JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. E4, PAGES 8679-8715, APRIL 25, 1999 Chemical,multispectral, and textural constraints on the compositionand origin of rocks at the Mars Pathfinder landing site H. Y. McSweenJr., 1 S.L. Murchie,2 J. A. Crisp,3 N. T. Bridges,3 R. C. Anderson,3 J.F. BellII1, 4 D. T. Britt,5 J. Briickner,6 G. Dreibus,6 T. Economou,7 A. Ghosh,1 M.P. Golombek,3 J.P. Greenwood,1,8J. R. Johnson,9 H. J.Moore, 1ø R. V. Morris,11 T. J. Parker,3 R. Rieder,6 R. Singer,12 and H. W•inke6 Abstract. Rocks at the Mars Pathfindersite are probablylocally derived. Textureson rock surfacesmay indicatevolcanic, sedimentary, or impact-generatedrocks, but aeolianabration and dustcoatings prevent unambiguous interpretation. Multispectral imaging has resolved four spectralclasses of rocks: gray and red, which occuron differentsurfaces of the samerocks; pink, which is probablysoil crusts;and maroon,which occursas largeboulders, mostly in the far field. Rocksare assignedto two spectraltrends based on the positionof peakreflectance: the primary spectraltrend contains gray, red, andpink rocks;maroon rocks constitute the secondaryspectral trend. The spatialpattern of spectralvariations observed is orientedalong the prevailingwind direction. The primary spectraltrend arises from thin ferric coatingsof aeoliandust on darker rocks. The secondaryspectral trend is apparentlydue to coatingby a differentmineral, probably maghemiteor ferrihydrite. A chronologybased on rock spectrasuggests that roundedmaroon bouldersconstitute the oldestpetrologic unit (a flood deposit),succeeded by smallercobbles possiblydeposited by impact,and followed by aeolianerosion and deposition.Nearly linear chemicaltrends in alphaproton X-ray spectrometerrock compositions are interpretedas mixing linesbetween rock and adheringdust, a conclusionsupported by a correlationbetween sulfur abundanceand red/bluespectral ratio. Extrapolationsof regressionlines to zero sulfurgive the compositionof a presumedigneous rock. The chemistryand normativemineralogy of the sulfur- free rock resemblecommon terrestrial volcanic rocks, and its classificationcorresponds to andesite. Igneousrocks of this compositionmy occurwith clasticsedimentary rocks or impactmelts and breccias.However, the spectralmottling expected on conglomeratesor brecciasis not observedin any APXS-analyzedrocks. Interpretationof the rocksas andesitesis complicatedby absenceof a "1 gm" pyroxeneabsorption band. Plausibleexplanations include impact glass, band maskingby magnetite,or presenceof calcium-and iron-rich pyroxenes and olivine which push the absorption band minimumpast the imager'sspectral range. The inferredandesitic composition is most sinfilarto terrestrialanorogenic icelandites, formed by fractionationof tholeiiticbasaltic magmas. Early meltingof a relativelyprimitive Martian mantlecould produce an appropriateparent magma,supporting the ancientage of Patlff•nderrocks inferred from their incorporationin Hesperianflood deposits.Although rocks of andesiticcomposition at the Patlff•ndersite may representsamples of ancientMartian crust,inferences drawn about a necessaryrole for water or plate tectonicsin their petrogenesisare probablyunwarranted. 1DepartmentofGeological Sciences, University ofTennessee, 1. Introduction Knoxville. The Mars Pathfinder landing site is strewn with 2AppliedPhysics Laboratory, Johns Hopkins University, Laurel, Maryland. semiroundedpebbles, cobbles, and bouldersthat comprise 3jetPropulsion Laboratory, California Institute ofTechnology, 16% of the surfacearea [Smithet al., 1997a]. The remainderof Pasadena. the site consistsof fines,partly as windblown"drift" forming 4Centerfor Radiophysics andSpace Research, Cornell University, various aeolian bedforms,and partly as depositstypically Ithaca, New York. strewnwith small pebbles. The measuredrock abundanceis 5Lunarand Planetary Laboratory, University ofArizona, Tucson. similar to an estimatefor the entire Pathfinderlanding ellipse 6Max-Planck-InstitutftirChemie, Mainz, Germany. (18-25%) based on thermal emissivityresults [Christensen, 7EnricoFermi Institute, University ofChicago, Chicago, Illinois. 1986]. The characteristicsof this rocky landscapeare 8Nowat Institute ofGeophysics andPlanetary Physics, University of consistentwith depositionby the Ares and Tiu catastrophic California, Los Angeles. floods, possiblyoverprinted by ejecta from a nearby impact 9U.S.Geological Survey, Flagstaff, Arizona. crater [Golombeket al., 1997a;Parker and Rice, 1997]. The 10Deceased[September 21,1998]. Pathfinder location was originally selected,in part, on the 11NASAJohnson Space Center, Houston, Texas. premisethat it might be a "grab bag" site containingrocks 12CatalinaTechnologies, Tucson, Arizona. carried by floods from the ancient southern highlands [Golombeket al., 1997b]. However,the 800-1andistance to Copyright1999 by the AmericanGeophysical Union. the southern highlands boundary, the recognition that Paper number98JE02551. terrestrialfloods do not carry large rocks great distances,and 0148-0227/98/98JE-02551 $09.00 the angularityof many blocks all suggestthat rocks at the 8679 8680 MCSWEEN ET AL.: MARS PATHFINDER ROCKS •o•o A-7 , Baker's Bench Yogi A-5 •. Lamb TheDice--•'.-t:[_ A-10 .i• Souffi•-•"A-4•!:' ScoobyIA-8 Doo PhotornetryA-3 •1%A'2 Flats Barnacl&- •:•:'......... ':". MintJulep Bill ...........,-'-:.:.:..•,• ::•;:••,,...:•: :•..,Mini-Matterhorn Giver ............ •ds + '¾.: A-23 •mbam •uash . BrokenStimpy +Flat lop Has•k Moe A-18,-19"•' ':' % 'Wedge [' Half•me A-15 A-17Sha MermaM TheGardenRock Charming• Dune A-27 .,,,,•,,,•North Chimp WindPrevailing Direction t • 2meters Figure 1. Sketchmap of the Pathfinderlanding site showingthe approximatelocations of rocksand other featuresdescribed or referencedin the text. Eight rockswhich were analyzedby the APXS are coloredblack, and six soil APXS analysissites are indicatedby small solid boxes. APXS chemicalanalysis numbers are also given. Off the map in the directionindicated by the arroware BakersBench and Seal,which are 8 and 37.5 m from the centerof the lander,respectively. The rovertraverse, spanning 105 m, is alsoillustrated; fine lines connectingsome traverse segments indicate repositioning of the rover'sinternal guidance system based on end-of-dayIMP images. Pathfinder site may be locally derived [Malin et al., 1997]. The nature of the Pathfinder site rocks is somewhat Some rocks might be related to domes, interpreted to be uncertainbecause of conflicting or ambiguousobservations volcanic [Greeley et al., 1977] or sedimentary[Parker and and measurements.Preliminary imaging results [Rover Team, Rice, 1997], locatedapproximately 100 km eastof the site and 1997; Smithet al., 1997a] suggestedthat a diverseassortment associated with a dark mottled unit that extends into the of rocksis present,an inferencethat appearsto be inconsistent westernpart of the Pathfinderlanding ellipse. More likely, with availablerock chemistry[Rieder et al., 1997a]. Here we the rocks are samplesof a ridged plains unit [Britt et al., describe chemical compositions measured by the rover- 1998] or of older crust that may constitutethe local bedrock mounted alpha proton X-ray spectrometer (^PXS), and exposed,streamlined islands. Rocks at the Pathfindersite multispectral imaging by the Imager for Mars Pathfinder (IMP), and observationsof structuresand surfacetextures by itself are Hesperian-age materials interpreted to be the the IMP and by rover cameras. These data allow new lowermostfloors of the outflow channel system[Rotto and inferencesto be drawn concerningthe mineralogy,petrology, Tanaka, 1995; Tanaka, 1997]. It is likely that the bedrock and origin of theserocks. Our interpretationsare guided,to beneaththis unit is lower Hesperianbut it could be partly someextent, by the geologiccontext of the Pathfindersite, by upperNoachian, or both, dependingon local stratigraphy previous observations at Viking sites, and by the The relative locations of rocks described or alluded to in petrogenesisof SNC meteoriteswhich are commonlythought thispaper, as well as the rovertraverse, are shownin Figure1. to be Martian rocks[McSween, 1994, and referencestherein]. This map can be cross-referencedwith a previouslypublished The integrated chemical, multispectral, and textural data panoramicview of the landingsite showingrock namesand suggestthat the Pathfinder site is apparentlydominated by a locations(Mars Pathfinder, 1997, Plate 6). distinctive,perhaps unusual lithology. MCSWEENET AL.: MARS PATHFINDERROCKS 8681 2. Rock Structures and Textures processesprobably have enlarged the originalvesicles and changedtheir shapes[Greeley and Iverson, 1985]. A 2.1. Nature of the Observations possiblealternative to vesiclesis chemicallyetched pits, such as thoseformed in Antarcticaduring brief periodswhen rock surfaceshave thin films of snow xnelt;this mechanismhas been Observationswere made using camerason both the lander proposedas an explanationfor the pits in rocksat Viking and rover that permit interpretationsof rock textures. The landingsites [Allen and Conca,1991]. Otherpossibilities front cameraon the rover providedclose-up images of rocksat includeetching of softerminerals in a matrixof harderminerals a spatialresolution of 0.7-1 mm per pixel at closestrange. For [Carr, 1981]or wind blastingto carveventifact pits. It is rocksfarther than 1.3 m from the IMP landercamera yet within unclearwhether vesicleson the surfaceof Stimpy (Figure 2) 40 cm of rover cameras, the spatial resolution of the mighthave exerted control on windabrasion