JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. E7, PAGES 14,621-14,665,JULY 25, 2001

Characteristicsof the Pathfinder APXS sites:Implications for the compositionof rocks and soils

NathanT. Bridgesand Joy A. Crisp JetPropulsion Laboratory, Pasadena, California

James F. Bell III Centerfor Radiophysicsand Space Research, Cornell University, Ithaca, New York

Abstract. An integratedstudy of spectral,elemental abundance, and image data from the Pathfinderalpha proton X-ray spectrometer(APXS) measurementsites has led to a better understandingof the natureof Martiansurface matehals. This rigorousanalysis provides a new level of detailthat forms the basisfor the resultsreported here and that can be usedby future scientiststrying to understandrocks and soilson . EachAPXS sitehas been precisely locatedby analyzingstereo Imager for (IMP) androver camera images to determine the positionof the APXS duringdeployment. Rover and IMP imagesare usedto assesslocal geology,namely, the presence of pebbles,clods, and aeolian features in soilsand surface textures anddust concentrations on rocks. IMP spectrausing the latestcalibrations for radianceand reflectanceare analyzedat a numberof differentillumination geometries for eachAPXS site. The three-dimensionalorientation of the siteshas been determined and then used to computethe true sunlight(incidence) and viewing (emission) angles. Where suitable photometric coverage is available,reflectances are fit to the Hapke-Irvinefunction. This is thenused to adjustspectral reflectancesto a normalizedillumination geometry common to all rocksand soils. Comparingall the datasets shows a positivecorrelation among red/blue reflectance ratio, SO3content, and dust abundanceon rocks,indicating that rocks are coatedwith varyingamounts of sulfur-rich,red dust. Otherelements, such as siliconand iron, are poortracers for dustbecause their abundances within soil are similarto thosewithin rock. Rock APXS targetsthat are bluer,poorer in sulfur,and have minimaldust coverage face toward the northeast,the directionin whichwinds may be capableof efficientlyremoving loose material under current Martian conditions. The IMP spectralproperties of the soilsshow poor correlations with APXS elementalabundances. On the basisof currently releasedAPXS data,bulk soilsat the Pathfinderlanding site are richerin silicaand sulfurthan the Viking landingsites, suggesting a differentcomposition. The APXS soilswith the mostapparent pebblesare the poorestin SiO2,indicating that either the pebblesare moremafic thanthe APXS rocksor thepebble-free soil componentis inherentlyenriched in SiO2compared to Viking soils. The mixtureof materialsat the APXS sitesis a reflectionof the overall complexityof Martian surfacematerials, a detailedunderstanding of whichshould drive the choice of futureinstruments and missions.

1. Introduction abundances(from here on, APXS "ite" refers to the region within the actual 5-cm APXS analysis spot or an area that The alpha proton X-my spectrometer(APXS) on Mars likely containsthe spot, as describedbelow). IMP furnishes Pathfinder's rover provided the first in situ similar information, generally at a lower spatial resolution, measurements of materials and the first andprovides visible andnear-IR spectrathat are relatedto the analyses of unequivocalMartian rocks. It acquired 16 oxidation state, mineralogy, and physical nature of the measurementsof geologic materials, of which 11 were of surface.As wasrecognized before landing, the complementary sufficient quality that they have been converted to oxide information providedby these instrumentscould serve as a abundances [Rieder et al., 1997a]. These 11 measurement useful tool for constraining the elemental chemistry, targetsconsist of :5 rocks and 6 soils. Supportingthe APXS mineralogy,and physical makeup of the materials[Golombek, measurementswere the rover camerasand the lander'sImager 1997; et al., 1997b]. for Mars Pathfinder(IMP). The images from the rover show The Pathfinder APXS and supporting data collectively the detailedmorphology of APXS sites, giving clues to the provide previously unavailable information regarding the components contributing to the measuredbulk elemental Martian surface. The only other successfulMars landers, and 2, had well-calibratedX-my fluorescence spectrometers(XRFS) and comparableresolution cameras. Copyright2001 by theAmerican Geophysical Union. However,the XRFS did not measurerocks [e.g., et al., Papernumber 2000JE001393. 1982] and the camerassampled fewer wavelengthsthan IMP 0148-0227/01/2000JE001393509.00 and could be calibrated only to within 10% uncertainty

14,621 14,622 BRIDGES ET AL.' PATHFINDER APXS SITES

[Guinnesset al., 1987]. The availability of an imaging rover iron-bearingminerals and their weatheredproducts [Smith et on Pathfinder provided greater spatial resolution for some al., 1997b]. The band centersof the 12 "geology" filters areasthan achievablewith the fixed camerasof Viking. Initial rangein wavelengthfrom 440 to 1000 nm [seeSmith et al., studiesusing complementary APXS data, rover imaging, and 1997b, Figure6]. IMP data are 12-bit encoded,providing a IMP spectra/imaging have proven very useful for 64-fold improvementin dataprecision over the 6-bit Viking understandingthe geology of the landing site [Bridgeset al., Landercameras. Spectral features of two mineral groups 19.97, 1998, 2000; Basilevskyet al., 1999; Bell and Bustani, expectedon the Martian surfacespan the IMP wavelength 1999; Bridgesand Crisp, 1999; McSweenet al., 1999; Bell et range. Crystallineferric (Fe 3*) oxides,oxyhydroxides, and al., 2000]. poorlycrystalline and nanophase ferric oxides have diagnostic This paperdocuments and exploresthe relationshipamong spectralsignatures from 440 to 860 nm, a region sampledby APXS, IMP imaging/spectra, and rover imaging data for the eight IMP geology filters [Morris et al., 1985, 1989, 1990; APXS sites. This is done in order to better understand and Smithet al., 1997b]. Ferrous(Fe 2*) silicates,principally characterize the rocks and soils at the Pathfinder site and Mars pyroxene and olivine, have absorption bands near 1 /•m in general,namely, their mixing relationships,physical state, [Cloutisand Gaffey, 1991; Gaffeyet al., 1993]. Thesebands spectralproperties, and geochemistry. In addition, the new were well sampled by the five filters spread over the level of detail reported herein should benefit future wavelengthrange of 860 to 1000 nm [Smithet al., 1997b]. investigationsof Martian surfacematerials, which will require Onceon the Martian surface,IMP acquiredmultispectral data progressively more sophisticated measurementsand data of the entire Pathfinderlanding site. Seven imaging sequences analysis techniques. First, a brief review of the IMP, rover can be ttsedfor spectralanalyses (Table 1). Except for the camera,and APXS instrumentsis presented.This is followed Multispectral Spots, each of these covers almost the entire by an overview of earlier related studies. The methods landingsite scene. The Superpanand Multispectral Spots are employedto analyze the data are then presented. This is of mostrelevance because they wereacquired in all 12 geology followed by a discussionof the location, geology, surface filters with losslessor minimal (2:1) compression. orientation, and spectralproperties of each APXS site. The IMP images also provided important information on illumination geometry of each measurementspot, combined geology and geomorphology. The Insuranceand Superpan, with simple photometricmodeling and assumptions,is then with losslessor 2:1 compression,acquired images of rocksand usedto normalize the reflectancesand spectral ratios to a soils with a resolutionof 1 mrad/pixel, comparableto the 0.7 commongeometry. The paper goes on to comparethe APXS, mrad/pixelfor the Viking Landercameras. Finally, several imaging, and normalized spectral data. The Pathfinder soil IMP sequenceswere devotedto determining the position, compositionsare then comparedto Viking analyses. Finally, orientation,and location of the rover. Thesedata were usedby inferences on the overall characteristics of rocks and soils on engineersto updateposition and orientationinformation in Mars are explored. the rover software,plan traverses,and assessthe successof previoustraverses and have beenttsed here to help determine 2. Background APXS site positions.

2.1. IMP Data 2.2. Rover Imaging Data The Imagerfor Mars Pathfindercamera provided visible-near The PathfinderSojourner rover camed one color and two IR multispectral,stereo imaging at the Pathfinderlanding site monochrome cameras [Rover Team, 1997a]. The color camera at a resolutionof 1 mrad/pixel [Smith et al., 1997a]. The IMP was located on the rear of the rover. It had pixels for red, filter set wasdesigned to searchfor the spectralsignatures of ,and infrared and a crosstrack by along track resolution

Table 1. PathfinderImage Sequences Relevant to GeologySpectral Studies

, ,, , Imaee Seauence Compressiona Fi![ers, gm Stereo (s) PredeployPan (SOO01) 6:1 (965)' 12:1 (670) 670, 965 no 1 (stowed position) Mission SuccessPan (S0002,8) 12:1,2x2 pix. ave.b (440, 670);24:1 (530) 440, 530, 670 yes (stowedposition) Insurance Pan (S0030-33) lossless 440, 530, 600, 750 yes (stowedposition) Monster Pan (S0069,70,73,74) 6:1, 2x2 pix. ave.b(all,except 670)' 6:1 (670) 440, 530, 670, 750, 965 yes (deployedposition) Gallery Pan (S0164-167) 6:1 440, 530, 670 no 8-19 (deployed position) MultispectralSpots (S0171-2) lossless all yes 4-35 (deployedposition) Superpan(S0181-188) lossless (440, 670); 2:1 others all yes 13-83 (deployedposition)

, , , , •:ompressionfor imagingof surface.Compression used for sky imageswithin samesequence may differ. bPixelaveraging factor; if notindicated, no pixel averaging was done. BRIDG• ET AL.: PATI-tFINDER APXS SITES 14,623

of 3.4 x 2.9 mrad/pixel. The monochromecameras were mountedon the front of the roverand provided stereo images at a crosstrack x along track resolutionof 2.9 by 3.4 mrad/pixel. Becauseof the ability of the rover to get close to targets, the resultingspatial resolution of imagesof APXS targetsacquired by the rover cameraswas generally significantly greaterthan IMP.

2.3. APXS Data

The alpha proton X-ray spectrometer(APXS) was designed to determinethe elementalcomposition of rocks and soils on Mars [Riederet al., 1997b]. Emissionof alpha particlesfrom a :44Cmsource causes Rutherford backscattering (alpha mode), nuclear reactions of the alpha particles with some light elements(proton mode),and ionization by the alpha particles, producingX-rays (X-ray mode). The three modestheoretically provideinformation on all elementsheavier than He. Mars' CO• atmosphereinterferes with the alpha modeand the proton datacan effectively be understoodonly with complementary alpha data [Riederet al., 1997a]. As a result, only data from the PathfinderX-ray mode have been published. This mode providesinformation on the abundancesof elementsZ= 11 (Na) and higher, characterizingthe most abundantcations, plus chlorine, foundin commongeologic materials. Furtherdetails of the APXS instrumentand datareduction are given by Rieder et al. [ 1997a, 1997b]. At the presenttime we are limited to these X-ray results. The APXS teamis engagedin ongoing work to producea more complete elemental library with scaling factors for each detectableelement. Alpha and proton modecalibration under Martian atmosphericconditions is also proceeding[Jet PropulsionLaboratory, 1999; Foley et al., 2000]. Thelocations of theAPXS sites are shown in Figures1 and 2 and Table 2. By the end of the mission, 27 APXS measurementshad been madeon Mars (Tables 3a and 3b). Elevenwere atmospheric measurements (deployment did not reachrock or soil), with a twelfth(A-9) beingof poorquality owing to incompletecontact of the APXSon the surface. Four measurements(A-19, 20, 23, and 27) had low signal/noise becausethey weretaken in daytimeafter the failureof the rover battery and have not yet been converted to elemental abundances.The remaining 11 measurementsconsist of 5 rocksand 6 soils. All siteswere imaged multispectrally by IMP and, exceptfor BarnacleBill rock (A-3), by the rover front or rear cameras.

2.4. Previously Published Relevant Science Results

Theelemental abundance data from the APXS, mineralogical andsurface coating inferences from IMP multispectraldata, and morphologicalinterpretations from IMP and rover images have provided important information on the Pathfinder landingsite. TheAPXS soil compositionsare similar to those foundby Viking [Riederet al., 1997a],although they may be more consistentwith a mixture of Viking sulfate/chloride cementsand andesiticrock fragments[McSween and Keil, 2000]. Theirspectral, elemental, and magnetic characteristics pointto a palagoniticcomposition with a magneticphase of Fe-Ti spinels or maghemite[Madsen et al., 1999; Bell et al., 2000; Morris et al., 2000]. Providinga chemicallink between rocksand soils that could explain soil formationthrough rock modificationhas proved elusive [Bell et al., 2000]. The soils 14,624 BRIDGESET AL.' PATHFINDERAPXS SITES

cannotbe chemicallylinked to the rocks throughhydrolitic or IMP spectrareveal four rock classes: "gray," "red," "pink," acid-sulfate weathering processes [Morris et al., 2000], and"maroon" [McSween et al., 1999, Table 2]. There is a indicating that they may have an independentorigin. The strongcorrelation between the depth of the530 nm absorption APXS rock compositions are rich in silica and potassium andthe 670/440 nm ratio anda goodanti-correlation between relative to Martian soils and SNC meteorites, leading some the900- and530-nm absorption bands for manygray and red researchersto classify them as andesites[McSween et al., rocks. A correlation also exists betweenthe 670/440 nm ratio 1999]. However,the rocks appeardust-coated in IMP images andreflectivity in a givenwavelength. Ferric minerals such as and are far richer in sulfur and chlorine than most igneous ferrihydrite,maghemite, and especiallyhematite exhibit rocks, indicating that the APXS rock analyses are likely prominent530-nm and 800- to 900-nmabsorption bands, sampling a mixture of rock and dust [Bridgeset al., 1997, havehigh red/blueratios, and have high reflectivities[e.g., 1998; McSween et al., 1999]. Morris et al., 2000], whereassome pyroxenes have absorption

Table 2. IMP Sequencesfor APXS Sites

APXS. Site , Position(x-y.-z) • SceneSequence Image112]' Sol/LST RadcalImage ID Sol/LST A-2 1.89, - 1.95, 0.31 Mission Success 0002210155 1/0917 0172040(X)3 10/0921 undisturbed 0002210153 1/0917 0172040015 10/0924 maybedisturbed Gallery 0164020783 10/1129 017204(X1)3 10/0921 0164020643 10/1129 0172040015 10/0924 maybedisturbed Superpan 0182010155 18/1532 0466020031 18/1440 0182010153 18/1531 0466020029 18/1440 0182010159 18/1533 0466020027 18/1440

A-3 1.30, -2.45, 0.18 0033030025 2/1 649 0466040015 55/1523 0033030067 2/1 649 0466040011 55/1522 MS Spot 0172020063 5/1042 0172020003 5/1034 0172020075 5/1 043 0172020015 5/1036 0172020073 5/1042 0172020013 5/1036 Superpan 0182010122 18/1521 0466020016 18/1438 0182010121 18/1520 0466020029 18/1440 0182010127 18/1522 0466020027 18/1440

A-4 2.79,-2.64, 0.28 Iredeploy 0001120121 1/1020 0049030003 3/0910 undisturbed undisturbed Mission Success 0002210155 1/0917 017204(X)03 10/0921 0002210153 1/0917 0172040015 10/0924 undisturbed Insurance 0033030023 2/1648 0466040015 55/1523 0033030065 2/1648 0466040011 55/1522 undisturbed Monster 0069063123 3/0937 017204(X)03 10/0921 0069063121 3/0936 0172040015 10/0924 0069063127 3/0937 0172040013 10/0923 disturbed Gallery 0164020573 10/11 28 017204(X)03 10/0921 0164020433 10/1127 0172040015 10/0924 undisturbed MS Spot 0172060123 13/1125 0172•3 13/1108 0172060135 13/1127 01720OX}15 13/1111 0172060133 13/1127 0172060013 13/1111 disturbed Superpan 0182010155 18/1532 0466020031 18/1440 0182010153 18/1531 0466020029 18/1440 0182010159 18/1533 0466020027 18/1440

A-5 3.29,-2.48, 0.28 1redeploy 0001120117 1/1019 0049030003 3/0910 undisturbed undisturbed Insurance 0033030021 2/1646 0466040015 55/1523 0033030063 2/1 647 0466040011 55/1522 undisturbed Monster 00690&3053 3/0935 017204(X)03 10/0921 0069063051 3/0935 0172040015 10/0924 0069063057 3/0935 0172040013 10/0923 disturbed,undisturbed/ Gallery 0164020571 10/1122 017204(X)03 10/0921 nearbylocation 0164020431 10/1122 0172040015 10/0924 disturbed Superpan 01820 10105 18/1514 0466(00031 18/1440 0182010103 18/1514 0466020029 18/1440 0182010109 18/1516 0466020027 18/1440 undisturbed/nearby Superpan 0182030107 55/1529 0466020031 18/1440 location) 0182030105 55/1529 O46(K}20029 18/1440 0182030111 55/1530 046(K}20027 18/1440 Table 2. (continued)

APXS Site Position(x-y-:[) a Scen•.Sequence ImageID b Sol/LST RadcalImage ID Sol/LST A-7 4.58, 2.91, -0.18 Predeploy 0001120117 1/1019 0049030003 3/0910 Insurance 0033030010 2/1641 0466040030 55/1525 0033030053 2/1641 0466040011 55/1522 Monster 0069063053 3/0935 0172040003 10/0921 0069063051 3/0935 0172040015 10/0924 0069063057 3/0935 0172040013 10/0923 MS Spot 0172060063 13/1117 0172060003 13/1108 0172•5 13/1120 0172060015 13/1111 01720(Klff73 13/1119 0172060013 13/111 l Superpan 0181060107 66/0939 00150(K)015 66/0928 0181060105 66/0938 00150(K)013 6610927 0181060111 66/0939 00150(K)011 66/0927 Superpan 0182010055 18/1458 0466020031 18/1440 0182010053 18/1458 0466020029 18/1440 0182010059 18/1459 0466020027 18/1440 Superpan 0182030106 55/1529 0466O4O030 55/1523 0182030105 55/1529 0466040013 55/1523 0182030111 55/1530 0466040011 55/1522

A-8 2.85, 1.13, 0.32 MS Spot 0172050033 12/1038 0172050003 12/1032 0172050O45 12/1039 0172050015 12/1035 0172050043 12/!039 0172050013 12/1034 Superpan 0188030237 38/1435 0466040015 38/1423 018803023 5 38/1434 0466040013 38/1422 0188030241 38/1435 0466040011 38/1422

A-10 3.74,-0.43, 0.28 Insurance 0030020029 2/1338 0466O40015 38/1423 undisturbed 0030020071 2/1338 0466040011 38/1422 undisturbed MS Spot 0172040123 10/0937 0172040003 10/0921 0172040135 10/0938 0172040015 10/0924 0172040133 10/0938 0172040013 10/0923 disturbed MS Spot 0172130033 22/1115 0172130003 22/1110 0172130045 22/1116 0172130015 22/1112 0172130043 22/1116 0172130013 22/1111 dismrbecl Superpan 0181060139 66/0947 00150(K•315 66/0928 0181060137 66/0946 0015(Y:AX)13 66/0927 0181060143 6610947 001506(Oll 66/0927

A-15 -5.87, 2.80, 0.52 MS Spot 0171020033 34/1 236 0171020003 34/1231 undisturbed/bumperring 0171020043 34/1237 0171020013 34/1233 0171020041 34/1237 0171020011 34/1232 undisturbed Superpan 0185020057 13/0844 0007170069 33/1111 0185020055 13/0844 0007170067 33/1110 0185020061 13/0845 0007170065 33/1110

A-16 -3.79, - 1.31, 0.12 0032030007 2/1553 04660,g)015 55/1523 0032030049 2/1553 •11 55/1522 MS Spot 017204(X)63 10/0930 0172040003 10/0921 0172040075 10/0931 0172040015 10/0924 0172040073 10/0931 0172040013 10/0923 Superpan 0184010123 32/0951 0007170069 33/1111 0184010121 32/0951 0007170067 33/1110 0184010127 32/0952 0007170065 33/1110

0466040015 55/152.3 A-17 -5.56, -3.25, -0.35 0032030007 2/1553 0032030049 2/1553 0466040011 55/1.522 MS Spot 0172130063 22/1119 0172130003 22/1110 0172130075 22/1121 0172130015 22/1112 0172130073 22/1120 0172130013 22/1111 MS Spot 0172160033 35/1008 0172160003 35/1004 0172160043 35/1009 0172160013 35/1006 0172160041 35/1009 0172160011 35/1005 33/1111 Superpan 0184010073 32/0934 0007170069 0184010071 32/0934 OO07170O67 33/1110 0184010077 32/0936 OOO717OO65 33/1110 14,626 BRIDGF_xq3E-TAL.: PATHFINDER APXS SITES

Table 2. (continued)

APXS Site Position(x-y-z) a SceneSequence Image ID b Sol/LST RadcalImage ID Sol/LST A-18 -4.81, -3.81, -0.54 Insurance 0032030009 2/1554 0466040015 55/1523 0032030051 2/1554 0466(093011 55/1522 MS Spot 0172160063 35/1012 0172160003 35/1004 0172160073 35/1013 0172160013 35/1006 01'72160071 35/1013 0172160011 35/1005 Superpan 0184010089 32/0940 0007170069 33/1111 01840100• 32/0939 0007170067 33/1110 0184010093 32/0940 0007170065 33/1110

aMarsLocal Level Coordinate Frame (x pointsnorth, y pointseast, z pointsdown; origin [0,0,0] is geometriccenter of basepetal). bPredeployPan is listedfor the 670-nm filter. TheMission Success and Gallery Pans are for the •440-nm (top) and 670-nm filter (bottom).The InsurancePan is for 440 nm(top) and 750 nm(bottom). The Monster, MS Spot,and Superpan sequences are for 440 nm(top), 670 nm (middle), and 750 nm (bottom). All imageslisted were taken through fight eye, except 0182010122 for A-3 and 0033030010 and 0182030106 for A-7.

bandsnear 900 to 1000 nm [e.g., Cloutis and Gaffey, 1991]. showthat many contain lineamentsthat couldresult from Thereforemany of the diagnosticspectral characteristics of the igneous,sedimentary, or aeolian processes[Parker, 1998; classesare explainedby differencesin the abundanceof ferric McSweenet al., 1999]. Other rock texturesinclude crusts and andferrous components, consistent with dustsurface coatings protrudingknobs associated with pits. Thelatter have been or weatheringrinds. interpretedas possibleconglomerates [Rover Team, 1997b; Eight spectral classes of soil were identified, each McSweenet al., 1999; Moore et al., 1999]. Soils appearto distinguishedby diagnosticred (670- or 750-nm) reflectances, rangefrom drift to cloddyto indurateddeposits, with various near-IRspectral slopes, and absorption features at 800 to 1000 amountsof admixedpebbles [Moore et al., 1999; Bell et al., nm [Bell et al., 2000]. The spectraof bright soils generally 2000]. The presenceof windtails,duneforms, barchan dunes, indicate a mixture of nanophaseor poorly crystalline ferric dust deposits,probable lag deposits,and possible soil oxides, with somevariable small amount of crystalline ferrous horizonsindicates that aeolian processeshave significantly and ferric components[Bell el al., 2000]. The spectral modifiedthe soils [Greeleyet al., 1999, 2000; Golombekand signatures of dark soils are consistent with a coatset Bridges, 2000]. granularity, greatercompaction, or a greaterabundance of ferrouscomponents (e.g., sandor pebbles)compared to the bright soils. 3. Methods Visualinterpretations of soils androcks show a rich variety 3.1. Locating APXS Target Sites of texturesand morphologies. Most rocksat the landing site exhibit pits andin that respectare similar to rocksseen at the The position of each APXS site was determined Viking Landingsites. The pits are commonly interpretedas throughcareful analysis of IMP images, rover images,and volcanicvesicles, although aeolian scouror chemicaletching contact sensor and motor encoder readings from the APXS cannot be discountedfor forming at least some of them deploymentmechanism. Because of variabilities in image [Bridgeset al., 1999; McSweenet al., 1999]. Unlike the coverageand viewing geometry, the exactmethods used varied Viking rocks,many rocks at the Pathfindersite have elongated for each APXS site. Common to all analyses was the use of flutes, which are probably aeolian in origin [Bridgeset al., stereoIMP images. The x-y-z positionsof points of interest 1999; Greeleyet al., 1999]. Superresolutionviews of rocks weredetermined using left andright image pairs andgeometric

Table 3a. PathfinderAPXS ElementalAnalyses Normalized to 98% OxidesWith IronCast as Fe203 a

Pathfinder Soils Pathfinder Rocks

Oxide A-2 A-4 A-5 A-8 A-10 A-15 A-3 A-7 A-16 A-17 A-18 SiO2 50.1_+2.547.3+2.4 46.9+2.4 _50.8+2.647.3+2.4 49.3-+2.5 57.8-+2.9 54.8+2.8 51.3+_2.660.4+_3.1 54.4-+2.8 A1203 7.3-+0.7 9.0•0.9 8.5-+0.9 9.0-+0.9 8.1+0.8 8.2_4:0.8 10.6-+1.1 9.0-+0.9 9.8-+1.0 9.8_+1.0 10.4-+1.1 Fe203 18.1+1.7 15.8+1.4 18.8+1.7 14.7-+1.3 19.0_+1.718.7_+1.7 14.1_+1.3 14.4•_ 1.3 16.8_+1.513.0•1.2 15.2-+1.4 MgO 7.8-+1.2 8.2_+1.2 7.3+_1.1 7.0-+1.1 7.8-+1.2 7.2-+1.1 3.0-+0.5 5.8-+0.9 4.8_+0.7 3.0•0.5 4.8_+0.7 CaO 6.8_+1.0 5.5-+0.8 6.4-+1.0 7.2_+1.1 6.3_+1.0 5.9-+0.9 5.2_+0.8 6.5-+1.0 7.3_+1.1 7.7_+1.2 5.9-+0.9 K20 0.2-+0.1 0.2_+0.1 0.3_+0.1 0.5-+0.1 0.2_+0.1 0.5-+0.1 0.'7_+0.1 0.5-+0.1 0.'7_+0.1 0.5-+0.1 0.8-+0.1 TiO2 1.2_+0.2 1.4-+0.2 0.9-+0.1 1.1_+0.2 1.1_+0.2 1.3+_0.2 0.8-+0.2 0.9-+0.1 1.0-+0.1 0.7_+0.1 0.9-+0.1 SO3 3.9-+0.8 6.4•1.3 5.5-+1.1 5.2_+1.1 6.1-+1.2 5.1-+1.0 2.2_+0.4 3.9-+0.8 2.8_+0.6 0.7_+0.3 2.6•0.5 CI 0.5-+0.1 0.6-+0.2 0.6-+0.2 0.7_+0.2 0.7_+0.2 0.6-+0.2 0.5•0.1 0.6-+0.2 0.5-+0.2 0.3_+0.2 0.6-+0.2 Na20 2.3_+0.9 3.7_+1.5 2.7_+1.1 2.0-+0.8 1.5•0.6 1.3_+0.7 3.2_+1.3 1.7_+0.7 3.0-+1.2 2.0-+0.8 2.4•1.0

•Originaldata from Rieder et al. [1997a].Uncertainties are the same as those reported by Rieder et al. [1997a]for iron cast as FeO. BRIDGES ET AL.: PATHFINDER APXS SITES 14,627

calibration cameramodels. In some casesthe APXS deployed on the surfacewas clearly visible from IMP, allowing an easy determinationof position (Figure 2, A-8, for example). The view of other sites was not as good, with the rover blocking IMP's view of the deployedAPXS (Figure 2, A-2, for example). In thesecases, coregistration between images of the areawith •o•m and without the roy er in the scene was used to determine APXS ,4oo0o position. For soils, if surfacematerials were disturbed to such an extentthat laterimages could not be easily correlatedto the original, undisturbedsurface, IMP images acquiredprior to APXS deploymentwere studied,if available. Rover camera imageswere acquired of all APXStargets except A-3. Someof these clearly showedthe APXS deployedon the target and provided positional information that could commonly be u5oo00c5 correlatedto IMP images, from which accuratepositional information was determined.

3.2. Visual Assessment of APXS Sites To studythe geology of eachsite, all relevantimages were ufoor--c5 catalogedand studied, including all rover frames(Tables 2 and 4). IMP imagesfrom the InsurancePan, MultispectralSpot, andSuperpan sequences were used for all sitesbecause of their • ,4 cSo 00 2:1 or losslesscompression and multispectralcoverage, with the 440-and 750-nm wavelengths being especially useful. ,4oo0oo The InsurancePan was particularly helpful becauseit imaged APXSsoil sitesprior to their disturbanceby the rover. Where additionalphotometric coverage of somesites was available with other panoramas,these were also used,despite their lower resolutionand spectralcoverage. The details of the dataused and methods employed for each APXS site are discussedin greaterdetail in section4.

3.3 Determining Spectral Reflectance of APXS Sites Using IMP For IMP data we used the latest calibrations for radiance and reflectance [Reid et al., 1999a, 1999b]. Raw data were convertedto radianceusing CCDCAL Version 2, provided by Bob Reid at the University of Arizona (UA). This version incorporatescorrections for saturatedpixels, shuttering, dark current,flat field, and bad pixels, modificationsnot provided in Version 1. These data were subsequentlyconverted to reflectancerelative to theradiometric targets (R*) using the UA program SPECTCAL Version 3. This relative reflectanceis whatis referredto as "reflectance"in this paper. Radiometrictarget sequencesused for the conversion were close in time and, in most cases, sol to the scene images (Table 2). This facilitated the computation of scene reflectancesusing targets undernearly the same illumination conditions,while also minimizing diurnaland secularchanges in optical depth. Using similarly timed sequencesprovides a first-order correction for diffuse illumination effects, especially for relatively flat surfaces. Postlanding flat field high-frequencyartifacts induced by dustcontamination on the lens and other problemswere considered small, and efforts to factor these out by subtractinga sky flat field image [e.g., Metzger et al., 2000] werenot pursued. Someof the resultingdata were comparedto spectrafrom spots in the same location using CCDCAL and SPECI'CAL Version 1, which was usedfor the initial IMP analyses by Smith et al. [1997a], McSween et al. [1999], and Bell et al. [2000]. Reflectancesgenerally differedby no more than 0.03 between the versions. However, because blue reflectances are 14,628 BRIDGESET AL.: PATHFINDER APXS SITES

A-3

A-16

Figure2. Imagesofeach APXS deployment asseen from IMP. Note that the body of the rover blocks a view of the APXS in several cases. low,these small differences translate into potentialdifferences containedmissing packets, necessitating the useof left eye in red/bluereflectance ratio of upto 0.3. Reflectancesfor each 440 nm reflectances(Table 2). The 670/440-nm ratio was spotwere retrieved using the softwareprogram IMPSPEC• alsocomputed, and for sequences in which there were no dataat (developedby coauthorJ. Bell). Themeasured pixels were 750 nm, this defines "red/blue." containedwithin a box proportionalin sizeto the APXSspot To addressthe photometriceffects of incidenceand as seen from IMP (Plates 1-11). To accountfor slight emissionangle on inclined surfaces such as rocks and Mermaid reflectanceoffsets between the left andright eyesof IMP, all Dune,the three-dimensionalgeometry of eachAPXS site was spectrawere scaled to theaverage of eitherthe 440-nm or the computed(Table 5). The x-y-z positionof four points 670-nm reflectances. Because reflectances of most IMP surroundingeach spot was determinedusing stereoIMP spectraat the Pathfinderlanding site peak near750 nm images.Four sets of threepoints were then fitted to a plane [McSweenet al., 1999; Bell et al., 2000], the 750/440-nm withstrike and dip components. The mean plane of the four ratiois generallyused to define"red"/blue for eachAPXS site. planeswas then determined. The completeprocedure for The 750-nm filter is containedin IMP's right eye, so in most computingthe planes is described in Appendix A. Phaseangle casesright eye440 reflectanceswere used to computethe g in thecenter of eachimage was read from the IMP image red/blueratio. However,in somecases the right eye 440-nm headers.This, combined with the plane geometry, was used to BRIDGES El' AL.: PATHFINDER APXS SITES 14,629

A-2

o 40

o 35

• o 30

•- o •5

• o •o

•> o15

670/445 • A-2, S0002U if' 010

o 050

oo 400 500 600 700 800 900 1 ioo

Wavelength (nm) b

[] - =

A-2, SO164B 440•30-õ70 o

c

.

d

;- 440-600-•50eobr A-2, 2 3 4 5 6 7

Plate 1, Viewsof the A-2 site: (a) IMP three-color(left) andred/blue ratio (right) images Colors and ratios ttsedare indicated below the bottomright comerof eachframe. The scaleat bottomright is in ratio units. The boxesshow the areassampled as possibleA-2 material,with the exact locationof the APXS spot being unknown.A possiblebumper ring mark is locatedbetween the two drift boxes. Abovethese is a rockthat is alsoprobably seen in thefront camera view in Platelc. (b) Spectrafor each box and sequence. (c) Roverfront cameraenhanced color image of the A-2 region. The rockat left center(arrow) is probablythe sameone seen abovethe drift sampleregions in Plate la (3-color,6-image mosaic produced from frames S003029-34). 14,630 BRIDGES ill AL.: PATHFINDER APXS SITES

"'-3

040

03�

g 03. � 0 .25 !ij i � B 20 a: � > .15 'i li a: . I. A-3.9Xl33C

o OSO

.0 500 600 700 800 900 10(1:0 7501+00 b Wavelength (nm)

c

Plate 2. Views of the A-3 site: (a) IMP three-color (left) and red/blue ratio (right) images. Yellowish material is interpreted as dust. which commonly is within cracks and pits. (b) Spectra for each sequence. (c) Superresolution image of Barnacle Bill (made by T .J. Parker. JPL). BRIDGES Er AL.: PATHFINDERAPXS SITES 14,631

A-4

OQ

O�

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. � . .. . . � .. # o 15 > 't,�:/�, .. , ...... � ��.. '. : .. • SOOOll.lTuo;o.st� � 'i/, , ' -- '.j)' ,jO�.. � 0.10 '//:1'. : ; ' :...::...: ��?i��; tr;::.�:t.. 1 . ,;��::. :':':'-:�g1g:�:��iSj� OO� ... ::: : .' •••• -SOHJ2A. t:I"'Ul st�1St\Xbe(j ;.' ----·S0182A,flI:.

0.0 400 500 600 700 BOO 900 1000

Wavelenth (nm) b

c

Plate 3. Views of the A-4 site: (a) IMP three-color (left) and red/blue ratio (right) images. (b) Spectra for each sequence. (c) Enhanced rover left front camera view (T021070) showing the A-4 A-5 dark a regions of the and sites. Note spots, which are interpreted as pebbles, although some could be soil clods. 14,632 BRIDGES Ef AL.: PATHFlNDER APXS SITES

A-5

0.40

035

15 0.30 � C 0.25 as U CD � 0.20 a: CD > 0.15 � Q) • SOOO IL. lrue SlIe a: 0.10 __ �nn . ',1 tn:,:. $;1,.,. --S0069F, true site -- -SOl 648, nearl1y stte (I€II ) ----- $01 648 , nearl1y sne (r1ghl) 0.050 .•••- SOI82A. true sneldlslur1>e

0.0 400 500 600 700 800 900 1001

Wavelength (nm) b

Plate 4. Views of the A-5 site: (a) IMP three-color (left) and red/blue ratio (right) images. (b) Spectra for each sequence. See Plate 3c for a rover view of the A-5 region. BRIDGES ET AL.' PATI-•NDER APXS SITES 14,633

• oo ß ¸ 0 ca

. •..,• 14,634 BRIDGES ET AL.' PATHFINDER APXS SITES

computethe trueincidence i' andemission e' angles relative to the inclined plane (Figure3). The Hapke-Irvine photometric functionwas then fitted to reflectancesof a given site where suitablephotometric coverage was available, as discussedin detail in section 4.

3.4. Spectral Characterization of APXS Sites The averageR' within each box was usedto definethe reflectancesfor a given wavelength. These values, and their standarddeviations bR within the box, werecompiled and are shown for 440-, 670-, and 750-nm wavelengthsin Table 6. All reflectanceswere graphedagainst wavelengthto produce the spectralplots shown in part b of Plates 1-11. Spectral ratios Z (670/440 and750/440) were also computed(Table 6). The variation within eachratio wascomputed by

bZ=Z•/[ LR••R•2 2 +-•-2•tR22J ß (1)

Three-colorand spectralratio imageswere produced(part a of Plates 1-11). The color images are false color contrast- enhanced,depicting 440-nm R'as blue,530- or 600-nmR' as green,and 670- or 750-nmR* as red, with the choiceof wavelengthsdepending upon the sequenceused and contrastin each wavelengthstretched over digital numbers(DNs) 0-255. Such images provide good discriminatorsfor distinguishing lithic anddust/soil components, in particular, pebble-rich soil and dust-coveredrocks. Because green wavelengths are relatively featurelesson Mars, ratio images, using either 750/440 or 670/440, werealso produced. The imagesare 8 bit, with DNs of 0 and 255 being equalto red/blueratios of 2 and7, respectively. Only two sites, A-4 andA-S, wereacquired with a sequence(S0001L) that did not have 440-nm data. We thereforebelieve that using ratios as opposedto intrinsic brightnessesboth representsthe suite of available data and parameterizesthe datato addressthe questionsof interest to this paper. We neverthelesspresent full spectraof eachAPXS site for all illumination geometries(part b of Plates 1-11) and tabulate reflectances at 440, 670, and 750 nm for readers who may wish to recompilethe datausing different methods.

3.5. Assessing the Effects of Illumination Geometry To compareproperly the reflectancevalues of the APXS sites, a considerationof illumination geometry is warranted. As seen in Plates 1-11 (part b) and Tables 5 and 6, the reflectancesand spectral ratios of each site are dependentupon incidencei', emissione', andphase angle g. This variancehas beenthe subjectof previousphotometric studies of Martian surfacematerials at the Pathfinderand Viking landing sites [Guinness, 1981; et al., 1986; Guinness et al., 1987, 1997; Johnsonet al., 1999]. Phaseangle is a function of the position of the Sun and viewer and is independentof the orientation of the surface (Figure 3). Surface orientation affects the angle of the incident (J) and emergent (1) rays relative to the surfacenormal (i' and e', respectively) and the orientationof theseangles relative to the principal plane (ql', where0 ø and 180ø are in the principal plane). It is therefore necessaryto considerboth phaseangle and surfaceorientation when comparing reflectance values of different surfaces measuredat differenttimes of day. The manner in which reflectance R* varies as a function of illumination geometry is dependent upon the scattering BRIDGES ET AL.' PATHFINDER APXS S1TES 14,635

Table $. PhotometricGeometry

Site .. Sequence Stnke•deg. Dip,deg g, deg i, deg i' •, deg e,deg ½,a.deg A-2 S0002U 75.7 6.3 W 47.0 39.2 40.8 70.6 65.2 S0164B 51.5 9.6 14.5 61.0 56.0 S0182A 105.4 51.5 50.7 65.7 71.0

A-3 S0033C 37.4 53.0 E 141.7 68.8 computedshadow 77.6 25.0 S0172B •44.6 19.3 -30.8 63.5 11.5 S0182A 114.0 49.0 computedshadow 67.4 16.0

A4 S0001L 66.7 6.0 E 65.4 24.3 21.4 83.9 78.3 S0002U 47.0 39.2 36.9 70.6 65.1 S0033C 133.5 68.6 71.7 77.4 72.0 S0069F 49.9 34.5 31.9 75.2 69.9 S0164B 62.8 9.8 4.6 72.6 67.3 S0172F 59.0 10.4 5.2 69.3 63.6 S0182A 105.4 51.5 53.9 65.7 60.2

A-5 S0001L •44.2 5.4 E 69.0 24.5 20.2 83.3 78.0 S0033C 123.3 68.3 72.7 77.6 72.3 S0069F 55.7 34.9 30.9 74.3 69.0 S0164B 62.8 9.8 4.5 72.6 67.2 S0182A 109.6 47.5 51.2 77.2 71.8 SO182C 98.5 53.5 56.7 76.3 71.1

A-7 S0001L -41.2 68 W 69.0 24.5 83.8/shadow 83.3 61.3 S0033C 130.6 66.9 8.8 90.4 79.1 S0069F 55.7 34.9 computedshadow 74.3 62.9 SO172F 68.4 11.9 48.5/partialshadow 79.3 63.3 S0181F 46.3 39.1 85.3/shadow 76.3 54.7 S0182A 106.5 43.6 -19.2 87.8 62.3 S0182C 98.5 53.5 -11.3 76.3 54.7

A-8 S0172E -5.4 16.1W 67.6 20.6 35.8 63.3 55.7 S0188C 49.9 39.4 23.4 61.8 55.3

A-10 S0030B 81.6 10.6E 75.7 23.8 24.1 77.7 67.2 S0172D 67.6 34.7 32.3 69.0 58.4 S0172M 59.0 13.6 6.1 68.1 57.5 S0181F 58.4 37.4 31.4 74.7 64.1

A-15 S0171B -63 8.5 E 83.8 14.0 22.3 77.6 72.2 S0185B 98.6 47.3 43.5 77.6 72.2

A-16 S0032C -37.4 26.3 E 108.1 55.6 75.7/shadow 90.6 66.7 S0172D 69.2 36.3 15.8 75.0 52.8 S0184A 68.9 32.8 15.7 68.4 45.9

A-17 S0032C -25.8 90 V 108.1 55.6 computedshadow 90.6 0.6 S0172M 87.0 12.8 -77.2/partialshadow 84.8 -5.2 SO172P 80.1 29.2 -60.8/minorshadowing 84.8 -5.2 S0184A 73.9 36.6 -53.4 80.3 -9.7

A- 18 S0032C -41.8 51.8 E 118.5 55.9 computedshadow 90.4 38.8 SO172P 78.3 28.4 -3.2 86.1 34.9 S0184A 67.9 35.5 -1.3 80.6 29.3

aNegativevalues of i' ande' are for surface normals ata greaterangle from vertical than i and e, respectively. Where calculations indicatea shadow should be present, it is observed and recorded as"computed shadow." Where a shadowisnot predicted, but observed,the value of i' is givenalong with an indication that a shadowis seen. propertiesof the surface.Several parameters determine this, 1997]. If thesefactors are known a priori, then it is possible amongthem the single scatteringalbedo, real and imaginary to choosean appropriatemodel that predictsreflectances as a indices of refraction, mean facet tilt angle, and opposition function of illumination geometry. However, the inverse effectwidth and height [e.g., Hapke, 1993; Guinnesset al., problemthat is facedin this study,that of using reflectance 14,636 BRIDGES ET AL.' PATHFINDER PXS SITES

N

J

g

Figure 3, Photometricgeometry for an inclined surface. J incident ray; I emergentray; N surfacenormal relativeto horizontalsurface; N' surfacenormal relative to inclined surface;g phaseangle; i incidenceangle relative to horizontal surface;i' incidenceangle relative to inclined surface;e emission angle relative to horizontalsurface; e' emissionangle relativeto inclinessurface; W the orientation of the emissionand phase anglesrelative to horizontal surface;and W' the orientation of the emission and phase angles relative to inclinedsurface (adapted from Hapke [ 1993], Figure8.1). datawith poorly constraineda priori informationon scattering reflectancechanges as a functionof surfaceorientation relative propertiesand a limited rangeof illumination geometriesfor a to the Sun(g0/[• + I•]). The reflectancedependence on phase given surface,makes determining R' for a nonmeasuredangle is determinedby the signand magnitude of at. The value illumination geometrydifficult. We thereforeuse a simple of at cannot be derivedfrom physical characteristicsand is modelto normalizethe reflectancevalues without going into a rather a complicatedcorrection factor. However, as the detailed photometric analysis. The effect of emission, purposeof this exerciseis to normalizethe reflectancesof incidence, and phase angles on reflectances can be different APXS spots to a common geometry and not to approximatedusing the Hapke-Irvine function, which is necessarily model the mechanismsfor changes in the applicable to many materials when the opposition effect is reflectances,the useof this simpleequation is appropriate. No insignificant [Guinnesset al., 1987], which encompassesall naturalmaterials, including those here, exhibit a single value observations here: of or, with the sign commonlybeing negative at low phase anglesand changing to positive at intermediatephase angles (see below). R = e'•s, (2) !.to+ Ix Equation(2) canbe rearrangedto give where!x0is cos/' and!• is cose'(see Appendix A for a derivation of thesevalues). The equationis arrangedsuch that reflectance (31 21lo increasesas incidence and emission angle decrease. The ln•R*{pø +I't}] =ln[B] +otg, amountof diffuseillumination is proportionalto the incidence angle and inversely proportional to (g0/[l,t0 + g]) and is By measuringvarious values of R*at givenvalues of i', e', and thereforeindirectly accounted for to first orderin this equation, g, the parametersct and B0 can be computedby linear least althoughnot explicitly modeledas in the work of Thomaset squaresfitting. Oncethis is done,R' canbe estimatedfor any al. [1999]. B0 and a are variablesthat dependupon the photometricangle, thereby allowing proper computationof propertiesof the materialand atmosphereand are wavelength- relative reflectancesand ratios for various materials. Here, (3) dependent.The parameterB0 determinesthe degreeto which is usedto computeB0 and a in the 440-, 670-, and 750-nm Plate 5. Views of the A-7 site: (a) IMP three-color(left) and red/blueratio (right) images.(b) Spectrafor eachsequence. Notethat areassurrounding the site are commonlybluer than the site itself, indicatingthat dust, as opposedto purely lightingby diffuse illumination, is contributingto theredness seen. (c) Enhancedthree-color, six-image rover rear camera view of the A-7 area on Yogi (S006117-22). Images were acquiredduring the first APXSdeployment attempt on Yogi, whichwas unsuccessful. Bright, orangematerial surrounding knobs is intervreted as dust.

A-7

O4O

• SO{:DIL true sde ß SOCI33C.Irue sile EI31369F. true s•te O35 IHO172F. true s•te S0181F, tru• •te -- -- - Lq[]ILa•A.neart• ß SD182C. lrue s•le O3O ,,._' _'i.' : _' '. •_L'_ ' ' ! 'i' 'i•

..,- ""- ,...., ..,, -" -_

015 // -

0 10

0 O5O

0.0 i I i J i 400 500 600 700 800 900 1000

Wavelength (rim) b

c a A-8

0.40

035 S0188C

.

0.30

0.25

0.20

0.15

0 10

0,050

00 500 600 700 800 900 1000

Wavelength(nm) b

Plate 6. Viewsof theA-8 site:(a) IMP three-color(left) and red/blue ratio (fight) images(b) Spectrafor eachsequence. BRIDGES El' AL.' PAT}WINDER APXS SITES 14,639

A-10

o 40 ..... i"" ...... 1 ...... i ...... i ...... I

0.35 . : sot 72E•

S0181F A-10, 80030B 'S'O'O

,-" 0.25

A-10, S017'2D • 0.20

> 0.15

A-10, S0172M or' o•o

O. 050

O0 500 600 700 BOO 900 ooo

Wavelength(rim) b

A-]O, S01S1F

2 3 4 5 6 7 a Plate 7. Viewsof theA-10 site:(a) IMPthree-color(left) andred/blue ratio (right) images. Notewindtails in S0030Band similar orientation of wispyfeatures near the A- 10 site. (b) Spectrafor each sequence. 14,640 BRIDGES ET AL.: PATHFINDER APXS SITES

A-15, SO171B

2 3 4 • 6 7 a

A-15 0.40 .,...,.,.• •

o35

0

0 25

0 20

0.15

0050

500 600 700 800 900 1000 b Wavelength(nm)

Plate 8. Views of the A-15 site: (a) IMP three-color (left) andred/blue ratio (right) images.Sample box in S0171B is within the APXS bumperring mark, which is barely visible here. (b) Spectrafor each sequence.(c) Enhancedrover rear camera three-color, six-image mosaic of the A-15 area (S027099-104). The speckled appearance of Mermaid duneformin the top right half of the framemay indicatesand- c size particles, although the speckles are at the limit of resolution. BRIDGES ET AL.' PATHHNDER APXS SITES 14,641

ZZZZZZZZZZ••• Z•ZZ••

Z•ZZ••

o o oo

•ooo••••• 14,642 BRIDGES El' AL.' PATHFI•E• APXS SITES

Z c,i •q BRIDGES El' AL.: PATHFINDER APXS SITES 14,643 bands for each APXS spot for which measurementsat more contents[Rieder et al., 1997a]. Therefore,a priori, we expect than one illumination geometrywere available. Equation(2) rocks with soil contamination to show a positive correlation is thenused to determinea value of R' for a given illumination betweenred/blue ratio and SO3 or C1 and soils mixed with geometry. For some APXS spots, as describedin detail in pebblesto showa negativecorrelation between the ratiosand section 4, there is insufficient photometric coverage for the SiO2. We thereforeplot red/blueagainst these data, as well as desiredgeometry such that leastsquares fitting cannotbe done. iron(expressed as Fe203*), a majorconstituent of Martiansoils In thosecases, original valuesof R* closeto the desired and rocks. photometricgeometry' are used. If suchdata are not available, then the original reflectancesirrespective of illumination 4. Results conditionsare employed. The effectof diffuseversus direct sunlight on red versusblue 4.1. Location and Characteristics of APXS Sites reflectancesis a recognizedproblem [Thomaset al., 1999]. Althoughnot addresseddirectly here, it is reducedby obtaining Reportedhere are detailedanalyses of the position and the calibrationtarget and scene images at nearly the sametime characteristicsof each APXS site. A shorter write-up of the under nearly identical illuminations and, in so doing, soils sites was previouslypresented by Bell et al. [2000]. minimizing atmosphericeffects. This, combined with the More derailed and additional information is provided here, normalizationprocedure described above, allows us to make togetherwith descriptionsof the rocksites. Plates1-11 show first-ordercomparisons of APXS site reflectances. eachIMP image cubeused for analysis of the APXS sites, as both false color andred/blue ratio images,the spectraextracted from these cubes,and, wherenecessary, rover cameraor IMP 3.6. Comparison of Spectral and APXS Data superresolutionimages. All spectrafor soils and rocksare APXS-denved elemental abundances are compared to showntogether in Plate 12, andphase curves are displayedin spectraldata from the APXS sites. The relativedifferences in Figure4. Therange of phaseangles available for analysisand the elemental abundancesamong the sites are the same as thoseused are shown in Figure5. The relative reflectancesand those originally reportedby Riederet al. [1997a]. However, red/blueratios of the sites are comparedin Figures6 and 7, whereasRieder et al. cast iron as FeO, the data here have been respectively.The headingsthat follow list the site and, in recastwith iron as Fe203,with the other elementalabundances parentheses,the x-y-z position (using the Mars Local Level adjustedaccordingly. This is done in orderto compareour CoordinateFrame [Mars Pathfinder, 1997]) and the sol and resultsto Viking soil analyses, recognizing that soils are time the APXS wasplaced on the site. probablyhighly oxidized[Banin et al., 1992] andthat many 4.1.1. A-2: Soil near base ofrearramp(1.89, rocksat the Pathfinderlanding site are coveredwith ferric-rich -1.95, 0.31; sol 2, 1413 LST; Plate 1). AresVallis dust [McSweenet al., 1999]. Becausethe Fe203 usedhere is measurement2 (A-2) wasthe first APXSanalysis of geological not a tme abundance,but rathera proxy for what is an uncertain material at the Pathfinderlanding site (A-1 was a background mixturewith ferrousiron, it is labeled Fe20•*, following reading). The APXS was placed on the surfacefor this previous convention [McSween et al., 1999; Bell et al., measurementon sol 2 at 1413 local solar time (LST). This 2000]. Both our andRieder et al.'s analysesnormalize the sum APXSspot has been one of the most difficult positions to pin weight percentoxides to 98%, with the assumptionthat the down. When the measurement was taken, the APXS was remaining 2% consists of unmeasuredoxides such as P205, blockedfrom IMP's view by the body of the rover (Figure 2). Cr205,and MnO. Imagestaken after the rover had movedshowed disturbed soil The 670/440 and 750/440 ratios, as opposedto the other amongscattered rocks and what couldbe an indentationin the spectralparameters that are available [e.g., McSween et al., soil formedby the APXS deploymentbumper ring (Plate l a). 1999; Bell et al., 2000], are used to compare the spectral However,this is unlikely becausethe contactsensors on the results to APXS-derivedchemistry. The red/blue ratio is front of the bumperindicated a lack of contactat the start and sensitive to the presenceand concentrationof ferric-bearing endingof the APXS measurement.A rover rearcamera image phases, with peak reflectancesfor many ferric oxides and of the site shows a rock that is similar to a rock viewed with oxyhydroxidesnear 750 nm [Smith et al., 1997b]. Both IMP (Plate lc). Studyof rover uplink and downlink files, the 670/440 and '750/440 ratios are usedbecause some sequences rover image, and stereoIMP images indicatesthat the APXS contained 670 nm without 750 nm and vice versa. Because one spot is located in a region consisting of several soil of the interestsof this studyis to gaugedust contamination on componentsand small rocks. The soil surfacenear the APXS rock surfaces,the red/blueratio on rocks shouldbe affectedby spot has a strike and dip of 75.7ø, 6.3øW, sloping slightly the concentrationof adheringferric-rich dust. Conversely, awayfrom the lander(Table 5). The heterogeneityof this area most clean rock surfaces should have lower red/blue ratios than andthe errorsassociated with pinpointing an accuratelocation soil and dust, suchthat lithic~rich soils are predictedto have without a view of the deployed APXS make assigning a lower ratios than lithic-poor soils. Using red/blue ratios materialunit to A-2 problematical.The regionsurrounding the therefore provides a means to assess the mixing of inferred location contains fine drift, rock, and windtail componentswithin soils, anothergoal of this study. These materials. It is not possible to determinewhether A-2 is ratios are even more effective discriminators when combined representedby any one of these materials,a combinationof with elemental abundance data. Martian soils are rich in sulfur two, or all three. So, to assessthe potential spectraldiversity and chlorine [Clark et ai., 19'77, 1982; Banin et al., 1992; of A-2, spectra were taken from five regions using three Rieder et al., 1997a; McSween et al., 1999; Bell et al., 2000] sequencesover a phaseangle range of 47ø- 105 ø. The diversity and have concentrationsof these elements greater than that of this areais evidentin the spectraand red/blueratios of each expectedfor typical rocks [Jambon, 1994]. Pathfindersoils component(Plates la and lb and Table 6). Becauseno single are poorer in silica than rocks and have overlapping iron soil or rock componentcan be assignedto A-2 and because 14,644 BRIDGES ET AL.' PATHFINDER APXS SITES

• E

(.El)e3 ue$3ellel:l (.Id)e::)ue$3ellel:l eAll•{e,{:{

E

o .... i .... i .... i .... i .... i .... i .... i ....

(.El)e3 UelOellel:l (.El)e::)UelOeliel:l eA}l•{e,{:{

(.Id)e::)ue•3elleEI eAll•lelJ (.Id)eOUelOelleEI e^ll•lelJ BRIDGES ET AL.' PATHFINT)ER APXS SITES 14,645

A-16

o •o ,$0032C, nearby site 301720, actual $ t $0184A, actual site I

•- o 30

• o25

• o20

:• o15 A-16, $0172D rr

o 050

O0 500 600 700 800 900 1ooo b Wavelength(nm) ,.

A-16, g0184A

a

c

Plate 9. Viewsof the A-16 site: (a) IMPthree-color (left) andred/blue ratio (right) images. Yellowish, high red/blueratio materialon Wedgeis interpretedas dust. (b) Spectrafor each sequence.(c) Enhancedrover right front cameraimage mosaic (N042079-80) of Wedge. Arrow shows A-16 region. Note abundantpits. Elongatedpits couldbe wind-carved. 14,646 BRIDGES ET AL.' PATHFINDER APXS SITES

o 35

•-- 0..0

• 8 2o

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Plate 1 0. Viewsof the A-17 site:a) IMP three-color(left) andred/blue ratio (right) images. Note that the regionis veryblue and has a low red/blueratio. No indicationof dustis obvious.b) Spectrafor eachsequence. c) Enhancedfive-imagemosaic of A-17 areaand much of Shark from the rover left front camera(S049070-4). Arrowpoints to A-17 area,located below a probablecrack. BRIDGES ET AL.' PATHHNDER APXS SITES 14,647

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Plate 1 1. Viewsof the A-18 site:(a) IMPthree-color(left) andred/blue ratio (right) images.(b) Spectrafor eachsequence. (c) Enhancedrover left front camerathree-image mosaic (S071041-3) of Half Dome and the A- 18 area, indicatedby the arrow. 14,648 BRIDGES ET AL.: PATtt•NDER APXS SITES

o

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.... I .... I .... I .... I .... I .... I .... I .... I .... I .... I .... I .... I .... 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Phase Angle (ø) Figure $. Phaseangles for all the potentialobservations of the PathfinderAPXS sites for multispectral sequences.Sequences that werecompressed or hadlow spectralcoverage and that also wereclose in phase angle to other sequenceswere not used.

eachhas a differentspectral signature, comparison of A-2 IMP and pitted. The sequenceavailable for spectral analyses, datato the APXSresults is not readilyachievable. In addition, S0172B, lies slightly to the left of the actual APXS the APXS oxide total for A-2 is only 68.6% [Rieder et al., deployment location but seems fairly characteristicof this 1997b], likely resulting from the APXS making insufficient overall regionof the rock (comparePlates 2a and 2c). A strip contactwith the soil, raising questionsabout the accuracyof of bright material, interpretedas dust within a small crack, the results. extendsvertically along the right side of the spectralanalysis 4.1.2. A-3: Barnacle Bill rock (1.20, -2.45, site andprobably indicates that somedust is within the actual 0.18; sol 3, 1504 LST; Plate 2). A~3 was the first A-3 spot as well. The strike and dip of the A-3 site are 37.4 ø, APXS analysis of a rock and, for that matter, the first 53.0øE, dipping toward IMP (Table 5). Unfortunately,only unambiguousgeochemical analysis of a rock on Mars. The one sunlit observationof A-3 in minimally compressedIMP rover blocks most of the APXS as seen from IMP when sequencesis available, a multispectral spot (S0172B) at a deployedon BarnacleBill, but enoughof the instrumentis phaseangle of 110ø (Figure5). The spectraare fairly flat, with visible to determine a fairly accurate location of the no obviousabsorption bands (Plate 2b). measurementsite (Figure2). No rover imagesof the A-3 spot 4.1.3. A-4: Textured soil (2.79, -2.64, 0.28; were taken. The site is located to the left of a crack within the sol 4, 1402 LST; Plate 3). A4 was acquiredon soil rock (Plate2c). BarnacleBill' s texturein this regionis craggy between Barnacle Bill and Yogi. The location is known 14,650 BRIDGESET AL.: PATHHNDERAPXS SITES

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(.H)o!ie}:l eouepelle}• e^!lele}:I 14,652 BRIDGES ET AL.' PATHFINDER APXS SITES

accuratelyfrom goodIMP views of APXS deployment(Figure 2). Although classified as a soil, the site contains an appreciablefraction of apparentpebbles that are visible in both IMP and rover images (Plate 3c). It was classifiedas a mixedsoil by Bell et al. [2000]. Soil mechanicstests by the Sojournershow that this region consistsof cloddy material [Mooreet al., 1999]. A pebble-poorzone is locatedjust to the right of A4. Rover trackshave disturbedthe site in imaging sequencesSO 164B andSO 182A. In thesethe disturbedmaterial dddddddddod ddodd andnearby undisturbed material were measured. The surfacehas an orientation of 66.7 ø, 6.0ø E, dipping toward the lander (Table 5). A wide phase angle coverage of 47ø-105ø is available.There is somephase angle dependence on R*, with observations at low phase angles for 670 and 750 nm generallyhaving reflectancesthat are greaterthan at higher phaseangles (Figure 4). The 440-nm phasecurve has a saddle shape,with the lowestvalues of R' at intermediatephase. In ß ' all cases the disturbed material is darker and bluet than

o undisturbedmaterial (Plate 3b, Figure6a, andTable 6). 4.1.4. A-5: Textured soil (3.29, -2.48, 0.28; dddd• 'd• 'd4 sol •;, 1404 LST; Plate 4). As with A-2, A-5's location was hard to determine becauseAPXS deployment was not visible from IMP. Rover front cameraimages show that it is nearthe boundarybetween a textured,pebble-strewn soil and a ddddd,-4dZ 'd,.4 Zd,.4d,4 pebble-poorzone close to Yogi (Plate 4c). Severalsequences wereimaged after disturbanceby rover tracks. A depression near the likely measurementspot probably is the APXS bumperring mark. This, togetherwith detailedanalysis of IMP framesbefore and after the rover was in the region, narrowed downthe position to that shownin Plate 4a. The shadowsand topography of the depressionand tracks make the region o o unrepresentativeof undisturbedA-5 material. Therefore, for thesesequences, data of nearbyareas are used. The soil at A-5 was classifiedas "Bright I" by Bell et al. [2000], a soil type inferredas drift rich in nanophaseferric oxides, with minor ferrous and crystalline ferric components. The surface is orientedwith a strike and dip of 44.2 ø, 5.4ø E, sloping toward ddd IMP (Table 5). Phaseangle coverageranges from 69ø to 124ø (Figure 5). The phase curve is complicated, exhibiting no linear trends among points (Figures 4a-am). Undisturbed materialat all phaseangles, exceptthe highest valueof 124ø , o '.• is brighter than the disturbedmaterial (g = 110ø) (Plate 4b, Figure6a, andTable 6). A hint of an absorptionfeature near 900 nm is apparent. o r4 4.1.•;. A-7: Yogi rock (4.•;8, 2.91, -0.18; sol 10, 1011 LST; Plate •;). APXS site A-7 was clearly

o imagedby IMP, providing unambiguousinterpretation of its position (Figure2). It is locatedon the lower left faceof Yogi as seenfrom IMP. Rover color imagesof the region near A-7 show a pitted, craggy surface. IMP images show a concentrationof bright, redmaterial, interpreted as dust, in the APXS site. The A-7 site has a strike and dip of-41.2 ø, 68øW (Table 5) and is seen at a grazing angle from IMP. Four o ',•d<5<5 c:Sr,,ic:Sd sequencesimaged Yogi whenit wasnot in shadow,providing widephase angle coverage of 68ø-131ø (Figure5; Tables5 and 7); oneof thesesequences (S0182A) doesnot containdata for the exactA-7 spot, so a region slightly above is used. The 750/440 ratio of the spot variesfrom 3.24 to 6.77, depending on photometricgeometry (Figure 7 andTable 6). This is redder than other rocks at the landing site (see discussionbelow). Becausethe A-7 site differs in brightnessand red/blueratio from other illuminatedparts of Yogi (Plate 5a), variations in dust coatings, in addition to some effect from diffuse BRIDGES ET AL.: PATHFINDER APXS SITES 14,653

illumination[Thomas et al., 1999], probablyhave a strong [Rover Team, 1997b; Greeleyet al., 1999; Moore et al. 1999]. effect. However, the morphology of the duneform is similar to 4.1.6. A-8: Scooby Doo hardpan soil (2.85, terrestrial whalebacksthat are often precursorsto barchan 1.13, 0.32; sol 14, 1027 LST; l•!ate 6). The dunes, indicating that it could have a significant sand position of the APXS for A-8 was clearly imagedby IMP, component[ et al., 1999]. Bell et al. [2000] classified providingcertainty in its coordinates(Figure 2). It is located A-15 as "Dark,"a soil type interpretedas more coarsegrained on top of "ScoobyDoo," a materialthat has beeninterpreted or more ferrous-richthan other soils. The measurementspot as a well-cementedsoil similarto a hardpan[Bell et al., 2000]. A-15 has an orientation of-63 ø, 8.5øE, sloping toward the Like A-5, it hasbeen classified as "BrightI" soil by Bell et al. Pathfinderlander (Table 5). Potential phase angle coverage [2000]. It has a strikeand dip of -5.4ø, 16.1øW, sloping to rangesfrom 73ø to 107ø, but becausethis is alreadya narrow the left as seenfrom IMP. Superpanand MS Spot data were range and compresseddata wouldprobably not provide much usedto studythis area,with a phaseangle coyerage of 500-68ø moreinformation, only MS Spot and Superpandata at g= 84ø (Figure5, Tables 5 and 7). Two other sequenceswere also and99 ø areused(Figure 5). Both sequenceshave virtually the available, but becausethey were compressedwith lower same spectrum,despite the slight disturbanceby the APXS spectralresolution and hadphotometric geometries similar to bumperring in S0171B (Plate 8b). A-15 is darker and bluer the Superpan/MS Spot, they were not used. A-8 is the than most of the other soils sampledby the APXS (Figures6 brightestsoil at the landingsite in the wavelengthscompiled and 7). (440, 670, 750 nm) and has high 670/440- and 750/440-nm 4.1.9. A-16: Wedge rock (-3.79,-1.31, 0.12; ratios (Figures6a and7a). sol 37, 1024 LST; Plate 9). A-16 is on the lower part 4.1.7. A-10: Soil near Lamb rock (3.74, -0.43, of the rock Wedge. The APXS deploymenton the rock was 0.28; sol 20, 1311 LST; l•!ate 7). APXS site A-10is seenby IMP, giving a good lock on its position (Figure 2). locatedon darksoil nearthe rock Lamb. The rover partially This location is usedfor the two sunlit sequencesused. Owing blocksthe view of the APXS deploymentas viewedfrom IMP to incompleteInsurance Pan coverage, a spot slightly above (Figure 2). However, enoughof the APXS can be seen to the actual site was examinedfrom this sequence,but because deduce a precise measurement location. The site is the rock face is in shadow,it ultimately was not includedin characterizedby dark,fairly homogeneoussoil. This soil has this study. The A-16 area, and indeed much of Wedge, is beenclassified as "BrightII" by Bell et al. [2000], imerpreted heavily pitted (Plate 9c). Someof the pits on other parts of asa soil like "Bright I," but with a moreabundant crystalline the rock are elongatedand may be wind-carvedflutes [Bridges component, On the basis of soil mechanics tests, the A-10 et al., 1999]. The IMP imagesshow some bright, yellowish region consists of cloddy material [Moore et al., 1999]. For materialwithin A- 16 that is interpretedas dust. The A- 16 spot IMP sequencesSO 172M andSO 181F the rover had disturbedA- is orientedat -37.4 ø, 26.3øE, sloping toward IMP (Table 5). 10 whenthese images were acquired, so a nearbybox, also on Phase angle coverage for unshadowedobservations using darkhomogenous soil, is used. Near A-10, but not within it, minimally compressedsequences is limited to observations arelinear streaks of bright soil that areoriented nearly parallel from the MS Spot and Superpansequences at g = 69.2ø and to local wind tails. These streaksmay be small wind tails 68.9 ø, respectively. A-16 is intermediatein brightnessand associatedwith rocks below the limit of resolution. They red/blueratio relativeto other APXSrocks (Figures 6b and7b). could also be aeolian ripples' however, becausesuch an 4.1.10. A-17: Shark rock (-$.$6,-3.25, -0.35; interpretationrequires a wind directionnearly orthogonalto sol $2, 1011 LST; l•late 10). A-17 is located on the that which formed the wind tails, a wind tail orientation is lower portion of the rock Shark. The body of the rover blocks favored. The dark soil upon which A-10 is located is a view of the APXS deployed location as seen from IMP. distributedsouthwest of Lamb,indicating that wind erosionor Analysis of rover front cameraviews of the deployedAPXS deposition in the lee of Lamb via the predominant combinedwith an examinationof the sameregion seen from northeasterlywinds at the Pathfindersite couldhave modified the stereo rear camera shows that the site is on a this area. The A-10 site has a slope orientation of 81.6 ø, semihorizontalcrack (Plate 10c). Alternatively, it is possible 10.6øE, dipping toward IMP (Table 5). Becausethe phase that this crack representsthe juncturebetween the true Shark anglecoverage is limited to 58o-78ø (Figure5), the usefulness rock on top andanother, smaller rock encasedbeneath it. The of examining the compressed sequences is minimal. A-17 area contains knobs and pits, an appearancethat led Therefore, only the uncompressedsequences were studied RoverTeam [ 1997b]to proposethat it maybe a conglomerate, (g=58ø-76ø). A-10 has intermediatereflectances relative to althoughother possible interpretationswere also given. No other soils and high 750/440-nm ratios (Figurers6 and 7). obvious bright, yellow-reddishmaterial is seen in the IMP The 750/440-nm ratio of 5.80 for sequenceS0172M is the images, nor is bright material seen in the rover images, highestmeasured for any soil. suggestinglittle adheringdust. A-17 is on a vertical face (dip 4.1.8. A-I$: Mermaid duneform (-$.87, 2.80, of 90ø) that facesIMP andhas a strike of -25.8ø. Phaseangles 0.$2; sol 28, 1011 LST; l•!ate 8). Site A-15 is on for the multispectralsequences that observedShark vary from Mermaidduneform. An APXS bumperring mark is clearly seen 72ø to 108ø. Coverage for unshadowedfaces with good after deployment,allowing unequivocalplacement of the site spectralsampling is within phaseangles of 74ø-87ø (Figure5). (Plate8a, top). A rover rearcamera view of an areaon Mermaid For all wavelengths,A-17 has the lowest reflectivity of any near A-15 shows what could be a granulartexture, although APXS rock site, save for a few observations of A-3 and A-7 at this is at the limit of resolution (Plate 8c). Observations of 440 nm (Figure6b). It also hasthe lowestred/blue ratio of any bright material where the rover tracks have disturbed the rock site (Figure7b). duneformmaterial indicate that Mermaidcould be composedof 4.1.1!J. A-18: Half Dome rock (-4.81, -3.81, an underlying,fine-grained, light colored,cohesive, dust-rich -0.$4; s,01$$,1246 LST; Plate 11). A-18 is on the material overlain by a dark, coarse, protective lag deposit rock Half Dome. As with A-17, the rover blocks IMP's view of 14,654 BRIDGES ET AL.: PATHFINDER APXS SITES

the deploymentsite. By comparing the rover's position in measurementsand encompasses all other soil sequencesexcept front of Half Domeand views of the rock without Sojournerin A-5/disturbedand A-2 at 750 nm. A •/[• + •t] of 0.65 falls the way, and through analysis of rover front and rear camera within or is close to the surface orientations relative to the Stm images, the position of A-18 can be determinedsomewhat for all the soil sites. Therefore,taking all of the above factors confidently. It lies on the right sideof the highly pitted and into account,median values for the available dataof g=70ø and protrudingportion of the left half of the rock (Plate 1 l c). •/[• + •t] = 0.65 are the best to use for normalizingsoil Areasof the rock aboveand to the right of this areaare heavily reflectances(Table 8a), with the following two caveats:(1) fluted, unlike the A- 18 area. The differences between the A- 18 Reflectances for A-2 at 750 nm and A-5/disturbed at all part of Half Domeand the rest of the rock couldmean that the wavelengthsare not changed,as only one phase angle is rock is composed of two zones that are differentially available, and (2) The A4/undisturbedphase function at 670 susceptibleto erosion. In this case, A-18 area might be a nm does not yield a good enough fit to computect and B0. residual crust that has not eroded off. As with Shark, it has Thereforethe reflectancevalues usedare the averagesof the been suggestedthat Half Dome could be a conglomerateand two sequenceswith the closestillumination geometrydesired that knobs on its surfaceare conglomeriticpebbles [Rover (S0001L [g=65ø, •/(• + •t) = 0.82] andS0172F [g=59ø, •/(• Team, 1997b]. Someyellowish materialin the false color IMP + •t) = 0.69]). imagesis interpretedas dust(Plate 11b). The A-18 portion of There are no drastic changes between the normalized Half Domestrikes -41.8ø and dips 51.8ø E, towardIMP (Table reflectancesfor soils and the spreadof the original values 5). Phase angle coverage ranges from 65ø to 119ø, but (Figure6a), giving confidencethat the methodemployed here unshadowedcoverage with good spectral sampling is limited at least approximately estimates reflectancefor a given to 68ø and 78ø (Figure 5). A-18 has intermediatereflectances geometry. Most of the normalizedvalues fall within the and red/blue ratios relative to other rock APXS sites. original spread,with only someof the A-2 and A-15 spectra falling slightly outsidethe original range. In thesecases the normalizedvalues reflect the trendsof the phase curvesfrom 4.2. Effects of Illumination Geometry the measureddata. A comparisonbetween the original and It is obviousthat phase angle has a significant effect on normalizedspectral ratios of the soils also shows a close reflectance(Figure 4). Wherespectral coverage is sufficient, agreement.Most normalizedratios are within the spreadof soils showa reflectancedecrease at low phaseangles of ~30ø- original ratios (Figure7a). The exceptionsare A-2-670/440, 60ø, followedby a shallowerdecrease or increasein reflectance which decreases relative to the original values, A- at higherangles (an exceptionis the A-5 undisturbedsoil at 5/undisturbed-750/440, which increases, and A-15, lbr which 750 nm, which has the highest values at intermediatephase both ratios increaseslightly. angles).The shapes of thesephase curves are broadlysimilar The phaseangle and •/[• + •t] coverage for rocks varies to thoseof Mermaidduneform and soils along the photometric from 45ø-131ø and0.18-0.86, respectively(Figure 5 and Table equatormeasured by Johnsonet al. [ 1999]. 7). The •t0/(•t0 + •t) ratio is normalized to 0.60, which falls Phaseangle coy erage is verylimited for theAPXS rock sites within the boundsof all rock observationsexcept those for A- (Figure5). This is, unfortunately,endemic to steeprock 3. A-7 on its own spans this range, with A-17 and A-18 surfaces,which have less phase coveragethan horizontal having rangesof 74ø-87ø and 680-78ø, respectively. The two surfaces,such as soils, with the exact rangedetermined by the sunlit observationsof A-16 have virtually the same phase plungeof the surfacealong the principal plane. OnlyYogi has angle of 69ø. It therefore seems reasonableto normalize broadphase angle coverage and showsa reflectancedecrease reflectancevalues to g=69ø, which spans the phase angle with increasingphase angle, exceptfor one point at 750 nm coverageof A-7, A-16, andA-18 andis close to A-17. Within (Figures4d-4f). SitesA-17 andA-18 alsoshow a reflectance this limited range, reasonable reflectance factors can be decreasewith increasingphase angle, but the phase angle computedfrom least squaresfitting for A-16, A-17, and A-18 coverageis too limited to have confidencethat this is (Table8b). For A-7, nonlinear behavior of the phasecurve at indicative of a broader trend. The decrease in reflectance with 750 nm precludesleast squaresfitting. Instead, values for phaseangle over the rangeshere (68ø-131ø) agreeswith S0172F are used for all wavelengths, which has a nearly photometricequator rock measurementsby Johnsonet al. identicalillumination geometry(g--68 ø, •/(p• + •t) = 0.60) to [1999], which show a decreasein reflectancefrom phase that desired(note that all Yogi sequenceshave red/blueratios anglesof--10 ø to 80ø and~10 ø to 120ø for "gray"and "red" greaterthan the other rock APXS sites, except for S0033C). rocks, respectively,followed by an increasein reflectance Unfortunately,A-3 wasobserved only at a phaseangle of 45 ø, with increasingphase angle up to phaseangles of 140ø and so it is out of the phaseangle range (68ø-131ø ) of the other 180 ø . rocks. Its reflectancesare therefore left intact, recognizing Plotsof the left sideof equation(3) (hereafterreferred to as that these couldchange if the desiredphotometric correction the reflectancefactor) versusphase angle are shownin Figure couldbe performed. 8. APXSsoil site observationsrange in phaseangle from 47ø As with soils, there is only a small differencebetween the to 124ø andin •/[p• + •t] from0.49 to 0.82 (Figure5 andTable normalizedreflectances and the original spreadof values for 7). A-5/disturbedhas only one observation,so its trendwith rocks(Figure 6b) (asjust discussed,A-3 is left unchanged,and phaseangle cannot be determined. The same is truefor the A-2 the one sequencethat closely matches the normalized spotsat 750 nm. Of the remainingsites and wavelengths, geometryfor A-7 (S0172F) is usedfor that site). The change only A-8 andA-10 haveno overlappingphase angle coverage. for A-16 is insignificant,as the geometryof its two sequences The decrease in reflectance factor for A-8 and A-10 toward areclose to that of the normalizedgeometry. The reflectances phaseangles of 70o-80ø is mirroredby the broadershapes of for A-17 andA-18 rise bemusethe normalizedphase angle g or the morecomplete phase curves for the other sequences.A the combinationof normalizedincidence and emissionangles phase angle of 70ø falls between the A-8 and A-10 (•/[g0 + •t]) decreasesrelative to the original values. The BRIDGESET AL.' PATHFINDERAPXS SITES 14,655

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Table 88. NormalizedSoil SpectralProperties a

Site 440 I/F 670 I/E 750 !/F 670/.440 .750/440 A-2, drift (fight) 0.066 0.189 (0.187)b 2.85 (4.81)b A-2, drift (left) 0.048 0.174 (0.177)• 3.64 4.391 A-2, windtail(fight) 0.036 0.175 (0.146)b 4.82 5.38• A-2, windtail (left) 0.045 0.176 (0.197)b 3.89 (4.84)b A4, undisturbed 0.050 (0.249)c 0.267 (4.98) 5.34 A4, disturbed 0.036 0.164 (0.168)b 4.59 (4.67)b A-5, undisturbed 0.044 0.196 0.242 4.44 5.49 A-5, disturbedd (0.044) (0.169) (0.198) (3.87) (4.53) A-8 0.059 0.295 0.327 4.98 5.53 A-10 0.041 0.185 0.221 4.53 5.42 A-15 0.034 0.129 0.140 3.77 4.11

ag= 70o, [t0/[Yt0 + [t] = 0.65. Valuesin parenthesesare subject to thecaveats describedin footnotesb throughcL •Becauseonly SO 182A is availableat 750 nm for A-2 andfor A4, disturbed, it is shownhere despite the different photometric geometry (g = 105ø, •/([t0 + [t) = 0.66). The unmodified750/440-nm ratio is used. CBecauseof poor line fit betweenln(R*[yt0/(•0 + it)] andg, thevalue shown is theaverage for S0(X)IL(g--65 ø, [t0/(lx 0+ [t) = 0.82)and S0172F (g=59 ø, •/([t 0 + it) = 0.69). aBecauseonly S0182A is available for A-5 (disturbed),it is shownhere despitethe different photometric geometry (g = 110ø, •/(g0 + g) = 0.67). resultsof thesechanges are slightly increasedspectral ratios R2 values of 0.07 and0.01, respectively,and a fairly flat slope for A- 16 and somewhat decreased ratios for A- 17 and A- 18 (Figure 9a). If A-2, whose site location is uncertain (see (Figure7b). The sequencefor A-7 chosento closelymatch the above),is removed,the R 2 values become even poorer, at 0.03 desiredphotometric geometry has the highestratios of any of and ~0. Removing disturbedmaterial from A-4 and A-5 does the rock observations. Therefore the 670/440 and 750/440 not improvethe fit either, andthe slope remainsnearly flat. ratios increase in the order A-17, A-18, A-3, A-16, A-7. The Thefit of red/blueversus SiO2 for rocksis slightly better, with greatestuncertainty is A-3, as there is no way with the R2=0.24and 0.21. However,A-7 (Yogi)is thereddest rock, observationsavailable to effectivelymodel its photometric yet hasan intermediatesilica content. IfYogi is removedfrom behavior. Changingits measured•/(• + [t) of 0.47 to the the plot (acknowledgingthe possible extreme influence of normalized value of 0.60 will increase reflectance on the basis diffuseversus direct illumination for this particularrock), R2 of the simple Hapke-Irvine function, but the amount is increasesto 0.84 and0.80 anda negative correlation is found, unknownwithout knowledge of the parameterB 0. Similarly, with A- 16 being the reddest,most silica-poor rock and A- 17 not knowing the phaseangle dependenceon reflectance((x) beingthe bluest,most silica-rich rock. precludesestimating R'at otherphase angles. Therelationship to Fe203*shows a poor fit for soils for 750/440(R 2 = 0.09) buta better(but still poor)fit for 670/440 4.3. Comparison of Spectral and APXS Data (R2=0.29) (Figure9b). WhenA-2 andthe disturbedmaterials Spectralratios (670/440 and 750/440) versus SiO2, Fe203* , of A4 and A-5 are removed, the fit for 670/440 and 750/440 CI, andSO3 are shown in Figure9, with end-memberstabulated increasesto still relatively poor values of 0.58 and 0.13, in Tables 9a and 9b. Soils do not show a good fit or respectively.Because of A-7 (Yogi), the rocks show a weak fit correlationof thesespectral ratios with silicacontent, having (R2= 0.05 and 0.04) and correlation. RemovingYogi

Table 8b. NormalizedRock Spectral Properties a

S•,t.e 440 I/F 670 I/F 750 I/F 670/44O 750/440 A-3b (0.042) (0.109) (0.114) (2.59) (2.70) A-73 (0.030) (0.142) (0.154) (4.68) (5.08) A-16 0.059 0.186 0.188 3.14 3.19 A-17 0.073 0.149 0.145 2.04 1.99 A-18 0.071 0.179 0.184 2.52 2.58

ag= 69o, [t0/[Yt0 + [t] = 0.60.Values in parenthesesare subject to thecaveats described in footnotes b and c. hA-3has coverage only at g=45 ø and [t0/(yt 0+ •t)= 0.47,so reflectance values areshown for thatgeometry. 3A-7has nonlinear behavior that cannot easily be fit tothe Hapke-Irvine function,so data for S0172Fare used, which has a geometrynearly identical to thatfound bythe line fits for A-16, A-17, andA-18 (g--68ø, •/(• + [t) = 0.60). BRIDGES El' AL.' PATHFINDER APXS SITES 14,657

Table 9a. Soil Site Propertiesa

Site APXS Pro•rtie$ GeologicProperties A-2 lowestSO3; lowest Cl fine drift or windtailsor pebbles A4 highestSO 3 manypebbles or possibleclods A-5 lowestSiO2 manypebbles or possibleclods A-8 highestSiO2; highest CI (tied);lowest Fe203* hard,cohesive; no pebbles A-10 highestC1 (tied); highest Fe203* probablyaeolian modified; possible small windstreaks; no pebbles A- 15 intermediatecompositions aeolianduneform; possible sand or pebbles

aSpectralproperties of thesoils are too similarto makeany conclusive statements about their differences. increasesR2 to 0.89 and0.84, withA-17 beingthe bluestrock wt % bins, three each for soils and rocks, with two bins site with lowestiron andA-16 being the reddestrock site with containingboth (Figure9c). Becauseof this, it is difficult to the most iron. The concentrations of chlorine were at the find meaningful trends with the ratio versusCI data, and in limit of detectionby the APXS [Riederet al., 1997a], resulting fact, there is no correlation for soils. The highest rock site in lumpingof the abundancesof the varioussites into four 0.1 chlorine contents are found for A-7, also the reddestrock, but

.... i ...... I ..... ; 'X,'l ...... I ...... I ...... I ...... 1"'

1 A-7

A-1

...... -ø"" A.ii16

liiA-18 ß "- - 670/440smlsR2=007 - 670/440rocks R • 05 - 670/440 rocks R2= 0 24 --©-- 750/'440soils R i11 --n----e-- 750/440750•440 rocksSoils R2 00409 --a-- 750/'440rocks R• _- 0 21 A- 17 A-17 I I I I I I I I I I I I I I iiilJ illill IllIll Ilill il]l,I IlllllJ Ill.. IllIll ...... IIJ'I illIll IJlll 46 47 48 49 50 51 52 53 54 55 56 57 58 59 61) 13 14 15 16 17 113 19 A SiO2(weight %) B Fe203 (weight%)

' i ...... i ...... i ...... I ' ß ' ...... i ...... i ...... I ...... i ...... I 1•' 'd, ..... A-2 A-½(u)

II ^'s 5 ......

o ß ,>

• ••A-15 A-15

..•eA-2 ß IIA-18 A-7 I : 0 j ; 670/440smls JR •= 0 • I = 670/440 rocks IR; = 0 30 • - • I --e -- 750/440soils I R• = 0 15 I'1A,7f7. =_;.. •/44• :soilsJR • = 0 -/,, •f' --o--= 670/440750/44o rocks ]R2IR 2 =- O 077•59 ,, ...... I.; T?, ',0,?,, ...... ,,,{#, I,,,,, 1,,,I,, ,,, ,,,, I 030 040 050 060 0 70 1 0 20 30 40 50 60 C cI (weightø/o) D SO3(weight %)

Figure 9. Relativereflectance ratios for rocksand soils compared to APXS-derivedabundances of (a) SiO2, (b)Fe203*, (c) CI, and(d) SO3. Most of theratios are from reflectances normalized to a commonviewing geometry,as discussed in the text. Letters"u" and "d" refer to undisturbedand disturbed material, respectively. 14,658 BRIDGES ET AL.: PATHFINDER APXS SITES

are also found for A-18, the second bluest rock. About the only thing that canbe saidwith confidenceis that A-17, the bluest rock, also has the lowest chlorine content. The670/440 and750/440 R2 fits to SO3forsoils are 0.32 and 0.05, respectively(Figure 9d). RemovingA-2 and the disturbedsoils of A-4 and A-5 doesnot improve the fits, and thecorrelation is fairlypoor. However, for rocks the fits (R2 = 0.79 for both ratios) and correlations are better. There is a cleartrend of increasingred/blue ratio with increasingsulfur, intersectingsoil valuesin the middleof the plot. Thereis only a slightdiscrepancy with A-3 andA-18, althoughthat is within the errors of the data (see _+ values in Table 6), especiallygiven the uncertaintyof A-3's reflectancesat the normalizedillumination geometry. Integratingall of theratio versus oxide/element plots, there are no significant correlations between APXS-derived chemistryand soil spectralproperties, as has beenfound in previousinvestigations [Bell eta!., 2000]. With rocks, however,a considerableamount of information is obtained. In all casesthe bluestrock site, A-17, is at the upperor lower end of the oxide/elementdistribution, being highest in silica and lowest in iron, chlorine, and sulfur (Table 9b). The reddest rock,A-7, is highestin sulfurand chlorine (along with A-18) but intermediate in silica and iron. A-16, the second reddest rock, is the lowestin silica, highest in iron, intermediatein chlorine(along with A-3), andsecond highest in sulfur.

4.4. Correlation of Morphologic Observations to APXS and Spectral Data 4.4.1. S o i ! s. Analysis of IMP and rover cameraimages allowsa qualitativeto semiquantitativecomparison between featuresseen in the imagesto the APXS data(Table 9a). The imagesshow variations of "pebble"abundance, cohesion, and degreeof aeolianmodification. Rover imagesshow that sites A-4 and A-5 contain an appreciably higher fraction of discernablepebbles, and similar looking featuresat the limit of resolution,compared to otherAPXS sites. All the other soil sites, with the possibleexception of A-2, appearpebble-free (as was discussedin section 4.1, A-2 may or may not consist of a rocky component). However,contrary to what might be expected,the "pebbly"A4 and A-5 sites have the lowest, not the highest, silica contents of the soil sites. They are intermediate in iron and chlorine. The sulfur content of A4 is the highestof all the soils whereasA-5 falls in the middle. Soil site A-8 looks somewhat like a rock in IMP and rover images(Plate 6a and6c). However,the APXSresults show that it is closer in composition to soils (Table 3) [Riederet al., 1997a]. The rover wheelswere unable to dig into Scooby Doo, uponwhich A-8 is located,indicating large cohesionsand an indurated,or hardpan-like,surface [Moore et al., 1999]. A-8 is highestin chlorine(along with A-10) and silica andlowest in iron amongthe soils. It hasan intermediatesulfur content. Sites A-10 and A-15 are probably aeolian or aeolian- modifieddeposits, with A-15 possibly having a significant sandor lithic component(see section 3.1). A-10 is locatedon soil nearthe rock Lamb that, althoughsomewhat dark (Figure 6a, Plates7 and 12), is classifiedas "Bright II" by Bell et al. [2000]. As discussedin section 3.1, the site contains streaks possiblyformed by the wind. Of the soils, A-10 containsthe most chlorine (along with A-8) and iron and shows intermediate abundances of silica and sulfur. A-15 falls in the midrangeof all oxideand element distributions. BRIDG •ES ET AL.: PATHYi•• APXS SITES 14,659

4.4.2. Roe k s. The surfacetexture, and apparent(visual) A-4 and A-5 and has a similar amount of SO3. A-15 has degreeof dust coverage, varies among the rock APXS sites intermediatesilica and fairly low sulfur and therefore cannot (Table9b). All the rocksites are roughwhen viewed close up, easily be correlatedto Pathfinderrocks (lower sulfur, higher consisting of pits, bumps, flutes, and lineaments. All the silica) or Viking soils (highersulfur, lower silica). sites have bumpsand pits, although pits on A-7, the reddest Reported errors in the APXS analyses may not be andmost sulfur-rich rock, do not appearwell developed.Flutes significantenough to discountdifferences between the Viking aremost well developedon A-18, the most chlorine-rich site, and Pathfindersoils [Bridgesand Crisp, 1999; Bell et al., and elongated pits or flute-like forms are seen in the A-16 2000]. As mentionedabove and seen in Figure10 andTable 3, region. Possible lineaments are seen on or near the A-7 the initially published Pathfinder values commonly fall (reddest,highest SO3) and A- 18 (hi ghestCl) sites [McSweenet outside the standarddeviation of Viking analyses. It is al., 1999]. possible that other unmeasuredoxides and components,such A qualitative assessmentof dust cover can be made by as P20$, Cr203, MnO, and H20, could be present in greater looking at the fraction of bright and yellow to red material in amounts than the 2 wt % assumed, such that the APXS assumed rock site IMP, rover color, and rover black and white images. totals of 98 wt % are too high. This wouldparticularly be the This materialcommonly fills in pits and other depressionsin caseif Na20, whichhas an uncertaintyof +40% of the reported the rock surface,indicating that it is probably airfall dust. No value [Rieder et al., 1997a], is over represented. Combined, dust is obvious at A-17, the bluest rock site, which also has thesefactors could potentially lowertotals to 93 wt % or more. the highestSiO 2 and lowest SO3, CI, andFe203 (Figures 9a-9c; This would place Pathfinder silica values for A-4 and A-5 below Table9b) andis the moststeeply dipping (Table 5). The most the highest valuesfor Viking but still outsidethe field of the dustis apparenton A-7, the reddestand most sulfur-rich rock other Viking soils. However, lowering the totals makes site. Many knobs on Yogi and within the A-7 areaappear to Pathfinder sulfur contents even lower and more different than protmdefrom a dust-richcover (Plates5a and5c). Viking. This arguesagainst APXS uncertaintiesbeing the sole reason for differencesbetween Viking and Pathfinder soils [Bridgesand Crisp, 1999; Bell et al., 2000]. 5. Interpretations One possibility is that the Pathfinder soils are richer in silica andpoorer in sulfurthan the Viking soils andthat this is $.1. Soils not dueto the effect of silica-rich rocky componentsthat are The analysis of the Pathfindersoils presentedhere can be observable by the IMP and rover cameras. It is not comparedto resultsfrom the Viking landers, which measured unreasonablethat soils in differentregions of the planet could, soil compositionswith X-my fluorescencespectrometers, and as a class, be different from each other. Soil mechanics Pathfinderrocks (Table 3). Unlike the APXS, the Viking experimentsshow that the most sulfur-richsoils, A-4 and A- XRFS measuredabsolute elemental abundances, leaving some 10, consistof cloddymaterial [Moore et al., 1999]. Cloddy uncertainty on comparisons between the two data sets. To material at the Viking sites was also the most sulfur-rich compare the relative differences in elemental abundances [Clark et al., 1982; Banin et al., 1992]. Some of the lumps betweenthe Pathfinderand Viking sites, Pathfinderrock and that have beeninterpreted here as pebblescould be clods, dust- soil compositions are divided by the average Viking coveredpebbles, or weatheredpebbles, which could explain compositionsfor each oxide or element (Figure 10) [see also the high sulfur abundancesin A-4 and moderate (not low) Bell et al., 2000, Table 1] [Bridgesand Crisp, 1999]. As a abundancesin A-5. The positive correlations between S and group, Pathfinderrocks and soils are richer in SiO2, TiO2, Mg in Viking and Pathfinder soils could be indicative of an MgO, and,except for A-2, A120• and poorerin SO3relative to enhancedabundance of magnesiumsulfate cement within the Viking soils. The Pathfinder values for these oxides are clodsand lesseramounts in other soil samples[Clark et al., generally outside the standarddeviation of the Viking 1982; Banin et al., 1992; Rieder et al., 1997a]. measurements. Silica and sulfur abundances in Pathfinder soils The Pathfinderlanding site, being at the mouth of Ares and are between those in Pathfinder rocks and Viking soils. , is likely to contain concentrationsof sand and Expressingiron as FeO instead of Fe203 increasesnon-iron lithie fragmentsdeposited by the putative floods that formed oxideabundance by only 2% anddecreases iron oxideby • 8%, the channels. Indeed,the presenceof barchandunes and the resultingin minimal changes. properties of Mermaid duneform(see above) are indicative of The Viking soils werepassed through a mesh with a 12.5 x sand [Greeley et al., 1999; Moore et al., 1999], and the 12.5 mm spacing to remove pebbles and clods prior to abundantventifacts argue for sandlikely being presentduring analysis [Clark et al., 1977]. It is thereforelikely that the at least sometimes in the past [Bridgeset al., 1999; Greeleyet Viking XRFS data more closely representunmixed soil than al., 1999, 2000; Golombekand Bridges, 2000]. On the basis the PathfinderAPXS data, which probably sampledat least of soil mechanics tests, morphology, and spectral several surfacesmixed with a variety of components. An measurements,it is likely that puredrifts were not analyzedby important question is, then, to what degree do the APXS the APXS at the Pathfinder site [Moore et al., 1999] and, analyses represent Viking-like soils mixed with other instead, mixtures of drifts, sand/lithics, and other materials materials?On the basis of compositionalinformation alone, were sampled. Any silica-rich sand within the soils should the nesting of Pathfinder soil sulfur and silica contents generally be below the spatial resolution limit of the betweenthose of Viking soils and Pathfinderrocks indicates Sojournercameras, but possibly within the sampling depthof that somemixing has occurred[Bell et al., 2000; McSween and the APXSdata. Many sandgrains and pebblesmay be covered Keil, 2000]. }towever,the most rocky looking sites, A-4 and with dust, further hiding their presence. Therefore the A-5, actually have the least silica, and A-4 has the highest Pathfinder site could consist of fine-scale mixes of lithie and sulfur. A-10, with no apparentpebbles, doeshave low silica soil-type materials. Alternatively, the weathering andfairly high sulfurabundances but is still richerin SiO2 than environmentat the Pathfinder landing site may have been 14,660 BRIDGES El' AL.: PATHIqNDI• APXS SITES

I I I I I I I

o A-2 n A-4 • A-5 EEl a A-8 v A-1D m A-15 © A-3 m A-7 ß A-16 ,,, A-17 ? A-lB

1.5 II

D.5 E

Si02 g1203 Fe203MgO Cato 'l'iO2 SOa CI

Oxid e or El em ent

Figure1 0. CompositionsofMars Pathfinder materials measured bythe APXS relative to averageViking XRFS soil compositions. Abundancesare expressedin weight percentoxides, except CI, with all iron assignedto Fe:O3. Pathfinderanalyses are normalizedto 98 wt % total oxides. Open symbolsare soils and solid symbols are rocks. Small black horizontal lines show the standarddeviation of Viking soil compositionsrelative to the average.Thick lines showthe changein rangeof SiO: and SO3compositions for Pathfindersoils if analysesare normalized to 93 % total oxides. BRIDGES ET AL.: PATHYINDER,M>XS SITES 14,661

differentthan that at the Viking sites, perhapsbecause of more Mars year (G. Landis, personal communication, 1998) waterpresent following the floods, resultingin soils richerin meaning that thicknessessufficient to reducethe APXS and silica and poorer in sulfur than elsewhereon Mars. A final spectralsignature of rockcan accumulate in only about 10-100 possibility is that the pebbles contain less SiO2 than the years. However,these do not representlong-term averages, APXS-measuredrocks, having a compositionmore consistent with dust devils, such as those measured or seen at the with the SNC meteorites and that inferred for rocks over much Pathfindersite [Schofieldet al., 1997; Metzger et al., 1999, of Mars' surface. 2000], or strongwinds removing dust on shortertimescales. Dustwithin pits andother depressions in the rock surfaceswill 5.2. Rocks be mostly shelteredfrom wind removal and may reach an equilibriumthickness above which it is removed. Rocks such The integration of visual, multispectral, and elemental- asYogi, whichappear heavily dustcovered yet havefew pits, abundancedata presentedabove indicates that rocks at the may in fact have many pits that are filled with dust. On the Pathfinderlanding site, and probably over muchof Mars, are basis of the trends of wind tails, wind streaks, and the Mars covered with varying amounts of red, sulfur-rich dust, an generalcirculation model (GCM), the strongestwinds at the interpretationin agreementwith previouswork [Bridgeset al., Pathfinderlanding site today are northeasterly,blowing along 1997, 1998; McSween et al., 1999; Bell et al., 2000; Morris downwindazimuths of 179ø-251ø [ et al., 1981; Smith et al., 2000]. This is supported by the following et al., 1997a; Greeleyet al., 1999]. Thereforeany rocks observations: dipping along upwind azimuths of 359ø-71ø should be 1. The reddestsite, A-7, has the most sulfur. subjectedto strongwinds capable of removingdust (Note: The 2. The bluest site, A-17, has the least sulfur. GCM predictsthat the strongestwinds occurin the winter and 3. Thereis a fairly linear trendof red/blueratio v s. SO3from haveshear stresses as greatas 0.012 N m-•, which, with an A-17, through A~3/A-18/A-16/A-7, to red, sulfur-rich soils atmosphericdensity of 0.01 kg m-• translatesinto a surface (Figure9d). frictionspeed of 1.1 m s-• [Pollacket al., 1981;Greeley et al., 4. Visually, A-17 appearsto have very little dustand A-7 1999]. White et al. [1997] found that dust can be removed seemsto have a lot. A-7 has knobs that appearto protrude fromthe surfacesof pebblesand put directly into suspensionat througha dustylayer that probablyinfills pits, accountingfor frictionspeeds of 2 m s-•. It is thereforenot unreasonableto the relative lack of pits on Yogi. assumethat dustcan be removedfrom even larger roughness 5. Sitesthat faceinto the directionof the strongestwinds, elements,such as the rocksat the Pathfinderlanding site, at which should remove dust, tend to have lower red/blue ratios, the friction speedspredicted by the GCM.). The reddest,most higher sulfurcontents, and less dust (see below). sulfur-and dust-rich site, A-7 on Yogi, has a dip azimuthof 6. A somewhatweaker point is that A-17 also has the 229ø (Table5) andis thereforesheltered from the strongwinds. lowest chlorine andhighest silica contents. This is considered A-3, of intermediatered/blue ratio, fairly low sulfurcontent, somewhatinconclusive because, although consistent with an and fairly low apparent dust, dips at an azimuth of 127ø, absence of dust, the overall correlations for the ratios versus causingwinds to blow acrossit at a grazing angle. The other SiO2 andCI aremuch weaker than they arefor SO3. three sites, A-16, A-17, and A-18, are directly facing the Sulfur serves as the best tracer for soil and dust because it is strong winds, with dip azimuths of 53ø, 64ø, and 48 ø, much more abundant in soil relative to rock than are silica, respectively. These sites have intermediatelevels of or no iron, 'andchlorine (Figure9 and Table 3). On average, soils observed dust and low to intermediate red/blue ratios and sulfur have a factor of 2.3 more SO3than rocks, with the sulfur (Figure9d andTable 9b). contentamong rocks varying by a factor of 5.6. In contrast, Weathering rinds, such as siliceous finds that form on the factorsfor rock to soil and variation among rocks tbr CI, Hawaiianbasalt [Farrand Adams, 1984; Crisp et al., 1990] or Fe203*, and SiO2 are 1.3/2.0, 1.2/1.3, and 0.9/1.2, on rocks in the presenceof water [Caseyet al., 1993; Dorn and respectively. The unmixedcomponents of pure rock and pure Meek, 1995], may be presenton the Pathfinder rocks. A few soil are therefore expectedto have roughly similar iron and Pathfinder rocks exhibit strongly forward-scattering silicon contentsbut very different sulfur contents, such that photometric functions, consistent with a smooth surface mixing betweenthem will show the strongest correlations consistingof a desertvarnish-like weatheringrind of a glassy with sulfur. This is consistentwith the finding that plots of matrix [Johnsonet al., 1999] or aeolian polish [Bridges et al., sulfur versus oxide concentration for rocks exhibit lines that 1999]. Evidence for thin coatings altering the spectral intersectsoil compositions[McSween et al., 1999]. signaturesof rocks has been foundat the Viking sites [Evans The thickness and areal distribution of dust cover on the and Adams, 1979; Sharp and Malin, 1984; Guinnesset al., rock surfaces are difficult to estimate. The dust need not be 1982, 1987, 1997]. Rock coatings, save for amorphous thicker than the wavelengthof light, 440 to 750 rim, or about silica, are predictedto abrademore quickly than the host rock 1/xmthick, to be opaqueand block reflectedlight off the rock on Mars, but the relative ratesof coating formation versusloss surface.The samplingdepth of the APXS is approximately 10- from aeolian abrasion are poorly constrained [Kraft and 100 /xm, increasing with the mass of the element [Crisp, Greeley,2000]. Irrespectiveof the possibility of weathering 1998]. Therefore very thin accumulationsof dust should finds anddespite the tact that nearly pure silica finds couldbe reduceor block the signatureof rock as seen from IMP and the present, it is not clear how rinds would producethe positive APXS. On the basis of 24 sols of materials adherence red/blueversus sulfur relationship seen. Sulfur-richcrusts on experimentresults, dustaccumulated at a rate of 0.28% areal Earthare rare and are generallyrestricted to evaporatesulfates, coverage per sol [Rover Team, 1997b; Landis and Jenkins, commonlygypsum [Dom, 1998], or ephemeralcoatings near 2000], which, dependingupon the size, shape, opacity, volcanic vents. Furthermore,the higher sulfur and red/blue backscatterfunction, and packing of the dust, results in valueswhere dust is seenand wherewind is low arguesfor a accumulationrates of the orderof < 1 to severaltens of/xm per surficialdust coating producingthe spectral/APXStrends, not 14,662 BRIDGES El' AL.: PATHFINDER APXS SITES weatheringrinds. Therefore,although rinds couldbe present, Improvedcalibration of analyticalinstruments, as is currently they are probably not influencingthe spectral/compositional occurringfor the APXS[e.g., Foley et al., 2000], is warranted relationshipsreported here. to removesome of the uncertaintyin these data. Finally, The dataand interpretations here reinforce the view that dust microscopicimagers capable of resolvingsubmillimeter-scale masksthe truerock chemistryas measuredby the APXSand the featuressuch as sand,dust clumps, and fine rock textures,also truerock spectralproperties as determinedby IMP. The sulfur- plannedfor MER, will beof tremendousbenefit in determining free rock composition, determinedby extrapolating to the the componentsthat contributeto the bulk spectral and interceptat 0% sulfuron sulfurversus oxide plots [McSweenet elementalsignature of a surface. al., 1999] may representthe truecomposition of the outer rock surfacelayer. This composition falls within the basaltic 7. Conclusions andesiteto andesire fields on alkali versus silica plots, a composition very different from SNC meteorites and that Analysesof the PathfinderAPXS sites using recalibrated inferred for most Martian rocks [McSween, 1994; McSween et IMP spectra,APXS data, and IMP and rover cameraimages al., 1999] and from orbital/telescopic spectral signatures show that each site is composedof one or more distinct indicative of mafic material [Mustard etal., 1993, 1997; componentsthat affect their bulk spectral and chemical Singer and McSween, 1993; Bell etal., 1997]. Rather, it is signatures.The Pathfinderrock APXS sites exhibit strong closer in composition to the andesitic signature found in correlationsamong apparent dust content, red/blue ratio, and thermal emissionspectrometer (TES) spectraconcentrated in sulfur and some weaker correlations with silica and chlorine. the southernhighlands [Bandfield etal., 2000]. Therefore,if The bluest, most sulfur-poorrock APXS sites face towardthe the APXS interpretationis correct, the rocks either could be northeast,the direction of the strongestwinds in the current basalticrocks covered with silicic weatheringfinds and dustor windregime. The most logical interpretation is that the rocks find-free andesitic rocks covered with dust. arevariously coated with sulfur-rich,red dust within pits and cragsin therocks' surfaces, especially on facessheltered from 6. Discussion strong winds. This indicatesthat the extrapolationto a "sulfur-free"rock composition,consistent with a silica-rich The results presented here reinforce the view that weatheringrind or silica-richrock of andesiticcomposition, is instrumentsviewing Martian rocks and soils do not sample a truerreflection of the actualcomposition of the outer rock end-membercomponents. The complex geologic history on layerthan that indicatedby the APXSdirect measurements. Mars, imprinted with aeolian, fluvial, and impact The soil APXS sites are composedof fines of various redistributionof fine-grainedmaterials, has produced variously cohesionsand states of aeolian modifications, along with mixed and sortedsurfaces. Therefore, although the IMP and small pebbles, clods, and possible sand. Sulfur-rich APXS are potentially very usefulinstruments for addressing compositionsare associated with somepossibly cloddy soils, centralquestions regarding Martian geology and chemistry, consistentwith Viking Landeranalyses. However,with this their integrateddata show that the natureof the Martian surface exception,the spectraland APXS signatures of the soil sites at the Pathfinder site limits their own effectiveness. arepoorly correlated to the geology. Thereforethe silica-rich This information can be used to guide strategies for and sulfur-poorcompositions relative to the Viking sites instrumentdevelopment and mission planing on futureMars indicatethat the Pathfindersoils are probably distinct, even landersand rovers. One of the most important problems is giventhe potentialerrors in the APXSresults. seeingbeneath the surfacelayer of dustor weatheringrinds on The characteristics of surface materials at the Pathfinder site rocks. The brute force methodof physically removing the are a broader reflection of complex mixing and sorting outer surface,as planned for the rock abrasiontool on the mechanismson the Martian surface,generally by aeolian Athenapackage of the Mars Exploration Rover (MER) and the activity. As such,they areindicative of what will likely be rock corer/grinder on 2, is a good approach. foundby futurelanders and rovers. Therefore the results of this Separatingcomponents in soils suchthat the end-membemcan studyshould guide the selectionand implementation of future be identifiedand characterizedis also a problem. Small drills instrumentsand explorationstrategies that can best sample or scoopsthat can be carefully manipulatedto sample well- unmixed materials at a small scale as well as rock interiors. definedregions of soils wouldbe of greatbenefit. The ability to sievesoils in orderto segregatesmall fines, pebbles,clods, andother components,such as wasused in basicform on the Viking Landers,would also be desirable. The ability to Appendix A separatemagnetic fractions, as plannedfor MER, will also be useful. A1. Determining Surface Orientation of APXS Sites Spectralanalysis of rocks and soils, and the components that truly composethese bulk categories,will benefit from a The surfaceorientations were computed by finding the x-y-z wider wavelength range. For example, thermal infrared positionsof three representativepoints using cameramodel instruments,such as Mini-TES on MER [Squyreset al., 1999], software. The Mars Local Level Coordinate Frame was used (a should be able to measure the thermal inertia of rocks and soils right handedsystem with x pointing north, y pointing east, andthereby constrain the amountof duston rocks, small rocks andz pointing downand 0-0-0 at the basepetal's top surface within soil, andthe bulk densityof soils. Suchdata, combined andgeometric center). The threepoints can be representedas with visual-near-IR-thermal IR spectra obtainable with PI(Xl, Yl, Zl), P2(X2,Y2, Z2), and P3(x3,Y3,Z3).Two vectors Athena's Pancam and Mini-TES, will provide improved originatingfrom the samepoint to two otherpoints are then chemicaland mineralogical information on the Martian surface defined.For example, vectorsfrom Pl to P2 and from P• to P3 than that obtainable with IMP or the Viking Landercameras. are BRIDGES ET AL.' PATHHNDER APXS SITES 14,663

PtPa =(x2-x•)l +(Y2-Y•)J +(z2-zt)k (Ala) Taking the height of IMP (0.92 and 1.54 m in the stowedand deployed position, respectively) and knowing the x-y-z PiPs =(X3 -x•)i +(y3-y•)j + (z3-zt)k, (Alb) coordinatesof the highest position in the three-point plane (which has the lowest z, becauseof the fight-hand rule in the where i, j, and k are basic vectors. These two vectors lie coordinatesystem), the look angle of IMP to the top of the within a plane. To definethis plane, a vectornormal to it, N•, plane of interestcan be computed.If this angle is greaterthan is computedfrom the crossproduct: p• then there is no possibility of the surfaceplunging away from IMP andthe uppersurface is viewed:if N 1 ----P•P:XPtP•- li j k I (A9) Ix2-x• Y2-Y• z2-z• I tan-1 >Pl , Ix 3-x• Y3-Y• Z3-Z• I x1.54z,,,,. 2+ y Zminz,,,,. 2 ]

li j k I then "upper." If this condition is false, then a test is done to determinewhether the surfaceis orientedtoward or away from lb 1 b2 b3 I the camera.If it is orientedtoward it, the uppersurface is seen. If it is orientedaway, then the lower surfaceis viewed. In the = (a2b3-c6b:)i + (aBb1 - alb3)j + (alb2 - a2bl)k, (A2) former case, the distanceto the highest point (lowest z) is fartherthan that of the lowest point: if wherearefers to the componentsof the first vector (P•P:) and b refersto the componentsof the secondvector (P•Pa). It is ¾Xzmin 2+ Yzmin 2> ¾ Xzmax 2+ YZmax 2 , (AlO) now necessaryto definethe angle • that this plane makeswith the horizontal. The angle betweenthese planes is the angle then "upper";otherwise, "ower." betweenthe perpendicularvectors to the planes (N• and N: The dip directioncan now be computed.For an uppersurface from the horizontal plane), which is found from their dot of a plane, if the y intercept(in this right-handedsystem) of product: the strike on the top of the plane (lowest z) is higher than the interceptat the bottom of the plane (highestz), then the plane COS fi = •N•- N2 .a•b•+ alb2+ asbs is dipping west:if (A3) N, {N• ¾[a,2+a22+a•q[b,2+b22+bs2J Yz•,, - xz•tan(o) > Yz•i, - xz•i,tan(o), (A11) wherea refersto the componentsof the first vector (N•) and b then "west"; otherwise, "east." If the camerais viewing a to the componentsof the secondvector (N:). BecauseN: is a lower surface,then the logic is reversed. horizontalplane at z = 0 (or z equalsto anything), bt = b2 = 0 Now the plane is completely defined in three-dimensional andb• = 1. The equationthen reducesto (3-D) space (e.g., N20øW, 15øE). With the inherent uncertaintyin this approach,four points wereselected and four cos• = N•. N2 _ a3 (A4) planes computed(using points 123, 124, 134, and 234). To N Ual2 + az2+ a32• computethe averageof these, the averageof the poles of the planes was convertedto a mean vector. The mean plane This is the dip of the planarsurface. having a pole equalto the meanvector was then found. To find the strike, the vector parallel to the plane containingboth N• and N: is found.Taking the crossproduct A2. Determining True Incidence and Emission as in A2 andrealizing that bt =b 2=0 andb3= 1 gives Angles The final stepis to computethe true incidenceand emission N•XN:= l i j k i angles relative to an inclined surface(Figure 3). Here the I a• a2 a• I = a2i - a•j. (AS) 10 0 1 I incidenceand emergentangles relative to a horizontal surface aredesignated i ande and those relative to the actualsurface i' ande', respectively. Valuesof i and e are in the image headers The strike o is then computedby of the IMP frames. The plungeangle on the planar surfacethat the sunlight makes,Ps, is {J= tan-1 (-O a/ Oa). (A6) To determinewhether the upper or lower planar surfaceis Ps=cos' t cos• (A12) being measured(e.g., the top or an overhangof a rock), it is lcosyVrtan2¾+cos2• ) where? is the anglebetween the solarazimuth r I andstrike' pt=aco• cos• (A7) • cos•I/tan2• + cos2• ¾= lB - ol. (A13)

first necessaryto computethe angle of plungeof the surfacein There are two geometries in which the surface can be the look direction(p). illuminated and thereby seen. The first and most easily visualized is the case in which the Sun is in the same strike- where[3 is the anglebetween the site azimuthcz and strike: definedhemisphere as the dip directionis (e.g., for a strike of N20øWand a dip to the west, the solar azimuthwould have to (A8) be from 160ø to 340 ø to meet this criteria). In this case: 14,664 BRIDGES ET AL.: PATHFINDER APXS SITES

i' =i -p,. (A14) Bridges,N.T., and J.A. Crisp,Constraints on compositionas inferred from Viking XRFS and PathfinderAPXS and IMP data In the second case the Sun is on the other side of the (abstract),Lunar Planet. Sci. XXX, 1927, 1999. Bridges,N.T., R.C. Anderson,J.A. Crisp,T. Economou,and R. Reid, hemisphere. If the incidenceangle is greaterthan the Separatingdust and rock APXS measurements based on multispectral complementof the solar plungeangle, then the surfaceis in data at the PathfinderLanding Site, Eos, Trans.,AGU 78(46), Fall shadow.Otherwise, the trueincidence angle is the horizontal Meet. Suppl.,F402-F403, 1997. incidenceangle plus the plungeangle: Bridges,N.T., J.F. Bell, J.A. Crisp, T. Economou,J.1L Johnson,S.L. Mumhie, and R.J. Reid, Comparisonbetween APXS and IMP multispectraldata at the PathfinderLanding Site: Evidence for dust if i > 90 -psthen"shadow"; otherwise,/'=/+ps. (A15) coatingson rock surfaces(abstract), Lunar Planet. Sci, XXIX 1534, 1998. Note that the position of the observer(in this caseIMP) does Bridges,N. T,, R. Greeley, A. F. C. Haldemann,K. E. Herkenhoff, M. not affect the value of the phase angle. Assumingan Kraft, T. J. Parker, and A. W. Ward, Ventifacts at the Pathfinder landingsite, J. Geophys.Res., 104, 8595-8615, 1999. illuminatedsurface, the phaseangle g' is simplycomputed by Bridges,N.T., J.A. Crisp,and J.F. Bell, The Mars PathfinderAPXS sites: the standard formula New insights from improved IhdP calibration and image analysis(abstract),Lunar Planet. Sci. XXXI, 1740,2000. g' =cos-•(cos[i']cos[e]+ sin[i']sin[e]cos[W]), (A16) Casey, W.H., H.R. Westrich, J.F, Banfield, G. Ferruzzi, and G.W. Arnold, Leaching and reconstructionat the surfacesof dissolving chain-silicate minerals, Nature, 366, 253-256, 1993. where• is the azimuthalangle betweenthe emergentand Clark, B.C., et al., The Viking X Ray Fluorescenceexperiment: Analyticalmethods and early results,J. Geophys.Res., 82, 4577- incidentplanes, going clockwise from the emergentplane, and 4594, 1977. is equalto the differencein solar and look direction azimuths Clark, B.C., A.K. Baird, R.J. Weldon, D.M. Tsusaki, L. Schnabel, and asfound in the IMP imageheaders. M.P. Candelaria,Chemical composition of Martianfines, J. Geophys. Res., 87, 10,05%10,067, 1982. Cloutis, E.A., and M.J. Gaffey, Pyroxene spectroscopyrevisited: Acknowledgments. The commentsof an anonymous Spectral-composition correlations and relationship to geothermometry,J. Geophys.Res., 96, 22,809-22,826,1991. reviewer are appreciated. This paper could not have been written Crisp,J.A., The effect of thincoatings of dustor soil on the bulk APXS withouthelp from several individuals. R. J. Reid providedsoftware and compositionof the underlyingrocks at the Pathfinderlanding site personalinstructions to perform the latest IMP calibrations. D.A. (abstract), Lunar Planet. Sci. XXIX, 1962, 1998. Alexanderand E. D. Duxburygeometrically corrected many images Crisp, J.A., A.B. Kahle, and E.A. Abbott, Thermal infrared spectral thatwere centralto the stereoanalysis. T. E. Litwinwrote the 'image characterof Hawaiianbasaltic glasses, J. Geophys.Res., 95, 21,657- 21,669, 1990. processingsoftware for stereolocation. F. S. Anderson'sassistance with Dorn, R., Rock Coatings,429 pp. ElsevierSci., New York, 1998. debuggingproblems with someof the softwareis greatlyappreciated. Dom, R.I,, and N. Meek, Rapid formationof rock varnishand other We acknowledgethe printingof highquality figures by K. S, Captaro. rock coatingson slag depositsnear ,California, Earth Surf Discussionson photometry with B. Hapke and R. M. Nelsonwere very ProcessesLandforms, 20, 547-560, 1995. useful.Finally, we posthumouslythank H. J. Moore, who locatedmany Evans, D.L., and J.B. Adams, Comparisonof Viking Lander of theAPXS sites and conducted related research. 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