Mass Extinction
ASTR 1420
Lecture 9
Sections : 4.6, 6.4, 11.3
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.
Mass Extinctions
Check
• http://en.wikipedia.org/wiki/Impact_event
• http://en.wikipedia.org/wiki/Mass_extinction
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
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.
Cause of Mass Extinction Events
• What caused these events?
 sudden temperature changes??
Average Global Temperature
Atmospheric CO2 content (ppm)
What’s the cause?
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
 ash + dust
 prevent photosynthesis
 destroy a food chain + CO2 emission and acid rain also.
Causes
• Sea-level falls (7 matches out of 12 cases)
Other Causes
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•
•
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Impact events (1-50)
Ice ages
Hothouse (methane gun)
Nearby supernova or Gamma ray burst
Methane clathrate (aka, methane ice)
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 high above the ground)
• About 1,000 times stronger than
the Hiroshima bomb.
• Knocked off about 80 million trees
within 15 miles
A football field sized asteroid passes by this week!
Shoemaker-Levy
Happens frequently…
• A chain of impact craters
on Ganymede (one of the
largest satellites of Jupiter)
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
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.
Holocene (since 8,000BC to present)
Late Heavy Bombardment (threat to alien civilization?)
• Short period (50-100 Myr) of
bombardment much later than the
formation of planet
Sample Returns
• Apollo Mission
• Six Apollo missions : 382 kg
• Three Luna missions : < 0.5kg
Moon Rocks
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
~4.0 - 3.8 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!
Ages of Apollo Rocks (in Gyr)
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.
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)
Gomes et al. (2005, Nature)
Clearing of Remnants  Late Heavy Bombardment
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
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 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?
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 the Solar System LHB impact rate
Sterilizing Impact simulation
Simulation of a slow impact by a 500km size asteroid…
http://www.youtube.com/watch?v=LlF8APEkh-E
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
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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