NASA Technical Reports Server (NTRS) 20150001341: Geologic Mapping of Vesta

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PlanetaryandSpaceScience103(2014)2–23
ContentslistsavailableatScienceDirect
Planetary and Space Science
journal homepage: www.elsevier.com/locate/pss
Geologic mapping of Vesta
R.A. Yingsta,n, S.C. Mesta, D.C. Bermana, W.B. Garrya, D.A. Williamsb, D. Buczkowskic,
R. Jaumannd, C.M. Pieterse, M.C. De Sanctisf, A. Frigerif, L. Le Correg, F. Preuskerd,
C.A. Raymondh, V. Reddyg, C.T. Russelli, T. Roatschd, P.M. Schenkj
aPlanetaryScienceInstitute,1700E.Ft.Lowell,Suite106,Tucson,AZ85719,UnitedStates
bArizonaStateUniversity,AZ,UnitedStates
cJHU-APL,MD,UnitedStates
dDLR,InstituteofPlanetaryResearch,Berlin,Germany
eBrownUniversity,RI,UnitedStates
fNationalInstituteofAstrophysics,Italy
gMaxPlanckInstituteforSolarSystemResearch,Germany
hNASAJPL,CaliforniaInstituteofTechnology,CA,UnitedStates
iUCLA,CA,UnitedStates
jLPI,TX,UnitedStates
a r t i c l e i n f o a b s t r a c t
Articlehistory: WereportonapreliminaryglobalgeologicmapofVesta,basedondatafromtheDawnspacecraft’sHigh-
Received14February2013 AltitudeMappingOrbit(HAMO)andinformedbyLow-AltitudeMappingOrbit(LAMO)data.Thismapis
Receivedinrevisedform part of an iterative mapping effort; the geologic map has been refined with each improvement in
25November2013 resolution.Vestahasaheavily-crateredsurface,withlargecratersevidentinnumerouslocations.The
Accepted21December2013 south pole is dominated by an impact structure identified before Dawn’s arrival. Two large impact
Availableonline3January2014
structureshavebeenresolved:theyounger,largerRheasilviastructure,andtheolder,moredegraded
Keywords: Veneneia structure. The surface is also characterized by a system of deep, globe-girdling equatorial
Dawn troughsandridges,aswellasanoldersystemoftroughsandridgestothenorth.Troughsandridgesare
Vesta
also evident cutting across, and spiraling arcuately from, the Rheasilvia central mound. However, no
Geologicmapping volcanic features have been unequivocally identified. Vesta can be divided very broadly into three
terrains: heavily-cratered terrain; ridge-and-trough terrain (equatorial and northern); and terrain
associated with the Rheasilvia crater. Localized features include bright and dark material and ejecta
(somedefinedspecificallybycolor);lobatedeposits;andmass-wastingmaterials.Noobviousvolcanic
featuresareevident. StratigraphyofVesta’s geologic unitssuggestsahistoryinwhich formationof a
primarycrustwasfollowedbytheformationofimpactcraters,includingVeneneiaandtheassociated
SaturnaliaFossaeunit.FormationofRheasilviafollowed,alongwithassociatedstructuraldeformation
that shaped the Divalia Fossae ridge-and-trough unit at the equator. Subsequent impacts and mass
wastingeventssubduedimpactcraters,rimsandportionsofridge-and-troughsets,andformedslumps
and landslides, especially within crater floors and along crater rims and scarps. Subsequent to the
formation of Rheasilvia, discontinuous low-albedo deposits formed or were emplaced; these lie
stratigraphicallyabovetheequatorialridgesthatlikelywereformedbyRheasilvia.Thelastfeaturesto
be formed were craters with bright rays and other surface mantling deposits. Executed progressively
throughoutdataacquisition,theiterativemappingprocessprovidedtheteamwithgeologicproto-units
in a timely manner. However, interpretation of the resulting map was hampered by the necessity to
provide the teamwith a standard nomenclature and symbology early in the process. With regard to
mappingandinterpretingunits,themappingprocesswashinderedbythelackofcalibratedmineralogic
information.Topographyandshadowplayedanimportantroleindiscriminatingfeaturesandterrains,
especiallyintheearlystagesofdataacquisition.
&2014ElsevierLtd.Allrightsreserved.
1. Introduction
Geologic mapping is a comprehensive investigative process
nCorrespondingauthor.Tel.:þ19203603627;fax:þ19204652376. thatorganizesdisparatedatasetsintogeologicunitswiththegoal
E-mailaddress:[email protected](R.A.Yingst). of revealing the underlying geologic processes and placing those
0032-0633/$-seefrontmatter&2014ElsevierLtd.Allrightsreserved.
http://dx.doi.org/10.1016/j.pss.2013.12.014
R.A.Yingstetal./PlanetaryandSpaceScience103(2014)2–23 3
processes into a global, contextual framework. The arrival of the (howardite,eucriteanddiogenite)meteorites(afamilyofmeteor-
DawnspacecraftattheasteroidVestaprovidesafirstopportunity ites believedtohaveoriginatedfromVesta (BinzelandXu,1993;
for this approach to be utilized for Vesta at the sub-km scale, at ConsolmagnoandDrake,1977;McCordetal.,1970)anddiscussed
whichfeaturessuchasimpactcraters,locallandslidesandtectonic in more detail in Section 3) to be placed in geologic context,
structurescanberesolved.TheinnermainbeltasteroidVestaisa shouldthesourcesbelocated.
particularly compelling target for this traditional investigative Thegoalsincreatinganygeologicmapdeterminethelevelof
processbecauseoflong-standingevidenceforitsbasaltic surface detail at which the map is created, and thus the requiredspatial
andlongitudinalmineralogicheterogeneitygatheredfirstthrough resolutionofdataselectedforthebasemap.Wherethegoalisto
Earth-basedpolarimetricandspectroscopicmeasurements(Degewij summarize the current state of knowledge for a region for
et al., 1979; Gaffey, 1997, 1983; McCord et al., 1970; Reddy et al., archiving, the presented map will differ from one where the
2010).Suchasurfaceindicatedadifferentiatedcrustand,potentially, purpose is toprovide a preliminaryoverviewof geologic context
volcanicactivityinVesta’spast. in a setting where data collection is in process, or where the
Prior to the arrival of the Dawn spacecraft, the highest- amountortypeofdataavailablevariesacrossthemappedregion.
resolution images of the surface of Vesta (38km/pixel) were Theselattermapsareofteniterative–thatis,multipleversionsare
provided by the Hubble Space Telescope (HST; Li et al., 2010). createdbecauseeachiterationisrefinedasdatabecomeavailable.
During favorable approach conditions in 1994 and 1996, the HST Anexampleofsuchasituationisthegeologicmappingthatmay
providedreflectancedataat0.439,0.673,0.953and1.042mm,and occur during field work, where a sketch map of local units or
from these data, albedo, elevation and mineralogical data were layers is created first to inform the choice of future sampling
derived,fromwhichmapsofmineralogiccompositionandlithol- locations, and is updated as those samples are collected and
ogywereproduced(Binzeletal.,1997;Gaffey,1997;Lietal.,2008, analyzed. The more comprehensive geologic map is generated
2006) These data revealed a surface dominated by regionally later, when all the available data has been acquired, refined and
distinct units interpreted to be impact-excavated pyroxene-rich analyzedindetail.
plutonic material, results that agreed generally with mineralogic An orbital mission to another planetary body is analogous to
mapscreatedfromEarth-basedspectroscopy(Degewijetal.,1979; thisscenariooffieldworkfollowedbydataanalysis,wheretimein
Gaffey,1997,1983;Reddyetal.,2010).Thoughnecessarilygener- the field mirrors the period of spacecraft data acquisition.
ated from images with a resolution no better than 38.5–52km/ Adetailedgeologicmapisoftengeneratedafterthemissionends,
pixel(Binzeletal.,1997;Lietal.,2010,2008;Zellneretal.,1997), onceallthedataareacquiredandhavebeenfullycalibratedand
these maps represented first steps in understanding Vesta’s refined.However,asinfieldwork,analysisofdatabeginsassoon
geologichistory. as it is acquired. Iterative mapping is a processthat provides the
NASA’sDawnspacecraftenteredVestanorbitonJuly16,2011, geologiccontextfor,andrevealstheinterrelationshipsof,geologic
and spent one year in orbit to characterize its geomorphology, characteristics revealed byeach emerging dataset. Further, it can
elementalandmineralogicalcomposition,topography,shape,and do so within a timeframe that allows the map to inform data
internalstructurebeforedepartingtoasteroidCeresonSeptember analysisofotherteammembersonthemissiontimeline.
5, 2012. Three orbital phases of the mission returned images at The global geologic maps presented here demonstrate the
successivelyhigherresolutions;thehighestofthesewas20–25m/ progression of lessons learned from generating each iteration
pixel. Preliminary geologic results from the initial orbital phase (Yingstetal.,2012,2011).Wherepossible,wehavereferredback
(“Surveyorbit”)arereportedbyRusselletal.(2012)andJaumann to units and surface features identified by mapping efforts that
etal.(2012). predate the Dawn mission (Binzel et al.,1997; Gaffey,1997); we
During the pre-encounter phase of the mission, the Dawn note that because the spatial resolution available for these map-
scienceteamfollowedtherecommendationsofBatson(1990)for ping efforts was (cid:2)500 times coarser than that available here,
planetary geologic mapping and divided the asteroid into 15 there are previously named and mapped regions that are not
quadrangles for geologic mapping. Preliminary global geologic includedbecausetheydonotexistasgeologicallydefinedfeatures.
maps were also produced in an iterative fashion as new data This includes Olbers Regio, identified in HST images as a dark
becameavailable(Yingstetal.,2012,2011).Theseiterationsofthe ovoidregionapproximately200kmacross.Forthefinaliteration
globalgeologicmapwereutilizedbythescienceteamduringthe ofthemapweincludetypeexamplesofunitsandlandforms,and
active phases of the mission to inform evolving hypotheses, descriptionsandinterpretationsofprimaryunits;wealsoattempt
correlate crater size-frequency statistics, mineralogic data and to deconvolve and interpret the basic stratigraphy in a relative
other products with preliminary geologic units, and place new senseutilizingstratigraphicrelationships.Weintendforthemap
datawithinabaselinegeologiccontext.Thisworkrepresentsthe to provide a contextual framework for more advanced composi-
compilationandanalysisoftheseiterativeefforts. tional and/or geomorphological mapping at large scales (smaller
regions).However,becausetheprocessofdataanalysisisstillinits
earlystagesasofthiswriting,weexpecttheconceptsfortheunits
2. Approach andstructurespresentedtoevolveasanalysisandunderstanding
mature.Ourgoalsforthisworkarethustwofold:firstly,toprovide
A geologic map is a visual representation of the distribution the community with a preliminary assessment of the geology of
and sequence of rock types and other geologic information. Vesta, using traditional geologic mapping methods as a primary
It allows observations to be organized and represented in an tool to perform this assessment; and secondly, to report on and
intuitive format, unifies observations of heterogeneous surfaces analyzethemappingprocessasitwasconductedduringanactive
made at different localities into a comprehensive whole, and mission,whereiterativeproductswerefeddirectlytotheteamto
provides a framework for science questions to be answered. informsubsequentdataacquisitionandanalysis.
A geologic map defines boundaries for the extent and overlap of
important characteristics such as mineralogy, topography, mor-
phology and elemental abundance. This information can then be 3. Geologicsetting
used to analyze relationships between these characteristics; this,
in turn, can inform models of thermal and structural evolution. Vesta is an ellipsoidal asteroid with dimensions estimated at
In the case of Vesta, a geologic map also would allow the HED 286.3(cid:3)278.6(cid:3)223.270.1km (Russell et al., 2012). Efforts by
4 R.A.Yingstetal./PlanetaryandSpaceScience103(2014)2–23
Fig.1. ColorshadedreliefmapofthesurfaceofVesta.TopographyisderivedfromDawnFramingCameradata.Thecoordinatesystemshowsisthe“Claudia”systemusedbythe
Dawnscienceteam.Themapshowsthelocationsofphysiographicprovinces,majorstructuralcentersandimpactstructuresgreaterthan30kmindiameter.Whiteboxesindicate
thelocationsoftypeFigs.6–22throughoutthepaper.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)
Imagecredit:NASA/JPL/DLR.
Binzeletal.(1997),Gaffey(1997)andLietal.(2010,2008)utilized dominatedbyalargeimpactstructureidentifiedbeforeDawn0sarrival
Earth-based and HST spectral data to identify and interpret low- (Thomasetal.,1997a,b);thenameRheasilviahasbeenapprovedby
resolutionalbedopatternsonthesurface.Spectralsignatureswere theInternationalAstronomicalUnion(IAU)forthisstructure(theIAU
alsoidentifiedbasedonground-basedspectroscopyandHSTWide- is the organization that certifies the nomenclature of planetary
FieldPlanetaryCamera(WFPC2)imagesthatresolvedVestaatupto features;onlyIAU-approved names areused throughout thismanu-
(cid:2)91/pixel. Characteristics used to discriminate between potential script).Thesurfacealsohasthreelargesystemsoftroughsandridges:
unitsincludedderivedalbedo,spectralshape,andvariationsinthe one around the equator, one confined to the northern hemisphere,
depth, position and width of the 1mm absorption band (Fe2þ, a and one cutting across, and spiraling arcuately from, the Rheasilvia
common component of basaltic minerals, has a 1mm absorption centralmound(Jaumannetal.,2012).Acolorreliefmapofthesurface
band). The lithologic maps derived from analysis of these data isshowninFig.1.
showed a surface composed of several discrete, spectrally-similar GlobalgeologicmappingandimageanalysisofVestausingdata
regions. The hemisphere that Binzel et al. (1997) noted as their ofincreasingresolutionacquiredatsuccessivephasesoftheDawn
“westernhemisphere”wasinterpretedasrelativelyuniform,similar mission has enabled the identification of several impact basins
to iron-rich pyroxenes, and comparable to surface basalts such as hundredsofkilometersindiameter(e.g.,Rheasilvia,Veneneia),an
eucrite meteorites. By contrast, their “eastern hemisphere” was ancientheavilycraterednorthernhemisphere,andregionalsetsof
morediverse,withmagnesium-richpyroxenesandseveralregions graben and ridge-and-groove structures (e.g,. Divalia Fossae and
of olivine-rich, diogenite, and low-Ca eucrite regions located near Saturnalia Fossae). We have also mapped and characterized a
theprimemeridian(Binzeletal.,1997;Gaffey,1997).Theaverage number of geologic units associated with impact basins and
surface ofVestawasnoted asanalogous to a mixofhowardite or craters,regionalcrateredplainsandhighlandsunits,andsurficial
polymict eucrite; these are regolith-derived members of the HED depositssuggestinglocalizedmasswastingofloosematerial.
meteorites(Gaffey,1997).TheseresultsindicatedthatVestahasan
old,differentiatedsurface,withspectrally-distinctregionsthatcan
be geochemically tied to the HED meteorites. Crystallization ages 4. Dataandmappingprocedure
measured by radiometric dating for HEDs in Earth-based labora-
tories document that rocks comprising Vesta were formed within EachofDawn’sseveralorbitalphasesatVestaprovidedincrea-
the first 100 million years of solar system history (4.43–4.55Gyr; singlyhigherspatialresolutiondatathatwereintegratedintothe
Lugmair and Shukolyukov, 1998; Nyquist et al., 1997; Tera et al., mappingeffort,assummarizedinTable1.Usingdatafromeachof
1997; see review by McSween et al., 2011). The mapping results thesephasesas“waypoints,”wecompletedthreemainiterations
revealsurfacefeaturesformedbyprocessesthatmustpostdatethe of the global map. The first was created during the approach
veryoldageoftheHEDs. phase, when data were taken to determine the Vestan pole
Data available prior to the Dawn mission indicated that impact (Rotational Characterization, or RC) and to support navigation
cratering was the dominant process on the surface of Vesta (OpticalNavigation,orOpNav).TheRC/OpNavmapwasbasedon
(e.g., Gaffey, 1997), and the Dawn data confirm that Vesta has a clear filter data from the Framing Camera (FC), which covered
heavily-crateredsurface(e.g.,Marchietal.,2012a).Thesouthpoleis the surface at 3–9km/pixel resolution. The second iteration was
R.A.Yingstetal./PlanetaryandSpaceScience103(2014)2–23 5
completedsubsequenttoSurveyorbitandwasbasedonFCclear 415mm in blue; here we use 440/750mm, 750/920mm and 750/
filterdataat(cid:2)200mresolutionandaDigitalTerrainModel(DTM) 440mm).
derived from Survey orbit image data (Jaumann et al., 2012; Forthefirstiteration(RC/OpNav),theportionofthesurfaceimaged
Preusker et al., 2012). The third was based on data from the was mapped in its entirety by four separate workers who then
High-AltitudeMappingOrbit(HAMO)withaspatialresolutionof compared and consolidated results. This method was adopted for
(cid:2)61m/pixel. For a summary of the navigational aspects of the tworeasons:Firstly,itwasimportantinthis earlystagetoallowall
Dawn at Vesta mission, including orbit tracks, see Russell et al. mappers to become familiar with the surface features as rapidly as
(2005)andPolanskeyetal.(2011). possible,asthe firstmap hadtobeproduced withinafew months.
Thepreparationofalliterationsofthegeologicmapfollowedthe Secondly, wewanted to calibratethe different approaches thateach
methods developed and described by Shoemaker and Hackman mapperutilized,sothatinlateriterationsthis would bearelatively
(1962), Wilhelms (1990, 1972), Tanaka et al. (1994, 2010) and knownfactor.
Greeley and Batson (1990). Units were defined on the basis of Fortheseconditeration, eachworker mapped oneoffourbroad
characteristicssuchasmorphologicfeatures,surfacetextures,color regions: 30–901S, or 0–1201,120–2401 or 240–3601 longitude, with
andalbedo;wherecolorisdefinedasthecolorratioschemeusedin eachofthelatterthreeblocksrangingfrom301Slatitudetothelimitof
Clementine multispectral images (Clementine data are often dis- coverageinthenorth.Someoverlapoccurredwherefeaturesorunits
playedasratiosof415/750mminred,750/950mmingreen,and750/ straddledtheselongitudinalblocks;thisoverlapallowedthemappers
tocompareresultsandaddressanypotentialareasofdisagreement.
Forthethirditeration,eachworkermappedadifferentoneofthese
longitudinally-definedblocks,tolessenbiasandtoalloweachworker
Table1 tobecomefamiliarwiththegeologyoftheentirebodyatsignificantly
Datasets utilized in the mapping effort. The resolutions acquired for each orbit
higherresolutionthanrevealedbyRC/OpNav.
arenoted.
RC/OpNav Survey HAMO 4.1. Iterativemapping
FramingCamera,mosaic 400m/pixel 250m/pixel 60m/pixel
4.1.1. RC/OpNaviteration
DTM 750m/pixel 445m/pixel 92m/pixel
Geologicmappingscale 1:20–25M 1:1M 1:500K Theinitialiterationsofthemap,basedonFCimagesfromthe
OpNavandSurveyphasesofthemission,weregeneratedinAdobe
Fig.2. MapofVesta’ssouthernhemispherebasedonFramingCameraRotationalCharacterization(RC)data.Thelatitude/longitudecoordinatesystemshownisanarbitrary
setoflongitudesoverlaid(theFCteamhadnotsettledonastandardlatitude/longitudesystematthistime).
6 R.A.Yingstetal./PlanetaryandSpaceScience103(2014)2–23
Illustratorbythemappingteam.ThemapproducedfromRCand easternrimsofthethreecraterscombined)andapotentialcomplex
OpNav data is shown in Fig. 2. This map covers latitudes from craterpeak(thehigh-standingrimofMarciaandCalpurnia,which
0 to 901; however, RC/OpNav data covered some areas further wereamongthehighestalbedofeaturesseenatthisresolution;see
norththantheequator.Unitsareinformedbythesemorenorthern Fig. 3). This combination of features likely represents what was
images. Hypotheses to be tested in this iteration were that the originally identified as the dark albedo feature “Olbers Regio” on
surfaceofVestawouldshowevidenceoftwoprocesses:(1)crater- previous maps, as noted by Reddy et al. (2012a,b, 2013). Notably,
ing, especially in the form of a large crater at or near the south though the ridge-and-trough system around the equator is very
pole; and (2) volcanism, in the form of some surface features, prominent,itwasnotidentifiedormappedatthisresolution(the
possiblyventsandflows(aspredictedbyWilsonandKeil,1996), northern latitudes were not imaged at this altitude). The walls of
orpyroclasticmaterial. thedeepesttroughs,whichcouldbeseeninsomeoftheearliestFC
Inthisiterationofthemap,wedividedthesurfaceintobrighter- images,weremappedascurvedcraterrims.Intermsofcolorratio
and darker-toned units, as well as circular features that were data(definedasFCmonochromedataandcolorratiodata,where
inferred at the time (and later confirmed) to be impact craters. red–blue tones capture the visible continuum and green tones
Large variations intopography were alsovisible, including several capture the relative strength of the ferrous absorption band at
prominent scarps. Other features identified included the south 1.0mm), the most prominent feature noted was a deposit that
polar impact crater (Rheasilvia) and two ridges identified in later appearsorange-toned.Thisfeaturewasfirstidentifiedintheimages
iterationsasthescarpdiscontinuouslyboundingthecrater(ridge1 showninFig.3,andinferredtobeassociatedwithanearbycircular
and ridge2 in Fig. 2); lower-albedo regions associated with Rhea- feature(confirmedasanimpactcraterinSurveydata,andnamed
silviaejecta(d1);ahighplateau(BT2,VestaliaTerra);andgrooves Octavia).Itslocationisbroadlysimilartothatofapotentialolivine
withintheRheasilviaimpactstructure(notedasGHTinFig.2,with signaturenotedbyBinzeletal.(1997).Inthisiterationwemapped
individual troughs outlined where they could be identified). Pro- the deposit as a surface mantling feature, pending more detailed
minent features on Vesta that could not be discerned at this morphologic data. We note, however, that another prominent
resolution included Veneneia and all other, smaller craters. The “orange”toneddepositthatwasrevealedintheSurveydataaround
large topographic variation from the top of Vestalia Terra to the the crater Oppia does not correlate with any spectral signature
bottomofthecratersMarcia,CalpurniaandMinuciawasnotedbut notedbyBinzeletal.(1997).Thesetypesofdepositsarediscussed
we mapped it incorrectly in this iteration as a crater rim (the inmoredepthinSection5.5.1.
Fig.3. FramingCameraRC1imagef2_362695687withanarbitrarysetoflongitudesoverlaid.MisidentifiedintheRC-basedmapasacraterrim(upperarrow)andpeak
(lowerarrow)aretheeasternandwesternrims,respectively,ofthecratersMarcia,CalpurniaandMinucia.
R.A.Yingstetal./PlanetaryandSpaceScience103(2014)2–23 7
The geologic map in Fig. 2 was used to assist the team in Ourgoalatthispointwastofacilitatethemappingprocess,which
highlighting and preparing for the types of features and terrains was most easily done by using the products created by team
thatwouldbeencounteredasthemissionprogressed.Outstanding membersassignedtothatwork.Wethususedthisteam-derived
issuestobeaddressedincluded:understandingtheuniquenature coordinatesystemasshowninFigs.4–22.Notethatatthetimeof
of Rheasilvia (whether there was a large extent of impact melt; thiswriting,thePDSisprovidingDawndatainalongitudesystem
why there was a central mound rather than a peak or an inner/ that can be obtained from the Claudia longitude by subtracting
outer ring complex; the processes that formed the ridge–trough 1501.
complex);confirmingcircularfeaturesasimpactcratersandthus FCcontinuedtobethebasemap,butimprovedcolorcoverage
beginning todeconvolvethe cratering historyand relative ageof and preliminary VIR data were also available. Though the inter-
the surface; clarifying the scale and extent of features so they pretationofVIRbandcombinationsisnotclearasofthiswriting,
could be compared to similar features on other bodies; and we include the data because it informed some boundaries and
characterizingthenatureofthehigher-andlower-albedoregions, interpretations. The geologic map was compiled in ArcGIS soft-
especiallyinrelationtotopography.Thedataalsoconfirmedsome ware(v.10.1)usingdigitalmappingtechniquesasoutlinedinthe
important previous observations, including the presence of the NASAPlanetaryGeologicMappingHandbook(Tanakaetal.,2010).
largesouth-polarimpactstructure(Thomasetal.,1997a,b). We chose to utilize ArcGIS instead of continuing to use Adobe
Illustrator because digital mapping facilitates unit characteriza-
tions, feature correlation, and crater counts. This iteration of the
4.1.2. Surveyiteration mapwasproducedat1:1,000,000scale.
ThegeologicmapresultingfromSurveydataisshowninFig.4. At this resolution and coverage, we were able to identify and
Forthisseconditeration,allbasemapproductswerecreatedusing describe the gross characteristics of many of the main geologic
a coordinate scheme developed by the science team (known unitsonVesta(crateredplainsandcrateredhighlands;equatorial
informallyasthe “Claudia” systemaftera craterat 01longitude). and northern trough terrains; equatorial cratered terrain; bright
Fig.4. Geologicmap(a)andlegend(b)basedonSurveyorbitaldata,producedat1:1,000,000.Yellowcirclesindicateareasidentifiedashavingadiogenite(twosmallcircles)
orolivine(largecircle)signature(Binzeletal.,1997).Notethelocationsof“orange”-tonedsurfacematerial(stippledpattern)mappedasdarkmantlingmaterial.Thisandall
followingfiguresshowcoordinatesinthe“Claudia”systemutilizedbytheDawnscienceteam.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderis
referredtothewebversionofthisarticle).
8 R.A.Yingstetal./PlanetaryandSpaceScience103(2014)2–23
and dark ejecta and crater materials; and Rheasilvia mound and TheSaturnaliaFossaeandDivaliaFossaewereidentifiedatthis
ridge-and-grooveterrain).Firstestimatesofcratersize-frequency stage as continuous structural features rather than as scattered
distribution were calculated based on those units (Marchi et al., grooves or as disconnected peaks and valleys. These features0
2012c;Schmedemannetal.,2012).Weconfirmedourobservations dimensions were mapped (northern cratered trough terrain and
from the previous iteration that no large melt sheet associated equatorial ridge and trough terrain respectively) and measured,
with Rheasilvia exists on the surface. Other crater features were and potential correlations with other terrains were assessed.
identifiedandmappedwiththisiteration,including:craterswith Specifically, the two large ridge-and-trough complexes outside
higher-albedo ejecta rays; craters with lower-albedo ejecta rays; thesouthpolarregionwerepreliminarilyassociatedwithtectonic
and so-called “bimodal” craters with one rim portion sharp and disruptions that occurred during the formation of Rheasilvia and
theothermoredegraded(Krohnetal.,2012). Veneneiaimpactstructures(equatorialandnorthernrespectively)
MajorcratersotherthanRheasilviawereidentified.Thelargest (Buczkowskietal.,2012;Jaumannetal.,2012).
of these was the Veneneia impact structure, lying north of the Remainingissuesatthisstageoftheiterativemappingprocess
younger Rheasilvia crater and identified by sections of rim scarp included clarifying the nature of the higher- and lower-albedo
associated with a semi-circular topographic low. The interpreta- regions,andtheirrelationtostratigraphyandtopography.
tion of this set of features as an impact structure was vigorously
debatedamongthemappersandthegreaterDawnscienceteam.
Argumentsagainstsuchaninterpretationincluded:(a)theshape 4.1.3. HAMOiteration
of the proposed impact structure was not circular and had an Forthisiteration,showninFig.5,weusedamonochrome(clear
irregularverticalprofile;(b)thecenterofthetopographiclowdid filter) FC mosaic as our basemap. Images in this mosaic have an
not correspond precisely with the center of the proposed struc- average spatial scale of (cid:2)70m/pixel for HAMO. This base was
ture; and (c) the rim scarp was not continuous. The presence of importedintoArcGISandsupplementedbytheSurveyDTM.FCcolor
Veneneiawas not confirmed until the acquisition of HAMO data, ratioimagesfromSurveyorbitwithaspatialscaleof (cid:2)250m/pixel
which revealed that the rim scarp was more complete than had andVisibleandInfraRed(VIR;DeSanctisetal.,2011)hyperspectral
previouslybeenbelieved. imagesfromtheSurveyandHAMOorbitswithspatialscalesof700
Fig.5. GeologicmapbasedonHAMOorbitaldata,asdescribedintext.Themapwasproducedat1:500,000,usingFramingCameradataasthebasemap.Vestaisdivided
latitudinallyintofourareallyextensiveunits:theRheasilviaFormation,theDivaliaFossaeandSaturnaliaFossaeunits,andcrateredhighlands.Theblankareatothenorth
representsregionsforwhichthereisnoHAMOdata.(a)Simplecylindricalprojection;(b)NorthandSouthpolarprojections;and(c)legend.
R.A.Yingstetal./PlanetaryandSpaceScience103(2014)2–23 9
and200m/pixel,respectively,providedinformationonsurfacecom-
position and were used to refine unit boundaries. The final map in
Fig.5wasproducedatascaleof1:500,000.
Inmappingthisiteration,weexpandedthelistofunitsintoa
more traditional Description of Material Units (DOMU). Names
were assigned to each unit, some associated with IAU-compliant
namesofthemostprominentorcharacteristicfeatureassociated
withthatunit(Roatschetal.,2012).
With this iteration, the data was sufficient to resolve differ-
encesinsurfacetexturedowntothe(cid:2)100mscale,andadifferent
illumination angle allowed albedo differences to be more clearly
discerned through comparison to global Survey data. For this
iteration, we mapped craters down to 2km diameter at the
requestoftheDawnscienceteam(forclarity,onlycraters46km
diameterareshowninFig.5).Usingthisinformationwewereable
to make several improvements to the geologic map, including
(1) differentiating some larger units into smaller ones based on
relative crater density and texture; (2) mapping the extent of
Rheasilvia-modified terrain, which extends to nearly the equator
in some places; (3) characterizing the extent of fine-textured
ejectamaterialsatfreshercraters;and(4)identifyingandanalyz-
ing the characteristics of unique small-scale (tens of m) features
such as units with lobate boundaries, and pitted terrain within
craterfloors.Withregardtodifferentiationoflargerunits,weused
improved surface texture information to divide portions of the
Rheasilviaridge-and-grooveterrain(Fig.4)intoRheasilviasmooth
material(less-heavilycratered,smootherthansurroundingRhea-
silvia materials) and the more heavily cratered highlands and
cratered plains north of it (Rs, cp and ch in Fig. 5 respectively).
Units added included mass wasting material and Rheasilvia
smooth material. We also revisited the boundaries of cratered
terrain (cratered highlands and cratered plains in earlier maps)
based on roughness of texture and relative crater density. Speci-
fically,thearearepresentedbycrateredhighlandsincreasedatthe
expense of cratered plains. Additionally, the boundaries of the
cratered highlands unit were expanded at the expense of ridge-
and-troughterrain,toincludeareaswithsimilarmeantopography
and FC color. The equatorial ridge-and-groove terrain and north-
erncrateredtroughterrainwererenamedtheDivaliaFossaeridge-
and-trough and Saturnalia Fossae cratered trough units, as the Fig.6. Masswastingmaterial(mw).(a)Portionofafan-shapeddepositinMarcia
crater,withdarklobatematerial(dl)totheeastinthecraterfloor.Thisdepositis
improvedresolutionrevealedthatgrooves(amoregeneralterm)
characterizedbysubtleridgesofmaterialradiatingdownslope.Notetheindividual
wereindeedtroughs. bouldersvisibleinthedeposit.ThecenterofthisLAMOmosaicisatlatitude9.71N,
longitude186.01EintheClaudiacoordinatesystem.Northisup.(b)Slumpdeposit
southofMatronaliaRupesthatliesbetweenRheasilviasmoothterrain(Rs)andthe
5. Materialunits Rheasilviaridge-and-grooveterrain (Rrg). Here,mw is comprised of subparallel
blockssliding downslope.Thecenterof thisLAMOmosaicisatlatitude54.51S,
longitude91.91E.LAMOmosaic.Northisup.
BasedondatafromtheDawninstruments,thesurfaceofVesta
Imagecredit:NASA/JPL/DLR.
is comprised of four major terrains: individual craters and asso-
ciated impact materials, widespread undifferentiated cratered
units, the Saturnalia and Divalia Fossae units, and materials benches separated by crescent-shaped cliffs or scarps beginning
associated with the Rheasilvia impact structure. Other more at the top of a slope (Fig. 6a). Lobate or fan-shaped, smooth-
localized units include lobate, smooth and tholus materials, and textureddepositsalsooccur,oftenassociatedwithimpactcraters
mass-wastingmaterials.Wedescribeeachoftheseunitsinterms (Fig. 6b). More irregularly bounded deposits tend to have a
ofmorphology,surfacetexture,relativecraterdensity,topography hummocky texture and often display subtle or more diffuse
andcolorratiodata.Wealsopresenttypelocalitiesforeachunit; boundaries.
images are from HAMO unless otherwise noted. The symbology
andnomenclatureusedareshowninFig.5.
5.1.1.2. Interpretation. Weinterpretthisunitasdebrisfalls,slumps
5.1. Surficialdeposits orslidesformedthroughslopefailurethatmaybeassociatedwith
a number of possible processes that involve mass movement of
5.1.1. Masswastingmaterial(mw) material downslope. Possible drivers include “seismic” shaking
5.1.1.1. Description. This unit takes the form of deposits along the associated with impact crater formation or slope failure due to
bases of steep slopes or crater walls due to mass movement of overburden. Theteam requestedthat these materials be mapped
material, indicating the mobility of the regolith (Jaumann et al., together by process rather than associated feature, in order to
2012; Pieters et al., 2012). There are several morphologies facilitate analysis of their distribution and thus how regolith
represented within this unit. Slumps occur as sequences of mobilitymayvarybylocation.
10 R.A.Yingstetal./PlanetaryandSpaceScience103(2014)2–23
Fig.7. Brightlobatematerial(bl).ThistypearealiessouthofAriciaTholus.The Fig.9. Smoothmaterial(s).ThistypeareaisinMarciacrater,lyingonasouthern
brightlobatematerialisthesmoothtongueofmaterialdrapingthewesternrimof benchbetweentherimandamasswasting(mw)depositdownslopetothenorth.
thecrater.ThecenterofthisHAMOmosaicisatlatitude1.61S,longitude163.61E. Notethedifferenceintexturebetweensmoothmaterialandthemorehummocky
Northisup. mwmaterial.ThecenterofthisLAMOmosaicisatlatitude3.61N,longitude1871E.
Imagecredit:NASA/JPL/DLR. Northisup.
Imagecredit:NASA/JPL/DLR.
asmassmovement.Yellowareasincolorratioimagestendtobe
smootherandmaybecomposedofimpactmelts.
5.1.3. Darklobate(dl)
5.1.3.1. Description. Dark lobate deposits (Fig. 8) are similar in
morphology to the bright lobate unit but have a distinctively
lower albedo. These materials extend from crater rims or scarps
onto crater floors or local topographic lows. Deposits have a flat
topography, with lobate margins and relatively smooth surfaces.
Deposits differ from mw deposits in their flat surfaces, very
smooth textures and relatively sharp boundaries. Dark lobate
deposits have low albedo in FC monochrome images, with a
yellow tone in color ratio images, though again, this color is not
unique to this unit. These deposits generally have lower crater
abundancescomparedtotheirsurroundings.
5.1.3.2. Interpretation. Similarly to the bright lobate unit, we
interpret the dark lobate unit to be impact-derived material;
Fig.8. Darklobatematerial(dl).ThistypearealiesinOctaviacrater,withinapatch
ofdarkcrater(dc)material.Here,dlappearsfine-textured,withrollingtopography. surface texture is consistent with impact melt (McCord et al.,
ThecenterofthisLAMOmosaicisatlatitude3.41S,longitude147.61E.Northisup. 2012;Reddyetal.,2012b).
Imagecredit:NASA/JPL/DLR.
5.1.4. Smoothunit(s)
5.1.4.1. Description. The smooth material unit is highly localized,
5.1.2. Brightlobate(bl)
consisting of several exposures found on the floor and rim of
5.1.2.1. Description. Thisunitischaracterizedbylobesthatextend
Marciacrater(Fig.9).Thisunitdisplaysoverallsmooth,darkand
from crater rims or local topographic highs (e.g., scarps) onto
relatively featureless surfaces at the tens of meters scale, except
crater floors or local topographic lows (e.g., Fig. 7). Deposits that
forvariableamountsofsmallimpactcratersandsomeclustersof
fall within this unit have a convex-up topography, with lobate
pits. Smooth unit deposits have low albedo in FC monochrome
margins andsmoothtohummockysurfaces. Depositsdiffer from
images, and are typically blue–green to green–brown in FC color
mw deposits in their convex, positive topography and relatively
ratioimages.
sharpboundaries.Brightlobatedepositshaveintermediatealbedo
inFCmonochromeimages,withayellowtoneincolorratioimages,
thoughthecolorisnotuniquetothisunit.Thesedepositsgenerally 5.1.4.2. Interpretation. We interpret this smooth unit to be very
havelowercraterabundancescomparedtotheirsurroundings. youngimpactmelt.Pitclustersareassociatedwithlowhydrogen
andOHlevels(DeSanctisetal.,2012;Prettymanetal.,2012)and
have been interpreted as pits formed when volatiles from a
5.1.2.2. Interpretation. We interpret the bright lobate unit to be volatile-rich impactor boiled off subsequent to crater formation
impact-derived material, younger than the surrounding surface. (Denevi et al., 2012). Mapping results are consistent with this
These are likely flow deposits and may be the result of either hypothesis. Alternately, the unit could be fine-grained materials
impactejectaflowlobesorimpactdebristransporteddownslope depositedbymassmovement.
R.A.Yingstetal./PlanetaryandSpaceScience103(2014)2–23 11
Fig.10. Tholus material (t). This type areais Aricia Tholus, The tholus is cone- Fig.11. Crateredhighlandsmaterial(ch).ThetypeareaisnortheastofNumisia
shaped,withacrateredsurface;thereisnoindicationofflowsorothervolcanic crater. Notetheabundance of overlappingcratersof varioussizesand statesof
products associated with this structure. The center of this LAMO mosaic is at degradation;manyhavesubcircular,ratherthancircularperimeters.Thecenterof
latitude101N,longitude1601E.Northisup. thisLAMOmosaicisatlatitude5.01N,longitude260.51E.Northisup.
Imagecredit:NASA/JPL/DLR. Imagecredit:NASA/JPL/DLR.
5.1.5. Tholus(t)
5.1.5.1. Description. Therearetwotholimappedatthisresolution:
Aricia and Lucaria Tholi (Fig. 10). These are defined as isolated
topographic highswithheavily-crateredsurfaces and darklobate
patchesassociatedwiththem.Theyhaveintermediatealbedosin
FCmonochromeimagesandappeardarkbluetopurpleinFCcolor
ratioimages.
5.1.5.2. Interpretation. We interpret the tholus unit to be impact-
sculptedcrust,possiblycontainingvolcanicdikesorintrusions,or
volcaniccones.Dark-rayedcratermaterialanddarklobatepatches
onAriciaTholusmayindicatebasalticmaterialexposedbyimpact
cratering. Alternatively, the dark-rayed crater could have an
exogenic source (i.e., carbonaceous meteorite), and the small
lobate patches could be impact ejecta flows or impact melts
(Reddyetal.,2012b).
5.2. Crateredterrains
Fig.12. Crateredplainsmaterial(cp).ThistypeareaissouthwestofDrusillacrater.
5.2.1. Crateredhighlands(ch) Whilemanyimpactstructuresarepresent,thecratersaresmallerandfewer.The
5.2.1.1. Description. This extensive unit has a heavily-cratered centerofthisLAMOmosaicisatlatitude23.51S,longitude239.51E.Northisup.
Imagecredit:NASA/JPL/DLR.
surface and a higher albedo and overall topography than the
surrounding plains (Fig. 11). The boundary between this and
otherunitsisoccasionallysubtlebutdiscernableasacombination
of steepening topographic slope and an increase in roughness of
highlands, the former interpretation is currently preferred. If this is
surface texture. This unit is concentrated along the equator and
thecase,VestaliaTerramaybetheoldestterrainonVesta.
includestheVestaliaTerrahigh.Anintermediatealbedoisseenin
monochrome FC images, with localized bright and dark patches.
InFCcolorratioimagescolorrangesfrompurple-redtobluetones.
5.2.2. Crateredplains(cp)
Early analysis of VIR data led to an interpretation of the spectral
5.2.2.1. Description. The cratered plains unit (Fig. 12) occurs as
signatureofthisunitashavinghowarditicmineralogy(DeSanctis
narrow, somewhat isolated regions of smoother, topographically
etal.,2012).
lower,relativelyslopingterrainwithlowercraterdensitythanthe
Divalia Fossae unit. All occurrences of this unit are bounded by
5.2.1.2. Interpretation. Weinterpretthisunittobeancientterrain. crateredhighlands,theDivaliaFossaeunitorboth.
The Vestalia Terra region may be a preserved section of ancient
crustalmaterials(e.g.,Raymondetal.,2013)oramoundofaccumu-
lated ejecta, modified by later impact cratering, producing a 5.2.2.2. Interpretation. Weinterpretthisunittobeancientcratered
distinctivetopographichigh.Becausethesurfacetextureofcratered terrain degraded or smoothed by either the emplacement of a
highlands is similar throughout, showing no distinct difference thinning layer of Rheasilvia ejecta, or the degradation of sloping
between the higher Vestalia Terra and the surrounding cratered materialovertime.