19 68MNRAS.138. .245A Mon. Not.R.astr.Soc.(1968)138,245-249. A substantialmajorityofresearchers,particularlyintheUnitedStates,has processes (see(1)foranhistoricalreviewofthiscontroversy).Recentevidence few suchcratershavebeenidentifiedwhichare greaterthan19kmindiameter. very littleattention.Adark-haloedcrater,unlikeatypicalrecentcrateronthe the twoprocesses,becausemorphologyofbothvolcanicandimpactcraters produced orextensivelymodifiedbyvolcanism;othersappeartohavebeen more balancedviewthatbothinternalandexternalprocessesareoperating,or provided byRangerandOrbiterspace-craftphotographyhasencouragedthe or comets.Othershavebelievedthatthecraterswereproducedbyinternalvolcanic surrounding surface. crater isrelativelyrareonthevisibledisk.Identificationmadedifficultbecause its surroundings.Thislow-albedomaterialmaybeeitherintheformofaroughly troversy continues. produced byimpact(2)-(4).Yetitisdifficulttodistinguishbetweentheeffectsof have operated,onthelunarsurface.Thus,somecratersappeareithertobeen believed thatmostofthelunarcraterswereproducedbyimpactmeteoroids geologic mapofthemooncompiledin1962by Hackman(5).Hemappedafew circular aureole,orintheformofrays.Whateveritshalo,thissort lunar surface,isgenerallysurroundedbymaterialofaloweralbedothanthat A latermapoftheKepler regionbyHackmanhimself,forexample, didnot haloed craters,andsometimes ignoredtheserelativelyminorfeatures altogether. origin. dark-haloed cratersaretypicallysmall(lessthan 5kmindiameter)althougha can beverysimilar.Thus,providedlargelywithambiguousevidence,thecon- of themostprominentsuchcraters,andlabelled themasprobablyvolcanicin In addition,thehalovariesbothindegreeof darknessandofcontrastwiththe show theprominentdark-haloed crateronawell-definedraywestof Kepler(6). been consistentlypostulated forthem(7). Despite thegenerallack ofattentionpaidtothesecraters,avolcanic originhas 16 © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Introduction. Theoriginoflunarcratershasbeendebatedformanyyears. Amidst thiscontroversy,onetypeofcrater,thatwithdarkhaloes,hasreceived One ofthefirstgeneticreferencestodark-haloed cratersisfoundinthephoto- Subsequent U.S.GeologicalSurveymapshave occasionallyshownthedark- A volcanicoriginhasconsistentlybeenhypothesizedfortherelativelyrare volcanic origin,andsuggestsarelationshipbetweendark-haloedcraters features hasbeenproduced,whichgivessupporttothehypothesisoftheir dark-haloed cratersonthelunarsurface.Amapofmostprominentsuch contemporary transientevents. John W.SalisburyyJoelE.M.AdlerandVernG.Smalley DARK-HALOED CRATERSONTHEMOON (Communicated byJamesRing) (Received 1967October9) Summary 19 68MNRAS.138. .245A Alphonsus. Again,avolcanicoriginwaspostulatedbytheexperimentersforthese 246 JohnW.Salisbury,JoelE.M.AdlerandVernG.SmalleyVol.138 features, primarilybecauseofobviousstructuralcontroltheirlocation(3,pp. result ofthehighresolutionphotographsobtaineddark-haloedcratersin terrestrial maars,orgasexplosioncraters(3,pp.134and275).Craterrimde- to thesharplyconcaveupwardrimsofwhataregenerallyassumedbeimpact vent (3,pp.122and318).Mostauthorsconsidertherimmaterialitselftobe posits ofthesedark-haloedcratersarecharacteristicallyconvexupward,incontrast constructional, ratherthancollapsefeatures.Theyprobablycorrespondto formed bytheimpactoflargeblockscohesivematerialejectedfromacentral pitted withmanysmall,superimposed,rimlesscraters,whichmayhavebeen craters (3,p.318).Therimmaterialsofthedark-haloedinAlphonsusare facing p.306). et al.suggestthatthedark-haloedcratersasaclassareolderthanCopernicus pyroclastic debris(3,p.275),butitispossiblethatcomposedinpartofmaterial maar orgasexplosiontype.Theirrelativeageisdoubtful,buttheyareyounger surface andmapsthemasyoungerthantheCopernicusraymaterial(7,map nicus, ontheotherhand,asbeingamongyoungestfeatureslunar ray system(3,p.228).Shoemakerreferstothedark-haloedcratersaroundCoper- derived fromthegasthatproducedcraterinfirstplace(8). than themaria.Theyareoftenignoredasminorlunarsurfacefeatures. is that,liketheonesinAlphonsus,theyareofvolcanicorigin,probably 122 and271).Inviewoftheirraisedrims,thesefeatureswereconsideredtobe transient phenomena. and tostudytheconnection,ifany,betweenthesefeaturescontemporary of dark-haloedcratersasaclass,todeterminetheagerelationshipsthesefeatures occurrences ofpossibleinternalorigin;i.e.lunarclouds,brightspotsandother in thelastfewyearsonU.S.NavalObservatory61-inchtelescope(Flagstaff, Aeronautical ChartandInformationCentre.Verylong(12-80s)exposure Arizona) inaco-operativeprogrammebetweenthatinstitutionandtheAirForce images areobtainedonextremelyfinegrainedplates (KodakType649-F).Careful than o•5arcsec)ontheplatesisaccompanied by anunusuallycleardelineationof use ofUFGdeveloperhasgenerallyresultedin maximumresolutionandalinear order thatthegeneraldistribution ofcraterscouldbemorecertainly determined. high resolutionphotographiccoverage,suchas thatprovidedbyRangerIX,in No othersourceofinformationwasused;i.e. advantagewasnottakenoflocal albedo differences. density response.Asaresult,theveryhigh spatialresolution(usuallybetter of differentplates.Inevitably, despitetheseefforts,thefinalresult istosome dark-haloed craters,andthecratersweremapped independentlybyeachofus. on thelunarsurfacethan weshowonourmaps.Theimportantpoint inthis extent subjective.Inaddition, thesearemanymoredark-haloed(butfaint) craters Discrepancies inourindividual mapswereresolvedthroughdiscussion andstudy © Royal Astronomical Society • Provided by theNASA Astrophysics Data System The recentRangerIXexperimentexcitednewinterestinthesefeaturesasa There issomedoubtabouttherelativeagesofdark-haloedcraters.Kuiper In summary,currentthoughtconcerningthedark-haloedcratersingeneral The purposeofthispaperistotestthevolcanichypothesisfororigin, Method andresults.ParticularlyfinefullMoonphotographshavebeentaken A selectionofcontactcopiesthesesuperior plates wasusedinmappingthe 19 68MNRAS.138. .245A 0 regard isthat,despitedisagreementonindividualcraters,thepatternofcrater tried toeliminatetheeffectsofvariablephotographicqualitybyusingtwelve affected bythequalityofphotographiccoverageagivenarea,anddegree distribution wasthesameforeachofourmaps.Therealitythispatternis No. 3,1968Dark-haloedcratersontheMoon247 which isusuallyfoundtooptimizethedetectionofdark-haloedcratersinagiven of contrastthedarkhaloeswithbackgroundonwhichtheylie.Wehave attempted toreducetheeffectsofvariablecontrastbyusingprintsdifferent different plates,themostusefulofwhichwere5818and3171-A.Wehavealso haloed cratersareshownonourmapsforanarea,suchascentralSerenitatis, the dark-haloedcratersmoreeasilydiscernable. area. Wehavealsofounditmostusefultoworkwithnegativeprints,whichmake densities madeondifferentcontrastphotographicpaper,onecombinationof because thereare,infact,fewsuchcratersthere.Itdoes,ofcourse,becomemore western OceanusProcellarum,whereexceptionallypoorcontrastaggravatesthe north andsouthlatitudeissurelytoolow.Again,however,webelievethatthe of dark-haloedcratersmappedbeyondabout50eastandwestlongitudeor difficult todetectthesecratersnearthelunarlimb.Consequently,totalnumber were seenonourphotographs. distribution patternofthesecratersisstillvalid,withthepossibleexception section. but anindicationofthegenerallocationsuch craterfieldsisgiveninthenext limb problem. that contrastisenhanced betweendark-haloedcratersandtheir background haloed cratersintheirvicinity. ThemostobviousexampleisTychowhich, ifwe be real,ratherthanthe resultofobservationalbias.Forexample,while itistrue and commonlyclusteredaroundrecentlight-rayed craters,suchasCopernicus several craterdiametersbeyondthehalo.Thelessprominentdark-haloedcraters diameter wide(seeFig.1).Someofthesecratershavedarkraysthatextend central crater,withawell-defined,verydarkhalo,whichisroughlyonecrater and Aristillus,oralong the marginsofmaria.Thisdistribution appearsto crater itselftoosmalltoberesolved.Thesecraters werenotindividuallymapped, surface (seeFig.1).Presumably,theseare also dark-haloedcraters,withthe such darkspotsarefoundtogetherinfields,usually onadarkerthanusualmare small (lessthan1kmindiameter)andpossessnodiscernablecrater.Often,many of verydarkrays(9),butthisisanunusualfeature. and thecraterslessbright. displayed thesamemorphologicalcharacteristics,buthaloeswerelessdark diameter wide—e.g.Picard.Onelargecrater,Dionysius,hasanextensivesystem around alight-rayedcrater, somelight-rayedcratershavenoprominent dark- Fig. 2isobviouslynon-random.Thesefeatures areconcentratedonthemaria, © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Having usedthesetechniques,webelievethatrelativelyfewprominentdark- Distribution ofdark-haloedcraters.Thedistribution ofdark-haloedcratersin (3) Intensedarkspotsoccuronthesephotographs,whicharetypicallyquite (1) Themostprominentdark-haloedcraterstypicallydisplayedabright (2) Thelargerdark-haloedcraterstypicallyhavehaloeslessthanonecrater Characteristics ofdark-haloedcraters.Threedifferenttypesdarkfeatures 19 68MNRAS.138. .245A ^ pearanceofwesternOceanusProcellarumnotedonseveralplates,wehavea judgement uncertain.Wealsohavethevisualimpressionthatthereareagreat were dealingwithobservationalbias,shouldbesurroundeddark-haloed that thereareroughly11darkspots,orfaintdark-haloedcratersoftheuncertain with themaria,supportsvolcanictheoryfortheirorigin.Itisconceivable, Procellarum tothewestofKeplerandAristarchusupintoSinusRoris.Both highlands alongitsentirewesternedge,continuesthemarginofOceanus such asaroundSerenitatisandNectaris.Judgingfromthedark,mottledap- nent dark-haloedcraters—e.g.themostprominentfieldsliebetweenCopernicus 248 JohnW.Salisbury,JoelE.M.AdlerandVernG.SmalleyVol.138 contrast andviewangleareunfavourableintheseareas,however,makingsucha strong impressionthattheprominentseriesoffieldsborderscentral in Fig.i.Generallythelocationofthesefieldsisconsistentwiththatpromi- because therearenoprominentdark-haloedcratersinanarea,oneshouldnot sort, withinonecraterdiameterofTychothatwedidnotmaphere.Thus,simply craters, asisCopernicus.Quitethereversetrue.Itshouldbenoted,however, however, thatwehavemappedthelocationofsmallareasdark,sub-surface craters followsthatoftheprominentdark-haloedillustratedinFig.2. many faintdark-haloedcratersineasternTranquillitatisandnorthernFecunditatis, and thehighlandstoeast.Theyalsotendfavourmarginsmaria, necessarily concludethatinternalprocesseshavebeenentirelyinactivethere. material thatwereexcavatedbyordinarymeteoroidimpact.Becausedark-haloed and large,however,thedistributionofbothdarkspotsfaintdark-haloed despite therelativepaucityofprominentdark-haloedcratersintheseareas.By that itismorelikely,especiallyconsideringthevolcanicnatureofdark-haloed material wouldhavetobesmallandsharplylocalized.Itispossiblethatthe intrusion ofstocksorlaccolithscouldfulfilltheserequirements,butwebelieve craters aretypicallysurroundedbylight-haloedones,theareasofdarksub-surface is probablyduetotheirrangeinage.Mostofthesecratersappearbemore craters inAlphonsus,thatdark-haloedasaclassarevolcanic. with mare-typefloors, whilebeingvirtuallyabsentfromthehighlands. The to beoverlappedbytheraysofCopernicus,and thusolderthanthatfeature,as (actually dark-rayed)craterinthehighlandsnearMaurolycuscutsoneofTycho’s recent thanthelight-rayedcraters.Forexample,moreprominentdark-haloed preferentially aroundthe marginsofthemaria,inornearraycraters, or incraters strong similarityofdistribution ofdark-haloedcratersandtransientevents suggests in transientevents.Middlehurst (10)hasreportedthattransientevents takeplace lunar history. that relativelyyoungfeature.Ontheotherhand, otherdark-haloedcratersappear rays, thusbeingyoungerthanoneofthemost recentlunarcraters.Asshownin Fig. i,thedark-haloedcratersclosesttoCopernicus arealsomorerecentthan changes andotherevidence ofinternallunaractivityhaveledtoincreased interest ducing dark-haloedcratershavenotbeenconfined toasinglebriefepisodein Kuiper etal.suggested(3).Consequently,itwould seemthattheprocessespro- © Royal Astronomical Society • Provided by theNASA Astrophysics Data System Fig. 2doesnotillustratethelocationoffieldsdarkspots,suchasthoseseen The non-randomdistributionofdark-haloedcraters,andtheirassociation Age relationships.Theconfusionwithregardtotheageofdark-haloedcraters Relationship totransientevents.Recently,well-substantiated reportsofcolour 19 68MNRAS.138. .245A © Royal Astronomical Society arrozv. older thanCopernicus.FieldsofdarkspotseastCopernicus areindicatedbythedouble Eratosthenes (arrozv,topleft)appearstobepartlyobscured byaCopernicanray,andthus the CopernicanraysystemandisyoungerthanCopernicus.Dark-haloedcraternorthof this print.ProminentcratersouthofCopernicus(arrow,bottomcentre)clearlyoverlies Fig. i.NegativeprintoftheareaaroundCopernicus.Dark-haloedcratersarewhitein Provided bytheNASA Astrophysics Data System [Facing page248 19 68MNRAS.138. .245A © Royal Astronomical Society •Provided bytheNASA Astrophysics DataSystem Base mapisUSAFLunar ReferencesMosaic,publishedbytheAeronauticalChart and Information Centre,St.Louis, Mo.,istedn,i960. Fig. 2.Mapofmoreprominent(stars)andless (squares)dark-haloedcraters. 19 68MNRAS.138. .245A the dark-haloedcratersmaystillbeatwork. surface, lendingsupporttothetheoryoftheirvolcanicorigin. a geneticrelationshipaswell.Thus,itappearsthattheprocessesproduced No. 3,1968 history. producing dark-haloedcratersarenotconfinedtoasinglebriefepisodeinlunar craters, suchasTychoandCopernicus,indicatingthatthevolcanicprocesses Air ForceAeronauticalChartandInformationCentreinsupplyingthephoto- lunar surface. temporary transienteventssuggeststhattheseprocessesmaystillbeactiveonthe graphy thatmadethisstudypossible. Prisner, U.S.NavalObservatory,RobertMaulfair,Lowellandthe (10) Middlehurst,B.M.,1967.Ananalysesoflunar events,Rev.Geophys.,5,173-89. (4) Fielder,G.&Marcus,A.,1967.Furthertestsforrandomnessoflunarcraters.,Mon. (3) Heacock,R.L.,Kuiper,G.P.,Shoemaker,E.M.,Urey,H.C.&Whitaker,A., (2) Ronca,L.B.,1966.Meteoriticimpactandvolcanism,Icarus,5,515-20. (1) Wright,F.E.,H.&H.,1963.TheLunarSurface:Introduction, (5) Hackman,R.J.,1962.GeneralizedPhotogeologicMapoftheMoon:MilitaryGeology (6) Hackman,R.J.,1962.GeologicMapandSections oftheKeplerRegionMoon: (7) Shoemaker,E.M.,1962.Interpretationoflunar craters:inPhysicsandAstronomy (8) Urey,H.C.,1961.Onpossiblesubstancesfor theC2moleculesobservedin (9) Smalley,V.G.,1965.ThelunarcraterDionysius, Icarus,4,433-5. Lunar-Planetary ResearchBranch, 1967 October. © Royal Astronomical Society • Provided by theNASA Astrophysics Data System (1) Thedark-haloedcratershaveanon-randomdistributiononthelunar Conclusions (2) Dark-haloedcratersarebothyoungerandolderthanthebright-rayed Acknowledgments. ThewritersgratefullyacknowledgetheassistanceofJohn (3) Thesimilargeographicaldistributionofdark-haloedcratersandcon- Air ForceCambridgeResearchLaboratory, Bedford, in TheMoonMeteoritesandComets,SolarSystem,ed.byMiddlehurst Not. R.astr.Soc.,136,1-10. Jet PropulsionLaboratoryTechnicalReportNo.32-800,Pasadena,California. Branch, U.S.GeologicalSurvey,Washington,D.C.,Scale1:3,800,000. Kuiper, Vol.4,pp.1-56,UniversityofChicagoPress. Alphonsus crater,Astrophys.J.,134,268-9. U.S. GeologicalSurveyMap1-355(LAC-57).Scale 1:1,000,000. of theMoon,ed.byZ.Kopal,AcademicPress,New York. 1966. RangerVIIIandIX,PartII.Experimenter’sAnalysesInterpretations: Massachusetts 01730. Dark-haloed cratersontheMoon References 249