An Asteroid Shower Over the Cretaceous Period

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1 An Asteroid Shower Over the Cretaceous Period William Bottke Southwest Research Institute (with thanks to David Vokrouhlicky and David Nesvorny)

2 Motivation 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).

3 The Recent Impact Flux on the Earth and Moon

4 Impact Rates on Earth and Moon Craters on N. American, European Cratons Craters show two distributions. D crater > 20 km Sample references: Grieve and Shoemaker (1994); Earth Impact Database; abase; Harnack and Kleppinger (1997)

5 Impact Rates on Earth and Moon Craters on N. American, European Cratons Erosion, Bias, or Surge? D crater > 20 km Craters show two distributions. 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. Sample references: Grieve and Shoemaker (1994); Earth Impact Database; abase; Harnack and Kleppinger (1997)

6 Impact Rates on Earth and Moon Additional terrestrial and lunar data supports factor of 2 change in crater production rate over last 120 My. Location of Crater Data Set The Moon, Australia US Mississippi Lowlands; N. American, European Cratons Time Period (My Ago) <120 Production Rate D > 20 km Craters (10( -15 km - 2 yr - 1 ) 3-4 ~6 Sample references: Grieve and Shoemaker (1994); McEwen et al. (1997); Shoemaker et al. (1998)

7 Asteroids Evolution and Delivery to the Inner Solar System

8 Collisions in the Asteroid Belt 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)

9 Collisions in the Asteroid Belt 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)

10 Yarkovsky Drift into Resonances Koronis family Observed Model Bottke et al. (2001)

11 Fraction of Material Reaching Earth 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.

12 Fraction of Material Reaching Earth 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.

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

14 The Baptistina Asteroid Family (BAF) BAF BAF The BAF has mostly been overlooked to date because: It is a dark, hard-to to-see C-complex C 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 S Flora family along the inner edge of the main belt.

15 The Baptistina Asteroid Family (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?

16 Age of the BAF Yarkovsky- YORP model used to get family s age. Model Observed Many Interlopers Best fit age is 160 ± 20 My.

17 Size Distribution of the BAF 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.

18 Yarkovsky Evolution of BAF Members Simulation: D > 10 km bodies near J7:2 and M5:9. Overall, 10-20% of all km-sized BAF members escape over 160 My.

19 Impact Flux on Earth The BAF produces a surge in the terrestrial planet impact flux that peaks My after the family-forming forming event.

20 Results We input our collisional and dynamical simulations into a Monte-Carlo code and found for the Earth: Projectile size # BAF impacts over 160 Ma # background impacts over 160 Ma Increased by Factor D > 1 km D > 5 km 200 ± 60 6 ± ± 20 3 ± D > 10 km 1 ± ± The BAF increased impact flux on the Earth and Moon by 2-3 times over last 160 My!

21 Implications

22 The Cretaceous/Tertiary (K/T) Impactor 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.

23 The Cretaceous/Tertiary (K/T) Impactor Typical CM2: Murchison Kyte (1998) K/T projectile was a CM2-type carbonaceous chondrite. Consistent with fossil meteorite found in North Pacific sediments s from K/T boundary.

24 The Cretaceous/Tertiary (K/T) Impactor Trinquier et al. (2006) K/T projectile was a CM2-type carbonaceous chondrite. Good match to 54 Cr isotopes taken from samples found at 3 well- characterized K/T boundary sites (i.e., all have strong Ir enhancement).

25 BAF as the Source of the K/T Impactor 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. S 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 My. > 90% probability that BAF is source of K/T impactor!

26 BAF as the Source of the Tycho Impactor 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.

27 Conclusions 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 It is currently responsible for 20% of all NEOs and 40% of dark, C-type C NEOs. The Baptistina family is the most likely source of: The K-T K T impactor (> 90% probability) and the Tycho impactor (> 70% probability). The CM meteorites

29 Collisional Evolution of BAF Members BAF members also undergo collisional evolution. The D > 1 km population drops by 40% within 160 My. The D > 5 km population is unaffected.

30 Dynamical Evolution of BAF Members in the Terrestrial Planet Region 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.

31 Motivation 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?

32 Motivation 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);

33 Age of the BAF 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)

34 Known Asteroid Impact Structures Many craters concentrated in particular areas of North America, Europe, and Australia.

35 Known Asteroid Impact Structures Craters often found on cratons,, stable continental crust regions that have survived plate tectonics for > 500 My.

36 Yarkovsky Drift into Resonances 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)

A: Planets. Q: Which of the following objects would NOT be described as a small body: asteroids, meteoroids, comets, planets?

A: Planets. Q: Which of the following objects would NOT be described as a small body: asteroids, meteoroids, comets, planets? Q: Which of the following objects would NOT be described as a small body: asteroids, meteoroids, comets, planets? A: Planets Q: What can we learn by studying small bodies of the solar system? A: We can