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Contents lists available at ScienceDirect

Icarus

journal homepage: www.elsevier.com/locate/icarus

Geologic

mapping

of

the

Urvara

and

Yalode

Quadrangles

of

Ceres

David

A.

Crown

a ,∗

,

Hanna G.

Sizemore

a

,

R. Aileen

Yingst

a

,

Scott C.

Mest

a

,

Thomas

Platz

a ,b

,

Daniel

C.

Berman

a

,

Nico

Schmedemann

c

,

Debra

L.

Buczkowski

d

,

David

A.

Williams

e

,

Thomas

Roatsch

f

,

Frank

Preusker

f

,

Carol

A.

Raymond

g

,

Christopher

T.

Russell

h

aPlanetary Science Institute, 1700 E. Ft. Lowell Rd, Suite 106, Tucson, AZ 85719 USA bMax Planck Institute for Solar System Research, Gottingen, Germany

cFreie Universitat Berlin, Berlin, Germany

dPlanetary Exploration Group, Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723 USA eSchool of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287 USA

fGerman Aerospace Center, DLR, Berlin, Germany gNASA Jet Propulsion Laboratory, Pasadena, CA 91109 USA

hInstitute of Geophysics and Planetary Physics, University of California at Los Angeles, Los Angeles, CA 90024 USA

a

r

t

i

c

l

e

i

n

f

o

Article history:

Received30November2016 Revised21July2017 Accepted1August2017 Availableonline5August2017

a

b

s

t

r

a

c

t

WeconductedgeologicmappingoftheUrvara(Ac-13)and Yalode(Ac-14) Quadrangles(21–66°S,180– 360°E)ofthedwarfplanetCeresutilizingmorphologic,topographic,andcompositionalinformation ac-quired by NASA’s Dawn mission. The geologic characteristics of the two large impact basins Urvara (170kmdiameter)and Yalode(260kmdiameter)andtheirsurroundingswereinvestigatedusingDawn FramingCameradatasets, includingSurvey(415m/pixel), HAMO(140m/pixel), and LAMO (35m/pixel) imagesandmosaics,colorandcolorratioimages,andDTMsderivedfromstereo-photogrammetry. Geo-logicmappingdemonstratesthatimpactcrateringhasdominatedthegeologichistoryoftheUrvaraand YalodeQuadrangles,withearlycrateredterrainformationfollowedbyformationofthelargebasinsand widespreademplacementofbasin-relatedsmoothmaterial.Impactcratersdisplayawiderangeof preser-vationstatesfromnearlycompletelyburied/degradedformstomorerecentpristinecraterswithterraced innerwallsandlobateejectadeposits.Cross-cuttingrelationshipsandmorphologicsignaturesshowthat theUrvaraimpactfollowedtheYalodeimpact,consistentwithagesderived fromcratersize-frequency distributions(580±40MaforYalodeand550±50MaforUrvara).Observeddifferencesinbasin materi-alsandrimmorphologysuggestheterogeneitiesinthesubstrateexcavatedbyimpact.Smoothdeposits thatcoverlargeareasofthequadrangles,includingthebasinfloors,rims,andexteriorzones,are inter-pretedtobedominatedbyUrvaraejecta butYalodeejectaandlocalizedice-richflowmaterialmaybe minorcomponents.GeologicmappingresultsandsimulationsofejectaemplacementsuggestthatUrvara andYalodeejecta depositsextendfor largedistances(more thantwo craterdiametersfromthe basin centers)andmayserveasimportantstratigraphicmarkersforthegeologicrecordofCeres.

© 2017TheAuthors.PublishedbyElsevierInc. ThisisanopenaccessarticleundertheCCBY-NC-NDlicense. (http://creativecommons.org/licenses/by-nc-nd/4.0/)

1. Introduction

NASA’s Dawn spacecraft, launched in September of 2007, or-bited the asteroid Vesta in2011 and2012 andwasinserted into orbit around Ceres on March 6, 2015, where it continues to re-turnimaging,spectroscopic,andelementaldatasetsfromthedwarf planet(e.g., Russell and Raymond, 2011; Russell et al., 2016 ). Cen-traltotheDawnMission ischaracterizationofearlySolarSystem

Correspondingauthor.

E-mail address: crown@psi.edu(D.A.Crown).

conditionsthroughgeologicanalysesofandcomparisonsbetween thesmall,differentiated,rockyprotoplanet VestaandlargerCeres (1/3ofthe massoftheasteroidbelt),whosespectral characteris-ticshadsuggestedawater-rich,carbonaceouschrondrite-like com-position(e.g., McCord and Sotin, 2005 ). Thisstudyreportson de-tailed geologicmapping oftwo 1:500,000-scale quadrangles (Ac-13andAc-14)in Ceres’southern hemispherecovering the region 21–66°S,180–360°EandcontainingthelargeimpactbasinsUrvara andYalode(Fig. 1 ).Wepresentamap-baseddescriptionofthe ge-ology, a synthesis of the geologic historyof the region of inter-est,and analyses of impact basin morphology on Ceres. We also http://dx.doi.org/10.1016/j.icarus.2017.08.004

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Fig.1. Global60pixel/degree(137m/pxl)DigitalTerrainModel(DTM)(Preuskeretal.,2016)overHAMOimagemosaic(Roatschetal.,2016)showingthelocationsofthe Urvara(Ac-13;21–66°S,180–270°E;leftbox)andYalode(Ac-14;21–66°S,270–360°E;rightbox)QuadranglesofCeres.DTMrangeinelevationis−7049to7499m.Simple cylindricalprojectionwithcenterlongitudeof180°E.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthis article.)

explore the implications of using these large impact basins on Ceresasstratigraphicmarkers.

2. Background

A key objectivefor the exploration of the dwarf planet Ceres by the Dawn Mission is to understand the role of water in the geologichistoryof a primitivebody. Priorto the mission several linesofevidencesuggestedthatCereswaswater-richandits sur-facegeologymighthavebeenstronglyinfluencedbywaterand/or water ice. The relatively low density of Ceres indicates that it is a differentiated body with water concentrated in outer layers abovearockycore(Thomas et al., 2005; McCord and Sotin, 2005; Castillo-Rogez and McCord, 2010 ). Ceres’ spectral characteristics havebeen attributedto silicate compositionsalong with carbon-atesandiron-richclays(Rivkin et al., 2006 ), OH-or H2O-bearing phases(Lebofsky et al., 1981, 1986; Rivkin et al., 2006 ),and aque-ousalterationofolivine-richmaterials(Milliken and Rivkin, 2009 ). Modeling studies showed the potential for recent or current hy-drothermal activity (McCord and Sotin, 2005; Castillo-Rogez and McCord, 2010 ). Inaddition,observations fromtheHerschel Space Observatory showed localized water vapor emissions from Ceres thatwereinterpretedtobeduetosublimationofcometaryice ex-posedatthesurfaceorcryovolcanism(Küppers et al., 2014 ).

Dawn observations of Ceres to-date have provided significant newinsightsintoitsgeologyandgeologichistory.Evidenceforthe presence,potentialabundance,and/oreffectsofwatericeinclude: a)measurementofvariationsingravityconfirminginterior differ-entiation,withpotential concentrationsofwaterinouter,weaker layers(Park et al., 2016 );b)mappingandmodelingofcoldtrapsin permanentlyshadowedareas incraters inthe northpolarregion (Platz et al., 2016a; Schorghofer et al., 2016 );c)imagingofthe vol-canicdome AhunaMons, whose morphologyhas beenattributed toextrusivecryovolcanicactivity(Ruesch et al., 2016 );d)spectral detectionofexposed watericeatOxoCrater(Combe et al., 2016 ); e)spectral signatures of ammonium phyllosilicates, the presence ofwhich suggests widespread alteration of the surface materials

ofCeres (Ammannito et al., 2106 );andf) deficits inthermal and epithermal neutron flux measured by the Gamma Ray and Neu-tronDector(GRaND)consistentwiththepresenceofsubsurfaceice athighlatitude (Prettyman et al., 2016 ).In addition,brightspots associatedwithOccatorandother craters onCereshavebeen in-terpreted toindicatethepresence ofsodiumcarbonateandother saltsthatmayresultfromhydrothermalactivity(De Sanctis et al., 2016 ;seealso Nathues et al., 2015, 2017 ).AnalysesofCeres’impact craters haverevealed that 1) the current surface of Ceres is de-pletedinlargecraters (i.e.,>100–150km)relativetotheexpected populationpresumablyduetoimpact-drivenresurfacing(Hiesinger et al., 2016; Marchi et al., 2016 ),2)polygonalformsandfractured floors reflect pre-existing structural patterns in fractured crustal materials (Buczkowski et al., 2016 ),and3)morphologicattributes ofsomecratersandthesimple-complexcratertransitiondiameter are consistentwithimpact into surfacematerialsthat are a mix-tureoficeandrock(Hiesinger et al., 2016 ).

TheDawnScienceTeamisleadingacoordinatedmapping pro-gram that will result in generation of (1) a global geologic map of Ceres (primarily based on HAMO(High Altitude Mapping Or-bit) images (Mest et al., 2016; 2017 )) and (2) 15 geologic maps of 1:500,000-scale quadrangles using LAMO (Low Altitude Map-ping Orbit) images (Williams et al., 2017a and other papers in thecurrentissue;seealso Nass, 2017 ).GeologicmappingofCeres hasbeenundertakentohelpintegrateandsynthesizeobservations madeusingDawnMissiondatasetsinordertoadvanceknowledge ofCeres’surfacegeologyandgeologicevolution,aswellasto pro-vide geologicandstratigraphiccontext forspectral andelemental data.

3. Dawndatasets

ForgeologicmappingoftheUrvaraandYalodeQuadranglesof Ceres,weutilizedindividualFramingCamera(FC)imagesand im-agemosaics,colorandcolorratiodatasets,anddigitalterrain mod-els(DTMs)derivedfromFCimages(Preusker et al., 2016; Roatsch et al., 2016; 2017 )(Fig. 2 ).OnboardtheDawnspacecraftthereare

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Fig.2. Urvara(left)andYalode(right)QuadranglesofCeres:a)LAMOmosaic(35m/pxl)withlocationsofFigs.4–10indicated,b)HAMODTM(60pixel/degree),c)HAMO photometrically-corrected,colormosaic(R=965nm,G=750nm,B=440nm;yellowshowsareanearsouthpolethatisunusableduetohighincidenceangles),andd) Surveycolorcompositeimage(usingbandratios:R=965/750nm,G=550/750nm,andB=440/750nm).AllimagesinCerespolarstereographicprojectioncenteredon 270°E.Imagemosaics(Roatschetal.,2016,2017),HAMOcolor,andDTM(Preuskeretal.,2016)courtesyofDLR;SurveycolorcompositecourtesyofMPS.

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twoidenticalFramingCameras(FCs)(Sierks et al., 2011 );FC2 pro-vided near 100% coverage of the surface of Ceres in clear filter mode (i.e., panchromatic; filter F1) during three planned orbital phases.ImageresolutionimprovedsignificantlythroughtheSurvey (415m/pixel),HAMO(140m/pixel), andLAMO(35m/pixel; Fig. 2 a) phases,andclearfiltermosaicsgeneratedateachphasewereused asbasesforgeologicmapping.Weexaminedmorphologicfeatures, surfacetextures,andalbedointhesemosaics.

The DawnFraming Cameraisequipped witheightfilters(one clear and seven multispectral) covering a range of 420–980nm, yielding globalcolordata.FC datawereradiometricallycalibrated andphotometrically corrected,straylight wasremoved, anddata were converted to reflectance (I/F) (Nathues et al., 2014 ). The resulting reflectance data were map-projected and co-registered to generate color cubes, which were then mosaicked (Fig. 2 c). A detailed description of the FC data processing pipeline is pre-sented in Reddy et al. (2012) . The result is a robust color mo-saic that wasused to refine unit boundaries and assist in inter-preting unit stratigraphy. Color ratios (e.g., ratios similar to the “Clementine color ratio” utilized at Vesta (Reddy et al., 2012 )) were also examined. To assist in and compare to geologic map-ping, we used the following band ratio combination, as out-linedin Reddy et al. (2012) :R=965/750,G=550/750,B=440/750 (Fig. 2 d). In this ratio mosaic, the red channel is meant to highlightvariationsinthespectralsignatureatlongerwavelengths, whereasgreentonesrepresentvariationsinthepeakwavelengths oftenassociated withalbedo. Theblueratioprovides an estimate of shortwavelength variations, particularly thecontinuum slope; bluertonesmaybeapproximatelyequatedtolessmatureregolith. The colorratio mosaicwasusedwithcaution becauseit appears to beaffected byshadowboundary issuesandisthus difficultto interpret.

Dawn Framing Cameraimages have been used to derive dig-ital terrain models (DTMs) for Ceres via stereo-photogrammetry (Preusker et al., 2016 ).Dawnacquired∼880clearfilterimages dur-ingSurveyorbitat415m/pixeland2350HAMOclearfilter im-ages at ∼140m/pixel. Images were acquired globallyunder simi-larilluminationconditionsbutdifferentviewingconditionsto cre-ate a stereo constellation that was used to produce Survey and then HAMO(∼140m/pixel spatialresolution; 10m vertical accu-racy; Fig. 2 b)DTMs.Weutilizedtopographicinformationfor land-formsonCerestoassistinunitidentificationandcharacterization andtoassesscontactrelationships.

4. GeologicmappingoftheUrvaraandYalodeQuadrangles Urvara and Yalode are adjacent impact basins located in the southern hemisphere of Ceres. These two impact basins domi-nate the geology of the 1:500,000-scale Urvara (Ac-13; 21–66°S, 180–270°E) and Yalode (Ac-14; 21–66°S, 270–360°E) Quadrangles (Fig. 1 ).Yalode is260kmindiameter,centeredat42.3°S,293.6°E; Urvarais170kmindiameter,centeredat45.9°S,249.2°E.Theyare the second- and third-largest distinct impact structures on Ceres after Kerwan (280km diameter, 10.8°S, 123.6°E). To the east of Yalode is the subdued expression of a similar-sized circular de-pression,presumablythesiteofadegradedimpactbasinthat pre-dates Yalode. Urvarahas a more well-preserved morphologic ex-pression than Yalode, and structures radial to Urvara (including BaltayCatena)extendtotheeastacrossYalode.Eachbasinisfully containedwithintheself-namedquadranglebutdistalimpact de-positsandstructuresextendintoneighboringquadrangles, includ-ingtheZadeniQuadrangletothesouthandtheOccatorandRongo Quadrangles tothe north. Urvaraand Yalode basinsoccur within the crateredterrain that dominatesmuchofthe surfaceof Ceres. AhunaMons, anapparently uniquevolcanicextrusiononthe

sur-faceofCeres(Ruesch et al., 2016 ),isfoundintheRongo Quadran-gletothenorthwestofYalodebasin.

We have produced geologic maps of the Urvara and Yalode Quadrangles usingArcGIS software andimaging andtopographic data acquired by the Dawn spacecraft (see Section 3 and Williams et al., 2017a thisissue).Basemapimagemosaics,DTMs, andcolor andcolorratiodatasets wereused toidentify,map the locations/distributionof,anddescribe geologicunitsandfeatures. Geologicmappinglayersincludedpointfeatures,geologiccontacts, andlinearfeatures.Geologiccontactpolylineswereconvertedinto polygonsto definegeologicunits.SurveyandHAMOmosaicsand aSurvey-basedDTM wereusedtodefine apreliminarysetof ge-ologicunitsforeachofthequadrangles(Crown et al., 2015; Size- more et al., 2015a, 2015b; Yingst et al., 2015 ).LAMOmosaics,the HAMODTM, Survey color ratiodata, andHAMOcolor datawere used to refine and finalize the map unit scheme and to provide detailedunitcharacterizations(Fig. 2 )(Crown et al., 2016a; 2016b; Sizemore et al., 2016a; 2016b; Yingst et al., 2016 ).

Fig. 2 shows the key datasets used for geologic mapping of Ceres.Both HAMO andLAMO (Fig. 2 a)mosaics show smooth to rugged surfaces across the region encompassing the Urvara and YalodeQuadrangles.Smooth surfacesare prominentonthefloors ofthelargebasinsandwithin basinrimzones, andalsooccur in thesurroundingcrateredterrain.Thecrateredterrainexhibits var-ied surface texture and morphology, controlled primarily by the abundance, size range, and degradation states of impact craters, whichareobservedtovaryspatiallyacrosstheregionofinterest.

The HAMO DTM (Fig. 2 b) clearly shows well-defined de-pressions in the interiors of Urvara, Yalode, Meanderi (diame-ter=103km),andMondamin (diameter=126km)aswellas vari-oussmall-tomedium-sizedcratersscatteredthroughoutthe quad-rangles.Locally high-standing,rugged terrainisevident surround-ingthesefourimpactstructures andmayrepresentpartsoftheir continuousejectablankets.Thecrateredterrainexhibitssignificant variationsinelevationacrossitssurfaceandseveraldiscrete high-standingzonestotheeast ofYalode. Theoverall rangesin eleva-tionforthe UrvaraandYalode Quadranglesare−5874to 5671m and−5819to5579m,respectively.

HAMOcolordata(Fig. 2 c)showsubtlevariationsacrossthe Ur-varaandYalodeQuadrangles.Smoothmaterial,andbasinfloorsin particular,exhibit morebrownishcolorsanddarkerareasrelative to the crateredterrain, which exhibits a mix ofbrown and blue colors.Anumberofimpactcraters (e.g.,23°S,279°E;54°S,272°E; 48.9°S, 322.5°E; 25.8°S, 191.5°E; 39°S, 238°E; 34.3°S, 266°E) have distinctlyblueejectamaterials.Brightejectadepositswithoutthe prominentbluecolorareobservedsurroundingcraterscenteredat 21.2°S,202.9°Eand42°S,308.2°E,anddarkbrowncratermaterials areevidentintheYalodeQuadrangleassociatedwithBesuaandan unnamedcraterat49°S,301.5°E.

Survey orbitcolorcomposites (Fig. 2 d)show significant varia-tionsacrossbothquadrangles.Strongcolorvariationsareoften as-sociatedwithindividualwell-preservedejectablankets,whichare apparentasbrightblueregions.Alargezone ofdeepredcoloris apparentwithinpartoftheUrvarabasinfloorandadjacentregions tothe east, west,andsouth. Blue, tan,andbrowncolors charac-terizethenorthern partsofthetwo quadrangles.Colorvariations revealedby thiscompositeimage are difficulttointerpret dueto lackofclearcorrelationbetweencolorvariationsandmorphologic ortopographic features;however,theymayindicatedistinct min-eralcompositions,grain-sizes,orsurfacematerialages,consistent withacomplexgeologicalhistory.

Fig. 3 shows geologic maps of the Urvara and Yalode Quad-ranglesofCeres. Thegeologicunit andsymbol scheme usedwas developedthrough iterativemapping asthe Dawn spacecraft ap-proachedandorbitedCeresthroughtheSurvey,HAMO,andLAMO phasesofthe mission,andwasrefined through consultationand

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collaborationwith other investigators conducting1:500,000-scale geologicmapping ofquadrangleson Ceresandthe DawnScience Team in general (e.g., Williams et al., 2017a ). Consistent appli-cation of this geologic mapping approach allows comparison of 1:500,000-scalequadranglemaps(and theirderived geologic his-tories)andfacilitatesintegrationofquadranglemapsintoregional and global mapping efforts (Mest et al., 2016; 2017 ). Across the UrvaraandYalodeQuadrangles,wehavemapped14geologicunits and14 geologicfeature types(including both lineandpoint fea-tures). Themain typesofgeologicunitsmapped includecratered terrain,UrvaraandYalode basindeposits(includingsmooth mate-rials),andvarioustypesofimpactcratermaterials.

4.1. Descriptionofmapunits

The geology of the Yalode Quadrangle is dominated by the 260-km diameter impact basin Yalode and related crater mate-rials extending to the northern and southern boundaries of the map region. Other named features include the impact craters Belun (33.7°S, 356.3°E, 36km diameter), Besua (42.4°S, 300.2°E, 17km diameter), Lono (36.7°S, 304.3°E, 20km diameter), Mon-damin (63.0°S, 353.6°E, 126km diameter), and Tibong (29.8°S, 352.2°E,36kmdiameter)aswell asNarSulcus(41.86°S,280.11°E) and the eastern part of Baltay Catena (49.34°S, 274.49°E). The 170-km diameter impact basin Urvara and its extensive crater materials dominate the geology of the Urvara Quadrangle. Ur-vara impact deposits surround the basin on all sides and ex-tendbeyondtheUrvaraQuadrangletothenorth, east,andsouth. OthernamedfeaturesintheUrvaraQuadrangleincludetheimpact craters Consus (20.7°S, 200.5°E, 64km diameter), Fluusa (31.5°S, 277.9°E, 60km diameter), Meanderi (40.9°S, 193.7°E, 103km di-ameter), Sekhet (66.4°S, 254.9°E, 41km diameter), and Tawals (39.06°S,238.02°E,8.8kmdiameter),aswellasfourcraterchains: BaltayCatena (49.34°S,274.49°E);GerberCatena(38.3°S,215.5°E), Pongal Catena (37.37°S, 267.66°E), and the southernmost of the Samhain Catenae (2.75°S, 252.85°E). Geologic units identified in these quadranglesalso occur elsewhere on Ceres(see other con-tributions tothisvolume),includingthecrateredterrainanda se-riesofimpactcratermaterialunits.Smoothmaterialsimilartothat foundelsewhere isalsoevident, butherein isgroupedwithbasin materialsbecauseofitsstrongassociationtotheUrvaraandYalode basins.Wealsoidentifiedandmappedgeologicfeatures,including structuralanderosionalfeaturesaswellasthoserelatedtoimpact craters. Geologiccontactsare typicallynotdistinct andare gener-allymappedasapproximate.Themostwelldefinedgeologic con-tacts occurinassociation withthedistalmarginsoflobate ejecta depositsassociatedwithwhatappeartobepristineimpactcraters.

Cratered terrain(unit crt) isthe oldest geologic unit identified in the Urvaraand Yalode Quadrangles; it dominates the western portion of theUrvara Quadrangleand theeastern portion ofthe Yalode Quadrangle(Fig. 4 ). Whilegenerallyexhibitingmoderately rugged,heavilycrateredsurfaces,thecrateredterrainexhibits vari-ablemorphologyandincludessmoothzonesinwhichcrater den-sityandsizeisdecreasedrelativetosurroundingareas.Cratersizes in the two quadrangles range from the limits of LAMO resolu-tion to the 260-km-diameter Yalode basin. Impact craters show a range in morphology from well preserved to highly degraded and/orburiedforms.Muchofthesurfacevariabilityofthecratered terraininbothquadranglescanbeattributedtolocalvariationsin the density, size range, andpreservation state of impact craters. Therimsofmediumtolarge,relativelywell-preservedcratershave the greatest localrelief. Localized concentrations ofsmall impact craters may be due in some cases to clusters or chains of sec-ondary craters. In addition to variations in the characteristics of thecontainedimpactcratersacrosscrateredterrainsurfaces,there arealsosignificantchangesinelevationwithinthecrateredterrain.

Fig.4. PartsofLAMOmosaicshowingcrateredterrainofCeresinUrvara(a)and Yalode(b)Quadrangles:a)crateredterraintothewestofUrvarashowingcraters exhibitingarangeofsizesanddegradationstates,including103kmdiameter Me-anderiandasimilar-sizedsubduedbasintoitsnorth;imagecenteris34°S,194°E, b)crateredterraintotheeastofYalodeshowingcratersexhibitingarangeofsizes anddegradationstates,includingTibong,Belun,andasegmentofadegradedbasin rimthatextendsintotheRongoQuadrangle;imagecenteris29.5°S,349°E.Note alsothesmallimpactcraterchains.Lineworkfromgeologicmap(Fig.3).Northis tothetopatcenterofbothimages.

Therearesmooth,low-lyingregionsthatmaydelineatethe interi-orsofancientimpactbasinsthathavebeenerodedand/or buried (e.g.,near∼26°S,334°EintheYalodeQuadrangleandnear∼36°S, 217°Ein theUrvara Quadrangle).To theeast of theYalode basin therearealsoirregularhigh-standingzonesnotclearlyassociated withindividual impact structures, although they roughly align to

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Fig.5. PartsofLAMOmosaicshowingrangeofimpactcratermorphologiesintheYalodeQuadrangle:a)freshcraterwithlobateejectaandrimterraces;imagecenteris 52.5°S,276.5°E,b)Besuacraterwithsmoothejectablanketandhummockyfloor;imagecenteris42.5°S,300°E,c)Tibongcraterwithdegradedrimandheavilycratered floor;imagecenteris30°S,352.5°E,andd)scarpsandlowhillsalignedincircularpatternindicatingburied/degradedcraterriminsmoothmaterial(outlinedwithburied impactcraterrimsymbol);imagecenteris45.5°S,326.5°E.Northistothetopatcenterofallimages.

what mayhave been thesouthern rimof the degraded basin to theeastofYalode.

ThecrateredterrainintheUrvaraandYalodeQuadranglesis in-terpretedtorepresenttheancientcrustalmaterialsofCeres, dom-inatedbycratermaterialsfromamultitudeofimpacteventsearly in the history of the dwarf planet. The cratered terrain is likely dominatedbyancientimpactejectaandcraterwallandfloor de-posits,butmayalsoinclude youngercratermaterials that cannot beclearlydelineatedandlinkedtoaspecificsourcecrater.

Impactcratersareobservedthroughoutbothquadrangles;they displayarangeofpreservationstatesandhavebeenmappedboth within the cratered terrain and the widespread units associated withUrvaraandYalode (Figs. 5 and 6 ). Pristine features (includ-ing Tawalsandunnamed craters onthe northwestern rimofthe

Urvara basin and near the NW and SW corners of the Yalode Quadrangle)havesharplydefinedinnerwallscoveredbytalusand show lobate ejecta patterns that cover and mute the underlying terrains. Ejecta lobes are narrow andelongate to broad in plan-formandareobservedtofillintopographiclows(suchastroughs or crater chains) and can be deflected by local topographic ob-stacles (such as mounds or hills). At the southeastern corner of the Yalode Quadrangle, lobes ofejecta extending froma pristine craterintheZadeniQuadrangle(Platz et al., 2016b; Williams et al., 2017a ) appear to have been deflected around and partially over thecentral peak ofMondamin. The lobateejectablankets associ-atedwithpristinecratersareoftenevidentincolordatasets.Other well-preservedcraters inthe region(including Urvarabasin, 126-km Mondamin, 17-km Besua, 20-km Lono) have well-defined,

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quasi-circularforms withprominentrimsandin some cases dis-cernible ejecta blankets. For example, Besua and Lono craters withintheYalodebasinhaverecognizableejectablanketsthathave beenmappedbutthedistalcontactsofthesedepositsaredifficult toidentifyastheejectadepositsthinandmergewithsurrounding materialswithdistancefromtheirsourcecraters.Degradedimpact features,includingYalodebasinandnumeroussmallercraters, ex-hibitsubduedrims,typicallylackdiscreteejectadeposits,andhave infilledinteriors.Somedegradedcraterswithinthecrateredterrain (e.g., Tibong and Meanderi) have heavily cratered floor deposits. Crater floor deposits are generally hummocky and may include slump materialfromcrater wall terracesaswell ascrater central peak material. Analyses of LAMO images andHAMO topography revealthepresenceofremnantdegradedand/orburiedbasinrims thatindicatethedominantroleofimpactcrateringthroughoutthe geologichistoryofthisregionofCeres.

Cratermaterial unitsmapped inthe UrvaraandYalode Quad-rangles include crater material (unit c), crater floor material hum-mocky(unitcfh),craterfloormaterialsmooth(unitcfs),craterterrace material(unitct),andcratercentralpeakmaterial(unitccp).Crater material includes the ejecta andrim materials of well preserved tomoderatelydegraded impactcraters,aswellascraterfloor de-posits in locations where the floor materials are not mapped as separateunits.Somebutnotallcraterswithmappableejecta blan-ketsalsoexhibithummockyfloordeposits.IntheUrvara Quadran-gle, two unnamed craters exhibit both hummocky floor deposits and distinct, asymmetric lobate components in their ejecta. An unnamed crater to the southwest of Yalode with distinct ejecta shows well-developed interiorterraces and a central peak. Mon-damin cratercontainssmooth craterfloormaterials andacentral peak. Centralpeaksarealsoevident inUrvarabasinandtwo un-namedcraters intheUrvaraQuadrangle.Southernportionsofthe Urvaracentralpeakcomplexexhibitclustersofoverlapping round-to-irregular, low-rimmed depressions. These depressions may be degraded examples ofpits observed ina varietyofmore pristine craters elsewhere on Ceres(e.g., Haulani,Ikapati,Dantu, and Ku-palo), whicharelikely analogoustopitted cratermaterials previ-ouslyidentifiedonVestaandMars(Denevi et al., 2012; Tornebene et al., 2012; Sizemore et al., 2017 ).

The prominence and large areal extents of the Urvara and Yalodebasinsandassociateddepositsaswellastheirpotential im-portanceasstratigraphicmarkers inthegeologichistoryofCeres support distinguishing thesebasin materials from therest ofthe impactcratermaterials inthetwoquadrangles.Theimpactcrater deposits associatedwiththe UrvaraandYalode basinshavebeen mappedasbasinmaterials,withsimilarsetsofunittypesashave been mapped forimpact crater materials across thequadrangles. Mapping the different sets of Urvara and Yalode basin materials effectively definesgeologic formations foreach ofthe basins, al-though,asdescribedbelow,asingleunitofsmoothmaterial asso-ciatedwiththetwobasinsisused.

Urvara and Yalode basins are distinct from one another in terms of morphology and preservation state. Yalode basin has a variably preserved rim, which is continuous and sharplydefined to the north/northwest and is irregular or degraded elsewhere. Yalode’seasternrimisdefinedbyaseriesofscarpsandlow hills aligned ina circumferential pattern. Hummocky depositsinterior toaprominentscarpareobservedalongYalode’snorthernrimand aremappedasYalodefloormaterialhummocky(unitYfh);these de-positsmaybe degradedterrace materials mixedwithlocal mass-wasting and perhaps some ejecta from Urvara. The western rim ofYalodeisdisruptedby Urvarabasin,andstructuralfeatures ex-tend from Urvara across Yalode’s floor, including Baltay Catena. Aseries ofroughlycircumferential scarpsdelineatethestructural trendofYalode’swesternrim.TheinteriorofYalodebasincontains an irregular ring of low hills that could represent a basin inner

Fig.6. PartsofLAMOmosaicshowingexamplesofimpactcratersinthe Urvara Quadrangle:a)freshcraterwithasharpeasternrimandhummockyfloormaterial; imagecenteris35°S,265°E.Thewesternrimiscollapsed,andejectatothe north-eastandcollapsedrimmaterialtothenorthwestexhibitlocalizedlobatemargins; b)Theyoungercrater(topleft)oftwolesspristinecratershasasharprim,central peak,hummockyfloormaterial,andlobateejectathatoverliestheoldercraterto thesoutheast;imagecenteris56°S,204.5°E.Northistothetopatcenterofboth images.

ring and/or reflect the presence of one or moresubdued impact craters within Yalode. Largeexpanses ofbasin materials, mapped asYalode ejecta material (unit Ye), are observed to the northand southofYalode’srimandareinterpretedtobetheprimaryejecta depositfromthe Yalode impactevent, perhaps mixedwithsome ejecta from Urvara. Within the Yalode Quadrangle and extend-ing to the north into the Rongo Quadrangle(Platz et al., 2016b, 2016c; 2017 ),theYalode ejectamaterialhasa hummockysurface

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that exhibits numerous surface features (e.g., scarps, ridges, and grooves) with dominant trends circumferential andradial to the basin andthat containsnumerous small impact craters. Medium andlarge craters arenoticeablylessabundantandwherepresent moredegraded/buriedrelativetothoseinthecrateredterrain.The Yalodeejectatothenorthofthebasinandextendingfor∼150km northdisplaysaquasi-radial patterninmorphology andlocal re-lief/ruggednessthathelpstodefinethespatialdistributionofwhat maybethenear-basinthickerandcontinuousejectamaterials.To thesouth, the Yalode ejectais similarly hummocky andcontains troughs,ridges,andscarpsthatarelarger,althoughfewerin num-berandlessclearlyorganizedintoradialorcircumferentialtrends thanto the north. Here, Yalode ejecta extendsfor 100+ km into theheavily crateredsouth polarregionin theZadeni Quadrangle (Platz et al., 2016b; Williams et al., 2017a ), aswell asacross the crateredterraintothesoutheastwhereitappearstohavepartially filledMondamin. To the southwest,the Yalode basin andin par-ticularYalodeejectamaterialappeartobeoverlainbyejectafrom theUrvarabasin.

In contrasttoYalode, Urvarabasinhasa distinct centralpeak, consisting of a 20km long east-west trending ridge. This ridge andsurroundinghigh-standing,rugged terrain aremappedas Ur-varacentralpeakmaterial(unitUcp).Urvaraalsohasasharply de-fined rim that is continuous along its full circumference. Urvara exhibitsabroadmorphologicaldichotomy,largelyduetoextensive smooth materials covering the northern basin interior, and exte-riorregionstothenorthandeast.Thesesmoothareasaremapped asUrvara/Yalode smoothmaterial (UYs),which is discussed in de-tailbelow. Tothenorthandnortheast,theUrvararimconsistsof asinglesteep scarp boundingnarrowmuted terracesoverlainby smooth material. The southern and western portions of the Ur-vararimare rugged;the basinwalls exhibit multiplebroad inte-riorterraceswithroughsurfaces,mappedasUrvaraterrace mate-rial(Ut). Portions ofthe northern Urvarainteriordisplay a hum-mockytexturethatdisrupts extensivesmooth material.These re-gionsaremappedasUrvarafloormaterialhummocky(Ufh)andare interpretedto bedegraded terrace andwall-slump materials.The southernmostportionofthe basin floorconsistsofsmooth, dark, flat-lying material, mapped as Urvara floor material smooth (Ufs).

Thismaterialexhibitsawell-definedcontactandlocalized embay-mentrelationshipswithmorebroadlydistributedsmooth materi-alstothenorthandeast.Italsodisplaysalowersmall-crater den-sityand distinct colorrelative to adjacent smooth materials. The originoftheUrvarafloormaterialsmoothunitisuncertain;onthe basis ofobserved characteristics and geologicsetting,hypotheses includelocalizedextrusion ofvolatile-rich materials on thebasin floorand remobilization of volatile-rich basin rimand/or terrace materials.

Like therimandwallmaterials ofUrvara,the cratermaterials external tothe Urvara rimare morphologically dichotomous. Ex-tensive rough-to-hummocky materials to the south andwest are mappedas Urvaraejecta material (unit Ue) andinterpreted to be part of the primary Urvara ejecta deposit. This unit also occurs alongthe northern boundary ofthe UrvaraQuadrangle, although withalessruggedmorphologicexpression,andextendsnorthward intotheOccatorQuadrangle(Buczkowski et al., 2107 ).

Alarge expanseofsmooth materialextendsacrossthe bound-arybetween the Urvaraand Yalode Quadrangles, which we map asUrvara/Yalodesmoothmaterial(unitUYs) (Fig. 7 ).Smooth mate-rialhasbeennotedinanumberofareasonthesurfaceofCeres; forexample, smooth material is mapped on the floor ofKerwan basinandbeyonditsdegradedrimandisinterpretedtobethe re-sultof“impact-inducedmelting” of volatile-richcrustal materials (Williams et al., 2017b ). Smooth material comprises a major por-tionofthefloordepositsoftheUrvarabasinandalargeregionto thenorthandnortheastoftheUrvarariminsidetheUrvara

Quad-rangle. Thisunit extendseastward, comprisingmuch ofthe floor deposits of the Yalode basin, andcontinues across Yalode’s east-ernrimandbeyond,finallyformingtheuppersurfacematerialsof someofthetopographichighsoutsideofYalode.Althoughthe Ur-vara/Yalodesmoothmaterialdisplayssomemorphologicvariability acrossitswideexpanse,acombinedbasinunitwasdefineddueto (1)thepresenceofsmoothsurfacesasitspersistentanddominant characteristicand(2)alackofdistinct andtraceablecontactsthat wouldsuggesttheneedforsubdivision.

Urvara/Yalode smooth material is identified in HAMO and LAMO mosaics based on relatively flat-lying andfeatureless sur-faces.However,itisimportanttonotethatwhilesmoothmaterial isclearlylessheavilycratered(andcontainsfewerlarge,degraded craters)than thecrateredterrain,it containslargeconcentrations ofsmallimpactcraters aswell assomeremnant craterrimsfrom features that are presumably buried. We attribute some of the morphologicvariabilitywithin the smoothmaterial todifferences inthe sizesandabundanceofsmall,superposedcraters andalso thepresenceofconcentrationsofsecondarycratersinclustersand chains. In particular,parts of the Yalode basin floor to thenorth andsouth/southeastappeartohavehighercraterdensities. Never-theless, thesurfacematerialsare smoothin andaround these ar-easandclearcontactsarenot evident.Surface morphologywithin theUrvara/Yalodesmoothmaterialalsoappearstovaryduetothe natureoftheunderlyingterrain.Smoothmaterialisinterpretedto drapeandburypre-existingterrainsuchthatwhererugged under-lyingtopographyexists,itgivesthesmoothmaterialsamore hum-mockyandirregularappearance.Forexample,smooth materialis observed to cover mesas and knobs that represent thedegraded westernandeasternrimsofYalodebasin.Similarly,smooth mate-rial covers portions ofthe northern andeastern rimandinterior wallsofUrvara.

Urvara/Yalodesmoothmaterial isfound onsomeofthelowest andflattestsurfacesinbothquadrangles(i.e.,thefloorsofthe Ur-varaandYalode basins);however,thedistributionofsmooth ma-terialdoesnot correlate witheitherlocalorregional topography. Smooth surfaces cover localtopographic highs (including the Ur-vara andYalode rimsand theYalode interiorring structure)and extend ontothe highest surfaces inthe Yalode Quadrangle. Con-tacts between the crateredterrain and smooth material are typ-ically not distinct, but the smooth material does superpose the crateredterrain.Basedonthissuperpositionrelationship,the dis-tribution of smooth material, andits lack ofcorrelation with to-pography,theUrvara/Yalodesmoothmaterialisinterpretedtobea combinationofdepositsfrombothimpacteventsthathavemerged togethertoformasingleunit.Thisunitwouldhavevariable thick-ness revealingthe ruggednessofthe underlyingterrain to differ-ent degrees as well as a variable impact crater density due to craterchains,secondaryclusters,medium-sized(2–5kmin diam-eter)impactcratersandtheagedifferencebetweentheUrvaraand Yalode impact events.Smooth materialon the Yalode basin floor would presumably be dominated by Urvara ejectaand the more easterlydepositswouldcontaingreaterabundances ofYalode im-pact deposits. This interpretation is supported by the qualitative observationsofcraterdensitydiscussedaboveandcraterretention agesderivedforpartsofthetwoquadrangles(SeeGeologicHistory section).Thecoalescenceofdepositsrelatedtothetwobasins,as well asperhaps othersmaller craters, couldhave beenfacilitated bytheinclusion ofwatericeintheimpact deposits.This hypoth-esisis consistent withthe lackof cleardefinitionof contactsfor ejectadepositsassociatedwithallbuttheyoungestimpactcraters. Continuedmodificationofthesurfacebysmallimpactsalsocould havecontributedto obscuringcontactsbetweenparts oftheunit derivedfromdifferentsources.

Thedistributionofsmoothmaterialmayhaveoncebeenmore widespread in the Yalode Quadrangle, withthe thinner outlying

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Fig.7. PartsofLAMOmosaicshowingexamplesofsmoothmaterialintheUrvaraandYalodeQuadrangles:a)Urvara/Yalodesmoothmaterialontheeasternrimofthe Urvarabasin;imagecenteris41°S,259.5°E,b)contactbetweenUrvara/YalodesmoothmaterialandUrvarafloormaterialsmoothonUrvarabasinfloor;imagecenteris 49.5°S,253°E,c)Urvara/YalodesmoothmaterialacrossthedegradedeasternrimofYalodebasin;smoothmaterialappearstocoverscarp-boundedmesas(southeastof center)thatareremnantsofthebasinrim;imagecenteris45.5°S,314.5°E,d)Urvara/YalodesmoothmaterialintheinteriorofYalodebasinandsurroundingNarSulcus; imagecenteris42.5°S,283.5°E.Notethatboththestructuralfabric(ofNarSulcusandtoeast)andimpactcraterchainshaveE-WtrendsthatareradialtotheUrvarabasin. Northistothetopatcenterofallimages.

depositsbecomingincorporatedintothecrateredterrainandother basin ejecta materials. Note that the smooth material may be a specific type ofbasin ejectadeposit,representing excavation ofa distinct typeofsubstrateand/or asegregationofless-rockyejecta materials from theprimary ejecta duringemplacement. Evidence forexcavation of a distinct substrateis mostapparent inthe Ur-vara Quadrangle, wherethe distribution of smooth material sug-geststhatmuchofitwasdepositedasfluidizedejectatothenorth oftheUrvararimandonthebasinfloor(post-impactmodification mayhavealsoplayedaroleintheUrvarafloor).Thedistinctly dif-ferent textureofthe Urvaraejectadepositsmapped to thesouth ofthebasinandinextendedoutlyingregionssuggeststhatthe Ur-varaimpactormayhaveencounteredaheterogeneoustarget.

Current knowledge of Ceres (as discussed in the Background section) suggeststhat the Urvara and Yalode impacts, as well as other crater-forming events,may have excavated or exposed rel-atively weak crustal materials due to enrichment in ices, phyl-losilicates, and/or salts.This issupported by observations of sur-facemorphology intheUrvaraandYalode Quadrangles,including widespreadsmoothsurfaces,thepresenceofsomelobatemargins, thegradationalnatureofejectacontacts,andinference of coales-cenceofmultipledepositstoformtheUrvara/Yalodesmooth mate-rial.Localizedextrusions ofice-richmaterials withinthe area en-compassedby Urvara/Yalode smooth material areconsistent with surfacecharacteristicsandcouldplayaminorroleinformationof thisunit.

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4.2.Geologicfeatures

Based on observations of the Urvara and Yalode Quadrangles inLAMOimages,a varietyoflinearandpointfeatures havebeen mapped to illustrate geologic characteristics that help to convey processes that have affected this part of the surface of Ceres. Manyofthemap symbolsused relateto impactcrater processes. Impact crater rims are mapped for large (10km andlarger) and small (5–10km in diameter) craters with well-defined rims. De-gradedandburiedrimsarealsoidentifiedwithdifferentsymbols. BuriedcratersareparticularlynoticeablewithintheUrvara/Yalode smoothmaterial,suggestingthatthey formedonpre-existing sur-faces andhavebeen covered by smooth material. Pits onimpact craterfloorshavebeenidentifiedandmappedwithinUrvarabasin (Sizemore et al., 2016a ). Numerous impact crater chains are evi-dent scattered throughoutboth the Urvara and Yalode Quadran-gles.Impact crater chainstypically consist ofa linearto curvilin-ear,elongategroupingofrelatively smallimpact craters,although cratersizecanvarywithinagivenchain.BaltayCatena isachain ofsignificantly largercraters thatextends eastwardfroma scarp-boundedtroughradial toUrvara(Fig. 8 a).PongalCatena has sim-ilarmorphology andisa curvilinearfeaturecomposed ofa prox-imaltroughwitha distalchainofcraters extendingradially from Urvaratothenortheast(Fig. 8 a).Impactcraterchainshavetypical lengthsof∼10–∼30kmandcanbeaslongat100km.The orienta-tionsofimpactcraterchainsarevariablebutasignificantnumber, particularlyintheUrvara/Yalodesmoothmaterial,appeartobe ra-dialtoUrvara.

Two long pit chains, one of the Samhain Catenaeand Gerber Catena (126 and143km,respectively), trend in a quasi-radial patternnorthwestofUrvara(Fig. 8 b).Thesearedistinguishedfrom impact crater chains because the contained pits do not exhibit raisedrimsandare elongateinplanformalong thetrendofeach catena. Other features mapped across the region include scarps, ridges,troughs, graben,grooves, andchannels.Scarpsoriented in a circumferential pattern reveal terrace blocks along the south-erninteriorrimofUrvara, andscarp-boundedtroughs extend ra-diallyfromthebasintothenorthandeast.Scarpsdelineateboth therelativelycontinuousnorthern rimofYalode aswell assmall blocksandmesas that define remnantsof Yalode’sdegraded and discontinuousrimstothenorthwestandeast(Fig. 7 c).Scarpsare alsoobserved within Urvara andYalode ejecta materials andthe crateredterrain. Lobate scarpsare mapped to identify lower re-lief and more arcuate scarp features observed in Urvara/Yalode smooth material(and to distinguishthem fromscarpsassociated directlywithbasin rimfeatures).Throughoutthetwo-quadrangle map area, ridge and trough symbols are used to identify elon-gatetopographicfeaturesthatprovideruggednesstothelandscape on a local scale. Ridges and troughs are abundant in ejecta ma-terialto both the north andsouth of the Yalode basin and may representsome combinationoftopographicfeaturesblanketed by ejecta,structuralfeatures resultingfromthe largeYalode and Ur-varaimpacts,andthepatternsofejectaemplacement.

A few small graben are evident in the Urvara/Yalode smooth materialon the Yalode basin floor and inthe crateredterrain to theeast ofYalode. Thesefeatures are typically∼10kminlength, areshallow,andhaveconstantwidths.Theeasternexamplesseem to be associated with a degraded impact crater. The graben on Yalode’s floor are found along the inner edge of a mound that includes the highest elevation interior to Yalode’s rim and that forms part of the irregular inner ring of hills in Yalode’s inte-rior. Also associated with thismound is a high concentration of groovesand some channels(Fig. 9 a). Grooves in the Urvara and YalodeQuadranglesare shallow,linearto curvilineartroughsthat oftenoccurincloselyspaced,paralleltosub-parallelsets.Channels are narrower, shallower, and typically shorter than grooves and

Fig.8. PartsofLAMOmosaicshowingprominentcatenaeintheUrvara-Yalode re-gion:a)PongalandBaltayCatenae;imagecenteris40°S,273°E,b)southernmost memberoftheSamhainCatenaegroup;imagecenteris26.5°S,227.5°E.Pongaland BaltayCatenaearecurvilinearandtransitionfromtrough-likestructuresnearthe easternrimofUrvaratoseriesofdistinctrimmeddepressionsattheirdistalends. Thismayindicateanoriginassecondarycraterchains.Incontrast,theSamhain Catenaetothe westofUrvaraandGerberCatena(notpictured)aremorelinear andconsistofelongated,rimlesspits.Northistothetopatcenterofbothimages.

havecurvilineartosinuousplanforms.Channelsmayhavevariable widthsandinsomecasestwochannelsegmentsmayexhibit junc-tions,formingimmatureorproto-networks.Channelsareobserved tofollowthelocalslopedirection,includingadjacenttotheinner ringofYalodeandalongYalode’ssouthernrim(Fig. 9 ).

IntheUrvaraQuadrangle, groovesarepredominantly foundin Urvara/YalodesmoothmaterialbutalsooccurinUrvaraejectaand Urvara terrace materials. Channels typically occur in association with grooves in Urvara/Yalode smooth material, but can also be seen in Urvara hummocky and smooth floor materials (Fig. 10 ). IntheYalode Quadrangle,groovesandchannelsoccur togetherin Urvara/Yalode smooth material, Yalode ejecta, and on the ejecta blanket ofthe craterLono. Grooves orientedinapproximatelyan

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Fig.9. PartsofLAMOmosaicshowing channelswithinsmoothmaterials. Along Yalodebasin’srim(Fig.9a;imagecenter53°S,257°E)andinnerhills(Fig.9b;image center44°S,280.5°E),small,narrowchannels(someexamplesnotedbywhitelines) thatmaybealignedwithstructuraltrendsdissectsmoothmaterials.Insomecases, channelsegmentsformtributarypatternsthatextendalonglocalslopes.Channel morphologysuggestsacomplexsequencethatmayincludeextensionandrelated collapse,perhapsfollowed bylocalizedsappingandflow.Northistothetopat centerofbothimages.

east-westpatternareevidentnorthofUrvarainejectaandsmooth material,along thenorthern rimofYalode, andinYalode’sejecta northofthebasin.Theseappeartobe partofatexturalfabric of the surfacematerials that alsoshow impact crater chains in this orientation andare attributedto impact sculpting of the surface and/or settling of surfacematerials. Prominent concentrations of

Fig. 10. Parts ofLAMO mosaicshowing featureson the Urvara basin floor:a) prominentcomplexofgroovesandridgessituatedbetweenthecentralpeak com-plexandtheeasternbasinrim;imagecenteris42.5°S,255°E.Pittedareastothe southofthecentralpeakarealsovisible(bottomright).b)detailofgroovesand channelsinahummockyregiontothenortheastoftheridge/groovecomplex; im-agecenteris41.5°S,253°E.Channels andgroovesinthislocationareassociated withsinuousformsthatextendoutofhummockyregiontothenortheast, suggest-ingdownslopemovementofsmoothmaterial.Northistothetopatcenterofboth images.

groovesandchannelsarefound adjacentto Urvara’scentralpeak insmoothmaterialandwithinsmoothmaterialsonYalode’sfloor. Thewesternpartofthering ofhigh-standing hillsonYalode’s floordisplaysahighconcentration ofgeologicfeaturesofvarious types and orientations, which are referred to collectively as Nar Sulcus(Figs. 7 d, 8 a, 9 b).Aclosely-spacedsetofroughly east-west trending grooves extends from lower-lying smooth materials in-wardofYalode’sdegradedwestern rimintothehillsoftheinner ring;thistrendcontinueswithanadditionalsetofgroovestothe

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westofBesuacrater.North-trendinggrabengenerallyoriented per-pendiculartothegroovesareobservedinsmooth materialonthe inneredgeofYalode’sinnerring.Channelsorientedapproximately north-southandfollowinglocalslopesareevidentwithinthisarea (Fig. 9 b).Theoverallcombinationoffeaturessuggestsazoneof fo-cuseddeformation/disruptionofthesurfacematerialsfollowedby localizedincision duetosmall-scale collapseandpossible migra-tionofvolatilesunder,through,oracrossthesurfacematerials.

Lono crater also exhibitssets of groovesinits ejecta that ex-tendtothenortheastandsouthwest.Grooves ofthenortheastset occur in closely spaced sets and are generally short. Grooves to thesouthwestare longer,curvilinearfeatures that give theejecta blanketadissectedappearance.TothenortheastofLono,thefloor and rim of a buried impact cater within Yalode ejecta displays groovesandafewchannelformsinapatternthatfollowstherim ofYalode,connectingscarp-boundedmesassouthofLonotoridges andscarpsofYalode’snorthernrim.

A prominent system of ridges and grooves is located on the Urvara floor, east of the central peak complex (Fig. 10 ). Two of the ridges and a subset of the grooves run in parallel, trending N-Swithin Urvara/Yalode smooth material (UYs). Grooves in this subset arelinear, andtypically wider anddeeper thanelsewhere in Urvara. Small channels are also present, mostly trending N-S nearthecenterofthecomplex.Atthenorthernedgeofthisarea, a third ridge runs NW-SE,with a second subset of grooves run-ningperpendiculartoit(NE-SW).Tothenortheast,thissecondset ofgroovesbecomesmore branchingandsinuous, transitioningto channelsinaregionmappedasUfh(Fig. 10 b).Thesechannels ter-minatetothe NEin an area wheresmall-scalestreamlining sug-gestsdownslopemotionandsettlingofsmoothmaterial.

5. CharacteristicsofimpactbasinsonCeres:insightsfrom UrvaraandYalode

DawnFramingCameradatasetshavebeenusedtoexaminethe morphology,morphometry,spatialdistribution,andsizefrequency ofimpactcraters onthesurfaceofCeresoversizesofafew kilo-metersindiameterandlarger(Bland et al., 2016; Hiesinger et al., 2016; Marchi et al., 2016 ).Cereshasaheavilycrateredsurfacewith aheterogeneouscraterdistributionattributedtotheeffectsof spa-tialvariations inthe ice-rock crust andimpact-induced resurfac-ing.Thetransitiondiameterfromsimpletocomplexcraters(7.5– 12km)adjustedforsurfacegravityfollowsthetrendoficymoons intheoutersolarsystem(Hiesinger et al., 2016 ).Smooth deposits commonly seen on the floors of craters with diameters greater than40kmareattributedtomobilizationofimpactmelt, impact-inducedvolcanism,orpost-impactmelting,andthefracturesand central pits observed indicate a weak substrate relative to the MoonandVesta(Hiesinger et al., 2016 ).Thereislimitedevidence forrelaxationat longwavelengths. Forexample, Kerwan exhibits a relaxed profile, whereas other large basins (including Urvara andYalode)showgreaterdepthsthanpredictedinrelaxation pro-files(Bland et al., 2016 ).HAMODTMsfromstereo-photogrammetry showthatUrvarahasanapparentdepth(depthrelativetothe sur-rounding terrain) of7km (range 5–9km) and Yalode has an ap-parentdepth of6.5km(range 2.5–8.5)(Bland et al., 2016 ).There isan apparent absenceof craters greater than 300km in diame-ter anda general depletion inlarge craters across thesurface of Ceres(Hiesinger et al., 2016; Marchi et al., 2016 ), withthelower craterdensitiesinthe southernhemisphere thoughttobe dueto resurfacingassociatedwiththeUrvaraandYalodeimpacts.

ThegeneralcharacteristicsoftheUrvaraandYalodebasinshave been described above in conjunction with definition of geologic units for 1:500,000-scale quadrangle mapping on Ceres. Dawn datasets provide views of Ceres that for the first time allow us toapply its surfacegeologyto broaden ourunderstanding ofthe

natureanddiversityofimpactprocesses.HereweuseUrvaraand Yalode as examples to explore themorphologic attributes of im-pactbasins on Ceresand to explore the useof impact basins as stratigraphicmarkersforthegeologichistoryofCeres.

UrvaraandYalode areadjacentimpactstructuresandpartofa groupingofthreelargebasinsinthesouthernhemisphereofCeres. Adegraded impactbasinthat predatesthe formationofYalode is evident to the east of Yalode based on the presence of a quasi-circulardepressionandthe presenceof degradedremnantsof its rim (particularly evident in the Rongo Quadrangle). Yalode is a roughlycircularstructurewithavariablypreservedrimand irreg-ularinnerring.Urvaraisawell-definedcircularbasinwitha cen-tralpeakandrimzonesdefinedbyscarpsandterracing.

BothUrvaraandYalode display morphologicdifferences along their rims, and their rims have different properties when com-paredtoeachother.Urvara’swell-definedrimconsistsofa singu-larsteepscarptothenorthandacomplexofconcentricterracesto thesouthandwest.Therimtothenortheastiscomposedofa se-riesofhillsalignedinabasin-concentricpatternwithsteepscarps facing the basin interior. These characteristics show that the in-nercraterrim(orwall)iswell defined,andgivesdefinitiontothe basinasawhole,butthereisnosignificantexpressionoftheouter partof therim. Thisis incontrastto other craters onCeres that show morphologic andtopographic signatures forboth the inner andouter craterrims.Forexample,Occator crater(92km diame-ter;19.8°N,239.3°E)(e.g., Buczkowski et al., 2017; Nathues et al., 2017 )tothenorthofUrvaraexhibitssignificantterracingalongits inner crater walls and also displays both inner and outer crater wallslopes;theinnerrimispredominantlycharacterizedbysteep andwell-defined scarps, whereas the outer rimis a moresubtle annularmoundof materialsthat extendoutward fromthe crater. InonelocationalongOccator’swalltotheeast,therimisdefined only by terracing. By comparisonwith Urvara, this suggests that either1) in some cases craterrims on Ceresare not constructed by deposition ofan overturned flap andthecrater margin is de-finedsimplybycollapseandterrace formationor2)thepresently observedcraterrimsdefinedpredominantlybyscarpssuggest sig-nificantenlargementofcrater structuressuch thatmuch orallof theoriginalrimhasbeenmodifiedordestroyed.

Yalode’srimtothenorthconsistsofasingularsteepscarp sim-ilartothatofUrvara.TheremainderofYalode’srimisnotwell de-fined.Tothesouthitcanonlybeinferredbasedonthetransition from Urvara/Yalode smooth material to the more rugged Yalode ejectaand thechange in topographyassociated withthat transi-tion.Totheeast,Yalode’srimconsistsofbasin-concentrichillsand moundswithscarpsontheirbasin-interiormargins.Tothe north-west,therimisexpressedasazonewithnumeroussmall(bothin lengthandexposedrelief)scarpswithvariablebutgenerally basin-concentric orientations. The western rim is largely non-existent; theseriesof low-reliefhillsobserved betweenUrvaraandYalode mayinclude remnantsofboth basins’ rims. Hummockyfloor de-positsadjacenttoYalode’snorthernrimareinterpretedtobesome combinationofdegradedterracesandlocalmass-wastingdeposits, perhapsmixedwithsome Urvaraejecta. Hummockyfloor materi-alsarealsoobserved inpatches interiortoUrvara’snorthern rim. Comparisonsbetween UrvaraandYalode and toother craters on Ceres such asOccator indicatea potential evolution ininner rim morphologyovertimefroma)well-definedterracestob)a combi-nationofdegradedterracesandadjacenthummockydepositstoc) moresubduedbutstillhummockyfloormaterialswithprogressive degradation. However, diversity in degradational sequences, per-hapsrelatedtosubstrateproperties,isalsoapparent.Forexample, Kerwanbasinisdefinedbya degradedrim(withinnerandouter rimslopes)butexhibitsnoterraces;thesamemorphologyis evi-dentalong thenorthern rimofthedegraded basinto theeastof Yalode.

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Fig.11. GeologicmapsoftheUrvara,Yalode,andZadeniQuadranglesofCeresshowingthemappedextentsofUrvaraandYalodeejectadepositsalongwithcontactsof theseunitsfromtheOccator(Buczkowskietal.,2017)andRongo(Platzetal.,2016b,2016c;2017)Quadranglestothenorth.ZadeniQuadranglefromWilliamsetal.(2017a). Polarstereographicprojectioncenteredon270°E.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

NumerouscatenaehavebeenidentifiedonthesurfaceofCeres and manyof these features are spatially associated with the Ur-vara andYalode basins.PongalandBaltayCatenaeextendradially fromUrvarato thenortheastandeast,respectively, andwe inter-prettheseto belargeimpactcrater chains.BaltayCatenadissects Yalode floordeposits andthusclearlyshowsthe youngerrelative age of Urvara. Uhola Catenae extendnorth fromnear the north-ernrimofYalode andconsistofchainsoflargetroughsand elon-gatedpits.AprominentwesternfeatureisradialtoYalodebutthe prominentfeaturetotheeastisparalleltothefirst,suggestingthat the Uhola Catenae in general are not radial to Yalode. Samhain Catenae include a large number of pit chains with northwest-southeast trends thatextend acrossa largearea of Ceres’surface north of Urvara and Yalode. The southernmost of the Samhain CatenaeandGerberCatenaareradialtoUrvara.TheSamhain Cate-nae have been interpreted to be tectonic features that pre-date the formation the Urvara andYalode impacts (Buczkowski et al., 2016; Marchi et al., 2016; Scully et al., 2016a,b ).Ourobservations of thesefeatures donot show clear burial by Urvaraand Yalode ejecta, suggestingformationmaynot haveprecededbasin forma-tion.JuninaCatenaearesecondarycraterchainstothewestof Oc-catorcrater (upto∼500kmfromtherims ofUrvaraandYalode) thatarebelievedtobeassociatedwithUrvaraand/orYalode;their non-radial trends to the basins are attributed to Ceres’ fast ro-tation and small radius (Scully et al., 2016b ). Some of the cate-nae nearUrvaraand Yalode occur in ejectamaterials from these basins but othersare observed to extend beyondmapped ejecta deposits.

Fig. 11 shows LAMO-based geologic mapping results for the Urvara and Yalode basins. This composite from five 1:500,000 quadranglesshowsthe extentof mappedejectadeposits andthe boundariesbetweenUrvaraandYalodeejecta.Asdiscussedabove, geologicfeatures in the region that may be basin-related extend beyondthedistaledgesofthemappedejecta,suggestingthat the effectsofbasinformationonthesurfaceofCeresarefar-reaching. Precise locations of geologic contacts for the Urvara and Yalode ejectadepositsare difficult todetermine andmap. Geologic con-tactsonCeresaretypicallyindistinct exceptforyoungcrater ma-terials, and the emplacement of distal ejecta by its very nature suggests thinning outward and merging gradually into the sur-rounding cratered terrain. Global mapping using HAMO data by Mest et al. (2016, 2017 ; see also Buczkowski et al., 2016 ) shows asimilar distributionofejectasurroundingUrvaraandYalode for themostpart,butmoreextensiveejectadeposits totheeastand southofYalodethanfromLAMO-basedmapping.

The spatial distributions of ejecta deposits, crater chains, and basin-relatedstructuralfeaturesassociatedwithUrvaraandYalode have the potential to provide important stratigraphic constraints for deciphering Ceres’ geologic history, even with the present uncertainties in ejecta distribution and thickness and the exact mode(s)offormationoflinearfeatures.Toexaminepossible theo-reticalboundingextents forUrvaraandYalode ejecta, wepresent simulations of the distribution and impact velocity of ejecta for eachofthebasins.Forthesesimulations,we assumeda45° ejec-tionangleforeach testparticleandasymmetricejection geome-try,andutilizedejectascalinglawsdescribedinHousenand

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Hol-Fig.12. GlobalmosaicofCerescenteredat180°longitude.ColoredpointsrepresenttheimpactlocationsofejectatracerparticlesfromtheYalodeandUrvaraimpactevents. Thecolorencodestheimpactvelocityofeachparticle.Theparticledensityisameasureoftherelativeprobabilitythatsecondarycraterscouldhaveformed.Yalodeparticles insideUrvaracratermaybeignored,sincethelaterformationofUrvararesurfacedthatarea.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderis referredtothewebversionofthisarticle.)

sapple(2011).Ceres’rotationandellipsoidalshapeweretakeninto account,thoughchanges inthegravityfield duetomass redistri-butionduringthe impactwere not. Results are shownin Fig. 12 . Deepbluedotsrepresenttheslowestejectaabletoformsecondary craters (∼200m/s (Bierhaus et al., 2012 )); the continuous ejecta blanketwouldoccurwithinthisdeepbluezone,whichalignswell withmappingresultstothenorthofYalode,whereruggedterrain interpretedasthecontinuousejectablanketisobserved.Basedon this simulation, continuous ejecta would be expected to appear roughly within one crater radius around Urvara and Yalode, but depositionofejectawouldoccuratsignificantly greaterdistances (i.e.,one craterdiameterandfarther).Basedonthesesimulations wewouldalsoexpectthatsomelinearfeatures(e.g.,craterchains) associatedwiththeUrvaraandYalode impacteventsmaynot be orientedradialtothesebasinsduetoCeres’rotation.

In addition, we can make comparisons to large impact struc-turesonothersmall,airless,rockybodiesinordertobetter under-stand the potential nature and extent ofeffects from the Urvara andYalodeimpact eventsonCeres’surfacegeology.Thisisuseful forassessingthegeologicrecordsofthesebasinsasboth regional and global stratigraphic markers. Numerous variables come into playduringandafteranimpacteventandthusinfluencethe char-acteristicsoftheresultingcrater;suchvariablesincludetheparent body’sgravityandrotation,thephysicalpropertiesofthebody,the physicalpropertiesofthe impactor,andthe conditionsatimpact (e.g., Melosh, 1996 ).Modelingeachoftheseindividuallyisbeyond thescopeofthismappingeffort.However,itisinstructiveto com-pare quantitative characteristics amongthe largest basinson dif-ferentbodiestoprovidecontextforunderstandingbasinsonCeres (Table 1 ).

Yalode basin, at 260km diameter, measures ∼9% of Ceres’ circumference (assuming a geometric mean radius of 469.7km (Russell et al., 2016 )). In comparison, the lunar Imbrium basin,

withadiameterof1145km(e.g., Wilhelms, 1987 ),is10%ofthe Moon’s circumference (assuming a 3475km lunar diameter). The Apollo16site,1600kmfromthebasinrim,iswidelyconsideredas influencedby Imbriumejecta(e.g., Wilhelms, 1987 ).Basin forma-tionmodelinghasindicatedthat Imbriumejectamayoccur upto distancesofthreebasindiameters(e.g., Housen et al., 1983; Haskin et al., 2003 )andmixtodepthsof100sofmeterseven600–700km from the basin rim (Petro and Pieters, 2008 ). Similarly, the Ori-entalebasin, at930km,is∼9% ofthe Moon’scircumference. The Hevelius Formation,which defines the continuous ejecta blanket of Orientale,extends from300–600kmfrom thering defined by MontesCordillera(Scott et al., 1977 );recentworktyingthe forma-tion of light plains to the Orientale impact suggeststhat the in-fluence ofOrientale actually extends up to four basin radii away (1800km,or17%ofthelunarcircumference)(Meyer et al., 2016 ).

We examined various characteristics of impact basins on the Moon,Mercury,Callisto,Vesta,andCerestobetterassesshowthe inferred extents of ejecta and basin-related features (e.g., crater chainsandcircumferentialfracturesandridges)onCerescompare tootherbodieswithabroadrangeofphysicalproperties(Table 1 ). UrvaraandYalodeareofsimilarsizeinrelationtothesizeoftheir parent body aslarge basinsonthe Moon andMercury.We eval-uated the extent of the influence of a basin on its host plane-tary surface by examining the extent of observed/mapped ejecta andidentificationofthedistances atwhich basin-relatedfeatures arerecognized.Valueswerecomparedamongplanetarybodiesby normalizingtocrater diameter.Rangesinvalues(ifgiven)extend fromlow,asdefinedbytheextentofejecta,tohigh,basedon ex-tentofbasin-relatedfeatures.The rangesforUrvara(2.5–3.4)and Yalode (2.1–2.8) are generally comparable to those of the other planetarybodies shown but mostsimilar to those ofthe Asgard and Valhallabasins on Callisto. These results are also consistent with the simulation resultsshown in Fig. 12 . The simulation

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re-Table1

Characteristicsoflargeimpactbasins. Impact Feature Parent Body Crater Diametera (km) ParentBody Diameterb(km) ParentBody Circumference (km)c CraterDiameter as%ofParent Body Circumference Approximate Maximum Extentof Ejectad(km) Approximate MaximumExtent ofBasin-Related Featurese(km) ExtentofBasin Influence NormalizedTo CraterDiameterf Referencesg (ejectaand basin structures) Imbrium Moon 1145 3475 10,917 10 1266 3435 1.1–3 a Nectaris Moon 860 3475 10,917 8 1360 1540 1.6–1.8 b, c Orientale Moon 930 3475 10,917 9 1680 1800 1.8–1.9 b, d Caloris Mercury 1500 4879 15,328 10 1850 – 1.2 e Rembrandt Mercury 716 4879 15,328 5 – – – Valhalla Callisto 525 4821 15,146 3 1165 1800 2.2–3.4 f Asgard Callisto 340 4821 15,146 2 740 1000 2.2–2.9 Rheasilvia Vesta 450 525 1649 27 550 – 1.2 g Veneneia Vesta 400 525 1649 24 – – g Urvara Ceres 170 940 2953 6 420 585 2.5–3.4 h Yalode Ceres 260 940 2953 9 540 730 2.1–2.8 h

a Sources:Moon-Wilhelms(1987);Callisto-Benderetal.(1997);Mercury,Vesta,andCeres-GazetteerofPlanetaryNomenclature(2016).

bSources:Moon,Mercury,andCallisto-NationalSpaceScienceDataCoordinatedArchive(2016);VestaandCeres-meanequatorialdiameterfromRusselletal.(2012,2016). c ParentBodyCircumference=πxdiameter.

dDistancesfromapproximatebasincenter;measuredongeologicmapswhereavailable;ImbriumextentdefinedbyFraMauroFm;NectarisextentdefinedbyDescartes

Fm;OrientaleextentdefinedbyHeveliusFm;CalorisdefinedbyOdinFm.

eDistances from approximate basin center; measured on geologic mapswhere available; Does notinclude antipodalterrains; Imbriumextent from estimates in

Haskinetal.(2003);NectarisdefinedbyJanssenFm;OrientaleextentfromestimatesinMeyeretal.(2016).

f Ifrangegiven,lowendisfromejectaandhighendfrombasinstructures.

gReferences:a.Housenetal.(1983),Haskinetal.(2003);b.Wilhelmsetal.(1979);Wilhelms(1987);c.WilhelmsandMcCauley(1971)d.Scottetal.(1977);Meyeretal.

(2016);e.GuestandGreeley(1983);f.Benderetal.(1997);g.Yingstetal.(2014);h.thiswork,Buczkowskietal.(2016),(Mestetal.,2017).

sults presentan explanation for whythese basinson Cereshave a relatively greater extent of influence than those ofsome other bodies– thelowergravitationmayallowforgreaterdistance trav-eledduetotherotationoftheparentbodyunderairborneimpact material.ThegreaterrelativeextentforUrvaracomparedtoYalode can be interpreted to be a consequence ofUrvara’syounger age; Urvaraejectaisfresherandthuseasiertounambiguouslyidentify andmap.

Our mapping demonstrates that Urvaraand Yalode ejecta de-positsextendtodistancesofmorethantwocraterdiametersfrom the centersof thesebasins, comparableto theextents associated withbasins onother planets ofsimilar size relative to their par-entbodies.Thecurrentlymappedejectablanketsandassociations interpreted betweendistal structuresandtheseimpactbasinsare thus consistent with what would be expected. Such basins on other bodies are used asstratigraphicmarkers, anchoring epochs andperiods inthegeologictimescale(e.g.,theImbrium and Ori-entale basins serve as markers for the beginning and ending of the Early Imbrian Epoch respectively (Wilhelms, 1987 and refer-ences therein)). We suggest that the large areal extents of Ur-varaandYalode’smappedejectablanketsandtheassociatedbasin structures are important globalstratigraphicmarkers that can be usedforestablishingthegeologictimescaleofCeres.Wealsonote that thesimilarityofejectaextentsforlarge impactstructureson

different Solar System bodies indicates that Ceres behaves in a grosslysimilarphysicalwaytoothersmall,airless,rockybodies. 6. Geologichistory

Our geologicmapping investigation of the Urvara and Yalode QuadranglesofCeresutilizesDawnFC datasetstoidentify,define, and map geologic units based on their morphology, topographic properties,andalbedoandcolorcharacteristics.Theseobservations alsodocumentstratigraphicandcross-cuttingrelationshipsthat al-low usto determine the relative ages of geologicunits and pro-cessesandderivethe geologichistoryofthisregionofCeres. Be-causeof the large size of the Urvara and Yalode basins and the great spatialextent of their associated deposits, the evolution of theregion canbe divided ina straightforward mannerand sum-marized in several distinct steps. Observed contact relationships aredocumentedin Table 2 andasynthesisofthegeologichistory isprovidedin Fig. 13 .

The oldest geologic unit is the cratered terrain, which repre-sentstheancientcrustofCeresthat recordeda multitudeof im-pactevents over much of the dwarf planet’s history. The Yalode basinformedintheseancientcrustalmaterialsandYalode’sejecta wasdepositedontopofthecrateredterrainsurroundingthebasin. Yalode’s ejectaextends to the south into the Zadeni Quadrangle Table2

UrvaraandYalodeQuadrangles:contactrelationshipsbetweenUnits.

Unit Contactsw/ Olderthanunit Overlapsw/unit YoungerThanunit crt Ue,UYs,Ye,c,ccp,cfh,cfs ccp Ue,UYs,Ye,c,cfh,cfs c Ue,Ut,Ufh,UYs,Ye,Yfh,crt,ccp,ct,cfh,cfs Ut,Ufh,UYs,Ye,Yfh,crt,ccp,cfs Ue,UYs,ct Ue,cfh

ccp crt,c,ct,cfh,cfs crt,c,ct c,cfh,cfs

ct c,ccp c,ccp

cfh crt,c,ccp crt,c,ccp cfs crt,ccp crt,ccp

Ue Ut,UYs,Ye,crt,c Ye,crt Ut,UYs,c UYs,c

Ucp Ufh,Ufs,UYs UYs UYs,Ufh,Ufs

Ut Ue,Ufs,UYs,c Ue,UYs Ufs,UYs,c Ufh Ucp,Ufs,UYs,c Ucp UYs Ufs,c Ufs Ut,Ufh,UYs Ut,Ufh,UYs

UYs Ue,Ucp,Ut,Ufh,Ufs,Ye,Yfh,crt,c Ue,Ucp,Ut,Ye,Yfh,crt Ue,Ut,Ucp,Ufh„c Ufs,c Ye Ue,UYs,Yfh,crt,c crt Ue,UYs,Yfh,c

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Fig.13. CorrelationofmapunitsforgeologicunitsintheUrvaraandYalodeQuadranglesofCeresbasedonobservedstratigraphicrelationshipsandgeologiccharacteristics. (Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

and to the north into the Rongo Quadrangle, where secondary craterchainsandfracturesare alsoapparent.Yalode ejectaisnot evidentmuchbeyondthebasin’s poorly-definedeastern rim; pre-sumablythe ejecta was originally less well developed here rela-tivetoother directions(perhapsduetoCeres’rotation) suchthat ithasmergedintothe crateredterrain closertothebasin inthis direction. Yalode’sejecta to the west is not apparent dueto de-posits associated with Urvara. The Urvara impact occurred after the Yalode impact, emplacing ejecta in all directions and cover-ingthe crateredterrain to thenorth, west,andsouthandYalode basin to the east. Secondary crater chains that extend from Ur-vara’srimacrossthewestern margin ofthe Yalodebasin andthe morphologic differences between the basins clearly indicate this sequence of basin formation. Yalode and Urvara both exhibit in-teriorunitsthatoccurredaspartoforlikelycloselyfollowedtheir impactevents, includingthe development ofterraces anda cen-tralpeak inUrvara andhummocky floordeposits inboth basins. The youngest regional unit is the Urvara/Yalode smooth mate-rial,which comprises much of floors ofboth basins butalso ex-tendstothe northofUrvaraandeast ofYalode. Giventhe distri-bution of this unit, its lack of topographic control, and the fact that smooth material is superimposed on remnants of underly-ing basin rimmaterials, this unit is interpreted to be dominated byUrvaraejecta butalsomaycontain Yalode ejectaandperhaps localized ice-rich mass-wasting and/or flow deposits. The Urvara floormaterialsmoothunitcoversalocalizedzoneonthesouthern partof Urvara’sfloor andisamong the youngestmap units. Im-pactcraters andtheir ejecta, rim, andfloor deposits aremapped acrosstheregion.Differencesinpreservationstateandrelativeage areinferred onthebasis ofthedefinitionandcharacter ofejecta margins.Some craters (e.g.,23°S, 279°E in NW Yalode and 54°S, 272°E in SW Yalode) show highly sinuous, distal ejecta margins and the presence of discrete ejecta lobes, as well as prominent talus deposits covering significant portions of their inner rims. These morphologic signatures indicate that the impacts forming thesecraters maybe among the youngestgeologicevents inthe region.

Crater populations in the Urvara and Yalode regions of Ceres havebeenexaminedforthepurposeofderivingageestimatesfor geologicmaterials andevents,including theformation ofthe Ur-vara andYalode basins. The crater database compiled by Adrian Neesemann(May 31, 2016 version)was usedto identify and ex-tract crater diameters forthe crater counting regions of interest. Asmallnumberofadditionalcraters were addedtothedatabase given examination of the LAMO mosaic. Given uncertainties re-gardingthenatureofthesurfacematerialsofCeresthatcould be includedinejectadepositsandtheapparent thinanddiffuse na-tureof ejectadeposits around impactcraters onCeres, itis very difficult to assess whether small craters with less than pristine morphologies are degraded craters superposedon the surfaceof interest orcraters that formed on the surfacebeneathand show through a thinejecta blanket.Similar issuesexist with large de-gradedcratersbuttheirstratigraphicpositioncanbemorereliably inferred. Effectsoflarge craters thatpre-date the unit ofinterest on modelageestimates ofthe superposedunit can be mitigated byexcluding specificcratersfromcountsand/orrestrictingthefit rangeatthe largersizes. In anycase, specificage determinations forgeologicunitsonCeresshouldbetreatedwithcautionasthey depend toa significant degreeon thepopulation ofcraters iden-tified and the fit range selected. Fit ranges are chosen based on visual inspection ofwhichrangeofbins bestmatchthe shapeof theproductionfunctiononadifferentialplot.Asunderstandingof thegeologyofCeresprogresses withcontinuedanalyses ofDawn missiondatasets,ageconstraintswilllikelybeimproved.

Cratercounting areas,craters includedin crater counts, crater size-frequencydistributions (CSFDs),andrelatedstatisticsand re-sultsare shown in Fig. 14 and Table 3 . Thecrater size-frequency distribution measurements are based on the LAMO mosaics and use of the CraterTools add-in to calculate crater diameters and countingareas(Kneissl et al., 2011 ).Craterplotsconform to spec-ifications in Crater Analysis Techniques Working Group (1979) . Derived ages are shown in Table 3 for both the lunar-derived (LDM) and asteroid-derived (ADM) chronology systems (Hiesinger et al., 2016 );thesubsequentdiscussion usesthe

References

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