TheThe ProductionProduction RateRate andand DistributionDistribution ofof SecondarySecondary CratersCraters onon MarsMars Nadine G. Barlow Northern Arizona University Department of Physics and Astronomy Flagstaff, AZ 86011-6010 [email protected] TheThe ControversyControversy

„ Distant, undistinguished secondary craters are a major contributor to the small crater production population and those results are questionable (McEwen eett al., 2005; Bierhaus et al., 2005; McEwen and Bierhaus, 2006) „ Secondary craters can easily be distinguished and avoided. Those which cannot be distinguished are a small contributor to the small crater production population and thus the results are valid (Neukum et al., 2004; Werner, 2005) „ It does not matter if undistinguished secondaries are a major contributor to the small crater production population since we also count them on the Moon, so extrapolation of the lunar crater curve to Mars makesmakes thethe resultsresults validvalid (Hartmann, 2005). IsIs SecondarySecondary CraterCrater ProductionProduction RateRate SameSame onon MoonMoon andand Mars?Mars?

„ Identify craters with obviousobvious secondary crater chains „ Measure diameters of

secondaries out to ~6 Rc „ Compute crater size- frequency distribution curves for secondaries of each primary crater studied. „ Craters >0.1-km-diameter included in analysis.

Edge of a 29-km-diameter crater on Mars. (THEMIS image V01990003) EstimateEstimate EnergyEnergy ofof ImpactImpact CreatingCreating MartianMartian CraterCrater ofof SpecificSpecific SizeSize

0.11 -1/3 0.22 0.13 1/3 1/0.22 EEk == [D/[1.8[D/[1.8ρρm ρρt gg (2R)(2R) (sin(sinθθ)) ]]]]

1/3 DD == 4.844.84 d[(d[(ρρm// ρρt)) (u/(u/√√gd)]gd)]

D = Diameter of crater d = projectile diameter = 2R -3 -3 ρm = 2500 kgkg mm ρt = 3000 kg m g = gravity = 3.69 m s-2 θ = angle of impact = 45° u = impact velocity (9.6 km s-1 at Mars)

After Melosh, 1999 ApplyApply toto MoonMoon

„ CalculatedCalculated sizesize ofof impactimpact cratercrater producedproduced byby equivalentequivalent energyenergy onon MoonMoon (u = 16.1 km s-1; g = 1.62 m s-2). „ LunarLunar OrbiterOrbiter andand ClementineClementine →→ identifyidentify lunarlunar craterscraters ofof correctcorrect sizesize andand withwith secondarysecondary cratercrater fields.fields. „ ComputedComputed cratercrater SFDsSFDs andand comparedcompared toto Mars.Mars. ResultsResults forfor LavaLava PlainsPlains Mars: Mars, Aristarchus, Harpalus Calahorra crater 0 26.5°N 321.4°E -1 D = 33.2 kmkm

-2 Moon: Aristarchus Moon: -3 Mars log R log Harpalus Aristarchus -4 23.7°N 47.4°W

-5 D = 40 km

-6 -1.5 -1 -0.5 0 0.5 Harpalus log D 52.6°N 43.4°W

R = [(D)3N] / [A (Db - Da)] D = 39 km ResultsResults forfor LavaLava PlainsPlains

Secondary Crater SFD Secondary Crater SFD

-1 -1

-1.5 -2

-2

-3 -2.5

Lambert Moon -3

Log (R) Log Mar s

log R log Mars -4 -3.5

-4 -5 -4.5

-6 -5 -1.5 -1 -0.5 0 0.5 _ -1.5 -1 -0.5 0 0.5 Log(D) log D OnsetOnset SizesSizes

AmazonianAmazonian 1010 kmkm

HesperianHesperian 1919 kmkm

NoachianNoachian 4545 kmkm

(Hartmann and Barlow, revised and resubmitted to MAPS)

„ ProbablyProbably duedue toto varyingvarying thicknessthickness ofof regolithregolith andand depthdepth toto layerlayer competentcompetent enoughenough toto produceproduce coherentcoherent blocksblocks forfor secondarysecondary cratercrater formation.formation. WhatWhat DoesDoes ItIt Mean?Mean?

„ PossibilityPossibility 1:1: NotNot comparingcomparing equivalentequivalent energyenergy impacts.impacts. „ PossibilityPossibility 2:2: EffectEffect isis real.real. MarsMars isis moremore efficientefficient atat producingproducing secondariessecondaries onon similarsimilar terrainterrain (approximately(approximately 2x2x higherhigher onon Mars).Mars). ConclusionsConclusions

„ SecondarySecondary cratercrater productionproduction raterate isis higherhigher onon MarsMars thanthan onon MoonMoon forfor diametersdiameters <0.3<0.3 km.km. „ IfIf trendtrend continuescontinues forfor distantdistant secondaries,secondaries, simplesimple extrapolationextrapolation ofof lunarlunar cratercrater densitydensity--ageage curvecurve toto MarsMars atat smallsmall diametersdiameters willwill overestimateoverestimate MartianMartian surfacesurface agesages ifif secondariessecondaries areare present.present. FutureFuture WorkWork

„ ExtendExtend studystudy toto highlandshighlands craterscraters toto constrainconstrain terrainterrain effects.effects. „ MoreMore examplesexamples →→ statisticalstatistical significance.significance. „ CompareCompare withwith Mercury,Mercury, wherewhere gg ≈≈ MarsMars butbut targettarget materialmaterial isis volatilevolatile--poorpoor likelike thethe Moon.Moon. „ TryTry comparingcomparing distantdistant secondariessecondaries ofof rayedrayed craters.craters.