Hyperspectral reflectance mapping of cinder cones at the summit of Mauna Kea and implications for equivalent observations on Mars

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi: /2006je002822, 2007 Hyperspectral reflectance mapping of cinder cones at the summit of Mauna Kea and implications for equivalent observations on Mars Edward A. Guinness, 1 Raymond E. Arvidson, 1 Bradley L. Jolliff, 1 Kim D. Seelos, 1 Frank P. Seelos, 1 Douglas W. Ming, 2 Richard V. Morris, 2 and Trevor G. Graff 3 Received 31 August 2006; revised 22 February 2007; accepted 7 May 2007; published 28 July [1] Atmospherically corrected, hyperspectral reflectance data (0.4 to 2.5 mm) derived from the Airborne Visible Infrared Imaging Spectrometer (AVIRIS), validated with fieldbased traverses and laboratory analyses, were used to map the distribution of ferric oxide, sulfate, and phyllosilicate minerals exposed on fresh and altered cones on the summit of Mauna Kea Volcano, Hawaii. Spectra from 0.4 to 1.1 mm exhibit charge-transfer and electronic transition features related to hematite, nanophase iron oxide, and possibly jarosite, whereas spectra from 2.0 to 2.5 mm are characterized by cation-oh features related to kaolinite, montmorillonite, and saponite. Unaltered cones exhibit concentric zoning of iron oxide signatures with crystalline hematite located near the summits and nanophase iron oxide signatures found at the base of cones and on flow surfaces. Altered cones (Puu Poliahu and Puu Waiau) have exposures of three phyllosilicate and possible jarosite units. In some cases there is zoning with a core of saponite surrounded by montmorillonite. Kaolinite is only found in two exposures on Puu Poliahu with one exposure coinciding with a possible jarosite unit. The relatively small spatial scale associated with alteration zones seen on Mauna Kea demonstrates the importance of obtaining high spatial resolution hyperspectral observations of Mars. Citation: Guinness, E. A., R. E. Arvidson, B. L. Jolliff, K. D. Seelos, F. P. Seelos, D. W. Ming, R. V. Morris, and T. G. Graff (2007), Hyperspectral reflectance mapping of cinder cones at the summit of Mauna Kea and implications for equivalent observations on Mars, J. Geophys. Res., 112,, doi: /2006je Introduction [2] The Mauna Kea summit region is dominated by cinder cones and lava flows of the Laupahoehoe Volcanic series, which formed both during and after the late Pleistocene Makanaka glacial episode [Wolfe et al., 1997]. In addition, a few Laupahoehoe cones have been glacially eroded as evidenced by oversteepened slopes [Porter, 1987], which suggests that they predate the Makanaka glacial period. Two notable examples of possible preglacial cones are Puu Waiau and Puu Poliahu. Deposits on these cones have also been significantly altered, most likely by hydrothermal activity that has produced well-developed crystalline sulfates (alunite and jarosite), phyllosilicates, and zeolites [Ugolini, 1974; Morris et al., 1996; Wolfe et al., 1997; Swayze et al., 2002]. In addition, palagonitic tephra, which have nanophase ferric oxide, allophane, and other poorly crystalline forms of weakly altered basaltic glass (i.e., no phyllosilicates) have been described at several locations on Mauna Kea [e.g., Morris et al., 2000, 2001]. 1 Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri, USA. 2 NASA Johnson Space Center, Houston, Texas, USA. 3 School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA. Copyright 2007 by the American Geophysical Union /07/2006JE The mineralogy of these altered cones is dominated by three types of alteration processes: (1) hydrolytic, low-temperature alteration of basaltic tephra to form palagonitic material and nanophase Fe-oxides; (2) hydrothermal, sulfatetic/ hydrochloric acid alterations of basaltic tephra to form sulfates (jarosite, alunite) and phyllosilicates (kaolinite, smectite); and (3) thermal, oxidative alteration of fresh basaltic tephra to form hematite on cinder cones [Morris et al., 2000]. [3] Recent discoveries of phyllosilicates and hydrated sulfate minerals on Mars from the Mars Express OMEGA hyperspectral mapper (0.35 to 5.08 mm) [Bibring et al., 2005; Gendrin et al., 2005; Arvidson et al., 2005; Langevin et al., 2005; Poulet et al., 2005] demonstrate that mineralogically Mauna Kea displays many similarities with Mars. Thus examination of hyperspectral data for Mauna Kea, coupled with field and laboratory analyses for verification, provide a powerful tool for calibration of what can be mapped from OMEGA. The Airborne Visible Infrared Imaging Spectrometer (AVIRIS) is a hyperspectral imaging instrument that covers the wavelength range from about 0.4 to 2.5 mm in 224 wavelength channels, with a channel spacing of about 10 nm and average width of about 10 nm [Vane et al., 1993; Green et al., 1998]. Reflectance spectra in this wavelength range contain electronic transitions and vibrational absorption features from ferric oxide, phyllosilicate, and sulfate minerals typically found in alteration zones [e.g., Crósta et al., 1998]. These absorption features 1of14

2 are useful for identifying and mapping units that contain alteration minerals, such as those reported on Mauna Kea. [4] In this study, AVIRIS calibrated-radiance data are first reduced to ground reflectance values by applying the Atmospheric Removal (ATREM) algorithm [Gao et al., 1993]. These corrections are tested by comparing spectra retrieved from the AVIRIS data to ground reflectance spectra collected in the field. Units are defined on the basis of spectrally distinct reflectance signatures extracted from atmospherically corrected AVIRIS data using two wavelength regions to separately identify materials dominated by charge transfer and electronic transition absorptions ( mm) [Burns, 1993] and by metal-oh and/or carbonate ( mm) absorptions [Gaffey et al., 1993]. Candidate minerals that are likely contributing to the spectral signature of each unit are validated by comparisons with laboratory reflectance data of known samples. The spatial distribution of the mm and mm units are mapped using a spectral matching algorithm. Field observations and laboratory data (reflectance spectra and XRD) of samples collected from specific units are then used to constrain the mineralogy and texture of surface material exposed within each unit. The paper concludes with a discussion of associations among the mm and mm units and an analysis of the AVIRIS-derived maps, field observations, and sample data to understand the processes that generated and emplaced the units and to understand the spatial and spectral constraints and complications for mapping Martian mineralogy from OMEGA hyperspectral data sets. 2. AVIRIS Atmospheric Correction and Reflectance Computation [5] The AVIRIS scene used in this study was acquired along a flight line that began northeast of the Mauna Kea summit and continued to the southwest to the summit of Mauna Loa. A subsection of the scene, which is approximately km in size, covers the Mauna Kea summit (Figure 1). The summit area is essentially devoid of any vegetation that might influence the AVIRIS analysis. Ground control points determined from handheld GPS measurements and from digital topographic maps produced by the United States Geological Survey (USGS) have been used to georeference the subscene. AVIRIS calibratedradiance data have been corrected for atmospheric scattering and attenuation and converted to ground radiance coefficient values [Hapke, 1993] using the ATREM algorithm [Gao et al., 1993]. AVIRIS radiance coefficient values are referred to simply as reflectance throughout the rest of the paper. ATREM is based on a radiative transfer model that computes the atmospheric transmittance of gases and scattering from aerosols. Table 1 lists the specific parameter values that have been used for the ATREM processing. A refinement of the ATREM results is achieved by applying the Empirical Flat Field Optimal Reflectance Transformation (EFFORT) algorithm, which is a spectral smoothing process based on the empirical line method [Boardman, 1998]. This refinement adjusts for minor errors in calibration and atmospheric correction by applying a linear transformation with a gain near unity and an offset near zero to each AVIRIS band. After the ATREM and EFFORT corrections, AVIRIS reflectance spectra for shadowed areas at the summit have values in the range of 0.04 to 0.06 and are spectrally featureless. [6] First order validation of atmospheric corrections can be examined by comparing the AVIRIS retrieved spectra with ground spectral reflectance observations measured over a homogeneous area. The area selected for validating the atmospheric corrections was a uniform, flat location at the summit that is covered by fresh to slightly altered cinders (see Figure 2 for calibration site location). Twenty-one reflectance measurements were collected in a grid covering an area approximately equivalent to one AVIRIS pixel using an Analytical Spectral Devices, Inc. (ASD) FieldSpec Pro spectrometer, which has a spectral range of 0.35 to 2.5 mm with a spectral resolution of 3 nm in the VNIR and 10 nm in the SWIR. The geographic coordinates of the calibration site, determined by handheld GPS measurements, were used to locate the calibration site within the AVIRIS scene. An AVIRIS spectrum for the site was retrieved by averaging a 2 2 box of pixels closest to the coordinates of the calibration site. The ASD and AVIRIS spectra are compared in Figure 3. Note that wavelength regions near 1.4 and 1.9 mm, which correspond to the atmospheric water bands, were omitted from the AVIRIS spectra. The retrieved AVIRIS spectrum is an excellent match to the average of the ASD ground reflectance spectra. The difference between the two is less than 11% at all wavelengths and the average deviation is about 2.6%. This comparison demonstrates that the atmospherically corrected AVIRIS data are representative of the reflectance for materials exposed on the surface. 3. Definition of Spectral Units [7] AVIRIS spectra for a significant fraction of the summit area are relatively dark and featureless and similar to the spectrum of the calibration site (Figure 3). These spectra are characterized by a uniform rise in reflectance from 0.4 to about 0.75 mm with a maximum reflectance value of about , a shallow ferrous absorption centered at about 0.95 mm, and a decrease in reflectance into the infrared region. This spectral reflectance pattern and field observations demonstrate that these areas are covered by fresh to slightly altered basalt tephra. Thus the AVIRIS reflectance data set has been searched for regions that are spectrally distinct from the commonly exposed fresh to slightly altered basalt. For the analyses, the wavelength range of AVIRIS has been divided into visible and nearinfrared (VNIR) and short-wave infrared (SWIR) wavelength sections to separate charge transfer and electronic transition features related to ferric minerals, such as hematite, goethite, and jarosite, from vibrational absorption features due to hydroxyl-bearing minerals, sulfates, and carbonates. Separate analyses of VNIR and SWIR wavelengths were based on preliminary results, which showed that VNIR features would dominate the definition of spectral units if the full wavelength region was used [Guinness et al., 2001]. Specifically, the wavelength range of 0.41 to 1.06 mm (corresponding to AVIRIS channels 5 to 75) was used in the VNIR analysis, and wavelengths between 2.09 to 2.45 mm (AVIRIS channels 182 to 218) were used for the SWIR analysis. 2of14

3 Figure 1. A single georeferenced AVIRIS band containing the Mauna Kea summit region is shown. The white box indicates the portion of AVIRIS frame that was analyzed in this study. Inset map locates the AVIRIS scene on the island of Hawaii. [8] Identification of distinctive spectral patterns in AVIRIS reflectance data with dozens of channels can be a difficult task, particularly if the depths of absorption features are shallow and therefore subtle. In this analysis, the approach used has been widely used for analysis of hyperspectral data sets and has been described by Kruse and Boardman [1994, 2000]. The dimensionality of the hyperspectral data set was first reduced to enhance subtle absorption features by applying a Minimum Noise Fraction (MNF) transformation on both the VNIR and SWIR portions of the AVIRIS data set [Green et al., 1988]. The MNF transform is computed as two successive principal component transforms [Lee et al., 1990]. The first principal component transform operates on an estimated noise covariance matrix for the data set, which is derived from the pixel-to-pixel variations in the data, to decorrelate noise in the data set. A second principal component transform is then applied to the data with decorrelated noise. The MNF result is a series of images ordered from highest to lowest eigenvalue. MNF images associated with higher eigenvalues typically display the highest variance and the least amount of noise, whereas MNF images with lower eigenvalues are typically noise dominated. Thus restricting further analysis to MNF images with the highest eigenvalues reduces both the dimensionality and noise in the data set. For the Mauna Kea AVIRIS data, the twelve MNF images with the highest eigenvalues were found to contain significant data variance for both the VNIR and SWIR wavelength ranges. In both cases these twelve MNF images were then analyzed with the Pixel Purity Index (PPI) technique [Boardman et al., 1995] to find the most spectrally distinct pixels. The PPI technique repeatedly projects n-dimensional vectors representing the AVIRIS spectra onto random n-dimensional unit vectors. Pixels that fall at the extremes of the distribution when projected along a given unit vector are tagged as being unique. Pixels that are most frequently tagged as unique are considered to be spectrally pure. The selection of spectral units was limited to the subset of the most spectrally pure pixels identified by PPI analysis. This subset of pure pixels was examined in n-dimensional space to select groups of pixels that were spectrally similar as possible spectral units. The final criteria for defining the spectral Table 1. Parameters Used in AVIRIS Atmospheric Correction Parameter Value Scene ID F000414T01_P03_R04 Date acquired 14 April 2000 Scene center location (deg, min, sec) N latitude W longitude Sensor altitude, km 20 Average surface elevation, km 4 Aerosol model midlatitude summer Gas species H 2 O, CO 2,O 3, NO, CO, CH 4,O 2 Total ozone, atm-cm 0.34 Visibility, km 50 3of14

4 Figure 2. False-color infrared aerial photograph shows the portion of the Mauna Kea summit examined in this study. The names of prominent cinder cones at the summit are indicated, along with the location of the calibration site used to test the atmospheric correction procedure. units were that a group of spectrally pure pixels had to be both spectrally similar and geographically co-located Visible and Near-Infrared Spectral Units [9] Five distinctive VNIR spectral signatures or units have been identified from the AVIRIS data within the study site. The best examples of these spectral signatures, as determined by the analysis method discussed above, occur on the cinder cones Puu Pohaku, Puu Poliahu, and Puu Lilinoe. The type locations for the five VNIR spectral units are shown in Figure 4. The spectral unit designations are based on the wavelength range (V for VNIR), an abbreviation for the cone where the reference sample was found, and a sequence number because multiple units are found on a single cone. These five VNIR spectral units have several common reflectance features (Figure 5). First, there is a well-defined reflectance increase from UV and visible wavelengths to near-infrared wavelengths most likely due to a strong ferric charge transfer absorption centered in the UV [Morris et al., 1985]. The spectra also contain a pair of absorption features with centers at mm and at mm that are likely due to Fe 3+ electronic transitions [Burns, 1993]. The five VNIR spectra tend to be distinguished by the steepness of the ferric charge transfer absorption edge and the depth and position of electronic transition features (Figure 5). The VLN1 unit identified on Puu Lilinoe has evidence for a third ferric absorption centered near 0.68 mm. Interpretation of the Fe 3+ absorption features in these spectra is complicated because the position and strength of the absorptions can be dependent on particle size, Fe 2 O 3 concentrations, and on matrix effects [e.g., Morris et al., 1989; Morris and Lauer, 1990], in addition to mineralogy. However, a number of laboratory studies have examined the VNIR spectral characteristics of Mauna Kea altered tephra samples [Bell et al., 1993; Golden et al., 1993; Morris et al., 1993, 1997, 2001]. These studies, together with data from spectral reflectance libraries [Grove et al., 1992; Clark et al., 1993], are used to make an initial Figure 3. The robustness of the AVIRIS atmospheric correction procedure is demonstrated by comparing AVIRIS and ASD field reflectance spectra of the same area. The field spectrum is the average of 21 spectra acquired over a relatively uniform flow unit near the base of Puu Poliahu that is approximately the size of one to two AVIRIS pixels. The AVIRIS spectrum was extracted from the same location as the field spectra. 4of14

5 Figure 4. The type locations of both the VNIR and SWIR spectral units are shown on a single-band (0.751 mm) AVIRIS image of the study area. The names of the cones where these units were found are also noted. The naming scheme for the spectral units consists of a letter for the wavelength region (V for VNIR and S for SWIR), a two-letter indicator for the cone where the unit was found (LN, Puu Lilinoe; WA, Puu Waiau; PL, Puu Poliahu; and PH, Puu Pohaku), and a sequence number because several units are defined from Puu Poliahu. interpretation of the likely minerals controlling the spectra of the five VNIR units. Additional interpretations about the mineralogy of these units will be discussed in the sample analysis section. [10] The three VNIR units labeled as VLN1, VPH1, and VPL1 have spectra with similar shapes and absorption features, but vary mainly in terms of the steepness of the ferric charge transfer absorption edge (Figure 5). The charge transfer feature and the ferric absorption features seen in these three spectra centered at and mm are consistent with hematite samples contained in spectral libraries from Grove et al. [1992] and Clark et al. [1993]. In addition, the AVIRIS spectrum for VLN1, which has the deepest absorptions, is very similar to a published laboratory reflectance spectrum for the coarse fraction of a highly oxidized Mauna Kea tephra sample [Golden et al., 1993]. Laboratory analyses of that tephra sample identified the presence of hematite. AVIRIS spectra for the VPH1 and VPL1 units are similar to that for VLN1, except that the ferric charge transfer features have steeper slopes and the other ferric absorptions are not as strong (Figure 5). The VPH1 and VPL1 spectra are consistent with laboratory reflectance data published for the fine fraction of several altered tephra samples collected from Mauna Kea that contain both hematite and nanophase ferric oxide [Bell et al., 1993; Golden et al., 1993]. [11] The remaining two VNIR spectral units have AVIRIS spectra that are somewhat different from the spectra of the three hematitic units. The spectrum for VPL2 is characterized by a relatively featureless reflectance rise starting from UV wavelengths through about 0.75 mm and relatively constant reflectance at longer wavelengths into the infrared (Figure 5). The reflectance characteristics displayed by this Figure 5. AVIRIS spectra are plotted for five VNIR units. Each spectrum is an average of the purest pixels as identified by the PPI analysis. 5of14

6 Figure 6. AVIRIS spectra are plotted for the three SWIR units. (a) Each spectrum is the average of the purest pixels as identified in the PPI analysis. (b) SWIR unit spectra are plotted with continuum removed. The continuum is a fit to spectra outside the absorption features. Spectra in Figure 6b are offset by 0.05 in reflectance for clarity. spectrum are typical of Hawaiian palagonitic tephra [e.g., Singer, 1982; Bell et al., 1993; Morris et al., 1993, 2001]. The fifth VNIR spectral unit, VPL3, has a shape characterized by a well-defined maximum at about 0.75 mm that separates an absorption feature centered at about 0.90 mm from a deep ferric charge transfer absorption at shorter wavelengths (Figure 5). The reflectance maximum at 0.75 mm and the 0.90 mm absorption feature are consistent with laboratory reflectance spectra of a yellowish-colored tephra sample collected from a thin layer exposed on a cinder cone near the summit of Mauna Kea that was determined to be jarosite by Morris et al. [1996]. However, the AVIRIS spectra from the type location of the VPL3 unit or elsewhere in the AVIRIS scene did not show any evidence of a 0.43 mm jarosite absorption band, possibly because the feature is too narrow and shallow. Further mineralogic interpretation of these units will be addressed later in the paper in conjunction with the spatial distribution of the units and field observations Short-Wave Infrared Spectral Units [12] Three separate spectral units have been identified on the basis of the analysis of the AVIRIS SWIR data (Figure 6). These SWIR spectral units are denoted with the same naming scheme as used for the VNIR units. These spectral types were identified on the cones Puu Poliahu, Puu Waiau, and Puu Pohaku (Figures 2 and 4). Reflectance values for the three SWIR spectral units are in the range of 0.2 to 0.5 at SWIR wavelengths, which is several times brighter than unaltered tephra exposed elsewhere on the summit. These spectra also exhibit a strong negative slope from 2.0 to 2.5 mm. Superimposed on the negative slope are several relatively shallow absorption features (Figure 6a) that are more clearly defined when the continuum is removed (Figure 6b). Prominent absorption features seen in these spectra include a relatively strong doublet feature centered at 2.2 mm, a single absorption feature also centered at 2.2. mm, and two weaker features centered at 2.3 and 2.4 mm. These absorption features are interpreted to be due to metal-oh vibrational features indicative of phyllosilicates [Hunt and Ashley, 1979; Clark et al., 1993]. The negative slope in the SWIR wavelength region is also common in phyllosilicates and is due to strong O-H or H 2 O vibrational bands located near 3.0 mm [Salisbury et al., 1991]. [13] The well-defined absorption doublet with reflectance minima centered at 2.17 and 2.21 mm seen in the SPL1 spectral type is characteristic of Al-OH bending and OH stretching combinations in kaolinite [Clark et al., 1993; Hunt, 1979]. The two weaker features centered near 2.3 and 2.4 mm seen in the same spectrum can also be attributed to kaolinite absorption features [Hunt, 1979] (Figure 6). For comparison, the library kaolinite spectrum displayed in Figure 7 is that of a well-ordered sample [Grove et al., 1992] that provides a good match to the features seen in the AVIRIS spectrum for SPL1. A second SWIR spectral type named SPH1 has an absorption feature with a single minimum centered at 2.2 mm (Figure 6). This feature is likely due to an Al-OH vibration that is typically found in smectite minerals. On the basis of comparisons with library spectra [Grove et al., 1992; Clark et al., 1993], we have interpreted this 2.2 mm feature in the AVIRIS spectra as due to montmorillonite (Figures 6 and 7). In support of this interpretation, Ugolini [1974] reported the occurrence of montmorillonite in an altered tephra deposit collected from Mauna Kea summit cinder cones. [14] The third AVIRIS SWIR spectral unit named SWA1 contains weak absorption features centered at about 2.3 and 2.4 mm (Figure 6). These features are generally consistent with the minerals nontronite and saponite, which are both smectites. Library SWIR reflectance spectra of nontronite 6of14

7 in shape, whereas the saponite feature is asymmetric in these library spectra due to a shoulder from a weaker absorption centered at about 2.29 mm [Swayze et al., 2002]. Detailed examination of the AVIRIS spectrum for the SWA1 unit suggests that the wavelength position and shape of the absorption features seen in this spectral unit are best matched by library spectra of saponite. In addition, saponite has been identified through XRD analysis of samples collected from Puu Waiau and Puu Poliahu [Ugolini, 1974; Swayze et al., 2002], which are the two cones where this spectral type is found in the AVIRIS data. The combination of the spectral evidence and the previously reported occurrences of saponite on Mauna Kea cones provide the basis for interpreting this third SWIR unit as containing saponite. [15] The systematic analysis of the SWIR wavelength region did not identify a spectral unit that could be attributed to alunite, which has been previously reported in samples collected at the summit of Mauna Kea [e.g., Morris et al., 2003] and in samples to be discussed later in this paper. SWIR spectra of alunite have an absorption feature centered at about 2.16 mm (Figure 7). In addition, jarosite, which was inferred from the VNIR data, has an absorption feature centered at about 2.26 mm [e.g., Hunt, 1979; Clark et al., 1993] that was not detected by the AVIRIS analysis. To further explore evidence for alunite and jarosite in the AVIRIS SWIR data set, AVIRIS spectra were searched for any evidence of absorption features at 1.75 and 2.16 mm for alunite or at 2.26 mm for jarosite by manually selecting and examining spectra throughout the AVIRIS data set. In no cases were spectra with these absorptions observed. For alunite, this negative result may indicate that alunite is not exposed over a large enough area at the scale of an AVIRIS pixel (15 15 m) or does not occur in high enough abundance to produce a detectable absorption in the AVIRIS data set. For jarosite, the absence of a SWIR absorption supports the observations made by Clark et al. [2003] that VNIR absorptions in jarosite are much stronger than its SWIR absorptions. Figure 7. Mineral spectra extracted from USGS spectral library [Clark et al., 1993] are plotted for comparison to SWIR unit spectra. (a) Spectra are offset for clarity. (b) Library spectra are plotted with continuum removed. The continuum is a fit to spectra outside the absorption features. Spectra in Figure 7b are offset for clarity. and saponite are very similar with only subtle differences in the positions and shapes of the 2.3 and 2.4 mm absorption features. As such, it can be difficult to distinguish among these two smectites with the spectral resolution of AVIRIS. Spectra of nontronite and saponite from Clark et al. [1993] suggest that the position of the 2.3 mm feature occurs at a slightly shorter wavelength in nontronite as opposed to saponite. In addition, the nontronite feature is symmetric 4. Spatial Distribution of Spectral Units [16] AVIRIS VNIR and SWIR spectral signatures discussed in the previous section were used to generate maps showing the distribution of these materials throughout the Mauna Kea summit region with separate maps created for the VNIR and SWIR data sets. The VNIR and SWIR maps were generated using the Spectral Angle Mapper (SAM) method [Kruse et al., 1993]. The Spectral Angle Mapper (SAM) method is one of many techniques (e.g., unmixing [Adams et al., 1993] and Tetracorder [Clark et al., 2003]) that have been used to map spectral units from multispectral and hyperspectral data sets. We have chosen to use the SAM technique because our objective is to show areas that are similar spectrally to the reference spectral units. This approach meets our need of showing the areal extent of highly altered regions. We expect to pursue more advanced techniques for identifying subpixel mixtures of spectral types, but these approaches is beyond the scope of the current paper. [17] The SAM method computes the angle between two spectra by treating the spectra as n-dimensional vectors, 7of14

8 Figure 8. Distribution of spectral units derived from SAM analysis is overlain on the single-band AVIRIS image of the Mauna Kea summit. (a) VNIR units with overlay of red for VLN1 (hematite), green for VPH1 (hematite), blue for VPL1 (hematite), purple for VPL2 (palagonitic material), and yellow for the VPL3 (jarosite) unit. (b) SWIR units with overlay of green for SPH1 (montmorillonite), red for SPL1 (kaolinite), and blue for SWA1 (saponite). where the number of dimensions is equal to the number of AVIRIS bands used in the analysis. The angle is determined from the dot product of two vectors where one vector is the reference spectrum for one of the VNIR or SWIR units and the other is the spectrum of a given AVIRIS pixel. Small separation angles mean that the two spectra are similar. The SAM method depends on the shape of the spectra and not on absolute reflectance values, thus minimizing illumination differences. The spectra from Figures 5 and 6 are used as reference spectra in mapping the distribution of spectral units. Pixels are assigned to a given spectral unit if the separation angle is less than an assigned threshold. Pixels with AVIRIS spectra that differ from all spectral units by more than the specified threshold are not classified. In this study, thresholds of 3 degrees (0.052 radians) and 2 degrees (0.035 radians) were used for the VNIR and SWIR data sets, respectively. The specific values used in this study represent a trade-off between the number of pixels classified and how closely the classified pixels match a given unit spectra. A smaller threshold was needed for the SWIR data set to avoid misclassifying likely noise in the AVIRIS spectra because of the relatively shallow band depths found in the SWIR units. [18] Results of the SAM analysis indicate that the distributions of VNIR and SWIR units are mostly restricted to the cinder cones at the summit (Figure 8). The exception is the palagonitic material unit (VPL2), which is found on both cones and associated lava flows. Relatively fresh, uneroded cinder cones, such as Puu Lilinoe and Puu Hau Kea (Figure 8a and Figure 2 for the locations of the cones) can have exposures of all three hematite units (VLN1, VPH1, and VPL1). In addition, cones that have all three hematite units exposed often exhibit a concentric pattern with the VLN1 unit, which has the strongest hematite absorptions, at the center, surrounded by the VPH1 unit, and then surrounded by the VPL1 unit, which has the weakest hematite absorptions. The concentric patterns also correlate with the topography of the cones with VLN1 8of14

9 found on the higher, central portions of the cone and the other two hematite units at lower elevations. Thus there is a trend of material with strongest hematite absorptions occurring at the summits of fresh cones through material with weaker hematite absorptions toward the bases of the cones. Some fresh cones have exposures of only the VPH1 and VPL1 hematite units without the VLN1 unit. In such cases, the VPH1 unit occurs at the higher portions of the cones and can be surrounded by the VPL1 unit at lower positions. Cones that are more eroded tend to have exposures of the hematite VPH1 and VPL1 units, along with the palagonitic material unit (VPL2) in more disorganized patterns. There is also one example of a concentric distribution pattern of hematite units on the eroded Puu Poliahu cone. The palagonitic material unit is also typically found widely dispersed across flow surfaces and is sometimes associated with the base of cinder cones. Last, the jarosite unit (VPL3) is only found in a few, small isolated exposures on Puu Poliahu and Puu Wekiu (Figure 8a). The size of individual VNIR unit exposures varies over a wide range, with length scales from as small as a few tens of meters across up to several hundred meters in the case of the VPL1 hematite and the VPL2 palagonitic material units. [19] The SWIR units are predominantly found on the Puu Pohaku, Puu Wekiu, and the more eroded cones Puu Poliahu and Puu Waiau (Figures 8b and Figure 2 for the locations of the cones). The few cases where the SWIR units are exposed near the flanks of a cone appear to represent down-slope transport of material from the cone based on the topography of the Mauna Kea summit region. The SPH1 (montmorillonite) unit is the most commonly occurring SWIR unit. It is exposed over much of the relatively fresh cone Puu Pohaku and in a small area on Puu Wekiu. The montmorillonite unit is also found on several areas of the eroded Puu Poliahu and Puu Waiau cones. The SWA1 (saponite) unit, in contrast, appears to be restricted to the eroded cones of Puu Poliahu and Puu Waiau. On Puu Waiau, the saponite unit occurs in two relatively linear zones about halfway up the outer flanks of the cone. These two saponite zones are surrounded by exposures of the SPH1 montmorillonite unit. The SWA1 unit exposure on Puu Poliahu occurs on the eastern flank of the cone and forms a concentric pattern with respect to a broader exposure of the montmorillonite unit. The distribution of the SPL1 (kaolinite) unit resulting from the SAM analysis is limited to two small isolated areas of Puu Poliahu. However, there are examples from the montmorillinite unit where it surrounds the kaolinite unit of AVIRIS SWIR spectra exhibiting a weak doublet absorption feature at about 2.2 mm, which would suggest the presence of kaolinite. The doublet absorption feature for these areas may be weakly defined, perhaps because kaolinite is only exposed over a small fraction of an AVIRIS pixel or well mixed with other materials. The SAM method probably assigned such pixels to the SPH1 montmorillonite unit because the absorption features for both montmorillonite and kaolinite occur at about the same wavelength. Thus the SPL1 kaolinite unit on Puu Poliahu may be more prevalent and cover a somewhat larger area than is shown on Figure 8b. The sizes of the SWIR units tend to be roughly the same magnitude as the VNIR units. The mapped kaolinite occurrences are both less than 100 meters across, whereas the saponite and montmorillonite exposures can vary from about one hundred to several hundred meters across. [20] Several trends can be seen when the distribution of the VNIR and SWIR units are compared to each other. For instance, relatively fresh, uneroded cones that have the VLN1 hematite unit do not appear to have any phyllosilicate unit other than a few small, isolated patches of the montmorillonite (SPH1) unit. In contrast, other fresh, uneroded cones that do not show evidence of the VLN1 hematite unit, but still expose the other two hematite units, generally have widespread exposures of the montmorillonite unit and some exposures of the palagonitic material unit. The more eroded cones generally have exposures of the VPH1 and VPL1 hematite units and the VPL2 palagonitic material unit, along with the two smectite units, SPH1 (montmorillonite) and SWA1 (saponite). Finally, it appears that the SPL1 (kaolinite) and the VPL3 (jarosite) units tend to occur together as localized exposures on eroded cones, most notably Puu Poliahu. 5. Field Observations and Sample Analysis [21] Field observations and laboratory analyses were used to better understand the detailed mineralogy of material exposed at the scale of centimeters to meters within the AVIRIS-defined spectral units. Field observations included two traverses across portions of Puu Poliahu with the ASD FieldSpec Pro spectrometer (described above) to make reflectance measurements. One traverse was conducted across part of the eastern flank of the cone that was mapped as the saponite and palagonitic material units. The second traverse covered a topographic saddle area near the center of Puu Poliahu where an exposure of the kaolinite unit is located. Materials along the traverses were characterized with spectral reflectance measurements by random sampling during the traverses. In addition, samples were collected from locations on Puu Poliahu within several of the mapped units. The samples were analyzed in the laboratory with reflectance spectroscopy and X-ray diffraction analysis (XRD). Figure 9 is an air photo showing Puu Poliahu with the sampling sites and traverse paths marked. [22] Surface materials along the eastern flank traverse consisted of a mixture of loose, dark to reddish gravelly material (centimeter to centimeter size cobbles) and lighter toned finer-grained tephra. There were also a few dark to reddish toned boulders exposed on the flank, along with an occasional outcrop patch of the lighter toned material. The spectral reflectance measurements made along the flank traverse were generally consistent with the area mapped as the saponite and palagonitic material units. Figures 10a and 10b show this match by comparing VNIR and SWIR spectra, respectively, for one of the field measurements acquired along the flank with an AVIRIS spectrum averaged over a 2 2 pixel area extracted from approximately the same area. However, examples of other spectral classes were also detected along the traverse. These included the hematite types, including the VLN1 type, montmorillonite, and relatively unaltered basalt (i.e., similar to the calibration site spectrum shown in Figure 3). The Puu Poliahu saddle area is characterized by a similar mix of both dark and light toned cobbles and tephra, along with whitish 9of14

10 Figure 9. False-color infrared aerial photograph provides a high-resolution view of the Puu Poliahu cinder cone. The field traverse paths where ASD field spectrometer data were acquired are indicated. The traverse across the eastern flank is labeled A, and the traverse in the saddle region is labeled B. In addition, the sample collection locations for laboratory spectra and XRD analysis are also noted. indurated gravelly material. There is also a complex of layered outcrop exposed in this area. As previously stated, the AVIRIS data suggest that kaolinite is exposed in this area. A comparison of a SWIR field spectrum with an AVIRIS spectrum covering approximately the same area supports the kaolinite interpretation (Figures 10c and 10d). In addition, the corresponding VNIR spectra suggest the presence of jarosite in both the field and AVIRIS data. This is consistent with other locations mapped by AVIRIS where kaolinite and jarosite units are spatially correlated. [23] Laboratory SWIR reflectance and XRD results are reported for samples collected at six Puu Poliahu sites that correspond to where jarosite and the three phyllosilicate units occur. At each of the six sampling sites, material within a roughly cm area approximately 1 2 cm deep (including rocks) was collected at each site. Samples were separated by dry sieving in the laboratory into > 2 mm, mm and < 0.5 mm fractions. Table 2 lists the sample identifiers, the AVIRIS spectral unit sampled, and a summary of the laboratory analyses results. Laboratory VNIR spectra for the samples are similar to each other and generally indicate the presence of palagonitic material or hematite with weak absorption features. As a result the VNIR laboratory spectral observations are not included in Table 2. Mineralogical and spectral properties of each size fraction were characterized by X-ray diffraction analysis and reflectance measurements using an ASD FieldSpec Pro spectrometer. X-ray diffraction analyses were conducted on powdered samples, which were compressed into a depression on an aluminum holder for random-oriented powder mounts. Samples were scanned from 5 to 70 2q using a scan speed of q/sec on a Rigaku X-ray Diffractometer with monochromatic CuKa radiation obtained with a graphite crystal monochrometer. [24] The mineralogy of six samples obtained by SWIR reflectance spectroscopy and XRD analyses is listed in Table 2. The spectral units mapped by AVIRIS are also listed in Table 2 for comparison to the laboratory analyses. Hematite, jarosite, and kaolinite were easily identified by XRD analysis. However, only smectite was confirmed via XRD analyses in some samples; XRD did not readily distinguish between the AVIRIS smectite classes, i.e., montmorillonite, saponite, and nontronite. For this study, laboratory SWIR reflectance measurements were used to distinguish between the three-smectite units by the identification of Al OH, Fe OH, and Mg OH combination bands in the mm region (Figure 11). As noted by Farrand and Singer [1990], SWIR reflectance spectra can be more sensitive than XRD in detecting poorly crystalline phyllosilicates. Phyllosilicates were most prominent in the fine fraction, i.e.,<0.5 mm as would be expected; however, phyllosilicates were also observed in the coarse fraction (>2.0 mm). These alteration phases have formed via hydrothermal alterations of basaltic tephra (see below) and hence these samples for the most part are strongly cemented by these alteration phases. [25] Kaolinite was easily identified by a 0.71 nm XRD peak and by a strong 2.21 mm SWIR band with a shoulder at 2.16 mm and weak bands at 2.32, 2.36, and 2.39 mm. Alunite was confirmed in samples by XRD peaks at of 14

11 Figure 10. ASD reflectance spectra acquired during field traverses on Puu Poliahu are compared to AVIRIS spectra for the same locations. AVIRIS spectra are averages of 2 2 pixel spots. (a) VNIR spectra of combined saponite and palagonitic material units from the eastern flank. (b) Corresponding SWIR spectra from the same location as Figure 10a. (c) VNIR spectra for the kaolonite unit from the saddle area traverse. (d) Corresponding SWIR spectra from the same kaolinite location as Figure 10c. SWIR spectra are plotted with the continuum removed. and nm and an absorption band at 1.47 mm, although alunite was not mapped as an AVIRIS spectral unit. Alunite also has an absorption band at 2.16 mm; however, the 2.16 mm kaolinite shoulder may have obscured this alunite band. Jarosite was not identified in SWIR laboratory spectra but major XRD peaks at 0.307, and nm indicated the presence of jarosite in several samples (e.g., HWMK276). Very broad XRD peaks around 0.3 nm suggested the presence of an amorphous Si or glass-like phase. While these amorphous phases could have been produced by several different processes, they have been previously interpreted to be palagonitic material in these samples [Morris et al., 2000, 2001]. Jarosite and palagonitic material were identified only in the VNIR regions in laboratory samples as described earlier for the AVIRIS data. [26] The identification of distinct smectite minerals was difficult in the laboratory samples. X-ray diffraction analyses easily confirmed the presence of smectite, i.e., broad XRD peaks around 1.5 nm, but XRD did not provide evidence for individual smectite end-members, i.e., montmorillonite, saponite, and nontronite. Clay fractions (<2 mm) of several smectite-rich samples were separated by sedimentation and centrifugation techniques. Mg- and K-exchanged samples confirmed the presence of 2-1 expandable phyllosilicates, i.e., smectite. Random powder mounts were also made of the treated clay fractions to determine if the 060 XRD peak could be used to distinguish between dioctahedral (i.e., montmorillonite and nontronite) and trioctahedral smectites (i.e., saponite) [e.g., Ozkan and Ross, 1979; Pevear et al., 1982]. Although there was an indication that most of the smectites analyzed by this method are dioctahe- 11 of 14

12 Table 2. Comparison of Laboratory Spectra, XRD Analysis, and AVIRIS Spectra for Puu Poliahu Samples a Sample SWIR Laboratory Spectra Results XRD Results Sample ID Location Fine Coarse Fine Coarse HWMK269 summit montmorillonite, montmorillonite plagioclase > plagioclase > augite > nontronite/saponite montmorillonite > augite olivine HWMK272 summit montmorillonite, weak kaolinite, weak plagioclase > augite plagioclase > kaolinite, nontronite/saponite alunite kaolinite > alunite > augite > and kaolinite montmorillonite, amorphous SiO 2 nh 2 O HWMK274 summit montmorillonite, weak nontronite/saponite montmorillonite, weak nontronite/ saponite and alunite HWMK276 west flank montmorillonite montmorillonite, weak nontronite/ saponite HWMK281 south flank montmorillonite, nontronite/saponite, weak kaolinite HWMK283 south flank montmorillonite, weak nontronite/saponite and kaolinite kaolinite, weak alunite kaolinite and montmorillonite(?), possible alunite amorphous SiO 2 nh 2 O plagioclase > augite > montmorillonite, amorphous SiO 2 nh 2 O plagioclase > augite > montmorillonite plagioclase > montmorillonite > augite > trace kaolinite plagioclase > augite > alunite, kaolinite, hematite/magnetite plagioclase > augite > montmorillonite, Amorphous SiO 2 nh 2 O plagioclase > augite > jarosite/alunite plagioclase > augite > montmorillonite > kaolinite, alunite, trace hematite/magnetite plagioclase > augite > alunite, kaolinite, hematite/magnetite AVIRIS Results montmorillonite kaolinite, palagonitic material kaolinite, palagonitic material montmorillonite, jarosite montmorillonite, jarosite, palagonitic material kaolinite, jarosite a Columns labeled Fine are for samples <0.5 mm in size, and columns labeled Coarse are for samples >2 mm in size. VNIR laboratory spectra generally indicated the presence of palagonitic material or hematite. dral in nature (i.e., approx nm 060 peak), there was not clear evidence to distinguish the dioctahedral smectites, i.e., montmorillonite from nontronite. Most likely, the principal smectite in samples on Puu Poliahu are nearer the montmorillonite end-member. A strong Al OH combination band near 2.21 mm (without a 2.16 mm shoulder) suggests montmorillonite (e.g., HWMK276). However, the presence of nontronite and/or saponite cannot be ruled out because of weak SWIR bands centered at 2.24, , and 2.39 mm in some samples (e.g., HWMK281). Nontronite and/or saponite identification is based upon the Fe OH combination band near 2.29 mm; however, ferruginous smectites may also account for the 2.29 mm Fe OH combination bands along with Fe 3+ Al-OH combination bands near 2.24 mm [Bishop et al., 1998, 1999, 2002a, 2002b; Gates et al., 2002]. Although beyond the scope of this study, further research is needed to place better constraints on the mineralogy of the smectite phases in these Puu Poliahu samples. [27] In summary, the field spectral reflectance measurements and laboratory sample analyses generally confirm the inferences derived from AVIRIS data that alteration minerals Figure 11. SWIR laboratory reflectance spectra are plotted for Puu Poliahu samples collected at the locations indicated in Figure 9. Samples were sieved into <0.5 mm (fines) and >2 mm (coarse) fractions. SWIR data are plotted with the continuum removed to enhance absorption features. Spectra were also offset in reflectance for clarity. 12 of 14

13 and zones with Fe 3+ oxides, sulfates, and phyllosilicates are exposed on the surfaces of many of the Mauna Kea summit cones. However, the field and laboratory data also show that the composition and mineralogy of exposed materials at the centimeter to meter scale are more complex and diverse. In some cases, several types of materials occur together in a single sample or within a small area on the ground. 6. Conclusions [28] In this Mars analog study, three spectrally distinct hematite units are seen in the AVIRIS data in terms of the depth of ferric absorption features and degree of crystallinity. Three units with spectral features related to phyllosilicates were also identified in the AVIRIS data on the basis of metal-oh combination bands in the short-wavelength infrared (SWIR) region of 2.0 to 2.5 mm. Spectral features of kaolinite, montmorillonite, and saponite characterize these three units. The montmorillonite unit is the most commonly exposed phyllosilicate unit on Puu Poliahu. In some cases there is zoning among the phyllosilicate units with a core of the saponite unit surrounded by montmorillonite. The kaolinite unit is only found in two discrete exposures on Puu Poliahu with one exposure coinciding with a possible jarosite unit. A palagonitic material unit is typically exposed at the base of cinder cones and is more widely located on flow surfaces. AVIRIS spectral data are consistent (in most cases) with laboratory mineralogical analyses, and thus provide ground truth for the AVIRIS-derived unit maps. [29] The altered summit cones on Mauna Kea underwent hydrothermal alteration from vapors and/or fluids percolating through the cones while the volcano was active [Wolfe et al., 1997]. The analyses of AVIRIS data, together with field observations, show that the exposures of Mauna Kea alteration zones are typically only tens of meters to several hundred meters in size and are for the most part confined to individual cones. These results from the Mauna Kea provide an argument against the bedrock diagenesis hypothesis proposed by McCollom and Hynek [2005] for the sulfaterich outcrops seen in Meridiani Planum on Mars. They suggest that the sulfate material observed in the Burns formation of Endurance crater [Squyres et al., 2006] was generated by sulfur-dioxide vapors emitted from fumaroles in a volcanic environment [McCollom and Hynek, 2005]. However, the Opportunity rover has made measurements of compositionally and mineralogically similar layered sulfaterich outcrops over its several kilometer long traverse across Meridiani Planum [e.g., Arvidson et al., 2006; Morris et al., 2006]. Further OMEGA hyperspectral reflectance observations, combined with Mars Orbital Camera and Odyssey THEMIS imaging data, show that these deposits unconformably overlie dissected Noachian Cratered Terrain, and exhibit homogeneous properties over tens of kilometers or more [Arvidson et al., 2006]. Thus the Mauna Kea analog and the widespread distribution of sulfate material in Meridiani suggest that hydrothermal alteration in a volcanic environment is not likely for the Meridiani deposits. [30] Acknowledgments. This work was partially supported by NASA grant NAG to Washington University as part of the Planetary Geology and Geophysics Program. We acknowledge the helpful reviews by W. Farrand and J. Bishop that significantly improved the manuscript. References Adams, J. B., M. O. Smith, and A. R. Gillespie (1993), Imaging spectroscopy: Interpretation based on spectral mixture analysis, in Remote Geochemical Analysis: Elemental and Mineralogic Composition, edited by C. M. Pieters and P. A. 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