1. ASTR 1420 Lecture 9 Sections : 4.6, 6.4, 11.3
Mass Extinction
ASTR 1420
Lecture 9
Sections : 4.6, 6.4, 11.3

2. Mass Extinctions in the Earth History
Mass extinction (a.k.a. extinction event) : is a sharp decrease in the number of species in a relatively short period of time.

3. Mass Extinctions Check http://en.wikipedia.org/wiki/Impact_event
Over 99% of species that ever lived are now extinct
During the past 550 Myrs, there were five mass extinction events ( )when more than 50% of animal species died

4. Permian Extinction : “Great Dying”
96% all marine species and 70% land species died.
The "Great Dying" had enormous evolutionary significance: on land it ended the dominance of mammal-like reptiles and created the opportunity for archosaurs and then dinosaurs to become the dominant land vertebrate

5. K-T Extinction : End of dinosaurs
65 Myrs ago, 75% of species died.
Ending the reign of dinosaurs and started the world of mammals and birds.

6. Cause of Mass Extinction Events
What caused these events?
 sudden temperature changes??

7. What’s the cause? Atmospheric CO2 content (ppm)
Average Global Temperature
Atmospheric CO2 content (ppm)
Not all major mass extinctions coincide with sudden changes in temperature!
 Then, why?

8. Asteroid Impact! (for some cases, but not for all!)
Cretaceous-Tertiary Impact (K-T impact)

9. Causes
Flood basalt event (11 occurrences all coincide with extinction events)
Large magma flood
ash + dust
prevent photosynthesis
 destroy a food chain + CO2 emission and acid rain also.
See-level falls : destroy continental shelf area!  disrupt weather pattern
Methane clathrate (aka, methane ice, methane is a 20 times more efficient agent for greenhouse effect)

10. Causes Sea-level falls (7 matches out of 12 cases)
Methane clathrate (aka, methane ice, methane is a 20 times more efficient agent for greenhouse effect)
destroy continental shelf area!  disrupt weather pattern

11. Other Causes Impact events (1-50) Ice ages Hothouse (methane gun)
Nearby supernova or Gamma ray burst
Methane clathrate (aka, methane ice)
Methane clathrate = Methane is 20 times more efficient agent for greenhouse effect than CO2)

12. Chicxulub Impact (= dinosaur killer, K-T impact)
~180km in diameter
Recent discovery (1978)
Equals to the energy of 10,000+ times of all nuclear weapon detonations

13. Some recent impacts! Arizona (Barringer Crater) ~4,000 ft diameter
50m size iron meteor collided at a speed of ~20km/sec.
~50,000 yrs ago
Tunguska (June 30, 1908, Siberia)
Burst meteor in the air
(~5 miles high above the ground)
About 1,000 times stronger than the Hiroshima bomb.
Knocked off about 80 million trees within 15 miles
Typical crater size ~ hundreds times the size of impactor

14. A football field sized asteroid passes by this week!
Probability of impact : <~ 0.02%

15. Shoemaker-Levy 21 distinct fragments.
Comet debris ranging up to 2km in size collides with Jupiter at speed 37 miles/sec in 1996 July over a week.

16. Happens frequently… A chain of impact craters
on Ganymede (one of the largest satellites of Jupiter)

17. Torino scale
A method for categorizing the impact hazard of near-Earth objects (NEOs).
 assessing the seriousness of collision predictions by combining probability statistics and known kinetic damage potentials into a single threat value.
Apophis: Highest ever Torino scale (“4”)
Initial calculation of 2.7% chance to hit the Earth in 2029.
Current calc = 1 in 12.3 million chance to hit the Earth in 2037.
the detonation energy of the strongest nuclear bomb ever-made
NEO contains asteroids and comets!
20,000 potentially hazardous NEOs in close proximity.
NASA can't pay for a killer asteroid hunt  cost to find 90% of asteroids, comets (larger than 1km) would be about $1 billion

18. Holocene extinction Man-made one?
Holocene (since 8,000BC to present)
Most biologists view the present era as part of a mass extinction event, possibly one of the fastest ever
predict that humanity's destruction of the biosphere could cause the extinction of one-half of all species in the next 100 years.

19. Late Heavy Bombardment (threat to alien civilization?)
Short period ( Myr) of bombardment much later than the formation of planet

20. Sample Returns Apollo Mission Six Apollo missions : 382 kg
Three Luna missions : < 0.5kg
The moon has been visited by 12 Apollo astronauts during missions in the 1960's and 1970's, and by numerous robot spacecraft. A total of ~382 kg of samples were returned by these Apollo missions, and and additional 321 g of samples were returned by three Russian Luna robot spacecraft. There are also several meteorites on earth that are of known lunar origin. Below is a summary of lunar geology gleaned from several references, cited below. 

21. Moon Rocks
Thong, Shovel, Hammer, Drill 

22. Late Heavy Bombardment (LHB)
LHB = lunar cataclysm = terminal cataclysm Proposed in 1973 by Tera et al. who noted a peak in radiometric ages of lunar samples ~ Gyr
Ages of Apollo Rocks (in Gyr)
Moon is too small to have geological activity, so most of rocks should be as old as the age of the Moon!
 However, most Apollo rocks are ~4 Gyr old!
Sharply declining basin-formation rate between Imbrium (3.85 Ga) and final basin, Orientale (3.82 Ga)
Few rock ages, and no impact melt ages prior to 3.9 Ga (Nectaris age)

23. Non-Lunar Evidence for LHB
Cratered uplands on Mars/Mercury (and even Galilean satellites!) inferred to be due to same LHB… but absolute chronology is poorly known or unknown.
ALH84001 has a ~4 Gyr age… but that is “statistics of one”.
Peaks in resetting ages noted for some types of meteorites…
but age distributions differ from lunar case.

24. What caused LHB?
Generally, any dynamical readjustment of the planets in a planetary system that “shakes up” remnant small-body populations…  could occur late, even very late.
Outer solar system planetesimals from late-forming Uranus/Neptune (Wetherill 1975)
Break-up of large asteroid (but big enough asteroids difficult to destroy)
Expulsion of a 5th terrestrial planet (Chambers & Lissauer 2002; Levison 2002)
Outer Solar System planetesimals & asteroids perturbed by sudden expulsion of Uranus & Neptune from between Jupiter & Saturn (Levison et al. 2001)
Late-stage post Moon-formation Earth/Moon-specific LHB (Ryder 1990)
You can skip this slides when you study for an exam!

25. Clearing of Remnants  Late Heavy Bombardment
Gomes et al. (2005, Nature)
Red = Jupiter, White=Saturn, Green (cyan) = Netune, Purple=Uranus

26. LHB effects on the Earth
Extrapolating from lunar craters (and size difference b/w Earth and Moon), the Earth must have experienced…
22,000 or more impact craters with diameters > 20 km
about 40 impact basins with diameters about 1000 km
several impact basins with diameter about 5,000 km

27. LHB issues for Extra-Solar System Astrobiology
It is plausible that similar, or even much more extreme, LHBs or VLHBs would affect planets in other systems.
any special configuration to promote/enhance LHBs?
What range of bombardments foster life (exchanging materials, spurring evolutionary change)?
How big an LHB surely sterilizes a planet?
Prevent or significantly delay a start of alien life
Do all stars go through the LHB phase?

28. Evidence of LHBs at other stars?
BD (Song et al. 2005, Nature)
1-2 billion year old Sun-like star about 300 Light years away
million times more dust particles than the current Solar System
Even 100 times higher impact rate than the Solar System LHB impact rate

29. Sterilizing Impact simulation
Simulation of a slow impact by a 500km size asteroid…

30. Chapter/sections covered in this lecture : 4.6, 6.4, 11.3
In summary…
Important Concepts
Important Terms
History of mass extinctions
Causes of mass extinctions
Late Heavy Bombardment and its implication to astrobiology
Dynamical instability of planets
K-T impact (Chicxulub Impact)
Mass extinction
Torino scale
Late Heavy Bombardment
Sterilizing impact
Chapter/sections covered in this lecture : 4.6, 6.4, 11.3