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Ichnofossils, Cracks or Crystals? A Test for Biogenicity of Stick-Like Structures from Vera Rubin Ridge, Mars

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New images from Mars rover Curiosity display millimetric, elongate stick- like structures in the fluvio-lacustrine deposits of Vera Rubin Ridge, the depositional environment of which has been previously acknowledged as habitable. Morphology, size and topology of the structures are yet incompletely known and their biogenicity remains untested. Here we provide the first quantitative description of the Vera Rubin Ridge structures, showing that ichnofossils, i.e., the product of life-substrate interactions, are among their closest morphological analogues. Crystal growth and sedimentary cracking are plausible non-biological genetic processes for the structures, although crystals, desiccation and syneresis cracks do not typically present all the morphological and topological features of the Vera Rubin Ridge structures. Morphological analogy does not necessarily imply biogenicity but, given that none of the available observations falsifies the ichnofossil hypothesis, Vera Rubin Ridge and its sedimentary features are here recognized as a privileged target for astrobiological research.
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Geosciences2020,10,39;doi:10.3390/geosciences10020039www.mdpi.com/journal/geosciences
Article
Ichnofossils,CracksorCrystals?ATest
forBiogenicityofStickLikeStructuresfromVera
RubinRidge,Mars
AndreaBaucon
1,
*,CarlosNetoDeCarvalho
2,3
,FabrizioFelletti
4
andRobertoCabella
5
1
DISTAV,UniversityofGenova,CorsoEuropa52,16132Genova,Italy
2
GeologyOffice,NaturtejoUNESCOGlobalGeopark,AvenidaZonaNovadeExpansão,
6060101IdanhaaNova,Portugal;carlos.praedichnia@gmail.com
3
InstitutoD.Luiz,UniversityofLisbon.FaculdadedeCiênciasdaUniversidadedeLisboa,
CampoGrandeEdifícioC1,Piso1,1749016Lisbon,Portugal
4
DipartimentodiScienzedellaTerra‘A.Desio’,UniversitàdegliStudidiMilano,ViaMangiagalli34,
20133Milano,Italy;fabrizio.felletti@unimi.it
5
DISTAV,UniversityofGenova,CorsoEuropa52,16132Genova,Italy;cabella@dipteris.unige.it
*Correspondence:andrea@tracemaker.com
Received:8October2019;Accepted:27December2019;Published:21January2020
Abstract:NewimagesfromMarsroverCuriositydisplaymillimetric,elongatestick‐likestructures
inthefluviolacustrinedepositsofVeraRubinRidge,thedepositionalenvironmentofwhichhas
beenpreviouslyacknowledgedashabitable.Morphology,sizeandtopologyofthestructuresare
yetincompletelyknownandtheirbiogenicityremainsuntested.Hereweprovidethefirst
quantitativedescriptionoftheVeraRubinRidgestructures,showingthatichnofossils,i.e.,the
productoflifesubstrateinteractions,areamongtheirclosestmorphologicalanalogues.Crystal
growthandsedimentarycrackingareplausiblenonbiologicalgeneticprocessesforthestructures,
althoughcrystals,desiccationandsyneresiscracksdonottypicallypresentallthemorphological
andtopologicalfeaturesoftheVeraRubinRidgestructures.Morphologicalanalogydoesnot
necessarilyimplybiogenicitybut,giventhatnoneoftheavailableobservationsfalsifiesthe
ichnofossilhypothesis,VeraRubinRidgeanditssedimentaryfeaturesarehererecognizedasa
privilegedtargetforastrobiologicalresearch.
Keywords:palaeontology;ichnology;ichnofossils;pseudofossils;astrobiology;biosignatures;Mars;
VeraRubinRidge;GaleCrater;Haroldswick
1.Introduction
NewobservationsatVeraRubinRidgebytheMarsSpaceLaboratoryRoverCuriosityshow
millimetric,elongatestructurespreservedinsedimentaryrocksdepositedinfluviolacustrine
environmentswithinGaleCrater.OnEarth,suchsettingsareinhabitedbyanenormousdiversityof
macro‐andmicroorganismsproducingtracesoflifesubstrateinteractions[1,2]thataresimilartothe
structuresofVeraRubinRidge.
Eversincetheywereannounced,thestructuresfromVeraRubinRidgehavebeencontroversial.
Inablogpost,NASAscientistslabelledthestructuresasenigmaticsticklikefeatures,proposingan
abioticoriginaserosionresistantmineralveins[3].Bycontrast,DiGregorio[4]suggestedapotential
biogenicnatureasthefossilizedproductsoflifesubstrateinteractions(ichnofossils).Thisbiogenic
hypothesiswasinformallydisputedinasetofblogpostsincludinginterviewsofEarthandplanetary
scientists[5–7].Despiteofthisdebate,morphology,sizeandtopologyofthesticklikestructuresare
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yetincompletelyknown,andtheirpotentialbiogenicityremainsuntested.Todate,thereisnopeer
reviewedpaperdiscussingtheabiotichypothesisforthesticklikestructures.Inthispaperweaimto
fillthesegapsbytestingtheirbiogenicity.Accordingly,thenullhypothesisisthattheobserved
featuresareabioticinorigin;thealternativehypothesisisthattheiroriginisbiological.
AllthestructuresthatwedescribeherecomefromtherocktargetHaroldswick,whichislocated
onthenorthernfoothillofAeolisMons(Mt.Sharp)atVeraRubinRidge,withinGaleCrater.Vera
RubinRidgeisanelevatedcrest,250mwide,onAeolisMons[8,9].Itischaracterizedbyahematite
signatureinspectra,forwhichreasonitisalsoknownasHematiteRidge[9,10].TheVeraRubinRidge
consistsofMurrayformationdepositsthatexperiencedenhancedcementationresultinginits
preservationasaridge[11].Understandingtheoriginoftheridgehasbeenakeyobjectiveforthe
MarsScienceLaboratorymissionsincebeforelanding[12].VeraRubinRidgehasbeenrecognizedas
anironbearingenvironmentappropriateforbiosignaturepreservation[13].
ThesticklikestructureshavebeenimagedbytheMarsScienceLaboratoryroverCuriosity
duringMartiandays(Sols)1905,1921and1923.Theirhostrockpertainstothefluviolacustrine
MurrayFormation,whichisevidenceofthepresenceofancientlakesintheGalecraterandindicates
theexistenceofhabitableenvironmentsbetween~3.8billionand3.1billionyearsago[14–18](fora
geologicaloverview,seealso[16,17,19,20]).
2.MaterialsandMethods
SourceimageshavebeenacquiredbyroverCuriosityusing(1)MastcamduringMartiandays
(Sols)1905and1921,(2)ChemCamonSol1921,(3)MarsHandLensImager(MAHLI)onSols1922
and1923.ImageswithsticklikestructureshavebeenanalysedwiththeimageanalysissoftwareFiji
1.51w(http://fiji.sc),animageprocessingpackagebasedonImageJ.Resultsofwidth,lengthand
angleanalysisarelistedintheAppendixA,FiguresA1–A3andTablesA1–A3.Sizeofthestructures
havebeenderivedfromsourceimagesusingtheMAHLIequations[21].
3.Results
11sticklikestructuresarepreservedaserosionenhancedreliefswithinasiltstoneoutcrop
(Figure1:specimenss1tos11).Theyconsistofhorizontal,straightsegmentsthatarefrequently
connectedbycurvedturns(Figure2a).Structuresareunlined.Contactwiththehostrockissharp,
buttherearenomanifestgrainsizedifferencesbetweenthesticklikestructuresandthehostrock.
OurinvestigationsusingimageanalysisandMAHLIequations[21]showanaveragewidthof0.7
mm.Theendsoffewstructures(s1,s11)appeartaperedbutitisunclearwhethertaperingresults
fromprimary(i.e.,structureproducing)orsecondary(e.g.,weathering,fragmentation)processes;as
such,endsofthesestructureshavebeenexcludedfromimageanalysis.Accordingly,eachstructure
retainsaconstantwidthalongitslength(averageperspecimenvariationinwidth:0.1mm).Widths
tendtobeconstantalsobetweenstructures(widthrange:0.5–0.9mm).Resultsofimageanalysisshow
anaveragelengthof4mm,butthisvalueunderestimatestheoriginallengthbecausesticklike
structuresshowclearevidenceoffragmentation(Figure2a).Someofthestructuresintersect
(specimenss4–s5;s8–s9;s10–s11;Figure2a–c)orcoalescewithoutintersecting(structuress1–s5;
Figure2a).Averageintersectionangleis63.1°.SeeappendixAforadditionalinformationabout
morphometricanalysis(FiguresA1–A3,TablesA1–A3).
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Figure1.ThesticklikestructuresatVeraRubinRidge,Mars.Labelsindicateindividualspecimensof
sticklikestructures(s1s11)andbulbousstructures(b).OutcropimagedbyroverCuriosityusing
MAHLI(image1922MH0001520010703174C00_DXXX).
Figure2.SticklikestructuresfromMars.(a)Sticklikestructures4intersectingtheSshapedforms5,
whichchangesitscurvatureatshortdistancefromspecimens1.(b)Tjunctionformedbyspecimens
s8ands9.(c)Intersectionbetweenspecimenss10ands11.SeeFigure1forspecimennumbers.
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4.Discussion
Inordertotestthenullhypothesis(abioticorigin),thesticklikestructuresarecomparedwith
similarnonbiologicalstructuresfoundonEarthandknowntooccuronMars,accordingtotheir(a)
morphology,(b)size,(c)geologicalcontextand(d)topologicalinterrelationships.
Themorphologicalfeaturesofthesticklikestructuresaresharedwithcrackfills.Desiccation
cracksareacommoncomponentofcontinentaldepositsonEarth(Figure3a),whereastheyhavebeen
reportedonMarsfromtheMurrayFormation[14]andMeridianiPlanum[22].Martiandesiccation
cracksmeetorthogonally,formingTjunctions[14];thiscomparesfavourablywiththeTjunction
observedinapairofsticklikestructures(s8,s9;Figure2b).Subaqueoussedimentarycracks,also
knownassynaeresiscracksorintrastratalshrinkagecracks[23,24],havebeendocumentedonMars
intheSheepbedmudstone[25].SimilarlytotheVeraRubinRidgestructures,subaqueous
sedimentarycrackscanformincompletepolygonsconsistingofcurved(curlicuelike;Figure3b),at
timesbranchedelements;unlikethesticklikestructures,theyareusuallyspindleshapedandcan
presenttaperedtips[23](Figure3b–d).OnEarthtaperingisanimportantfeatureofichnofossillike
structuresbecauseitarguesagainstanichnologicalorigin[26].MostoftheVeraRubinRidge
structuresarenottapered;theonlyexceptionisrepresentedbyspecimenss1ands11,butitisunclear
whethertheirnarrowtipsresultedfromprimary(i.e.,structureproducing)orsecondary(e.g.,
weathering,fragmentation)processes.
Thecommonlysteepflanksofthesticklikestructures(Figure2a–c)areconsistentwithcrack
fills(Figure3b–d).Becauseofmorphologicalsimilaritiesandgeologicalcontext,sedimentarycracks
areaplausibleanalogueforthesticklikestructures.OnEarth,formationofcracksiscommonly
mediatedbymicrobialmats,whereasonMarscracksareassociatedtostructuresresembling
microbialites[27].Itshouldbehowevernotedthatabioticprocessescanexplaintheproductionof
cracks[23,24].
Nonsedimentarycrackingisanadditionalcandidateprocesstoexplainthesticklikestructures
becauseoftheabundantfracturesobservedonMars,wheresulfatemineralizedfracturesattributed
tohydraulicfracturingarefoundintheMurray[14]andYellowknifeBayformations[16].The
immediatesurroundingsofthesticklikestructuresshowabundantnonsedimentarycracks,but
theirsize,lackoffillandorientationaremarkedlydifferentwithrespecttothesticklikestructures.
Filledfracturescanalsoberejectedbecauseoftheirtypicaltabularshape,whereassticklike
structuresareroughlyprismatic.Tensiongashesarealsoexcludedbecausetheyhavetaperedends
andbecausetheytendtoformconjugatesets.
Figure3.Sedimentarycracksandtheirsalientfeatures.(a)Mudcracksshowingtapering(Ta)andT
junctions(Tj).GorgesdeDaluis,France(Permian).(b)Shrinkagecrackswithcurvedshape(Cu),
taperededges(Ta)andsteepflanks(Sf).Kalshanehsection,Tabas,CentralIran(LalunFm.,Lower
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Cambrian).(c)Shrinkagecrackswithsteepflanks(Sf).ValedaMuasection,Portugal(Armorican
QuartziteFormation,LowerOrdovician).(d)Shrinkagecrackswithsteepflanks(Sf)andTjunctions
(Tj).PenhaGarciaIchnologicalPark,Portugal(LowertoMiddleOrdovician).
Thesticklikestructuresmightbeexplainedassinglemineralcrystalsbecauseoftheirelongate
shape,thecommonlypolygonalcrosssectionandtheflatfacesmeetingat~90°.Thesefeaturesare
shared,forinstance,withgypsumcrystalsgrowninsabkhaenvironmentsonEarth(Figure4a,b;see
alsoFigures9aand11ain[28]).Crosscuttingbetweens4ands5remindsofpenetrationtwinning,
i.e.,theprocessbywhichmineralsintergrowandresultininterpenetratingindividuals[29].Curved
morphologyofthesticklikestructuresdoesnotdisprovethenull(abiotic)hypothesissincecurved,
needlelikegypsumcrystalsofcomparablesizehavebeenreportedfromterrestrialevaporitic
settings[30,31].Sulfateminerals,includinggypsum,havebeendocumentedontheMartiansurface
withdatafromsatellites,landers,androvers[32].Amongotherprocesses,curvatureofmillimetre
sizedgypsumcrystalscanresultfromimpurityincorporationfollowedbycrackformationand
mechanicaltwinning[33].Nevertheless,euhedralmonocrystalsandtheirpseudomorphsarenot
perfectanaloguesofthesticklikestructures.Infact,thetipsofmoststicklikestructuresdonotshow
thesmoothflatfacesandthesharpcrystallikeoutlinesthattypifyeuhedralmonocrystals[29]and
theirpseudomorphs(Figure4c,d).Itshouldbenotedthattipsofspecimenss4–s6aresmoothand
flat,buttheycouldreflectsecondary(i.e.,fragmentation)processes.Inaddition,cleavageisnot
detectable.Forinstance,thereisnoevidenceoftheeasy[010]perfectcleavagethatistypicalof
gypsumandanhydrite[29](Figure4a).
Tectographs,i.e.,pseudofossilsproducedbythemovementofrocklayersrelativetoeachother
[34],canberejectedasanexplanationfortheVeraRubinRidgestructuresbecausethesticklike
featuresarenotparallelbetweeneachother.
Figure4.Crystalsandcrystalpseudomorphs.(a)Exhumedlargegypsumcrystalsfromsabkha
deposits.Crystalsshowsanelongateshapeandthetendencytosplitalongdefiniteplanesof
weakness(cleavage:Cl).Gypsumcrystalsformedearlierthanthosedevelopingtoday.SabkhaofBarr
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AlHikman,SultanateofOman.Largercrystalsare12cmlong.Imagefrom[35].(b)Nestoffibrous
gypsumcrystals10–20cmbelowthesabkhasurface.Gypsumcrystalsshowanelongateshape(El).
SabkhaofBarrAlHikman,SultanateofOman.Imagefrom[35].(c)Computedtomographyscan(CT
scan)ofcrystalpseudomorphs.Crystalsshowapolygonalcrosssection(Po).Largearrowsshow
pointedspursclosetosomevertices,whichareatypicalfeatureofaragonitecontacttwins.3.48Ga
oldDresserFormation(PilbaraCraton,Australia).Imagefrom[36].(d)Glendonitemouldsfromthe
shallowmarineRurikfjelletFormation[37]showingsharpcontactwiththehostrock(Sh).Early
CretaceousofSvalbard.
Althoughthenullhypothesis(abioticorigin)cannotbereadilydisproved,itshouldbenoted
thatthemorphologyofthesticklikestructuresresemblesthatofhorizontalburrowssuchas
Helminthoidichnites(Figure5a)andPlanolites,whicharecommoncomponentsoffluviolacustrine
settingsonEarth[38–40].ConstantwidthisatypicalfeatureofburrowsonEarth(Figure5a–d)since
tracemakerstendtominimizeenergyexpenditure,producingburrowsaswideastheirwidth[41].
SticklikestructuresarewithinthesizeandshaperangeofichnofossilsonEarth(seeFigure5).Most
specimensfromVeraRubinRidgepresentpolygonalcrosssections,assuggestedbyfragmented
specimensandtheprevailinglyflatuppersurfaces(Figure2a–c).Polygonalcrosssectionis
apparentlynonadaptiveforburrowingbehaviour,buttherearefewexamplesofichnofossilson
Earthwithsquarishsections,e.g.,Arthrophycus[42],(Figure5e,f)Bulbichnnus[43]andOblongichnus.
Subcylindricalspecimenswithroughlyellipticalcrosssectionarealsofound(e.g.,s2),althoughthey
arerarerthanpolygonalones.
Figure5.Ichnofossilsandtheirsalientfeatures.(a)Fossilburrows(Helminthoidichnites)showing
constantwidth(Co),curvedmorphology(Cu)andcolorcontrastwiththehostrock(Cc).Upper
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CarboniferousofthePramolloBasin,ItalyAustria[39,40].(b)Fossilburrows(Taenidiumisp.)
displayingconstantwidth(Co)andintersections(In).CortePereirosection,TocadaMouraComplex,
OssaMorenaZoneinsouthernPortugal(Carboniferous:Mississipian).(c)Helminthoidichnitesand
Cruzianaproblematicadisplayingconstantwidth(Co)andsteepflanks(Sf).IchnologicalParkofPenha
Garcia,Portugal(Ordovician).(d)Nereites(=Helminthoida)irregularisshowingconstantwidthand
thigmotaxis,i.e.,abehaviouraccordingtowhichanorganismstaysclosetoanexistingentity.Piani
diCreto,Italy(Cretaceous).Specimen24175/TF16oftheUniversityofGenova.(e)Arthrophycus
alleghaniensisdisplayingpolygonal,squarirshcrosssection(Po).Mação,Portugal(LowerOrdovician).
Specimen13,184oftheGeologicalMuseumofLisbon[44].(f)Detailofthesquarecrosssection(dashed
lines)ofArthrophycusalleghaniensis.ThemagnifiedareaisindicatedbyPoinFigure5e.
Moststicklikestructuresdonotgeometricallyinteractwitheachother,butsomepairsare
coalescingandintersecting.AprominentexampleconsistsofaSshapedstructure(s5)thatabruptly
changesitscurvatureatshortdistancefromanothersticklikefeature(s1),stayinginclosecontact
withitbutwithoutintersecting(Figure2a).Thisindicatesthatthedevelopmentofsticklike
structurescanbeinfluencedbyexisting(syngenetic)ones.OnEarth,thisisatypicaloutcomeoftaxes,
i.e.,theorientationandturningofanorganisminresponsetoaperceivedexternalstimulusfield[45].
SimilarlytotheichnotaxaHelminthoidichnites(Figure5a),HelminthorhapheandNereites
(=Helminthoida)irregularis(Figure5d),theSshapedstructurefromVeraRubinRidgecanbemodelled
byusingathigmotaxisrule,compellinganagenttostayclosetoanexistingentity[46,47].
Nevertheless,evaporiticmineralscangrowveryrapidly,henceweexpectthatacrystalcanactasa
physicalconstraintforthegrowthofsuccessive,almostsyngenetical,ones.Thisphenomenoncould
replicatethigmotaxisbehaviour,posingsignificantproblemsindisprovingthenull(abiotic)
hypothesis.
Softsedimentdeformation,fluidescapingandsedimentarycrackingmayaccountforthe
genesisofichnofossillikesedimentarystructures[48]butsuchabioticexplanationsoftheobserved
thigmotaxisliketopologicalrelationshiparedifficulttoproveforthoseprocesses.Verticalfluid
escapecanproducetinyverticaltubes(pillars)slicingsteeplythroughmassiveorlaminatedsand
[49].Alternatively,sandfilledfractures(sanddykes),commonlyrootedinastructurelesssandbed,
canbeformedbytheintroductionofmaterialfromabove,eitherunderpressureorbysimplefilling
ofpreexistingcracksorfissures.Sanddykesarestraighttosinuousinverticalview;theirsinuosity
canbeincreasedbypostdepositionalcompaction,withthedevelopmentofptygmaticfolds[50].
However,theseprocessesarenotknowntoproducethetopologicalrelationshipsobservedin
Martianspecimens.Similarly,toourknowledgetherearenoexamplesofsedimentarycracksthat
changetheircurvaturewhenencounteringothers.
TheSshapedstructureiscrosscutbyanothersticklikefeature(Figure1:s4),showingthatthe
herestudiedstructurescanalsointersectduringtheirdevelopment.Thisfeatureissharedwith
ichnofossils(Figure5b).Activedisplacementofsediment,includingcrosscutting,isconsidereda
strongindicatorofanichnologicalnatureforterrestrialstructures[26].Itshouldbehowevernoted
thatcrosscuttingisalsosharedwithcracksandsyngeneticalcrystalintergrowth.Ourimageanalysis
revealsanaverageintersectionangleof63.1°(intersectionanglerange:39.8°–85.6°).SomeoftheVera
RubinRidgestructurespresentangularterminations(e.g.,s5),whichisexpectedfrombreakageof
anorganismwithamodulargrowth[26].Similarsizeofdifferentstructuresandsinuousorientation
arealsocompatiblewithbodyfossilsofflexibleorganisms,whichhowevercannotbepreservedas
crosscuttingstructures.Crosscuttingprecludesthehypothesisofsticklikestructuresasbody
fossils.Inaddition,carbonizedremains,whichareacriterionforrecognizingthebodyfossilorigin
ofsedimentarystructures[26],arenotdocumentedfromtheVeraRubinRidgestructures.
Thestickshapedfeaturesareunlined;contactwiththehostrockissharpandwelldefined,
althoughthereisapparentlynodifferenceingrainsizebetweenthesticklikestructuresandthehost
rock.Sharpcontacts,andtheelevationofthesticklikestructuresrelativetothehostrock,suggest
thatthehostrockweatheredatafasterratethanthesticklikestructures(differentialweathering).
Hence,thesticklikestructuresarelikelytobemoreresistantthanthehostrock,possiblybecauseof
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moreintensecementation.Resistancetoerosionisconfirmedbythepresenceofanisolatedfragment
(Figure1:s2)thatexperiencedshortdistancetransport.
SticklikestructuresfromVeraRubinRidgearedarkertonedthanthealteredhostrockbutshow
similarhueswithrespecttotheunalteredsubstrate.Unlikethesubstrate,brokensegmentsoften
displaymetallichues(s5inFigures1and2a).Thisdoesnotprecludeanichnologicaloriginofthe
sticklikestructuresbecauseichnofossilswithmetallichuescanresultfromearlydiagenetic
processes,e.g.,onEarthpyriteandbaritefilledburrowshavebeendocumentedfromavarietyof
depositionalsettings,includingpostglaciallacustrineclays[51,52].Thesticklikestructuresarefound
overanareaofapproximately15cm2.Suchclustereddistributionimpliesthattheprocesses
responsibleforthedevelopmentand/orpreservationofthestructuresshouldhavebeenactive
locally.Thisiscompatiblewithbioticprocesses,e.g.,onEarthstressedenvironmentsare
characterizedbylowdiversityassemblages[53]withunevendistributionofichnofossils[54,55].
However,thisdoesnotprecludeanabioticoriginforthesticklikestructures.Forinstance,in
sedimentaryrocks,crystalsmaygrowinaggregatesdependingontheunevenlyhighconcentration
ofdissolvedions.
Thesticklikestructuresoccurinageologicalcontextthatfitswithevaporiticcrystalsand
sedimentarycracks.Thegeologicalcontextalsofitswiththeacceptedcriteriafortheidentificationof
ichnofossilsonEarth[56],i.e.,theyaresyngeneticwiththeprimaryfabricofthehostrockwhichis
sedimentaryandwellunderstoodintheregionalgeologicalcontextofGaleCrater;sticklike
structuresarecrosscutbylaterstagefracturesandarefilledwithmaterialcapabletosurvivetothe
diagenetichistoryoftherocks.Inaddition,sticklikestructuresarerelatedtoanancienthabitable
environment[14–18].Bulbousstructures(e.g.,structurebinFigure1)areembeddedinthehostrock
ofthesticklikestructures.Theyarehereinterpretedasconcretions,intendedasconfinedbodiesof
clasticsedimentlithifiedbyauthigenicminerals[57].OnEarth,formationofconcretionsiscommonly
mediatedbybiogenicprocessessuchasmicrobialactivityandthepresenceoforganicmatter
includingmucusonburrowwalls[58]—butfullyabioticprocessescanalsoexplaintheoriginofsome
concretions[59].Nevertheless,toourknowledgetherearenoexamplesofabioticconcretionssharing
themainmorphologicalfeaturesofthesticklikestructures,whichareroughlypolygonalshape,with
constantwidthandcurvedorientation.OnEarth,curvedburrowsrepresentthemain
physicochemicalsettingforthedevelopmentofcurvedconcretions[58].Therearenoconclusive
evidencesshowingacommonoriginoftheconcretionsfromHaroldswickandthesticklike
structures.Infact,concretionsshareasimilartexture,sizeandcolourwiththesticklikestructures,
buttheylackthemetallichuesthataresometimesobservedinsticklikestructures.Inaddition,they
differfromthesticklikestructuresinhavingapredominantlyellipticalcrosssectionwithalarger
size(majoraxisistypically~1.5mm).OtherpostdepositionalstructureswithintheMurray
Formationincludeenhancedrelieffeatures,darkraisedridgesandveins[60].Amongthese,
enhancedrelieffeaturesshareseveralfeatureswiththesticklikestructuresofHaroldswick,i.e.,they
areembeddedinthehostrock,showtopographicreliefabovethesurface,theyarenotclearlyrelated
tofracturepatterns,theyaredarkerthanthealteredhostrock[60].Unlikethesticklikestructures,
enhancedrelieffeaturestendtobedendritic,theirwidthisnotconstantand,mostimportantofall,
thecontactswiththehostrockshownoevidenceofdistinctboundaries.
Thehypothesistestingapproachweappliedfindsastrongparallelintheprotocolproposedby
BrasierandWacey[61,62]fordemonstratingbiogenicityofcandidatebodyfossilsandstromatolites.
Followingthisapproach,candidatefossilstructuresshouldnotbeacceptedasofbiologicalorigin
untilallplausibleexplanationsoftheirnonbiologicaloriginhavebeentestedandfalsified,thatis,
untilthenullhypothesisofanabiogenicoriginisrejected.Ourresultsshowthatnoneoftheplausible
nonbiologicalexplanationsofthesticklikestructuresisfullysatisfactory.Itmaybetherefore
temptingtoclaimthebiogenicityofthesticklikestructures,alsobecausenoneoftheavailable
observationsfalsifiesaneventualichnofossiloriginofthem.
Nevertheless,ourresultsdonotallowtoclaimthebiogenicityofthesticklikestructures.Infact,
availableevidencecannotdisprovethenullhypothesis,thatis,theabioticnatureofthesticklike
structures.CarlSagan’stenet(“Extraordinaryclaimsrequireextraordinaryevidence”;[63])doesnot
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onlysuggestcautionwheninterpretingextraterrestrialichnofossillikestructures,butitalso
encouragesthesearchforextraordinaryevidence(e.g.,regularlymeanderingichnofossils)onMars.
Insum,ourobservationsdonotnecessarilyimplythatthesticklikestructuresarebiogenicbut,
giventhatnoneoftheavailableobservationsatVeraRubinRidgefalsifiestheirichnofossilorigin,the
sticklikestructuresarehereidentifiedasaprivilegedtargetforastrobiologicalresearch.Assuch,the
sticklikestructuresfromVeraRubinRidgefollowtherecommendationofNASA’sMars2020Science
DefinitionTeam[64]inthattheymighthaveabiologicaloriginandthuscompelresearcherstogather
moredatabeforereachingaconclusionastothepresenceorabsenceofpastlife(seealso[65]).
Ourresultsalsohighlighttheurgentneedofestablishingarobustsetofquantitativecriteriafor
demonstratingthebiogenicityofichnofossillikestructures,microscopicandmacroscopic.Inother
words,thereistheneedofdefiningthemorphospaceofichnofossilsandabiogenicichnofossillike
structures.Thiswillallowtorejectthenullhypothesisofanabiogenicoriginforcandidate
ichnofossilsviamorphospaceanalysis.Thisclearlyparallelsthecellmorphospaceapproach[61].
Theestablishmentofbiogenicitycriteriawouldbenefitnotonlythesearchforextraterrestrial
life,butalsothesearchforPrecambrianlifeonEarth.Infact,thePrecambriangeologicalrecord
comprisesseveralproblematicstructuresthathavebeenrejectedastracefossilsandreinterpretedas
eitherabiogenicstructuresorbodyfossilsinsomecases[26,66].Ichnofossillikepseudofossilshave
alsobeenrecognizedinmorerecentdeposits[67].Ametazoanlikebodyplanisnotessentialfor
producingmacroscopictraces,asevidencedbymicrobialrelatedstructuresinOrdovicianlow
oxygenenvironments[68].Microorganismscanevenproducemacroscopictrailsorburrows,as
evidencedby2.1Gaichnofossilsattributedtomulticellularorsyncytialorganismabletomigrate
laterallyandvertically[69].Thesenonmetazoanmacroscopicichnofossilsencourageresearchof
extraterrestrialanalogues,i.e.,ichnofossilsproducedbytheaggregationofamoeboidcellsintoa
motilesluglikephase,similartothatofterrestrialslimemolds.
Inaddition,fivepropertiesofichnofossilshighlighttheastrobiologicalpotentialofthese
biogenicstructuresandthereforeencouragetheestablishmentofbiogenicitycriteriaforichnofossil
likestructures[70].First,theypreservetheactivityofsoftbodied(e.g.,[71])orpoorlymineralized
(e.g.,[72])organisms,togetherwithskeletonizedforms.Second,theyareresilienttoprocessesthat
obliterateotherbiosignatures,asevidencedbytracefossilspreservedinmetamorphosedandhighly
tectonizedrocks[73–75].Becauseoftheirpreservationpotentialandabundance,ichnofossilsare
amongthemostabundantevidenceofmicroscopicandmacroscopicpastlife,asexemplifiedby3.7
billionyearsoldstromatolites[76],2.1billionyearsoldburrows[69],andca.551millionyearsold
trackways[77].Third,theyareveryvisiblebiosignatures,eitherbecausetheytendtobeenhancedby
diagenesis[78]orbytheeffectsoftracemakingitself[79].Fourth,themorphologyofichnofossils
oftenreflectenvironmentalconditionsatthetimeofdeposition[80].Fifth,ichnofossilsarenotonly
directevidenceofbiologicalbehaviour[81,82],buttheirmorphologyiscommonlyindependentfrom
themorphology,sizeandbiochemistryofthetracemaker.Sincethatextraterrestriallifemaydiffer
significantlyfromtheoneonEarthinmorphology,sizeandbiochemistry,ichnofossilsare
biosignaturesthatareideallysuitedfordetectinganytypeoflife[70].Thesepropertiesshowthe
astrobiologicalpotentialofichnofossils,henceestablishingbiogenicitycriteriawouldbeof
fundamentalhelpforthesearchofextraterrestriallifeandofearlylifeonEarth.
5.Conclusions
Thenewobservationalandmorphometricdatareportedherehavefarreachingimplicationsfor
thesearchofextraterrestriallife.Ourmorphometricandtopologicdataareuniquetothesticklike
structuresamongMartiangeologicalfeaturesandshowthatichnofossilsareamongtheclosest
morphologicalanaloguesoftheseuniquefeatures.
Infact,thesticklikestructuresshowmorphological(elongateshape;presenceofcurves;
constantwidth;clustereddistribution)andtopological(crosscutting,coalescingrelationships)
featuresthatarecommonlyobservedinichnofossils(Table1).Geologicalcontextandsizearealso
compatiblewithichnofossils.Bycontrast,steepflanksandthesomewhatpolygonalcrosssectionare
Geosciences2020,10,3910of18
rarelyobservedinichnofossilsonEarth,withfewexceptions(Arthrophycus,Oblongichnus,
Bulbichnus).
Table1.Characteristicsofthesticklikestructuresandtheiroccurrenceintheirclosestmorphological
analogues.Fiveoccurrenceclassesareused(fromthemostfrequenttothelessfrequent:always,
usually,often,rarely,never).Colorscalereflectsfrequencyofoccurrence.
StickLikeStructuresSynaeresis
Cracks
EuhedralSingle
Crystals
Bioturbational
Ichnofossils
Geological
context
syngeneticwiththehostrockalwaysalwaysalways
fluviallacustrineoftenoftenoften
Morphology
elongateshapeinbeddingplane
viewusuallyusuallyusually
curvedusuallyrarelyusually
constantwidth(notapering)rarelyusuallyusually
Tjunctions,incompletepolygonsalwaysoftenoften
polygonalcrosssectionusuallyalwaysrarely
notipswithsmoothflatfacesusuallyrarelyusually
notendencytosplitalongdefinite
planesofweaknessalwaysrarely(cleavage)often
sharpcontactwiththehostrockalwaysalwaysusually
steepflanksalwaysalwaysrarely
Topology
crosscuttingrarelyusually
(twinning)
usually(false
branching)
coalescing(changetheircurvature
whenencounteringothers)rarelyrarelyoften
straightsegmentsarenotparallel
betweeneachotheralwaysusuallyusually
Other
metallichuesneveroftenrarely
millimetricwidthrarelyoftenoften
clustereddistributionoftenoftenoften
Followingtheinterpretationofotherextraterrestrialbiogeniclikestructures[65],theabsenceof
unequivocalevidenceforlifeonMarsistheonlyevidencethatfavoursanabioticoriginofthestick
likestructures.Nevertheless,availabledatacannotfullydisprovetwomajorabiotichypotheses,that
aresedimentarycrackingandevaporiticcrystalgrowthasgeneticprocessesforthestructures.The
polygonalsectionandthesteepflankssupportthecrystalhypothesis,althoughthesticklike
structureslackthesmoothflatfacesandthesharpcrystallikeoutlinesthattypifythetipsofeuhedral
monocrystals.TheTjunctionsandincompletepolygonsformedbythesticklikestructuresremind
offilledsedimentarycracks,althoughshrinkagecracks,unlikethesticklikestructures,areusually
spindleshapedandcanpresenttaperedtips(Table1).Accordingly,theavailableobservationsat
VeraRubinRidgedonotallowtoclaimthebiogenicityofthesticklikestructures,buttheyencourage
thecollectionoffurtherdata,especiallygeochemical,aboutthiskindofsedimentarystructureson
Mars.
Consequently,thiscasestudyraisesthequestionofhowtodisprovetheabioticoriginofan
ichnofossillikestructureinancientsedimentarydepositsonEarthandbeyond.Theanswertothis
questionnecessarilyliesinthestudyofmorphologyandgeologicalcontext,sincethechemical
compositionofpassivelyfilledichnofossilsisnotnecessarilyrelatedtobiologicprocesses.This
question,anditsconverse(howtoprovebiogenicityofichnofossillikestructures)areoffundamental
importancenotonlybecauseichnofossilsarethemostpersistentmacroscopicevidenceforlifeon
Earth,butalsobecausetheyareindependentfrommorphology,sizeandbiochemistryofthe
producer.Assuch,ichnofossilswouldallowtorecognizelifethatdiffersfromknownterrestrial
life.BydescribingtheuniqueVeraRubinRidgestructuresfromMars,thisstudyemphasizesthe
potentialofichnologyasanewfrontierinastrobiology.
Geosciences2020,10,3911of18
AuthorContributions:Conceptualization,A.B.;Datacuration,A.B.,C.N.D.C.,F.F.andR.C.;Formalanalysis,
A.B.,C.N.D.C.,F.F.andR.C.;Fundingacquisition,A.B.;Investigation,A.B.,C.N.D.C.,F.F.andR.C.;
Methodology,A.B.;Projectadministration,A.B.;Supervision,A.B.;Validation,A.B.;Visualization,A.B.;
Writing—originaldraft,A.B.;Writing—review&editing,A.B.,C.N.D.C.,F.F.andR.C.
Funding:TheworkofA.B.hasbeensupportedbytheUNIVERSITYOFGENOVA,projectsALANX,
PALEOGIANTS,CURIOSITY.
Acknowledgments:Twoanonymousreviewersarethankedfortheirconstructivecommentswhichgreatly
improvedthepaper.DerekBurgess(MartinMarietta,Raleigh,NC, USA)andDavidLoope(Universityof
NebraskaLincoln,Lincoln,NE,USA)isthankedfordiscussiononconcretions.WethankBarrydiGregorio
(UniversityofBuckingam,Buckingham,UK)fordiscussiononthebiogenicityofthestructuresandwe
acknowledgehimasthefirsttomaketheparallelbetweenichnofossilsandthesticklikestructures.Lorenzo
Bonini,AngeloDeMin,RomanaMelis,StefanoFurlani,MaurizioPonton,FrancescoPrincivalle,NevioPugliese
(UniversityofTrieste,Trieste,Italy)arethankedfordiscussiononthemorphologyofthestructures.Six
anonymousreviewersarethankedforrevisingapreliminaryversionofthemanuscript.AramBayetGoll(IASBS
ofZanjan,Zanjan,Iran)coordinatedthefieldworkinIran;GilMachado(Inst.D.Luiz,Lisbon,Portugal)and
NoelMoreira(Univ.Évora,Évora,Portugal),aswellasSebastiãoFraústo,arewarmlythankedforthelocation
ofCortePereiroandValedaMuasections,respectively.Theauthorsgreatlyappreciatetheaccesstothe
collectionsoftheGeologicalMuseumofLisbon,especiallytoitsexecutivedirectorProf.Dr.MiguelRamalho.
MoniqueMettraux(Geosolutions,Pau,France)andEliasSamanakassou(IAS,UniversityofGeneva,Geneva,
Switzerland)arethankedforprovidingpicturesofevaporitecrystals(Figure4a,b).JuanManuelGarcíaRuiz
(CSIC,UniversidaddeGranada,Granada,Spain)isthankedforprovidingapseudomorphpicture(Figure4c).
MadelineVickers(UniversityofCopenaghen,Copenaghen,Denmark)isacknowledgedforprovidinga
glendonitepicture(Figure4d).AlexanderE.S.VanDriessche(ISTerreCNRS&Univ.GrenobleAlpes)and
MaciejBąbel(UniversityofWarsaw,Warsaw,Poland)arethankedforcrystalimages.
ConflictsofInterest:Theauthorsdeclarenoconflictofinterest.
AppendixA
FigureA1.Widthanalysismap.Numbers1–34refertothemeasurementIDinTableA1.
Geosciences2020,10,3912of18
FigureA2.Lengthanalysismap.
FigureA3.Angleanalysismap.
TableA1.(followingpage).WidthmeasuresofthesticklikestructuresfromVeraRubinRidge,Mars.
SeeFigureA1forlocationofmeasurements.
MeasurementIDMeasuredSpecimenWidth(cm)
1s10.08
2s10.08
3s10.08
4s10.07
5s10.06
6s30.09
7s40.07
8s40.06
Geosciences2020,10,3913of18
9s40.06
10s50.09
11s50.09
12s50.08
13s50.08
14s50.09
15s60.06
16s60.06
17s60.07
18s70.07
19s80.05
20s80.07
21s80.07
22s90.08
23s90.08
26s90.08
24s90.08
27s100.07
25s90.08
28s100.08
29s100.07
30s100.07
31s100.07
32s110.04
33s110.05
34s110.04
Mean0.07
Min0.04
Max0.09
TableA2.LengthmeasuresofthesticklikestructuresfromVeraRubinRidge,Mars.SeeFigureA2
forlocationofmeasurements.
SpecimenLength(cm)
s10.40
s20.30
s30.26
s40.22
s51.05
s60.25
s70.24
s80.26
s90.43
s100.81
s110.23
Mean0.40
Min0.22
Max1.05
Geosciences2020,10,3914of18
TableA3.AnglemeasuresofthesticklikestructuresfromVeraRubinRidge,Mars.SeeFigureA3for
locationofmeasurements.
AngleIDRaysoftheAngleAngle(°)
α s4–s563.1
β s8–s985.6
γ
s10–s1139.8

Mean63.1
Min39.8
Max85.6
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... Joseph et al. (2021a,b) discovered these tubular specimens within Endurance Crater and hypothesized these are tube worms that had been dwelling within hydrothermal vents when the cater was filled with water. Microscopic images of Non-linearized Full frame EDR ©NASA | Stitcher and assembling ©Eco Astronomy Inc. (Right) | Julie, 1985;Sun j et al., 2012;Kupriyanova et al., 2015;Baucon et al., 2020). A. Most probably borehole and type of tube worm opercula at Endurance Crater. ...
... F. This is Sol 1905, Outcrop imaged by rover Curiosity using MAHLI at Vera Rubin Ridge, Mars presented mold likes Ichno fossils. (Baucon et al., 2020;Joseph et al., 2020aJoseph et al., , 2021a. The structure reported here is simple flattened, branched, oblong, to sub rectangular in cross section. ...
... G246 SAM and AM W.43887 respectively (Kupriyanova et al., 2015). (Baucon et al., 2020). Sol 1905, Outcrop imaged by rover Curiosity using MAHLI at Vera Rubin Ridge, Mars presented Labels indicate individual specimens of stick-like structures (Baucon et al., 2020). ...
Article
Full-text available
The observation of tubular structures within Endurance Crater, Mars, has been reported by Joseph et al (2021a,b) who hypothesized these may be mineralized and fossilized remnants of tube worms that in the ancient and recent past flourished within lakes of water heated by thermal vents. The discovery of what may be spherical hematite in this same vicinity supports the hydrothermal vent scenario, whereas the claims by Joseph (2021; Joseph et al. 2021c) that these spherules are fungal puffballs does not. This evidence from Endurance Crater and associated mineralogy and chemistry is reviewed. We conclude that the ancient lakes of Endurance Crater may have been heated by thermal vents and inhabited by tubular organisms that became mineralized, as hypothesized by Joseph et al; and that these same hydrothermal vents formed hematite spherules as hypothesized by the rover Opportunity team.
... This indicates that surface temperatures can reach highs of 47 °C (116 °F). On the other hand, lows of -127 °C (-189 °F) have been reported as calculated by orbiting craft The Journal of Cosmology, 31,2021 The Journal of Cosmology, 2021. 162 Rhawn Joseph Copyright © 2021 Cosmology.com ...
... Specimens resembling algae [26][27][28][93][94][95][96], domical concentric stromatolites, lichens, fungi, [27][28][29]30,97], and tube worms [31][32][33][34]98] have been photographed by the NASAs Mars rovers. Of course, similarities in morphology are not proof of life. ...
... On Earth, estimates are that between 200 to 600 million years ago "tube worms" had colonized these vents [117][118][119][120] along with their bacteria-symbionts, and eventually more complex species including crustaceans; as are commonplace today [121][122][123]. Likewise, it is believed that hydrothermal vents on Mars [124][125][126][127][128][129] and associated biological communities [124, The Journal of Cosmology, 31,2021 The 130], had been established and were also flourishing over 3 billions of years ago [57][58], in the recent past these organisms may have included tube worms and crustaceans [28-29, 31-33, 98]. ...