Feb 25, 1993 - of the ferric-ferrous absorption) are mapped and coregistered to Viking digital photomosaics. Analysis of these maps shows that although there ...
JOURNALOF GEOPHYSICAL RESEARCH, VOL.98,NO. E2,PAGES3387-3400, FEBRUARY25, 1993
The Surfaceof Syrtis Major: Compositionof the VolcanicSubstrateandMixing With Altered Dust and Soil JOHNF. MUSTARD, 1 S.ERARD, 2 J-P.BIBRING, 2 J.W.HEAD,1 S.HURTREZ, 3 Y. LANGEVIN, 2 C.M.PIETERS, 1ANDC.J.SOTIN 3 SyrtisMajoris an old,low reliefvolcanicplateauneartheequatorial regionsof Mars. It is a persistent low-albedofeatureon the planetand is thoughtto containa high abundance of exposedbedrockand/or locallyderivedsurfacematerialanddebris. Spatiallyresolvedvariationsin surfacespectralproperties, and therefore composition,are investigatedwith data from the Imaging Spectrometerfor Mars (ISM) instrument.ISM obtained128 wavelength channelspectrafrom 0.76 to 3.16 •m for contiguous pixels approximately 22 x 22 km in size acrossmuchof the plateau. The value and spatialdistributionof four primary spectralvariables(albedo,continuumslope,wavelengthof the ferric-ferrousbandminimum,area of the ferric-ferrous absorption) are mappedandcoregistered to Viking digitalphotomosaics.Analysisof thesemapsshowsthatalthoughthereis a highdegreeof overallspectralvariabilityon theplateau,the key indicators of maficmineralogy are relativelyhomogeneous. Detailedexamination of reflectance spectra from representative areasacrossthe plateauindicatethe volcanicsurfaceis dominatedby augire-bearing basaltsandthe pyroxenecomposition in the basaltsis estimated to be 0.275+ 0.075 Ca/(Ca+Fe+Mg)and 0.3+ 0.1 Fe/(Fe+Ca+Mg).Additionalmineralcomponents may includeolivine,feldspar,andglass. Most of the spectralvariabilityon the plateauis interpretedto resultfrom mixing of volcanicbedrockand/or locally derivedsurfacematerialanddebriswith highlyaltereddustand soil. In westernSyrtisMajor the alteredmaterialis a transientcomponent on the surfaceor occursin largespat/allycoherent patches(e.g., craterrims). In eastemSyrtisMajor it is apparentthatthe dustcomponents arefirmly fixed to the basaltic substrateas a stableoxide find or coating.
INTRODUCTION
Syrtis Major Planurn is a low relief shield volcano,
typically have greaterinherentspectralvariability than bright regions, and that the spectral variability is correlated with
approximately lx106 km2 in area, constructed onancient crust geomorphic
units [Soderblom et al., 1978; McCord et al.,
1982; Pinet and Chevrel, 1990], they are thought to contain west of the Isidis impactbasin. At least two eruptiveperiods exposed bedrock or mobile material that is locally derived are recognizedin the surface morphologyof the plateau: an [Soderblom, 1992]. Syrtis Major is therefore a strong early phaserepresentedby a ridgedplain unit with no evidence candidate for detailed analyses with remote observationsto of flow fronts or scarpsand a later phase characterizedby determinethe compositionof Martian volcanic material. extensive flows emanating from two central caldera with Reflectancespectraat visibleto near-infraredwavelengths fissuresand lava tubeson the shield flanks [Schaber, 1982].
Crater densitiesindicate an age of formationas post-heavy bombardment, similar to other low relief shield volcanos in the
easternhemisphereof Mars [Barlow, 1988]. These volcanic regionsare in clear contrastto the large volcanicconstructsin the western hemisphereof Mars (i.e., Tharsis province) and suggest a fundamental change in the nature of volcanic processesat this time.
SyrtisMajor has beenrecognizedfor manyyearsbecauseit is a persistent low albedo feature on the planet [e.g., Antoniadi, 1930] but is notablebecauseit is known to change in albedoand shapeseasonally. In general,the easternborder of Syrtis Major changesmore drastically than the western border [Slipher, 1962]. Syrtis Major has been observedto brighten and shrink in size after major dust storms then increasein size and darken during the ensuingmonths. This indicates that the processesof deposition and erosion are occurringwith greatregularity.Becauseequatorialdarkregions
can provide composition information. Telescopic observationsof Mars at these wavelengthsover the last 20 yearshaveprovidedcluesto the ferric andferrousmineralogy [e.g.,McCord et al., 1982;Singer, 1985;Bell et al., 1990] and have beenusedto searchfor evidenceof claysand carbonates
[e.g.,Clark et al., 1990;Blaney and McCord, 1989;Singer, 1985]. Bright materialsshow a strongultravioletto visible absorptionand a steeprise in reflectanceto the near-infrared thoughtto be largely the results of ferric crystal field and chargetransferabsorptions.Subtleinflectionsalongthe steep risein reflectance andin thenear-infrared indicatethepresence of poorly crystallineferric oxides [Bell and McCord, 1989;Bell
et al., 1990;Morris et al., 1990;Singeret al., 1990] probably mixed with a weakly absorbingor amorphouscomponent. This material is thoughtto be a palagonitederived from alteration and weathering of volcanic glass of basaltic composition. Reflectancespectrafrom dark regions show evidencefor mafic mineralabsorptions near 1.0 and 2.0 •m which have been interpreted to indicate the presence of augite 1Department of Geological Sciences, BrownUniversity, bearingmafic volcanics[Adamsand McCord, 1969;Singer, Providence, Rhode Island.
2Inst/tut d'Astrophysique Spatial, Universit6 Paris-Sud, France. 3Laboratoire dePhysique dela TerredePlan•tes, Universit6 ParisSud, France.
Copyright 1993by theAmerican Geophysical Union. Papernumber 92JEO2682 0148-0227/93/92JE-02682505.00 3387
1980].
The Imaging Spectrometerfor Mars (ISM) instrumenton boardthe SovietPhobos2 spacecraftacquireddata for several equatorial regions on Mars. Imaging spectrometer data combinethe spatialinformationinherentin imageswith the compositional information of high resolution reflectance spectra. These data provide the opportunityto expandand
3388
MUSTARD ET AL.: SURFACE OF SYRTIS MAJOR
extend the resultsfrom analysisof telescopicobservationsand windows, are up to 120 lines long and 25 sampleswide. The in particular, examine the spatial variationsin surfacespectral instantaneousfield of view of the instrument is 12'x12' which, to a surface properties and composition with geomorphic characteristics. from the altitudeof orbit of 6300 km, corresponds Previousanalysesof ISM datafor the surfaceof Mars [Bibring resolutionof approximately22x22 km at normal incidence. In et al., 1989, 1990; Erard et al., 1991] have identified the the rest position, the viewing plane of the instrument was presenceof mafic silicates,mapped the distributionsin several orientedparallel to the Sun'srays and thereforeat the subsolar regionsof Mars, identified and investigatedvariationsin the point, the incidenceangleequalsthe emergenceangle and the strength of the water of hydration band, and searched for phaseangle is 0ø. The scanningmirror permitssomevariation indications of weak carbonatebands. These telescopicand from this geometry,and the phaseangle varies =5ø for each orbital observations have demonstrated that the surface of Mars windowand is alwayslessthan20o. Incidenceand emergence exhibits complex variations in spectral properties that, in anglesare a functionof longitudeandlatitude. The imagedata many cases,are tightly coupledto bedrockcharacteristicsand for a given window were acquiredover a period of 25 minutes provide importantinformationfor lithologic compositionand and therefore the effects of temporal variability in the alteration history. atmosphereare minimal. There is a systematicincrease in SyrtisMajor is investigatedin this analysiswith ISM data atmosphericpath length with distancefrom the subsolarpoint. to characterizethe spectralpropertiesof the surface,to identify The groundtrack of the SyrtisMajor ISM window overlainon the major mafic mineralogy of the volcanic materials, and to the Viking orbiter digital photomosaics MC13SW and derive estimatesof the chemistry of these minerals. This is MC13SE is shownin Figure 1. achievedby (1) calibrationof ISM data to a spectralmodel for Mars, (2) identification and mapping of major spectral Data Reduction variables to determine the associationsof spectralvariability Detailed discussions of generaldata reductionare presented with surface morphology,(3) detailed examinationof ISM in the appendix. Briefly, all well-characterizedinstrumental, spectra from surface regions which display end-member solar, and atmospheric effects are removed in a series of combinationsof spectral properties, (4) interpretationof these additive and multiplicativesteps,includingnew correctionsfor spectra to characterize the major mafic mineralogy of the the third-orderoverlap. As part of this process,an improved volcanicplateau,(5) estimatethe chemistryof the major mafic spectral model for Phobos, based on laboratory meteorite minerals, and (6) discussionof possible source of spectral spectra with an additive thermal component,is used. The variability observed that are unrelated to differences in ferric atmospheric contribution to the data is removed using a and ferrousmineralogy. atmosphericmodel and an atmosphericdepth estimatedby the ISM INSTRUMENT AND DATA REDUCTION
strengthof the 2.0-1xmCO2 absorptionmeasuredby ISM.
Absoluteradiometricaccuracyis estimatedto be 10% [Erard et al., 1991]. Detailed examination of calibrated ISM data from Instrument Characteristics the Isidis-SyrtisMajor regionindicatesthat there are small (1Detaileddescriptions of the ISM instrumentare providedby 3 %) systematic offset errors in addition to radiometric Bibring et al., [1989, 1990] and Erard et al., [1991]. The ISM inaccuraciesrelated to the Phobosspectralmodel. The offset instrumentis a scanningimaging spectrometerthat coversthe errorsare remarkablyuniform and are only visible becauseof spectral range 0.76 to 3.16 txm. For each pixel, the 128 the extraordinarysignal to noise performance(=500:1) of the spectral measurementsare acquired simultaneously. A two- instrument[Erard et al., 1991]. To correct for these remaining calibration concerns, a dimensionalimage of the surface is obtainedby rotating the spectralmodel for bright and dark regionsof Mars basedon the entrance mirror to scan in the cross track direction for the of McCord et al. [1982] is used(Figure image samples and the forward motion of the spacecraft telescopicobservations providesthe image lines. The spectraldispersionis obtained 2). The ISM data were searchedto find the closestmatch to the by using a grating,whose the first and second orders are spectralmodel in both albedoand generalslope. Spectrafrom exploited.Thesetwo ordersare separatedby a beamsplitterand the matchedregionsare averagedand thenregressedagainstthe filters and measuredby four groupsof 32 cooledPbS detectors. model spectrato determinea set of gain and offset corrections Thesehave beendesignatedfirst and secondorderodd andeven. which are then applied to the entire image. Inspectionof the The odd channelsacquire data 23' perpendicularto the scan correcteddata indicates that systematicand persistentoffset direction from the even channels. Extensive evaluation of data errorshave beenremovedand the shapeof the spectraare more quality and integrity has shown that the even detectors are consistentwith the large body of telescopicdata. superior overall to the odd detectors [Erard et al., 1991]. Therefore
the even channels
for the first and second order are
used which resultsin 64-channelspectrafor each point on the surface. The signal to noise of the even channel data is extremelyhigh and averagesgreaterthan 500:1 for data from 0.77 to 1.51 txm and 1.68 to 2.6 txm. The detectorsensitivity drops off slightly at the extremes of the wavelength ranges used here but only drops below 100:1 at wavelengthslonger than 2.6 •m.
SPECTRALASSOCIATIONSON SYRTIS MAJOR
The high spectraland spatial resolutionof the ISM data permits detailed examinationof the spatial variability in surface spectral properties, and the investigation of associationswith surface morphology and features. This approach is taken in the following analyses by first determiningthe principal spectroscopic variablesin the ISM The ISM experimentacquired11 imagingspectrometer data dataandthenmappingtheirspatialdistribution.Althoughthe setsfor the surfaceof Mars. The atmosphericopacityduring physical causes of the spectroscopicvariation can be the observationswas uniformly low, estimatedto be 0.2-0.3 nonunique, the mapped distributions, interrelationships [Combes et al., 1991; Erard et al., 1992]. These data sets, or betweenvariables,and associations with surfacemorphology
MUSTARD ETAL.:SURFACE OFSYRTIS MAJOR
3389
Fig. 1. Groundtrack of the ISM instrumentoverlainon U.S. GeologicalSurveydigitalphotomosaics MC13SE and MC13SW. The datafor the digitalphotomosaics havebeenfilteredsuchthat regionalalbedocharacteristics have been surpressed in favourof morphologic characteristics. Major geographic featuresIsidis,SyrtisMajor, Arabia,andNili and
Meroepatera areindicated. Regions fromwhichISM spectra havebeenextracted for detailed analysis arealsoindicated by numbersand are presentedin Figure5. '
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Wavelength In Microns Fig. 3. Schematicrepresentation of the methodsusedto parameterize the four primary spectralvariablesin the ISM data. Albedo, not shown in the figure, is an averageof the first 32 wavelengthchannels. The precisemethodsused and the physical causesfor the variations are discussed in the text.
can be usedto narrow the possibilities. Furthermore,detailed
examination of absorption bandshapein reflectancespectra fromareasof thesurfacewithdistinctcombinations of spectral spatialvariability in surfacecompositionand properties.The properties allowsfor increased refinementof the interpretation methodsused to quantitativelymeasurethe spectralvariables, of surfacecomposition.
and interpretationsfor the physical causesof the variables are
discussed below.
The method of computing the spectral parameters is illustratedconceptually in Figure3. Albedois a simpleaverage Examination of representative ISM reflectance spectrafor of the reflectanceat all channelsbetween0.77 and 1.51 pro. In many differentmorphologicregionsindicatethe range of general,albedois a fundamentalparameterof a surface,but it spectral properties of thesedataaredominated by fourprimary dependson many factors including lithologic assemblages, variables:albedo,spectralslope,wavelengthof the ferric or mineral composition, relative abundancesof soil and rock, ferrousbandminimumbetween0.85 and 1.0 pro, andthe area weatheringstate, and grain size. Slope is measuredas the. (strength) of thisband[Mustardet al., 1990]. The spectral differencein reflectancebetweenthe averageof wavelength variablesare a functionof the composition andphysicalstate channels 2.44, 2.49, and 2.53 gm and the average of of the surfaceandindirectlyprovidea meansfor mapping wavelength channels 1.68 and 1.73 gm divided by the
SpatialVariationsin SpectralProperties
3390
MUSTARD ETAL.:SURFACE OFSYRTIS MAJOR
intermediateband minima is causedby mixing between the ferric and ferrous dominatedsurfacesrather than representing distinct surface compositions. The constancy in band minimum in the Isidis Basin and crateredhighlands, as well as the Syrtis Major volcanic plateau suggest that the primary mineralogyof each area is relatively homogeneous. Fischer et al., 1991; Drossart et al., 1991]. Band area is The image of band area (Plate lb) shows a similar surface measuredby first dividingthe spectraby a linear continuum, distributionas band minimum. Bright areasassociatedwith the determinedbetweenthe averagesof wavelengthchannels0.77 Isidis Basin and cratered rim have smaller band areas and0.80 ixmand wavelengthchannels1.25 and 1.27 ixm. The (representedin blue) and the volcanics of the Syrtis Major areais then computedas the sumof the differencebetweenthe plateau have larger band areas (representedin red). There are continuumandthe spectrumfor all wavelengthchannelsin this large regions with intermediateband areasvalues, represented interval. Band area variationsare expectedfrom differencesin in yellow, that are primarily associatedwith the easternpart of mineral assemblages or compositions,grain size differences, Syrtis Major. The central axis of Syrtis Major, which to the topographiccrestof the plateauand includes or mixing with componentsthat are spectrallyneutralor have corresponds low spectralcontrast.The bandminimumis foundby fittinga the calderaNili and Meroe Patera, is distinctly characterizedby cubicsplineto the continuumremoveddata and searchingfor the largest band area values. This is in agreementwith the theminimumvalue in the cubicspline. Bandareaandminimum observations of Pinet and Chevrel [1990] that indicated the are the best indicatorsof surfacemineralogy. For Mars, band central regions of Syrtis Major had the strongest relative minima less than 0.9 Ixm are anticipatedfor surfacesdominated absorptions in 0.91/0.73, 0.98/0.73, and 1.02/0.73 by ferric minerals,while bandminimagreaterthan0.9 Ixm are ratios. Although variations in band area can be related to several anticipatedfor surfacesdominatedby ferrousminerals. Ferric mineralsare most commonlyassociatedwith heavily weathered mineralogic and surface phenomena,the primary difference in soils that are typical of bright terrain [Singer, 1985] while band areabetweenthe Isidis basin and the volcanic plateauis a ferrous minerals are more strongly associatedwith unaltered mineralogic effect. The larger band areas associatedwith the volcanic plateau reflect the presence of mafic minerals. crustal components. The four spectralvariableswere computedfor each ISM However, on the plateau there is an additional level of pixel. Imagesof the spectralvariablesare coregistered to the variation, compared to the image of band minimum, that is Viking orbiter digital photomosaics MC13SW and MC13SE probably unrelated to simple differences in mineralogy. The using ISM pixel locationsdeterminedfrom the orbital and specific causes of these variations are likely to be a scanningparametersof the spacecraft. The locationswere combinationof grain size and mixing. The spectral slope variable, shown in Plate lc, shows the further refined using topography[Erard et al., 1991]. The images are then superimposedon the digital photomosaics greatest complexity. The bright areas in the Isidis basin are using a red, green,blue (RGB) transform. A separateRGB characterized by flat spectral slopes while the cratered transformis performedfor the variablesslope,center,and area. highlands and basin rim show slightly negative values. The For each variable, the digital photomosaic is assigned to plateau is bisectedalong the central axis with steep, negative intensity, ISM albedo is assigned to saturation, and the spectralslopesconcentratedin easternhalf and flat to slightly spectral variable is assignedto hue. The results of this negativespectralslopesin the west. There are areasalong the integrationof Viking and ISM data are presentedin Plate 1. topographicaxis of Syrtis Major near Nili Patera that have Color indicates the value of the variable and saturation spectralslopesas flat as the bright areasin the Isidis Basin. indicatesthe albedo of the surface. Red colors indicate higher If spectral slope were due primarily to atmospheric values than blue and saturated colors have a higher surface scattering, then the slope magnitude should be positively albedo. correlated with atmospheric path length and therefore Examination of Plate la indicates there is an apparent emergenceangle. Also, since aerosol scatteringis an additive bimodal distribution of band minima. Areas with short componentof the measuredradiation, it will result in a larger wavelengthband minima near 0.86 Ixm are representedin apparent slope for low-albedo surfaces compared to highgreen,and areaswith long wavelengthbandminima near 0.98 albedo surfaces. Therefore low-albedo surfaces would have ,amarerepresented in red. This divisionis relatedto albedoand higher magnitude slopes than high-albedo surfaces, and the also surfacemorphology. High-albedoregionsof the Isidis magnitude of the slope would be negatively correlated with basin and crateredrim have short-wavelengthband minima, and albedo. Neither effects are observed in the ISM data. The low-albedo areas associatedwith the Syrtis Major volcanic scatterplot of albedo versus spectralslope shown in Figure 4 plateau have long-wavelengthband minima. Intermediate illustrates the lack of negative correlation between these colorsare primarily observedat the boundariesbetweenthese variables. And, as can be seen in Plate lc, there is no major divisions. The two main areas of short- and long- correlationof slope magnitudewith atmosphericpath length wavelength band minima exhibit little variation and appear either as a function of altitude or emergenceangle. Viewing geometryis also unlikely to be a primary controllingfactor on homogeneous. The observed distributions of band minima are interpreted spectral slope. Incidence angle increasessystematicallyas a to indicated that the Isidis basin and adjacentcrateredrim and function of longitude from the subsolarpoint, but there are no highlandsare dominatedat the surfaceby ferric materialswhile concomitantchangesin slope magnitude observed. Although the volcanicson the SyrtisMajor Plateauindicatea significant aerosol scattering does contribute to the ISM measurementsof presenceof ferrous-bearing mineralsat the surface. The zone of the Martian surface reflectances [Drossart et al., 1991; Erard et
wavelengthinterval (2.49-1.70 ixm). This is thereforea true slopewithoutscaling.The slopeparameter is a measure of the long-wavelength spectralcontinuumwhich can result from grain size effects,spatial associations of surfacecomponents (i.e., linear mixing versuscoatings),composition,or aerosol scattering[e.g.,Singerand Roush,1983;Morris et al., 1990;
MUSTARD
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SURFACE OF SYRTIS MAJOR
3391
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Plate 1. Spatialdistributionof the valuesfor the spectralvariables,illustratedin Figure3, bandminima(a), bandarea(b), andcontinuum slope(c) superimposed on Viking digitalphotomosaics MC13SW andMC13SE (Figure1). Colorbar along the left sideof the imagesindicatesthe relativescalefor the variablesfrom high values(blue-green)to low values(redmagenta). Surfacealbedoas measuredby ISM is represented by the saturationfrom high albedo(saturatedcolors,e.g., IsidisBasin)to low albedo(weaklysaturated, e.g., centralSyrtisMajor). Bandminimum(a) rangesfrom 0.85 (green)to 0.98 gm (red),bandarea(b) from 15 (blue)to 250% (magenta)andspectralslopefrom -- 0.0 (blue)to -0.001 (magenta) %/•m.
al.,
1992], here it is a second order effect relative to the
properties, ISM reflectance spectra from seven regions, indicatedin Figure 1, havebeenextractedfor detailedanalysis. Plate lc). Theseregionsbracketthe rangesand combinations of spectral propertiesobserved. They are (1) Isidis Basin (bright, shortwavelengthband minimum, small band area, flat spectral Analysisof ReflectanceSpectra slope), (2) Cratered Basin rim (short-wavelength band Fromthe analysisof the spatialvariationsin broadspectral minimum, small band area, negative spectral slope), (3)
variations related to physical properties of the surface (i.e.,
3392
MUSTARD ET AL.: SURFACE OF SYRTIS MAJOR
Arabia/Syrtis (intermediate albedo, intermediate band have been normalized to the reflectance at 0.8 Izm and the firstminimum,small band area,flat spectralslope),(4) East Syrtis
order data (1.68-2.55 Izm) have been normalized to the
A (long-wavelength bandminimum,moderate bandarea,large reflectanceat 1.68 Izm in Figure 5b. Normalizationscalesthe negativespectralslope),(5) East SyrtisB (long-wavelengthreflectancesto an equivalent albedo and allows a more direct bandminimum,largebandarea,largenegativespectralslope), comparisonof absorptionband strengthas well as shape. As a (6) West Syrtis (long-wavelength band minimum,moderate further aid in comparing spectral properties independent of band area, small negativespectralslope),and (7) Nili Patera continuum characteristics, a simple straight-line continuum (long-wavelength bandminimum,largebandarea,flat spectral has been removed from each spectral segment and these continuumremovedspectraare presentedin Figure 5c. slope).Reflectance spectra for eachareshownin Figure5. These spectraare dominatedby very general spectral properties like slope,albedo,andthestrength of theadsorbed 0.35 , , • , , i , , waterbandnear3.0 Izm. To emphasize therelativestrengthand ISM Spectrafrom Syrtis Major and Isidis a shapeof spectral features, thesecond-order data(0.77-1.51Izm)
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