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)