Transcript Mass Extinction

Mass Extinction
ASTR 1420
Lecture 9
Sections : 4.6, 6.4, 11.3
Mass Extinctions in the Earth History
Mass Extinctions
Check
• http://en.wikipedia.org/wiki/Impact_event
• http://en.wikipedia.org/wiki/Mass_extinctio
n
• Over 99% of species that ever lived are now
extinct, but extinction occurs at an uneven
rate.
• During the past 550 Myrs, there were five
mass extinction events when more than 50%
of animal species died.
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
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.
What’s the cause? Temperature?
Not all major mass extinctions coincide with sudden
changes in temperature!  Then, why?
Asteroid Impact! (for some cases, but not for all!)
Causes
• Flood basalt event (11 occurrences all coincide with extinction events)
Large magma flood  ashes, dust  prevent photosynthesis  destroy food
chain ; CO2 emission and acid rain also.
Causes
• Sea-level falls (7 matches out of 12 cases)
destroy continental shelf area!  disrupt weather pattern
Causes
• Impact events (1-50)
• Ice ages
Nearby supernova or Gamma ray burst
Hothouse (methane gun)
Methane clathrate (aka, methane ice, methane
is a 20 times more efficient agent for
greenhouse effect)
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
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)
• About 1,000 times stronger than
the Hiroshima bomb.
• Knocked off about 80 million trees
within 15miles
Shoemaker-Levy
Happens frequently…
• A chain of impact craters
on Ganymede
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.
NASA can't pay for a killer asteroid hunt  cost to find 90% of
asteroids, comets (larger than 1km) would be about $1 billion
Holocene extinction
Man-made one?
• 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.
Late Heavy Bombardment
• short period (50-100 Myr) of
bombardment much later than the
formation of planet
• Apollo Mission
Sample Returns
• Six Apollo missions : 382 kg.
• Three Luna missions : <
0.5kg.
Moon Rocks
Late Heavy
Bombardment
LHB = lunar cataclysm = terminal
cataclysm
Moon does not have plate-tectonics, so
all rocks formed by various impacts
should be concentrated on earlier
ages!
• Proposed in 1973 by Tera et al. who
noted a peak in radiometric ages of
lunar samples ~4.0 - 3.8 Ga
• 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)
Proposed Dynamical Origins for LHB
dynamical readjustment of planets in a planetary system
can “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)
Gomes et al. (2005, Nature)
Clearing of Remnants  Late Heavy Bombardment
LHB effects on the Earth
• Extrapolating from lunar craters (and the 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
Sterilizing impact :
Impact on a planet which wipes out all life forms.
Depends on the size and velocity of an impactor (about 200300 km diameter?)
Sterilizing Impact simulation
Simulation of a slow impact by a 500km size asteroid…
http://www.youtube.com/watch?v=LlF8APEkh-E
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.
o any special planetary 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
• Does all stars go through the LHB
phase?
Evidence of LHBs at other stars?
BD+20 307
(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 Solar System LHB…
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
•
•
•
•
•
Mass extinction
K-T impact (Chicxulub Impact)
Torino scale
Late Heavy Bombardment
Sterilizing impact
Chapter/sections covered in this lecture : 4.6, 6.4, 11.3
Extreme Life Forms: next class!!