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An Asteroid Shower Over the Cretaceous Period

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An Asteroid Shower Over the Cretaceous Period AnAn AsteroidAsteroid ShowerShower OverOver thethe CretaceousCretaceous PeriodPeriod WilliamWilliam BottkeBottke SouthwestSouthwest ResearchResearch InstituteInstitute (with thanks to David Vokrouhlicky and David Nesvorny) MotivationMotivation Question. How do big disruption events in the asteroid belt affect the impact flux on the Earth and Moon? The answer involves understanding these issues: – Nature and timing of breakup events in the asteroid belt. – Asteroid evolution and delivery to the inner Solar System. – Impact record on the Earth and Moon. Is there anything unusual? Sample references: Zappala et al. (1999); Bottke et al. (2002). TheThe RecentRecent ImpactImpact FluxFlux onon thethe EarthEarth andand MoonMoon ImpactImpact RatesRates onon EarthEarth andand MoonMoon Craters on N. American, European Cratons Craters show two distributions. Dcrater > 20 km Sample references: Grieve and Shoemaker (1994); Earth Impact Database; Harnack and Kleppinger (1997) ImpactImpact RatesRates onon EarthEarth andand MoonMoon Craters on N. American, European Cratons Craters show two distributions. Erosion, Bias, or Surge? Possible reasons: – Erosion for craters older than 120 My. – Biases in crater record – Surge in number of big impacts on Earth starting >100 My ago. Dcrater > 20 km Sample references: Grieve and Shoemaker (1994); Earth Impact Database; Harnack and Kleppinger (1997) ImpactImpact RatesRates onon EarthEarth andand MoonMoon Additional terrestrial and lunar data supports factor of 2 change in crater production rate over last 120 My. Time Period Production Rate D > 20 km Location of Crater Data Set (My Ago) Craters (10-15 km-2 yr-1) The Moon, Australia 500-3200 3-4 US Mississippi Lowlands; <120 ~6 N. American, European Cratons <120 ~6 Sample references: Grieve and Shoemaker (1994); McEwen et al. (1997); Shoemaker et al. (1998) AsteroidsAsteroids EvolutionEvolution andand DeliveryDelivery toto thethe InnerInner SolarSolar SystemSystem CollisionsCollisions inin thethe AsteroidAsteroid BeltBelt Asteroids strike one another and create ejecta. Most fragments are ejected at low velocities (V < 100 m/s). Sample references: Benz and Asphaug (1999); Michel et al. (2001); Durda et al. (2004) CollisionsCollisions inin thethe AsteroidAsteroid BeltBelt Proper elements by Knezevic & Milani (2006) Interval between D > 100 km breakup events is ~200 My. Sample references: Bottke et al. (2005); Durda et al. (2007) YarkovskyYarkovsky DriftDrift intointo ResonancesResonances Koronis family •• Observed •• Model Bottke et al. (2001) FractionFraction ofof MaterialMaterial ReachingReaching EarthEarth Gladman et al. (1997); Bottke et al. (2006) 2:1 9:4 11:5 7:3 5:2 8:3 3:1 IMC Region υ6 The inner main belt (a < 2.3 AU) is much more efficient at producing impactors than the remaining belt. FractionFraction ofof MaterialMaterial ReachingReaching EarthEarth Gladman et al. (1997); Bottke et al. (2006) 2:1 9:4 11:5 7:3 5:2 8:3 3:1 IMC Region υ6 Big family forming events in the inner main belt have best chance to modify impact flux on Earth and Moon. TheThe BaptistinaBaptistina AsteroidAsteroid Family:Family: SourceSource ofof anan AsteroidAsteroid Shower?Shower? TheThe BaptistinaBaptistina AsteroidAsteroid FamilyFamily (BAF)(BAF) BAF BAF The BAF has mostly been overlooked to date because: – It is a dark, hard-to-see C-complex family in the inner main belt with only one D > 20 km member (298 Baptistina; D ~ 40 km). – It partially overlaps the large S-type Flora family along the inner edge of the main belt. TheThe BaptistinaBaptistina AsteroidAsteroid FamilyFamily (BAF)(BAF) BAF overlaps the 7:2 and 5:9 MMR with Jupiter and Mars (J7:2 and M5:9). Few asteroids are near the J7:2/M5:9! What happened? AgeAge ofof thethe BAFBAF Yarkovsky- YORP model used to get family’s age. – Model – Observed – Many Interlopers Best fit age is 160 ± 20 My. SizeSize DistributionDistribution ofof thethe BAFBAF The initial BAF had many small members. – 300 objects with D > 10 km – 140,000 with D > 1 km. SPH/N-body modeling indicates the parent body was D ~ 170 km. – ~88% of the BAF’s mass was initially in the form of D < 10 km bodies. YarkovskyYarkovsky EvolutionEvolution ofof BAFBAF MembersMembers Simulation: D > 10 km bodies near J7:2 and M5:9. Overall, 10-20% of all km-sized BAF members escape over 160 My. ImpactImpact FluxFlux onon EarthEarth The BAF produces a surge in the terrestrial planet impact flux that peaks 40-60 My after the family-forming event. ResultsResults We input our collisional and dynamical simulations into a Monte-Carlo code and found for the Earth: Projectile # BAF impacts # background impacts Increased by size over 160 Ma over 160 Ma Factor D > 1 km 200 ± 60 250 ± 20 1.8 D > 5 km 6 ± 2 3 ± 2 3 D > 10 km 1 ± 1 0.5 ± 0.7 3 The BAF increased impact flux on the Earth and Moon by 2-3 times over last 160 My! ImplicationsImplications TheThe Cretaceous/TertiaryCretaceous/Tertiary (K/T)(K/T) ImpactorImpactor Chicxulub, 65 Million Years Ago The K/T mass extinction event/Chicxulub crater was caused by impact of a D > 10 km projectile 65 My ago. TheThe Cretaceous/TertiaryCretaceous/Tertiary (K/T)(K/T) ImpactorImpactor Typical CM2: Murchison Kyte (1998) K/T projectile was a CM2-type carbonaceous chondrite. – Consistent with fossil meteorite found in North Pacific sediments from K/T boundary. TheThe Cretaceous/TertiaryCretaceous/Tertiary (K/T)(K/T) ImpactorImpactor Trinquier et al. (2006) K/T projectile was a CM2-type carbonaceous chondrite. – Good match to 54Cr isotopes taken from samples found at 3 well- characterized K/T boundary sites (i.e., all have strong Ir enhancement). BAFBAF asas thethe SourceSource ofof thethe K/TK/T ImpactorImpactor BAF impactors: – ~1 D > 10 km projectile hit Earth over last 160 My. Background impactors: – Prior to BAF formation event, >70% of D > 10 km NEOs were S-types. These bodies have the wrong composition to produce K/T impact! – Only 40% of all carbonaceous chondrite meteorite falls are “CM”. – We estimate the interval between CM impacts was 1800-2600 My. > 90% probability that BAF is source of K/T impactor! BAFBAF asas thethe SourceSource ofof thethe TychoTycho ImpactorImpactor Tycho Crater The age of Tycho crater (109 ± 4 My) falls in the peak of the Baptistina asteroid shower. Using a Monte Carlo code, we find ~70% chance that BAF projectiles made 85 km Tycho crater. ConclusionsConclusions The breakup of the 170 km Baptistina parent body ~160 My ago triggered an asteroid shower. It increased the impact flux of D > 1 km bodies on the terrestrial planets by a factor of 2-3. It is currently responsible for 20% of all NEOs and 40% of dark, C-type NEOs. The Baptistina family is the most likely source of: –The K-T impactor (> 90% probability) and the Tycho impactor (> 70% probability). –The CM meteorites CollisionalCollisional EvolutionEvolution ofof BAFBAF MembersMembers BAF members also undergo collisional evolution. The D > 1 km population drops by 40% within 160 My. The D > 5 km population is unaffected. DynamicalDynamical EvolutionEvolution ofof BAFBAF MembersMembers inin thethe TerrestrialTerrestrial PlanetPlanet RegionRegion We tracked >9000 particles in J7:2/M5:9. Bodies take time to get out of slow resonances. 1.7% strike Earth in 200 My. MotivationMotivation Bambach (2006) K/T Impact The nature of impact flux on Earth/Moon over last several Gy has been subject to considerable debate. – Constant, cyclic, or punctuated with random “showers”? – Does it come from mostly comets or asteroids? MotivationMotivation Question. How has the impact flux been affected by big disruption events in the main asteroid belt? Current NEOs produce impact flux similar to long term average flux over last 3 Gy. – Showers must be short & intense or prolonged & limited. Comets showers are expected to be only a few My in duration (if they exist at all). Sample references: Zappala et al. (1999); Bottke et al. (2002); Dones et al. (2006); AgeAge ofof thethe BAFBAF Astrid Merxia Massalia Erigone The BAF has same two-lobed shape as many young families (< 300 My old). This is a “fingerprint” of young families evolving by Yarkovsky/YORP evolution. Vokrouhlicky et al. (2006) KnownKnown AsteroidAsteroid ImpactImpact StructuresStructures Many craters concentrated in particular areas of North America, Europe, and Australia. KnownKnown AsteroidAsteroid ImpactImpact StructuresStructures Craters often found on cratons, stable continental crust regions that have survived plate tectonics for > 500 My. YarkovskyYarkovsky DriftDrift intointo ResonancesResonances Evolution of D = 1 m bodies Asteroids drift into resonances via Yarkovsky thermal drag. Resonances drive their eccentricities to Earth- crossing values. Sample references: Farinella et al. (1998); Bottke et al. (2000; 2002).
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