An Asteroid Shower Over the An Asteroid Shower Over the Cretaceous PeriodCretaceous Period

William BottkeWilliam BottkeSouthwest Research InstituteSouthwest Research Institute

(with thanks to David (with thanks to David VokrouhlickyVokrouhlicky and David Nesvorny)and David Nesvorny)

MotivationMotivation

Question.Question. How do big disruption events in the asteroid belt How do big disruption events in the asteroid belt affect the impact flux on the Earth and Moon?affect the impact flux on the Earth and Moon?

The answer involves understanding these issues:The answer involves understanding these issues:–– Nature and timing of breakup events in the asteroid belt.Nature and timing of breakup events in the asteroid belt.–– Asteroid evolution and delivery to the inner Solar System.Asteroid evolution and delivery to the inner Solar System.–– Impact record on the Earth and Moon. Is there anything unusual?Impact record on the Earth and Moon. Is there anything unusual?

Sample references: Sample references: ZappalaZappala et al. (1999); Bottke et al. (2002).et al. (1999); Bottke et al. (2002).

The Recent Impact Flux on the The Recent Impact Flux on the Earth and MoonEarth and Moon

Impact Rates on Earth and MoonImpact Rates on Earth and Moon

Craters show two Craters show two distributions. distributions.

Sample references: Grieve and Shoemaker (1994); Earth Impact DatSample references: Grieve and Shoemaker (1994); Earth Impact Database; abase; HarnackHarnack and and KleppingerKleppinger (1997)(1997)

Craters on N. American, European CratonsCraters on N. American, European Cratons

DDcratercrater > 20 km> 20 km

Impact Rates on Earth and MoonImpact Rates on Earth and Moon

Craters show two Craters show two distributions. distributions.

Possible reasons:Possible reasons:–– Erosion for craters Erosion for craters

older than 120 My.older than 120 My.–– Biases in crater record Biases in crater record –– Surge in number of Surge in number of

big impacts on Earth big impacts on Earth starting >100 My ago. starting >100 My ago.

Craters on N. American, European CratonsCraters on N. American, European Cratons

Sample references: Grieve and Shoemaker (1994); Earth Impact DatSample references: Grieve and Shoemaker (1994); Earth Impact Database; abase; HarnackHarnack and and KleppingerKleppinger (1997)(1997)

DDcratercrater > 20 km> 20 km

Erosion, Bias, or Erosion, Bias, or Surge?Surge?

Impact Rates on Earth and MoonImpact Rates on Earth and Moon

~6~6<120<120US Mississippi Lowlands; US Mississippi Lowlands; N. American, European CratonsN. American, European Cratons

33--44500500--32003200The Moon, Australia The Moon, Australia

Production Rate Production Rate DD > 20 km > 20 km Craters (Craters (1010--1515 kmkm--22 yryr--11))

Time Period Time Period (My Ago)(My Ago)Location of Crater Data SetLocation of Crater Data Set

Sample references: Grieve and Shoemaker (1994); McEwen et al. (1Sample references: Grieve and Shoemaker (1994); McEwen et al. (1997); Shoemaker et al. (1998)997); Shoemaker et al. (1998)

Additional terrestrial and lunar data supports factor of 2 Additional terrestrial and lunar data supports factor of 2 change in crater production rate over last 120 My. change in crater production rate over last 120 My.

Asteroids Evolution and Delivery Asteroids Evolution and Delivery to the Inner Solar Systemto the Inner Solar System

Collisions in the Asteroid BeltCollisions in the Asteroid Belt

Asteroids strike Asteroids strike one another and one another and create ejecta.create ejecta.

Most fragments Most fragments are ejected at low are ejected at low velocities (velocities (VV < 100 < 100 m/sm/s).).

Sample references: Benz and Sample references: Benz and AsphaugAsphaug (1999); Michel et al. (2001); Durda et al. (2004)(1999); Michel et al. (2001); Durda et al. (2004)

Collisions in the Asteroid BeltCollisions in the Asteroid Belt

Interval between Interval between DD > 100 km breakup events is ~200 My.> 100 km breakup events is ~200 My.

Proper elements by Proper elements by KnezevicKnezevic & & MilaniMilani (2006)(2006)

Sample references: Bottke et al. (2005); Durda et al. (2007)Sample references: Bottke et al. (2005); Durda et al. (2007)

KoronisKoronis familyfamily

Bottke et al. (2001)Bottke et al. (2001)

•• Observed Observed

•• ModelModel

Yarkovsky Drift into ResonancesYarkovsky Drift into Resonances

Fraction of Material Reaching EarthFraction of Material Reaching Earth

2:12:1

3:13:1

9:49:4

5:25:2

8:38:3

7:37:311:511:5

IMC RegionIMC Region

υυ66

The inner main belt (The inner main belt (aa < 2.3 AU) is much more efficient at < 2.3 AU) is much more efficient at producing impactors than the remaining belt.producing impactors than the remaining belt.

GladmanGladman et al. (1997); Bottke et al. (2006)et al. (1997); Bottke et al. (2006)

Fraction of Material Reaching EarthFraction of Material Reaching Earth

2:12:1

3:13:1

9:49:4

5:25:2

8:38:3

7:37:311:511:5

IMC RegionIMC Region

υυ66

Big family forming events in the inner main belt have best Big family forming events in the inner main belt have best chance to modify impact flux on Earth and Moon.chance to modify impact flux on Earth and Moon.

GladmanGladman et al. (1997); Bottke et al. (2006)et al. (1997); Bottke et al. (2006)

The Baptistina Asteroid Family: The Baptistina Asteroid Family: Source of an Asteroid Shower?Source of an Asteroid Shower?

The Baptistina Asteroid Family (BAF)The Baptistina Asteroid Family (BAF)

The BAF has mostly been overlooked to date because:The BAF has mostly been overlooked to date because:–– It is a dark, hardIt is a dark, hard--toto--see Csee C--complex family in the inner main belt with complex family in the inner main belt with

only one only one DD > 20 km member (> 20 km member (298 Baptistina298 Baptistina; ; DD ~ 40 km).~ 40 km).–– It partially overlaps the large SIt partially overlaps the large S--type Flora family along the inner edge type Flora family along the inner edge

of the main belt.of the main belt.

BAFBAFBAFBAF

The Baptistina Asteroid Family (BAF)The Baptistina Asteroid Family (BAF)

BAF overlaps the 7:2 BAF overlaps the 7:2 and 5:9 MMR with and 5:9 MMR with Jupiter and Mars Jupiter and Mars (J7:2 and M5:9).(J7:2 and M5:9).

Few asteroids are Few asteroids are near the J7:2/M5:9! near the J7:2/M5:9! What happened?What happened?

Age of the BAFAge of the BAF

YarkovskyYarkovsky--YORP model YORP model used to get used to get family’s age.family’s age.

–– ModelModel–– ObservedObserved–– Many InterlopersMany Interlopers

Best fit age is Best fit age is 160 160 ±± 20 My.20 My.

Size Distribution of the BAFSize Distribution of the BAF

The initial BAF had The initial BAF had manymany small members.small members.

–– 300 objects with300 objects with DD > 10 km> 10 km–– 140,000 140,000 with with DD > 1 km.> 1 km.

SPH/SPH/NN--body modeling body modeling indicates the parent indicates the parent body was body was DD ~ 170 km.~ 170 km.

–– ~88% of the BAF’s mass ~88% of the BAF’s mass was initially in the form of was initially in the form of DD < 10 km bodies.< 10 km bodies.

Yarkovsky Evolution of BAF MembersYarkovsky Evolution of BAF Members

Overall, 10Overall, 10--20% of all km20% of all km--sized BAF members escape over 160 My.sized BAF members escape over 160 My.

Simulation: Simulation:

DD > 10 km > 10 km bodies near bodies near

J7:2 and J7:2 and M5:9.M5:9.

Impact Flux on EarthImpact Flux on Earth

The BAF produces a surge in the terrestrial planet impact The BAF produces a surge in the terrestrial planet impact flux that peaks 40flux that peaks 40--60 My after the family60 My after the family--forming event. forming event.

ResultsResults

0.5 0.5 ±± 0.70.73 3 ±± 22

250 250 ±± 2020

# background impacts # background impacts over 160 Maover 160 Ma

336 6 ±± 22DD > 5 km> 5 km

331 1 ±± 11DD > 10 km> 10 km

1.81.8200 200 ±± 6060DD > 1 km> 1 km

Increased by Increased by FactorFactor

# BAF impacts # BAF impacts over 160 Maover 160 Ma

Projectile Projectile sizesize

We input our collisional and dynamical simulations into a We input our collisional and dynamical simulations into a MonteMonte--Carlo code and found for the Earth:Carlo code and found for the Earth:

The BAF increased impact flux on the Earth and Moon by 2-3 times over last 160 My!

ImplicationsImplications

The Cretaceous/Tertiary (K/T) ImpactorThe Cretaceous/Tertiary (K/T) Impactor

The K/T mass extinction event/The K/T mass extinction event/ChicxulubChicxulub crater was crater was caused by impact of a caused by impact of a DD > 10 km projectile 65 My ago.> 10 km projectile 65 My ago.

Chicxulub, 65 Million Years Ago

The Cretaceous/Tertiary (K/T) ImpactorThe Cretaceous/Tertiary (K/T) Impactor

K/T projectile was a CM2K/T projectile was a CM2--type carbonaceous chondrite.type carbonaceous chondrite.Kyte (1998)

–– Consistent with fossil meteorite found in North Pacific sedimentConsistent with fossil meteorite found in North Pacific sediments from s from K/T boundary.K/T boundary.

Typical CM2: Murchison

The Cretaceous/Tertiary (K/T) ImpactorThe Cretaceous/Tertiary (K/T) Impactor

K/T projectile was a CM2K/T projectile was a CM2--type carbonaceous chondrite.type carbonaceous chondrite.–– Good match to Good match to 5454Cr isotopes taken from samples found at 3 wellCr isotopes taken from samples found at 3 well--

characterized K/T boundary sites (i.e., all have strong characterized K/T boundary sites (i.e., all have strong IrIr enhancement).enhancement).

Trinquier et al. (2006)

BAF as the Source of the K/T ImpactorBAF as the Source of the K/T Impactor

BAF impactors: BAF impactors: –– ~1 ~1 DD > 10 km projectile hit Earth over last 160 My. > 10 km projectile hit Earth over last 160 My.

Background impactors: Background impactors: –– Prior to BAF formation event, >70% of Prior to BAF formation event, >70% of DD > 10 km NEOs were S> 10 km NEOs were S--types. types.

These bodies have the wrong composition to produce K/T impact!These bodies have the wrong composition to produce K/T impact!–– Only 40% of all carbonaceous chondrite meteorite falls are “CM”.Only 40% of all carbonaceous chondrite meteorite falls are “CM”.–– We estimate the interval between CM impacts was 1800We estimate the interval between CM impacts was 1800--2600 My. 2600 My.

> 90% probability that BAF is source of K/T impactor!

BAF as the Source of the Tycho ImpactorBAF as the Source of the Tycho Impactor

The age of Tycho crater (109 The age of Tycho crater (109 ±± 44 My) falls in the peak of My) falls in the peak of the Baptistina asteroid shower. the Baptistina asteroid shower.

Using a Monte Carlo code, we find ~70% chance that Using a Monte Carlo code, we find ~70% chance that BAF projectiles made 85 km Tycho crater.BAF projectiles made 85 km Tycho crater.

Tycho CraterTycho Crater

ConclusionsConclusionsThe breakup of the 170 km Baptistina parent body The breakup of the 170 km Baptistina parent body ~160 My ago triggered an asteroid shower. ~160 My ago triggered an asteroid shower.

It increased the impact flux of It increased the impact flux of DD > 1 km bodies on the > 1 km bodies on the terrestrial planets by a factor of 2terrestrial planets by a factor of 2--3. 3.

It is currently responsible for 20% of all NEOs and It is currently responsible for 20% of all NEOs and 40% of dark, C40% of dark, C--type NEOs.type NEOs.

The Baptistina family is the most likely source of:The Baptistina family is the most likely source of:

––The KThe K--T impactor (> 90% probability) and the T impactor (> 90% probability) and the Tycho impactor (> 70% probability).Tycho impactor (> 70% probability).

––The CM meteoritesThe CM meteorites