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Morphometry of a glacier-linked esker in NW Tempe Terra, Mars, and implications for sediment-discharge
dynamics of subglacial drainage
Frances Butcher, Matthew Balme, Susan Conway, Colman Gallagher, Neil Arnold, Robert Storrar, Stephen Lewis, Axel Hagermann
To cite this version:
Frances Butcher, Matthew Balme, Susan Conway, Colman Gallagher, Neil Arnold, et al.. Morphom- etry of a glacier-linked esker in NW Tempe Terra, Mars, and implications for sediment-discharge dynamics of subglacial drainage. Earth and Planetary Science Letters, Elsevier, 2020, 542, pp.116325.
�10.1016/j.epsl.2020.116325�. �hal-02614757�
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Earth and Planetary Science Letters
www.elsevier.com/locate/epsl
Morphometry of a glacier-linked esker in NW Tempe Terra, Mars, and implications for sediment-discharge dynamics of subglacial drainage
FrancesE.G. Butchera,b,∗,Matthew R. Balmea, SusanJ. Conwayc, Colman Gallagherd,e, Neil S. Arnoldf,Robert D. Storrarg, Stephen R. Lewisa,Axel Hagermannh
aSchoolofPhysicalSciences,TheOpenUniversity,WaltonHall,MiltonKeynes,MK76AA,UK bDepartmentofGeography,TheUniversityofSheffield,Sheffield,S102TN,UK
cCNRS,UMR6112LaboratoiredePlanétologieetGéodynamique,UniversitédeNantes,France dUCDSchoolofGeography,UniversityCollegeDublin,Dublin4,Ireland
eUCDEarthInstitute,UniversityCollegeDublin,Dublin4,Ireland
fScottPolarResearchInstitute,UniversityofCambridge,Cambridge,CB21ER,UK
gDepartmentoftheNaturalandBuiltEnvironment,FacultyofSocialSciencesandHumanities,SheffieldHallamUniversity,Sheffield,S11WB,UK hBiologicalandEnvironmentalSciences,UniversityofStirling,Stirling,FK94LA,UK
a r t i c l e i n f o a b s t ra c t
Articlehistory:
Received23September2019 Receivedinrevisedform5March2020 Accepted4May2020
Availableonlinexxxx Editor:W.B.McKinnon
Keywords:
Marsgeomorphology glaciersonMars wet-basedglaciation esker
glacialhydrology
We present asystematic, metre-scale characterisation ofthe 3D morphometry ofan esker on Mars, and the first attempt to reconstruct the multi-stage dynamics of esker formation on Mars. Eskers are sinuousridgescomprisingsedimentdepositedbymeltwater drainingthroughice-confinedtunnels within orbeneath glaciers. Detailed morphometricinsights intoeskers onMarsare important for (i) informingmorphometrictestsofwhethersinuousridgeselsewhereonMarsareeskers,and(ii)informing modellingexperimentswhichaimtoreconstructtheglaciologicalandenvironmentalcontrolsonesker formation onMars. WeuseadigitalelevationmodelgeneratedfromHighResolutionImaging Science Experiment(HiRISE)imagestocharacterisetheheightandwidthofanextremelyrareeskerassociated with a late-Amazonian-aged viscous flow feature (debris-covered glacier) in NW Tempe Terra, Mars.
Our measurements suggest that the NW Tempe Terra esker is a ‘stacked’ formation comprising an underlying‘lower member’ridge that issuperposed byanarrower‘uppermember’ridge. Weused a novelmorphometricapproachtotestwhethertheapparentstackingrecordstwodistincteskerdeposition regimes (either withinthe samedrainageepisode, orwithin temporally-separateddrainageepisodes).
Thisapproachpositsthateskercrestmorphologyiscontrolledbyprimaryeskerformationprocessesand, byextension,thatportionsofeskerswithsimilarcrestmorphologiesshouldhavesimilarmorphometric relationships. We predicted the morphometric relationships described by the constituent upper and lower member ridgesbased on‘referencerelationships’ observedfor morphologically-similarportions of the esker where no evidence ofstacking was observed. Ourobservations corresponded well with the predicted relationships, supporting our stacked esker hypothesis. We proposeconceptual models, whichinvokespatialandtemporalvariationsinsedimentsupplyandmeltwaterdischarge,toexplainthe stacked morphology.Thesemodels are informedbymorpho-sedimentary relationshipsobservedalong eskersonEarth.
©2020TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).
1. Introduction
Sinuous ridge landforms in the Phlegra Montes (162.96◦E, 32.68◦N; Gallagher and Balme, 2015) and NW Tempe Terra (83.06◦W,46.17◦N; Butcher et al., 2017) regions of Mars’ north-
* Correspondingauthor.
E-mailaddress:f.butcher@sheffield.ac.uk(F.E.G. Butcher).
ern mid-latitudeshave beeninterpreted aseskersbased ontheir morphologiesandassociationswithglaciallandsystems,including extant debris-covered glaciers (viscous flow features, VFF; Gal- lagher and Balme, 2015; Butcher et al., 2017). Eskers are ridges comprisingglaciofluvialsedimentdepositedinmeltwaterdrainage conduits within or beneath glacial ice. The mid-latitude glacier- linkedeskersonMarsindicatethattheirparentglaciersunderwent rare,localisedbasalmelting∼110–150Ma,despitetheextremely cold,aridconditionsofthemostrecent(lateAmazonian)periodof https://doi.org/10.1016/j.epsl.2020.116325
0012-821X/©2020TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).
2 F.E.G. Butcher et al. / Earth and Planetary Science Letters 542 (2020) 116325
Fig. 1. Theglacier-linkedeskerinNWtempeTerraesker,Mars.(A)ContextCameraimagemosaicoftheglacier-linkedeskerinNWTempeTerra.Whitearrowsmarkthe southernandnorthernmostvisibleextentoftheesker,blackarrowsmarkheadsoftwopossibletributaryeskers,thewhitedottedlinedelineatespresentVFFterminus,black dottedlinesdelineatemorphologicalsubzonesI–IVdefinedbyButcheret al.(2017),andwhiteboxesshowextentsofpanelsC–F.Extentshownbyblackboxinpanel(B),a colourisedMarsOrbiterLaserAltimeter(MOLA)elevationmapoverlainonThermalEmissionImagingSystemDaytimeIRimagemosaicshowingthelocationofthetectonic rifthostingtheglacier-linkedesker.ExtentshownoninsetMOLAelevationglobe.PanelsC–Fshowportionsof(C)subzoneI,(D)subzoneII,(E)subzoneIII,and(F)subzone IVoftheesker.PanelsC,DandFareorthorectifiedHiRISEimages,andpanelCisaCTXimage.SeeAppendixAforlistofdataproducts.
Mars’geologichistory(GallagherandBalme,2015;Butcheret al., 2017). Both eskers arelocated within tectonicrift/graben valleys.
These geologiccontexts, combinedwith the results of modelling experiments, suggest that locally-elevated geothermal heat flux wasa pre-requisiteforgeologically-recentbasalmeltingofglacial ice on Mars (Gallagher and Balme, 2015; Butcher et al., 2017;
Arnoldet al.,2019;SoriandBramson,2019).
In thisstudy, we characterise the metre-scale 3D morphome- try ofthe glacier-linked esker identified by Butcheret al. (2017) inNWTempeTerra,Mars(83.06◦W,46.17◦N;Fig.1).Ourprincipal aim isto document its morphometry andprovide a comparative dataset for future studies aiming to test whether sinuous ridges elsewhereonMarsareeskers.Ourrawmeasurements(Sections4.1 and4.2)highlightcomplexityinthemorphologyoftheNWTempe Terraesker,andsuggestthattheridgeisinfactacompositeesker formation comprising two ‘stacked’ ridge members (Section 4.3).
We apply a novel morphometric approach to our raw measure- ments to explore this hypothesis (Section 4.4), and present the morphometriesoftheconstituentridgemembersthat weidentify
(Section4.5).BasedoninsightsfromeskersonEarth(Section5.1;
e.g., Banerjee andMcDonald,1975; Brennand,1994; Burkeet al., 2008, 2010, 2012, 2015; Perkins et al., 2016), we then develop conceptual models forthe possible sediment-discharge dynamics ofmeltwater drainageepisode(s)that couldexplain the observed morphometriesandeskerstacking(Section5.2).
2. TheNWTempeTerraesker
Theglacier-linkedeskerinNWTempeTerra(Fig.1A–B),andits associatedlandsystemandgeologiccontext,wasmapped andde- scribedindetailbyButcheret al.(2017).Tosummarise,theesker emerges from within the tongue ofits parent glacier(lobate de- bris apron-type;e.g.,Squyres, 1979),andextends northwardinto the glacierforeland.It comprisesa singletrunk ridgesystemand two short (∼400m long) tributary ridgesconnectingto its east- ern flank. Butcheret al.(2017) defined fourmajor morphological subzonesalong theesker.SubzoneI(∼0–5km;Fig.1C)isa low- relief,texturedbandcomprisingridgesandtroughswithinthesur-
faceoftheremnantparentglacier.SubzoneII(∼5–8km;Fig.1D) comprisesa∼150mwide,sharp- tomulti-crestedcentralpromi- nenceatop abroader500–1500 mwide ridge,the gently-sloping lower flanks of which are mantled by remnant glacier materi- als of unknown thickness. Subzone III (∼8–11 km; Fig. 1E) is a round-crestedridgeportionthatisupto1300mwideandextends northwardintotheglacierforelandfromtheglacierterminus.The ridgenarrowsgraduallytowardsthesubzoneIII–IVtransition.Sub- zoneIV(∼11–17km;Fig.1F)hasanarrower(150–200m wide), sharp-crestedmorphology that extendsa further∼4kmintothe glacierforeland.Thenorthernmost∼2kmofsubzoneIVcomprises fragmentedanddegradedridgesegments.
3. Dataandmethods
3.1.Basemapdata
We extracted measurements of the esker from the 1 m/pixel digital elevation model (DEM; vertical precision ±0.492 m) and 50 cm/pixel orthorectified images generated by Butcher et al.
(2017) fromHighResolutionImagingScienceExperiment(HiRISE;
McEwenet al., 2007)images ESP_049573_2265andESP_049639_
2265.Giventheresolutionoftheinputimages,weconsidertheef- fectivehorizontalresolutionoftheDEM tobe 2m.We imported thesedataintoESRIArcGIS10.1softwareinasinusoidalprojection centred on 276.5◦E. (the centre of the valley hosting the esker).
We then derived slope, shaded relief, and aspect map products fromthe DEM to aid visual inspection and interpretation of the 3Dsurface. Alist ofdataproducts usedinthisstudy is provided inAppendixA.
3.2.Measurementof3Dmorphometries
Following the methods of Storrar et al. (2014a) and Butcher et al. (2016), we digitisedthe crests of the individual, unbroken ridge segments that form the esker system onto the integrated basemap.Forthepurposesofthisstudy,wedigitisedonlythema- jortrunk systemandexcluded two∼400mlongtributaryridges identifiedbyButcheret al.(2017).
To measure the height and width of the esker, we manually digitisedridge-transverse transects exceeding the ridge width, at 20m intervalsalong theeskercrestline.Weconvertedeachtran- sectto1mspacedpointsandviewedthemintopographicprofile (withtentimesverticalexaggeration)andinplanformagainstthe basemap.Weclassifiedtheleftandrightridgebasepointsforeach transectasthemajorbreaksinslopeeithersideoftheridge,and thecrestasthepointofmaximumelevationbetweenthesepoints.
FollowingButcheret al.(2016),wethencalculatedridgeheight H foreach cross-sectionaltransect asthe difference inelevation betweentheridgecrestpointandthebed.Wecalculatedthebed elevation by averaging the elevations of the left and right base points.Wecalculatedridgewidthforeachcross-sectionaltransect asthehorizontaldistancebetweentherightandleftbasepoints.
Post-processing of our raw 3D measurements is described in Section4.4.Thecrest pointsandassociatedrawheightandwidth measurements, in addition to processed height and width data generatedduring the analyses presentedin Sections 4.4and 4.5, areincludedinsupplementary dataset1.Weperformedstatistical analysesusingRsoftware(RDevelopmentCoreTeam,2008).
3.3.Sampling
We excluded cross sectional transectsfromthe sample if:(1) thebase ofthe ridge was obscured by proximal tributary ridges, (2)theyhadbeenvisiblymodifiedbysecondaryprocessessuchas impactcratering orsuperpositionby moraineridges(see Butcher
et al., 2017), or(3) they were within 40 m ofthe tapering ends oftheridgesegment.WeexcludedsubzoneI(Butcheret al.,2017) from ourmorphometric analyses owing to its near-completeob- scurationbysuperposingVFFmaterials.Ourrawdatasetcomprised measurementsfor457cross-sectionaltransects(seeAppendixB).
3.4. Measurementuncertainties
Appendix C describes calculations of measurement uncer- tainties. These uncertainties consider the vertical precision of (±0.5 m), and noise within (±1.3 m), the DEM. We found un- certaintiesinducedbymapprojectiondistortiontobeinsignificant (0.001m). Theuncertaintiesinrawheightandwidthmeasure- mentsare±2mand±3m,respectively.
TheridgeflanksinsubzoneIIarepartiallymantledbyremnant VFF materials. This probably resultsin underestimations ofridge dimensions, possibly on the order oftens of metres. The lack of knownbedtopographypreventsquantificationofthemagnitudeof thisuncertainty,butasignificantbreakinslopevisibleattheridge basepointsinsubzoneIIsuggeststhatthesuperposingmaterialis relatively thin; thick ice orice-rich debris would be expected to haveequilibratedthisbreakinslopeovertime.
4. Resultsandanalysis
4.1. Rawheightandwidthmeasurements
The distributions of the rawheight andwidth measurements for the NW Tempe Terra esker are shown in Fig.2A–B. Descrip- tive statistics forthese measurements are displayed in Table D.1 (AppendixD).
Whenplottedtogether(Fig.2C),therawheightandwidthmea- surements are positively correlated but exhibit complex trends.
Subzone IV measurements follow a simple linear trend, while the subzone II andIII measurements exhibit multiple non-linear trends. In the analyses that follow (Sections 4.3 and 4.4), we demonstratethatthiscomplexityisprobablyan artefactofstack- ing of two morphologically-distinct esker ridges under a multi- stage deposition regime (Section 5). Hence, the height-width trendsillustratedinFig.2andFig.3shouldnot beinterpretedas scalingrelationshipsarisingfromasingle,representativeregimeof meltwaterdrainageandeskerdeposition.Instead,simplerheight- widthtrendsdescribedbytheindividualstackedcomponents(ex- tracted in Section 4.4.3), are more likely to reflect their primary depositionregimes.
To simplify our analyses, we exclude transects covering the transitions between morphological subzones (n = 43; crosses in Fig.2C,seealsoAppendixB), sincetheir morphometriesmaynot berepresentativeofindividualmorphologicalsubzones.
4.2. Definitionofmorphometricdomainsandtheirmorphologies Ourraw measurements ofthe NW TempeTerra esker (Fig. 3) illustratemorphometriccomplexitybelowthescaleofmorphologic subzones I–IVidentified by Butcheret al.(2017). We identifysix groupingsinthescatterplotofrawheightandwidth(Fig.3),which wetermdomains(codedA–F).AsillustratedinFig.4,eachdomain isrelatedtoasetofspatially-associatedtransectsalongtheesker.
Fivedomains(A–E)correspondtotransectswithinsubzonesIIand III.DomainFcontainsalltransectswithinsubzoneIV.
Domains A and E have similar height-width trends, while in domainsCandD,theridgeissignificantlytallerforagivenwidth.
Domain Bexhibitsnocorrelation betweenrawheight andwidth;
transectsinthisdomainclusterinarelativelynarrowwidthrange (697–825m),buttheirheights(intherange17–37m) bridgethe gap between transectswith similar widths in domains A and E, andC–D.
4 F.E.G. Butcher et al. / Earth and Planetary Science Letters 542 (2020) 116325
Fig. 2. DistributionsofrawheightandwidthmeasurementsalongtheNWTempe Terraesker.Panels A–Bareboxplots,inwhichboxesrepresenttheinterquartile range,dashedwhiskersrepresentmaximumandminimum,solidbarsrepresentthe medians,andpointsrepresentthemeansof:(A)rawridgeheight,and(B)rawridge width.Panel(C)isascatterplotoftherawheightandwidthmeasurements.Crosses representtransectsthatoccurattransitionsbetweenmorphologicalsubzonesand areexcludedfromsubsequentanalyses.Errorbarsrepresentuncertaintiescalculated inAppendixC.Inallpanels,coloursrepresentsubzonesdefinedbyButcheret al.
(2017).
4.2.1. DomainsA–B
Domains A–B (Fig. 3) are located in subzone II of the esker (Fig.4),whichButcheret al.(2017) describedas‘multi-crested’.In domainA,theridgehasabroad,roundedcentralcrest,andasec- ond,parallelcrestpartwaydownitsgently-sloping westernflank (Fig.4B andFig. 5B–C).The crestsareseparatedbya 100–200m wide, 2–5 m deep,flat-bottomed trough (Fig. 5B)which is itself partiallyinfilledbyasmallridge(Fig.5C)towardsthetransitionto domainB.
Thebroad,gently-slopingflanksofdomainAcontinueintodo- mainB(Fig.4andFig.5D–E).However,theridgecrest indomain Binsteadhasanarrow,moresteep-sidedcentralprominenceatop the broader ridge (Fig. 5D). The central prominence has a sharp crestwhichtransitionstotwosharp,parallelcrestswhicharesep- aratedbya50–60mwide,∼5mdeep,V-shapedtrough.
ThecentralprominenceindomainBappearsto becontinuous withthesmallridge that partiallyinfillsthe northernendofthe inter-crest trough in domain A. The sharp to multi-crested cen-
Fig. 3. DefinitionofmorphometricdomainsalongtheNWTempeTerraesker.Scat- terplotsofrawheightandwidthmeasurementssampledforfurtheranalysisshow- ing: (A)domainsofspatially-associated transects(outlinedbydottedloops);(B) transectswherethereisnovisiblecentralprominence,whichfollowtwodominant, approximatelylineartrends;and(C)transectswithavisiblecentralprominence.In allpanels,errorbarsrepresentuncertaintiescalculatedinAppendixC,andcolours representsubzonesdefinedbyButcheret al.(2017).
tralprominence thatcharacterisesdomainBcontinuesnorthward tothesouthernmarginofdomainC.However,asamplinggapbe- tweendomainsBandC(necessitatedbyproximityofthisportion of theridge to a possibletributary ridge, seeSection 3.3) means that we do notspeculate on themorphometry ofthisridge por- tion,despiteitsmorphologicalsimilaritytodomainB.
4.2.2. DomainsC–F
Domain C (Fig. 4) corresponds to a shortportion ofthe ridge extending north from the subzone II–III transition identified by Butcheret al.(2017).ItismorphologicallysimilartodomainB(i.e., comprising broad lower flanks and a steep-sided central promi- nence),exceptthatthecentralprominenceisentirelycharacterised by a single crest (Fig. 5F). The sharp-crestedcentral prominence gradually becomeslower,wider,andmoreround-crested(Fig.5G) into domainD, transitioning toa spatulateformthat grades into the surface of the broaderridge at the northernmostend ofdo- mainD(Fig.4).
DomainEhasarelativelysimple,wide,round-crestedmorphol- ogy(Fig.4;Fig.5H),andlacksacentralprominence.Itnarrowsand lowers towards its northern end, tapering towards the transition
Fig. 4. MorphometricdomainsoftheNWTempeTerraeskerincontext.(A)CTXimagemosaicshowingextentsofmorphometricdomains(pointsjoinedbycoloureddotted lines),relativetomorphologicalsubzonesdefinedbyButcheret al.(2017) and theupperandlowermemberridges(whitearrowedbrackets).Theblackboxshowstheextent of(B),anorthorectifiedHiRISEimageofdomainsA–D,andthesouthernportionofdomainE.WhitearrowsindicatethelocationwherethespatulateportionoftheUmb becomestopographicallyindistinct(i.e.theUmbterminus).SeeAppendixAfordataproducts.ColoursinallpanelsrepresentsubzonesdefinedbyButcheret al.(2017).
to domain F. Domain F has a narrow, sharp-crested morphology (Fig.4;Fig.5I).LikedomainE,domainFexhibitsnodistinctcen- tralprominence.Visually,thedomainFridgeissimilarinwidthto boththecentral prominenceindomains BandC(Fig. 5DandF), andtotheinter-cresttroughindomainB(Fig.5B).
4.3.Thestackedeskerhypothesis
Theobservations described above lead ustopropose that do- mains A–D of the NW Tempe Terra esker form a ‘stacked’ es- kerformationcomprisingtwomorphologically-distincteskerridges (whichwetermmembers)whichformedunderamulti-stageesker depositionregime.Wehypothesisethattherawheightsmeasured fordomainsB–D(Fig.3)representthecompositeheightsofthese stacked ridgemembers: (1) abroad underlyingridge in domains B–DwhichiscontinuouswithdomainsEandF, andwetermthe lowermember(Lmb)ridge;and(2)anarrowersecondaryridge(the uppermember,Umb)thatwasdepositedatop theLmbindomains B–D. Following this hypothesis, we propose that domain A was originallyround-crested,andthat theinter-cresttroughpart way
down its gently-sloping western flank was eroded by meltwater supplyingtheUmbindomainsB–D.
4.4. Testsofthestackedeskerhypothesis 4.4.1. Approach
To explore the stacked esker hypothesis, we first made pre- dictionsabouttheheight-widthtrends thatwouldbe observedif thesignals oftheUmb andLmbridgeswere separated. TheLmb comprises two domains where there is noevidence for a super- posed ridge (domains E and F); domain E has a simple, broad, round-crestedmorphology,while domainFhas asimple,narrow, sharp-crested morphology.We used the morphometries of these domains as‘reference morphometries’, uponwhich webased our predictions ofthe height-widthtrends ofmorphologically-similar portionsoftheconstituentUmbandLmbridgesinthestackeddo- mains (Fig. 6A).Observations ofeskerson Earthsuggest thatthe crest morphologies and geometries of eskerscould be related to thedynamicsoftheirformation(e.g.,Burkeet al.,2015).Ourpre- dictionsassumethateskermorphometryisprocess-controlled,and thereforethat portions of martianeskerswithsimilar crest mor-