erogeneity on small spatial scales. Dark, dis-rupted terrains as small as 100 m imaged by SSIhave not been measured independently by NIMSbecause of its large pixel size (Fanale et al., 1999).All NIMS observations are averages of light re-flected from both icy and “non-icy” terrain units(McCord et al., 1999a). Third, the spectral resolu-tion of the NIMS observations is not sufficient todetect the fine structure that can in some casesdistinguish the candidate materials from eachother. Future missions and instruments areneeded to resolve this structure.
Interpretation of remotely sensed spectral in-formation hinges upon the availability of refer-ence spectra of known candidate materials mea-sured under controlled conditions (Fink and Sill,1982; Gaffey et al., 1993; Jamieson and Dalton,2002). Examination of cryogenic reflectance spec-tra of hydrated salts under simulated Europantemperature (80–130 K) and pressure conditionsprovides important design criteria for spectrom-eters intended for deployment to Europa. Of theEuropa candidate materials that have been char-acterized at relevant temperatures thus far, thehydrated sulfate salts remain the most popularsurface components in the published literatureand are likely to make up a significant propor-tion of the surface materials (McCord et al., 1998,1999a, 2001, 2002). These materials, as with manyhydrates, exhibit pronounced spectral variationsat temperatures below 150 K (Dalton and Clark,1998; McCord et al., 1999a, 2002; Dalton, 2000). Asthe temperatures on Europa typically range from80 to 130 K, such cryogenic reflectance spectra areimportant references for the study of Europa’ssurface composition.
EXPERIMENTAL APPROACH
In order to investigate the spectral behavior ofhydrates, a number of measurements were per-formed. A complete description of the cryogenicenvironment chamber, high vacuum pumpingsystem, and spectrometer can be found in Dal-ton (2000). Room temperature spectra were ac-quired for a suite of hydrated magnesium sul-fate salts. Low temperature spectra for twohydrated salts, hexahydrite (MgSO4?6H2O) andbloedite [Na2Mg(SO4)2?4H2O], were measuredusing a cryogenic environment chamber. With
one exception, all hydrate spectra in this paperwere measured using an Analytical Spectral De-vices (ASD) portable field spectrometer operatingover the 0.35–2.5 mm wavelength range. The vis-ible and near-infrared portion of the spectralrange (up to 1 mm) utilizes a silicon detector witha spectral sampling of 1.4 nm and a resolution of6 nm. The remainder of the spectral range is cov-ered by a pair of indium antimonide (InSb) de-tectors with spectral sampling of 2 nm and reso-lution of 11 nm (Goetz et al., 1998). The ASDspectrometer was programmed to average 100measurements, each having an integration timeof 1 s, together for each spectrum; after checkingfor consistency, five such spectra acquired at eachtemperature were averaged to produce the re-sults presented here. This corresponds to an in-tegration time of 500 s per observation. All roomtemperature spectra were acquired using Spec-tralon® (Labsphere, Inc.) as the reference mater-ial, while all low temperature measurementswere performed using Halon® (Allied Chemical)as the reference; the influences of minor spectralvariations in the reference materials were cor-rected using National Bureau of Standards mea-surements for Spectralon and Halon. Because ofthe long integration times and high signal inher-ent in the laboratory measurements, the errorbars of the resultant spectral measurements aretoo small to be clearly visible in the figures, andare therefore omitted for clarity. Calculated vari-ances typically ranged from 1025 to 2 3 1024,with a few values as high as 1023 longward of 2.4mm, where the ASD InSb detector sensitivity isless than optimal. Digital processing of spectra re-ported in this paper was accomplished using theUSGS SpecPr program (Clark, 1993), and all ab-sorption feature positions, widths, and banddepths have been calculated using the standardcontinuum-removal methods described in Clarkand Roush (1984).
The cryogenic chamber is constructed of stain-less steel, and consists of three concentric cylin-drical layers. The outer two layers are welded andsealed to provide a dewar. Liquid nitrogen is in-troduced through a small welded port, and cre-ates a bath above which a third, inner cylindersits. This innermost cylinder forms a samplechamber approximately 30 cm deep 3 18 cm di-ameter, and is supported from above by a lip thatrests upon the dewar. A 6-cm-diameter circularcopper plate is attached to the bottom of the sam-ple chamber via a 3.5-cm-diameter copper cylin-der and extends into the liquid nitrogen bath.
SPECTRAL BEHAVIOR OF HYDRATED SALTS 773
This serves as a coldfinger, which conducts heataway from the 3.5-cm-diameter copper sampleholder within the chamber. The sample chamberinterior is painted with carbon lamp black paintto absorb infrared radiation and can be evacuatedto 1025 mbar using a combination of roughing,sorption, and ion pumps. Two 49-mm-diametercustom sapphire (Al2O3) viewports in the cham-ber lid are arranged with incidence and emissionangles of 5°, giving a phase angle of about 10°. Il-lumination is provided via a tungsten halogenlamp with a regulated DC power supply. Sampletemperatures were monitored using a Lake ShoreCryotronics DT-470 gold-coated silicon diodetemperature sensor interfaced with a model 200temperature monitor and power supply externalto the chamber via a four-lead configuration.
Samples for the magnesium sulfate hydrate se-ries (MgSO4?nH2O, where n 5 1, 1.5, 2, 3, 4, 6, and7) were prepared from reagent-grade epsomite(MgSO4?7H2O) according to established temper-ature profiles and time intervals (Kamecki andPalej, 1955; Windholz et al., 1983) using a Barn-stead/Thermolyne High Performance 1700°CFurnace. The epsomite was ground and sievedprior to dehydration, and the 50–100-mm sizefraction selected. Some moisture was releasedupon grinding, and grains exhibited a degree ofcohesiveness, occasionally forming looselybound aggregates as large as 500 mm. Examina-tion of samples using an optical microscope be-fore and after each experiment established thatgrains were generally irregular in shape, with nu-merous facets which served as scattering centers,giving the overall appearance of a fine whitepowder. Sample compositions were verified us-ing a Perkin Elmer Pyris 1 differential scanningcalorimeter (DSC). The spectrum of pentahydrite(n 5 5) is the only one not measured with theASD spectrometer and was taken from Crowley(1991). The pentahydrite spectrum has been con-volved to the same wavelength set as the ASDspectra to facilitate comparison. In the case of n 50, a sample was synthesized by drying reagent-grade “anhydrous” MgSO4 at 800°C for 72 h inan NDI Vulcan A-550 furnace. The sample wastransferred directly from the oven into a desicca-tor containing Drierite® (anhydrous CaSO4 with3% CoCl2) (W.A. Hammond Drierite Co., Ltd.)crystals in a matter of seconds. The sample wasthen brought to the environment chamber, whichhad been purged continuously with N2 gas forhalf an hour, and transferred into the chamberagain in a matter of seconds, and the chamber
was rapidly evacuated to 1027 Torr. Optical ex-amination before and after drying and measure-ments revealed no evidence for fracturing orother physical changes such as air bubbles thatcould alter the scattering properties of the crys-tals, although qualitatively the dried MgSO4grains appeared slightly less transparent. For n 56 no heating was necessary; the sample wasplaced in a desiccator with Drierite crystals for 48h, and its composition was verified by DSC. Thereagent-grade epsomite was found to be partiallydehydrated, and therefore was placed in a high-humidity vessel for 24 h prior to measurement.Samples were kept in sealed containers inside adesiccator after DSC scanning, and checked forcompositional changes by a second DSC scan im-mediately after spectral measurements. Thesodium-magnesium sulfate hydrate, bloedite, wastaken from the Ward Collection at the U.S. Geo-logical Survey in Denver, CO. A section of a largercrystal was selected, crushed, and then examinedunder an optical microscope. Grains bearing visi-ble impurities were painstakingly removed byhand with a dental pick. The remaining grainswere ground and sieved to a 100–200-mm fractionand measured under nitrogen purge the same day.Microscopic examination revealed translucentgrains of irregular shape and rough, heavilyscratched faces but few signs of internal fractur-ing. Spectra of the sample acquired both beforeand after the experiment showed no appreciabledifference from the original specimen, and goodagreement with the published room temperaturespectra of bloedite from Crowley (1991).
SPECTRAL BEHAVIOR OF HYDRATED SALTS
Water and water-bearing compounds gener-ally exhibit characteristic absorption features atthe frequencies corresponding to permitted vi-brational transitions of the water molecule. Thenear-infrared spectrum of water is dominated byovertones and combinations of these transitions(Herzberg, 1991). The water molecule has threefundamental vibrational modes: an H—O—Hbend near 1,645 cm21, and both asymmetric andsymmetric O—H stretching modes near 3,450 and3,490 cm21, respectively. These correspond to fun-damental spectral absorption features at 6.08, 2.90,and 2.87 mm (Eisenberg and Kauzman, 1969). Inwater ice, these are shifted somewhat and occurcloser to 6.2, 3.05, and 3.0 mm (Ockman, 1957;
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Gaffey et al., 1993). Overtones and combinationsof these fundamentals, as well as interactions witha crystal lattice in water ice and other solids suchas minerals, produce additional absorption fea-tures in the near-infrared (Pauling, 1935). The elec-tric fields of the hydrogen and oxygen orbitalsoverlap, distorting the configurations of their os-cillating neighbor molecules, altering the oscilla-tion frequencies and combining to spread the nar-row transitions into broad and easily recognizableabsorption features. Many of these absorptionsare temperature-dependent, notably the 1.65-mmwater ice transition (Grundy and Schmitt, 1998).
The magnesium sulfate molecule has no corre-sponding near-infrared features. There are twoS5O stretching modes, with frequencies near1,330 (asymmetric) and 1,160 (symmetric) cm21,which give rise to absorption features at 7.5 and8.6 mm, well outside the near-infrared range(Chaban et al., 2002). It is important therefore tonote that the near-infrared spectral features of hy-drated salts arise from the water incorporatedinto the molecular structure, and are not intrin-sic to the host compound. The primary featuresof liquid water at approximately 1.0, 1.25, 1.5, and2.0 mm (values rounded for simplicity) are shownin the lower spectrum of Fig. 2. In order to en-hance the shapes of these features and preventsaturation, a mixture was made of water with50% (by weight) of finely ground quartz (SiO2),which is spectrally bland in this wavelengthrange. Grains of pure quartz were ground, sievedto ,1 mm diameter, and then suspended in wa-
ter. The spectrally neutral quartz particles serveto scatter some of the incident photons, reducingthe mean optical path length and preventing sat-uration in the long wavelength bands. Note thesymmetric, “U”-shaped character of the primarywater absorptions, and the displacement from thepositions of these absorptions in water ice in Fig.1. The middle spectrum in Fig. 2 is of reagent-grade magnesium sulfate, sold as “anhydrous”but apparently containing a small quantity of wa-ter physically bound to the MgSO4. This is sug-gested by the absorption features at 1.25, 1.5, and2.0 mm. The top spectrum in Fig. 2 is of the sameMgSO4 sample, after being dried at 800°C for 72h. The 1.25-mm absorption feature has virtuallydisappeared; the 1.5- and 2.0-mm feature banddepths have been reduced by 44% and 35%, re-spectively, while the reflectance levels at 1.2 and1.7 mm, between the water absorption positions,have only increased by 11% and 15%, respectively.The reduced intensity of the absorption features,compared with the continuum levels outside thefeatures, and the slight increase in overall re-flectance are consistent with a reduction in watercontent. Similar effects might be achieved by re-ducing the grain size or otherwise increasing theamount of scattering, but examination with anoptical microscope showed no evidence of frac-turing, pitting, or breakdown of the grains, whichremained translucent at visible wavelengths. Thissuggests that the mean optical path length wasnot significantly altered by heating. Also, theslopes of the two lower spectra are consistent
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FIG. 2. Spectra of magnesium sul-fate before and after drying for 72 h at800°C (top two curves) shown with thespectrum of liquid water.Note the po-sitions of the water absorptions at 1.25,1.5, and 2.0 mm in the MgSO4 spectra,caused by physisorbed water in thesample. These spectra demonstrate thatthe absorption features in the magne-sium sulfate salts at these locations arenot intrinsic to the MgSO4, but to theH2O associated with the MgSO4 mole-cules. For ease of viewing, the “anhy-drous” and “dried” MgSO4 curveshave been vertically offset by 10.25and 10.35, respectively.
with that produced by a strong water absorptionnear 3.0 mm, while the “dried” MgSO4 spectrumis nearly level. The reduced intensity of the wa-ter absorption features shows that most, thoughnot all, of the water has been driven off by heat-ing. It may be that some water was incorporatedinto the sample during its brief exposure to am-bient atmosphere, or that the heating was not suf-ficient to drive off all of the water; either possi-bility underscores the strongly hygroscopicnature of magnesium sulfate. Even exposed to thehard vacuum at the surface of Europa, magne-sium sulfate would not be expected to persist ina purely anhydrous state. Although the hydratedsulfate salts are only metastable against dehy-dration over long time scales for a wide temper-ature range (Linke, 1965; Marion and Farren,1999), McCord et al. (2001) have predicted thatsome highly hydrated sulfate salts originally de-rived from an interior ocean could persist into thecurrent epoch under Europan conditions. If pre-sent, sulfate salts are likely to coexist in a varietyof hydration states.
The reduction of intensity in the 1.25-, 1.5-, and2.0-mm absorption features in the top two curvesof Fig. 2 as compared with the continuum re-flectance levels demonstrates that these featuresare due to the incorporated water. Although themagnesium sulfate molecule does not give rise toits own near-infrared spectral absorptions, it doesexert considerable influence on the structure andappearance of the water features in the hydratedmagnesium sulfate salts. While hydrates typicallyexhibit absorption features at the primary waterfeature locations of 1.5 and 2.0 mm, these are gen-erally somewhat distorted and asymmetric com-pared with their appearance in pure water or wa-ter ice. This is due to the interactions of the hostmolecule and crystal lattice with the water incor-porated into the structure. Figure 3 illustrates theeffects of adding or removing water of hydrationto a sulfate salt. The top spectrum of dried MgSO4
(n 5 0) is the same as in Figure 2. Each successivespectrum in Fig. 3 adheres to the formula MgSO4
? nH2O, with n increasing from top to bottom. Aswater is incorporated into the structure, the shapeof the crystal is altered, and the orientations andbond lengths are changed. The attendant changesin electromagnetic fields and vibrational modesresult in changes to the spectrum. As the numberof possible transitions increases, the spectral fea-tures begin to merge and combine, creating broadabsorptions centered near 1.5 and 2.0 mm. Also,
as the number of waters of hydration increases,the broadened spectral features begin to resem-ble those of the hydrated material on Europa inFig. 1. It has been speculated (McCord et al.,1999a, 2001, 2002; Dalton, 2000) that magnesiumsulfates of even higher hydration states, such asMgSO4 ? 12H2O (dodecahydrate), might providea more nearly perfect spectral match.
At cryogenic temperatures, the transitions thatmake up the broad 1.5- and 2.0-mm H2O absorp-tions are less influenced by the energies of theirneighbors, and these individual absorption fea-tures become more distinct (Ockman, 1957;Hobbs, 1974). In the hydrates, this effect is am-plified by the presence of the host molecule,which acts to further separate the water mole-cules. At low temperatures, the hydrated salts ex-hibit discrete and easily separable absorption fea-
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FIG. 3. Spectra of the magnesium sulfate hydrate se-ries MgSO4 ? nH2O for n 5 0, 1, 1.5, 2, 3, 4, 5, 6, and 7.For viewing purposes, these have been offset verticallyby 11.8, 11.7, 11.55, 11.3, 11.0, 10.7, 10.5, 10.2, and10.2, respectively. No multiplicative scaling has been ap-plied. The absorption features near 1.0, 1.25, 1.5, 2.0, and2.5 mm are due to water incorporated into the crystalstructure. The n 5 0 curve (top) contains some water, giv-ing rise to the absorption features at 1.5 and 2.0 mm. Notehow the shape of the 1.5- and 2.0-mm features changes aswater of hydration is added. All spectra measured atroom temperature. The spectrum of MgSO4?5H2O is fromCrowley (1991).
tures (Dalton and Clark, 1998; McCord et al.,1999a, 2001; Dalton, 2000), which can be exploitedto distinguish among them.
SPECTRAL BEHAVIOR AT CRYOGENIC TEMPERATURES
The separation of hydrate features at low tem-peratures is exemplified by the cases of hexahydrite(MgSO4?6H2O) and bloedite [MgNa2(SO4)2?4H2O],which together may make up as much as 60% byweight of the surface salts on Europa (McCord etal., 1998). At temperatures below 150 K, the 1.5-mm water absorption feature in these salts sepa-rates into several distinct narrow features, pro-ducing a complex spectral signature (Dalton andClark, 1998; McCord et al., 1999a, 2001; Dalton,2000). Since the surface temperature varies be-tween approximately 80 K at the poles to 130 Kat the sunlit equator, hydrated salts at the surfaceof Europa should display these features. McCordet al. (2002) have pointed out that in flash-frozenbrines, many of these features are subdued rela-tive to their counterparts in pure crystalline hy-drates. This provides a potential means of distin-guishing between the brines or their analogs andother hydrated forms. At room temperature (Fig.4) both hexahydrite and bloedite exhibit similarspectral features because of water of hydration,despite the differences in their chemical compo-sition. Both have significant absorptions centerednear 1.0, 1.25, 1.5, and 2.0 mm. The absorptionband centers, approximate widths, and approxi-mate depths of the major features appear quitesimilar and are difficult to distinguish at low spec-tral resolution. The primary differences are in theweaker and narrower absorptions that make upthe distorted and asymmetric features at 1.5 and2.0 mm. At 120 K several spectral features becomenarrower and deeper, but the most striking dif-ferences are evident in the 1.5-mm band. Thesemay appear subtle when compared with the ex-tremely strong and broad H2O absorptions at thescales of Figs. 1–4, but they can be readily distin-guished at the spectral resolution and signal-to-noise ratio of the ASD spectrometer.
To investigate these differences, the spectra ofbloedite and hexahydrite at 120 K have been plot-ted on an expanded scale in Fig. 5. These spectrahave been linearly scaled and offset vertically toimprove visibility. Center positions, absorptionband depths, and full width at half maximum
(FWHM) have been calculated for each absorptionfeature using the continuum-removal method ofClark and Roush (1984). The largest of these fea-tures is the 1.45-mm absorption feature in hexa-hydrite (left side of lower curve). This feature ac-tually is comprised of two smaller features centeredat 1.46 and 1.48 mm. Together they produce aFWHM of 54 nm. The depth of this band is nearly20% of the signal level (continuum) at these wave-lengths. The other two weaker features at 1.54and 1.60 mm have FWHM values of 37 and 25 nm,respectively. Though weak, their respective banddepths of 7% and 11% are commensurate withband depths routinely utilized to discriminateminerals in terrestrial remote sensing investiga-
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FIG. 4. Scaled spectra of the hydrated sulfate salt min-erals hexahydrite and bloedite at room temperature andat 120 K. Note the cation-OH stretch feature at 1.35 mm,which appears as an indentation upon the short-wave-length shoulder of the 1.5-mm band. Broad features dueto water of hydration near 1.5 and 2.0 mm separate intoseveral smaller, discrete features, which are more pro-nounced at low temperatures, a phenomenon that can beutilized to produce reliable identifications of surface ma-terials at Europa. To maximize the dynamic range in thisfigure, the bloedite spectra have been scaled by a factorof 1.75 and then vertically offset. The 120 K spectrum hasbeen offset downwards by 20.1, while the 300 K spec-trum has been offset upwards by 10.25. The hexahydritespectra have been offset upwards by 10.82 (120 K) and11.03 (300 K) to improve visibility.
tions (Dalton et al., 2000; Clark et al., 2003) of sur-face regions containing large numbers of mixedminerals. Widths for the much stronger C-H ab-sorptions prevalent in organic molecules, whichare also of interest for Europa, range from one-half to five times this value (Dalton, 2002; Daltonet al., 2003). Another absorption feature diagnos-tic of hydrates is the 1.35-mm cation-OH stretch(Fig. 4; see also Dalton and Clark, 1999). Thisweak feature, superposed on the short wave-length shoulder of the 1.5-mm water feature, isabout 40 nm wide and occurs in most hydratedmaterials, including flash-frozen brines (McCordet al., 2002). Thus, while it may serve to set lim-its on the presence and abundance of hydratedcompounds, it may not be particularly useful fordiscriminating between them.
The cryogenic spectrum of bloedite in Fig. 5 hasfour narrow yet distinct absorption features thatare not apparent in room temperature spectra.These are centered at 1.47, 1.495, 1.52, and 1.55mm. While there may be yet finer structure thatcannot be distinguished at this resolution, thesefour diagnostic features are sufficient to identifybloedite. The most difficult feature to distinguishin a remote sensing application would be the 1.47-mm feature at left, which has a depth of only 1.1%relative to continuum, and a FWHM of 7 nm. Thisis still sufficient to span 5 channels on the ASDspectrometer (data points in Fig. 5), which is justenough to plot the shape of the feature, thoughthe absorption is likely too weak to calculate use-ful quantitative estimates from remotely senseddata. The next feature to the right, at 1.495 mm, ismore easily distinguished, with a band depth of1.7% and FWHM of 14 nm. The third feature isonly slightly narrower, with a FWHM of 13 nmbut a depth of only 1.1%, similar to the leftmostfeature (1.47 mm). The fourth and deepest feature,at 1.55 mm, has a band depth of 3% and FWHMof 15 nm, well within the resolving power of mod-ern laboratory instrumentation. While detectionof single percent absorption features in remotelysensed reflectance spectroscopy is pushing thepresent state of the art, minerals have been suc-cessfully identified using 3% features in terres-trial imaging spectroscopy (Dalton et al., 2000).These subtle yet measurable features can be usedas a standard to constrain the instrument para-meters necessary to reliably discriminate betweenhydrated materials on the cold surface of Europa.
SPATIAL CONSIDERATIONS
The asymmetric and distorted absorption fea-tures interpreted as evidence of hydrated surfacematerials are highly correlated with dark, dis-turbed terrains on Europa (McCord et al., 1998,1999a,b; Greenberg et al., 1999; Dalton, 2000; Fanaleet al., 2000). Spectral identification of this materialcould provide insight into the processes that havecreated and modified these terrains. This is con-tingent upon the ability to spatially resolve discreteterrain units of interest. Spectra of adjacent regionswithin an instrument field of view will combine toproduce an average spectrum having contributionsfrom each region. If these regions are composed ofdifferent materials, then the combined spectrummay contain absorption features from all materials,
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Hexahydrite 120K
FIG. 5. Cryogenic spectra of bloedite and hexahydritemeasured at 120 K. At low temperatures, the absorptionsassociated with certain transitions become discrete, en-abling discrimination between various hydrates. Thesefeatures have FWHM values ranging from 7 to 50 nm.The total width (wavelengths or channels spanned) ofeven the narrowest feature (at ,1.48 mm in bloedite) ex-ceeds 10 nm, well within the capabilities of modern in-strumentation. These spectra have been scaled and offsetto facilitate comparison. The bloedite spectrum has beenscaled by a factor of 5 and then offset by 21.5; the hexa-hydrite spectrum has been scaled by a factor of 3 and thenoffset by 20.75.
but at reduced spectral contrast than in the spectraof the individual surface units. According to thelinear mixing model (Gaffey et al., 1993; Mustardand Sunshine, 1999), the contribution of each com-ponent spectrum is proportional to the area of thesource region but also is proportional to the albe-dos. If, as in the case of Europa, a small patch ofan unknown material is surrounded by water ice,the average spectrum will be dominated by the wa-ter ice. This complicates detection of minor con-stituents of interest.
For this reason it is desirable to sample contigu-ous areas of a given composition in order to collectseparate spectra of spatially segregated surfaceunits. While these may turn out themselves to bemixtures, acquiring spectra of dark terrain free ofthe strong and complex absorption features of wa-ter ice will facilitate identification of componentmaterials. For reasonable statistical sampling, it isalso desirable to have multiple pixels within agiven surface unit. Adjacent measurements canthen be compared with each other and even aver-aged together to boost signal and reduce noise.
Investigations using Galileo SSI imaging haveidentified a number of intriguing surface featuresthat exhibit albedo variations on the order of hun-dreds of meters. Blocks and matrix materialswithin chaos regions exhibit recurring patches ofhomogeneous, dark red material ranging fromtens of meters to a few kilometers in width (Spaunet al., 1998; Greenberg et al., 1999). Ridged plainsunits are crisscrossed by linea having widths inthe tens to hundreds of meters (Kadel et al., 2000;Phillips et al., 2000). Some of these linea are darkerand redder than the surrounding terrain, indi-cating compositional variations. Spatially andspectrally resolved observations of dark linea ma-terials would provide valuable constraints on en-dogenic processes and their products.
Consider the linea depicted in Fig. 6. This SSIimage has a nominal resolution of 6 m/pixel andshows a number of small-scale features. At thetop of the image and in the foreground are sev-eral linea whose central troughs are filled withdark material. These troughs are approximately100 m in width and present an opportunity for
SPECTRAL BEHAVIOR OF HYDRATED SALTS 779
500 m
FIG. 6. Europan ridged plains(near 13°S, 235°W) imaged duringorbit 12 with the Galileo SSI clearfilter at a nominal image resolutionof 6 m/pixel. The linea in the upperand lower portions of the image havecentral troughs containing depositsof dark material ,100 m wide, butwith bright, presumably icy ridgesand walls close by. Spectral observa-tions with at least 100 m spatial res-olution would be needed to inde-pendently measure the bright anddark materials. Such measurementscould address outstanding questionsregarding the composition and ori-gins of the dark material.
spectral measurements of contiguous deposits ofthe dark material. This material may be linked tointerior processes, or possibly the result of radi-olytic processing, or a combination of both, whilethe bright icy material may be redeposited waterfrost. Spectral imaging of ridged plains at coarserspatial resolution than about 100 m/pixel will notbe able to measure these bright and dark materi-als in isolation from each other.
Figure 7 is an SSI image of the crater Mannan-nàn (3°N, 240°W) taken with the red, green, andinfrared (0.968 mm) filters. The crater floor andejecta exhibit a number of color heterogeneities,down to the resolution limit of 79 m/pixel. Muchof this material has the dark red color associatedwith the hydrated materials, and may representeither impactor-contaminated melt, or materialexcavated from a darker layer beneath Europa’ssurface (Moore et al., 1998, 2001). The white boxindicates the position of Fig. 8, which shows a pitjust east of the center of the crater at a resolutionof 20 m/pixel. Moore et al. (2001) have suggestedthat the well-defined spider-like feature at thecenter of the pit may be an extrusion emergingfrom radial fissures. Because this feature does notexhibit any noticeable surface relief in stereo views,or an ejecta blanket, a separate impact origin has
been ruled out. The strikingly dark nature of thematerial argues strongly for compositional differ-ences compared with the surroundings. Spectralobservations of such deposits have the potentialto provide definitive information about Europa’ssurface and subsurface composition, if they canbe separated from their surroundings. As withthe ridged plains, spectral imaging at less thanabout 100 m/pixel will not be capable of inde-pendently measuring the discrete compositionalunits that make up these intriguing surface fea-tures. On the other hand, as spatial resolution in-creases beyond 100 m/pixel, the ability to samplesmaller-scale deposits increases, as does the num-ber of contiguous measurements that can be per-formed within a given surface unit, enhancing thepotential science return of the observations.
DISCUSSION
Resolving narrow features requires severalspectral channels covering the wavelength spacefrom the centers to the wings of the features; a sin-gle spectral channel at the center is not sufficient.For example, a spectral feature spanned by onlythree channels is indistinguishable from noise be-
DALTON780
Mannannan`
FIG. 7. Color image of the crater Mannannàn (3°N, 240°W) acquired during orbit E14 using Galileo SSI red,green, and infrared (0.968 mm) filters. Image resolution is 79 m/pixel. The sun angle is moderately high and to theright. Color variations can be seen down to the resolution limit, but discrete, nearly homogeneous patches of darkmaterial are evident near the crater rim. This material appears much darker than the other red materials in the im-age, suggesting a difference in composition. As many as 10 adjacent 100–m pixels could be placed across the mosthomogeneous portions of these deposits, allowing unique compositional information to be extracted with high sta-tistical confidence. The white square just east of the crater center indicates the position of Fig. 8.
cause while the two channels at either side definethe continuum, only one channel describes the ab-sorption—and one channel of variation could bedue to random noise. Experience with the GalileoNIMS instrument has shown that noise due to ra-diation hits on the spacecraft electronics producesa large number of single-channel spikes, whichmust be dealt with in calibration and postprocess-ing (McCord et al., 1999b; Dalton, 2000). With fourchannels, a spectral feature becomes more believ-able, but it is doubtful that the exact band centeror edge will fall directly upon a channel position.Accurate knowledge of band shape and depth isdesirable for accurate discrimination and deriva-tion of quantitative material abundances. A mini-mum condition is to have five channels across aroughly Gaussian-shaped feature: even if the bandcenter is not at the central channel, there are twochannels available to describe the shape of eachside, one near the continuum level and one within
the absorption well. As the number of channels in-creases, accuracy of complete band-shape map-ping from the observations improves (Clark et al.,1990, 2003). With only five channels, a best-casesituation would be to have the central channel di-rectly on the band center, the adjacent pair of chan-nels near the FWHM, and the outermost pair atthe continuum level. This is not likely to occur inpractice, and so it is desirable to have as manychannels as possible.
With advance knowledge of surface composi-tion, it is possible to select wavelength positionsto coincide with absorption features of materialsof interest. However, in the case of Europa, pre-sent knowledge of surface composition is not ma-ture enough to make all the required predictions.There remains the distinct possibility of surfacecomponents that have not yet been considered.Further laboratory work on the spectral behaviorof additional materials, including hydrates, atlow temperatures is clearly warranted. It is alsopossible that some surface materials may exhibitfiner spectral structure than that discussed in thispaper. Since this possibility remains regardless ofthe resolution under consideration, it should bepointed out that there is no maximum desirableresolution: The potential to identify new materi-als increases with improved resolution. The goalof this paper is to describe the minimum resolu-tion necessary to discriminate among the pro-posed hydrated materials, both alone and in com-bination with each other. Given the radiationenvironment, with its attendant disequilibria, itis likely that the surface contains a number of ma-terials, with various states of hydration. Since theexact nature of the hydrated material is not yetknown, accurate identification hinges upon hav-ing sufficient spectral resolution to reliably dis-criminate among a number of compounds.
Examination of laboratory spectra of Europacandidate materials reveals a number of uniquefeatures at this level of detail (Dalton, 2000). Basedupon the considerations above, robust composi-tional interpretation will require a spectrometerhaving a 5 nm or better sampling interval. Put an-other way, there are known spectral features inhydrated salts that cannot be resolved by an in-strument having less than 5 nm resolution.
The spectral bandpass defines the width of thesmallest feature that can be resolved. The maxi-mum information can be extracted from the spec-trum if the sampling interval is half the bandpass(Nyquist sampling) or smaller (oversampling)
SPECTRAL BEHAVIOR OF HYDRATED SALTS 781
FIG. 8. Galileo SSI image of an intriguing feature justeast of the center of Mannannàn. This image was alsoacquired during E14, but with an increased resolution of20 m/pixel. The sun angle is moderately high and to theright. Even with some shadowing, the darkest material atthe center of the image can be seen to have a differentalbedo than its surroundings. This material could be ei-ther impact melt, or endogenous material that has seepedthrough radial fractures; some level of radiolytic pro-cessing may also have occurred. The origin, composition,and evolution of this deposit are not fully understood.Spectral observations much larger than 100 m/pixelwould not be able to address these issues because theywould not be able to resolve the dark central materialfrom its surroundings.
(Slater, 1980; Richards, 1994; Clark, 1999). How-ever, most contemporary imaging spectrometerspresently use critical sampling (sampling intervalequal to or commensurate with bandpass) be-cause signal-to-noise ratios in array detectors areproportional to the sampling interval (Swayze etal., 2003). Taking this into consideration, a spec-trometer having 5 nm bandpass and sampling(these quantities are often considered togetherand referred to simply by the term “resolution”)would help constrain the identities of most of thesurface materials considered here. As demon-strated in Fig. 5, an instrument operating at 2 nmresolution should be capable of unambiguouslyidentifying all of them.
CONCLUSION
The family of hydrated salts predicted to makeup a large proportion of the surface material atEuropa exhibit distinct fine structure in the near-infrared region. In at least some of these salts,these distinctions are even more pronounced atthe low temperatures of the icy satellites. If aspectrometer capable of at least 2–5 nm spectralresolution were deployed at the surface it wouldbe able to definitively answer many of the re-maining questions regarding both surface andsubsurface compounds, provided that it was ableto move around enough to sample many differ-ent terrain types. If such an instrument wereplaced in orbit it could also address these ques-tions, if it was capable of spatially resolving dis-crete surface features on a scale of 100 m orsmaller. The spatial and spectral resolutions rec-ommended here should be regarded as minimumcapabilities. An instrument meeting or exceedingthese specifications would allow spectral mea-surements of contiguous sections of disruptedand discolored terrains, enabling deciphermentof the enigmatic Europan surface composition.
ACKNOWLEDGMENTS
Sincere thanks to Roger Clark and Ted Roushfor access to the cryogenic and spectrometer sys-tems. I am grateful to Jim Crowley for useful dis-cussions and the pentahydrite spectrum. Also, toCorey Jamieson who synthesized the magnesiumsulfate hydrate series and assisted in the spectralmeasurements. This paper benefited from help-ful reviews by Tom McCord and an anonymousreviewer.
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