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RESEARCH POSTER PRESENTATION DESIGN © 2012
www.PosterPresentations.com
©2013PosterPresentations.com2117FourthStreet,[email protected]
CRACKORIENTATIONSOFBOULDERSONRYUGU:THERMALLYINDUCEDORIGIN?
•画像クレジット※：JAXA, 東京大, 高知大, 立教大, 名古屋大, 千葉工大, 明治大, 会津大, 産総研
2
ABSTRACT
Results
C
A
B
7750mx50m 15pc
�crack_11�?crack_152�nocrack_17
S.Sasaki1,S.Kanda1,H.Kikuchi2,T.Michikami3,T.Morota4,C.Honda5,H.Miyamoto4,R.Henmi4,S.Sugita4,E.Tatsumi4,M.Kanamaru1,N.Sakatani2,S.Watanabe6,N.Namiki7,P.Michel8,M.Hirabayashi9,N.Hirata5,T.Nakamura10,T.
Noguchi11,T.Hiroi12,T.Irie3,K.Matsumoto7,S.Kameda13,T.Kouyama14,H.Suzuki15,M.Yamada16,R.Honda17,Y.Cho4,K.Yoshioka4,M.Hayakawa2,M.Matsuoka2,R.Noguchi2,H.Sawada2,Y.Yokota17 ,andM.Yoshikawa2,
1OsakaUniv.,2ISAS/JAXA,3KindaiUniv.,4Univ.Tokyo,5Univ.Aizu,6NagoyaUniv.,7NAOJ,8NiceObservatory,9AuburnUniv.,10TohokuUniv.,11KyushuUniv.,12BrownUniv.,13RikkyoUniv.,14AIST,15MeijiUniv.,16ChibaInst.Tech.,17KochiUniv.
Imagecreditinthispresentation:JAXA,UniversityofTokyo,KochiUniversity,Rikkyo University,NagoyaUniversity,ChibaInstituteofTechnology,MeijiUniversity,UniversityofAizu,AIST
Imagesusedincrackanalysis(alldatain2018)
Diurnal thermal cycles would grow cracks alongmicrostrucure ofrocks.(Difference ofthermal expansionamong minerals would producethermal stress.)Thermalskin depth (κt)1/2 ~afew- 10cmforRyugu
Delbo,etal2014cmscaledesruptionHazeli,etal2018TFmay explain rounded rocks onItokawa.Molaro,etal2017Larger rocks may bebroken down.-- Microstructureofrocksmaydeterminefracturedirection.MacFadden etal2005Terrestrialrocksondesert.-- Meridionalfracturesarefrequentlyobserved.èEppes etal2015FracturesonMartianrockshavepreferredorientation.
Thermalfatigue
Sizeofrocks20cm– afewmeter- 100m?- MuchlargerthanthescaleofthermalfatigueCrackdirection– closetomeridionalone(N-S)- Effectofannual(+diurnal)thermalvariation?
RockcracksbythermalstressonRyugu’s surface?
1m
Styleofcracksandtheirdirections
• Wefound538crackedboulders(20cm– 20minsize)from104images,andanalyzeddirectionof765cracks.
• IfcracksareformedbyimpactsonRyugu,cracksmaynothaveapreferredorientation.
• Wecarefullylookatdarklinessothatwedonotcountpseudo-crackssuchastheshadowoflayering.
•• Meridionaldirectionsareabundant.• Irrespectiveofcracktypeandrocksize(20cm--20m)
• Sinuoustype• - Effectofpre-existinglayeredand/or
brecciatedstructure.
straight25%
sinuous23%arrested
20%
complex32%
CRACKNUMBERSstraight sinuous arrested complex
hyb2_onc_20190308_021228_tvf_l2c50mx50mRed:Crackconfirmed11Orange:Questionable157White:Nocrack17Imagesizeis1024x1024pixels.Inmostimages,differencebetweenverticaldirectionandN-Sissmallerthan10deg.
Imageresolutionis3mm/pixelatbest.Equatorialregioniscoveredby30cm/pixeldata.Imagesizeis1024x1024pixels.Inmostimages,differencebetweenverticaldirectionandN-Sissmallerthan10deg.
Hayabusa 2spacecraftrevealedthatthesurfaceof162173Ryugu iscoveredwithvarioussizesofboulders.OnRyugu,4400boulderslargerthan5mareobserved;therelativeabundanceoflargeboulders(>20m)isabouttwiceasthatofItokawaorBennu [1,2].ThoselargebouldersonRyugu canbeconsideredasimpactfragmentsfromsizedistributionandtheiraxialratio[2].
Recently,thermalfatiguehasbeenadvocatedforthedisintegrationprocessofsurfacerocks[3],wherediurnal(andannual)thermalcyclemaypromotecrackgrowthintherocksonregolithovervariousspatialandtemporalscales[4].WenoticedthatcracksonRyugu bouldershavepreferredorientation.Cracks/fractureswithmeridional(north-south)directionarefrequentlyobserved[5]inhighresolutionimageswherebouldersizeisbetweenafewtenscmandafewm.SinceRyugu hasalmostretrograderotation(172deg),Ryugu surfacewouldexperiense east-westasymmetryofsolarheating.DesertrocksoftheEarthandMarshavepreferredorientationofcracks[6,7].Thiswouldbeexplainedbythermalprocess,includingfreezing.Hereinthisstudy,weanalyzedmorethan500cracksonRyugu bouldersandcheckedtheirorientations.
Weanalyzed124images(takenfrom50-4000mheightatproximityoperationphasesuchasroverdeploymentandtouch-downsampling,ortheirrehearsals)byHayabusa-2ONC-T.Imageresolutionis3mm/pixelatbest.Imagesizeis1024x1024pixels.Weconfirmtheimagepositionandresolutionfromshapemodelmatching(SPC)and/oraltimetrydatabyLIDAR.Hayabusa 2usuallyobservesthesurfacefromthedirectionofthesun,whichprovidelowphaseangledatawithshortshadowwidth.Wecarefullycheckimagessothatwedonotpickuptheshadowedsurfacestructureasacrack.Somecracksareconfirmedusinganimagewithdifferent(larger)solarphaseangle.
Tocheckifarockhasacrackornot,15-20pixelsarenecessary.Atthehighestresolution,wemaycheckarockassmallas20cm.Assumingthesamerangesize,about2-5%ofbouldershavecracks.Sofar,wedonotobservechangesoftheabundanceratioofcrackedrocksontheRyugu surface.Westernbulgeregion(160E-70W)wouldhavefewerabundanceoftherocksbothwithandwithoutcracks.Weclassifiedcracksintofourstyles:examplesandhistogramsofcrackorientationareshown.(a)Straightcracks:Somecracksarerunninglinearlywithoutbendingorkinking.Somestraightcracksareassociatedwithopenfracture.(b)Sinuouscracks:Somecrackshavebowing,bending,andwavystructure.(c)Arrestedcracks:Weobservedmanyrockshaveacrackwhichdoesnotgothrough.Itlookslikeagrow-ing crackorstopofcrackpropagation.(d)Complex(typicallybranched)cracks:Sometimesaboulderwouldhavebeenbrokenintoseveralpieces.
Weseparatedthestrikeofcracksinto18directionswith10degbin.Weanalyzed538bouldersandfound60%oftheircrackshavethemeridionaldirection(+-15degfromN-S)exceptcomplextype.Thistrendiscommonamongcracktypesaswellasrocksize.
IfbouldercracksonRyugu areformedbyimpactprocesses,whetherimpactsoccurbeforeorafterRyugu formation,thedirectionofcracksshoulddistributemorerandomly. Sofar,solar-inducedthermalstressonasurfaceboulderbydiurnalrotationandannualrevolutionofRyugu mightbeapossibleprocessforthegrowthofbouldercracksinthemeridionaldirection,asdiscussedforthepreferredcrackorientationofdesertrocksoftheEarthandMars[4,6,7].However,weneedtoexplainwhylargeboulders(>10m,largerthanthermallengthscale)havepreferredcrackorientation.PreferredorientationofcracksisalsoobservedonbouldersofBennu [8];theywouldbedrivenbysolar-inducedthermalstress.
References:[1]Sugita,S.etal(2019)Science364.[2]Michikami,T.etal(2019)Icarus,331,179-191[3]Delbo M.etal.(2014)Nature 508,233–236.[4]Molaro,J.L.etal.(2017)Icarus 294,247-261.[5]Sasaki,S.etal.(2019)LPSCL,#1368.[6]MacFadden etal.,(2005)Geol.Soc.Am.Bull.117,161-173.[7]Eppes,M.C.etal.(2015)Nature Comm.,6,6712,[8]Delbo,M.etal(2019)EPSC-DPS-176-2.
