Cosmic Catastrophes
A Survey of Some Examples

posted June 4, 2001, jeb

OUTLINE

Impact
Supernova
Gamma-ray Burst
Ice Ages
Summary

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Reading Assignments

• Interlude 14-1, "What Killed the Dinosaurs?", pages 322-323, Chaisson and McMillan
• Cometary Impact, pages 325-326, Chaisson and McMillan
• Interlude 21-2, "Nearby Supernovae", page 483, Chaisson and McMillan
• Gamma Ray Bursts, page 498-500, Chaisson and McMillan
• Illinois State Museum web presentation on the Ice Ages

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The hazards of the universe are inescapable

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Impact




On March 11, 1998, the international astronomical agency that tallies the orbits of asteroids and comets announced that an asteroid is likely to pass within 30,000 miles of Earth on Oct. 26, 2028, a Thursday, and there is a possibility that it would hit Earth.


Dr. Brian G. Marsden, director of the Central Bureau for Astronomical Telegrams at the Smithsonian Astrophysical Observatory, Cambridge, Mass., cautioned in an interview that calculations of the asteroid's progress are approximate and that there is no immediate cause for alarm.

The next day( March 12, 1998) the likelihood of apocalypse seemed to shrink when Marsden's colleagues contended that the chance of an impact was zero and he said that based on additional calculations, the chances were smaller than he had originally thought.


One official estimated the chances for a major event during our lifetime
"We have calculated on the basis of what we know of the Earth's impact history that during a human life span of 66 years, there is a 1 in 5,000 chance of a major impact," Dr. Gehrels said


Near misses are fairly common. For example:
On March 22, 1989, a gigantic asteroid just missed the Earth, and was not noticed until March 31
(It passed Earth at twice the Earth-Moon distance, about 500,000 miles)

Potentially hazardous asteroids

None are know which rise above 0 on the Torino Impact Hazard Scale




Fossil Record

Where depths of the Earth are exposed (for example, in the Grand Canyon) remains of extinct organisms (fossils) appear

Various species disappear at the same depths, all of the world

Extinction
Lesson - extinction is the rule
Most species that ever lived on Earth are now extinct
There are several spectacular extinctions




Mass Extinctions Revealed from the Fossil Record

millions of Percent of Marine Extinctions . Period years ago Families Genera Species examples Late Ordovician 435 27% 57% . . Late Devonian 365 19% 50% . armored fish Late Permian 245 57% 83% >90% trilobite Late Triassic 220 23% 48% . . Late Cretaceous 65 17% 50% >75% dinosaurs Late Ecocene 35 2% 16% . .



BBC site on Mass Extinctions




Extinction of the Dinosaurs




The dinosaurs vanished as a result of some enormous catastrophe 65 million years ago


Dinosaurs were gigantic (100 ton) masters of the Earth for about 160,000,000 years

Then, suddenly (in less than a million years) all of them were lost
(along with most other species)
This is something like wiping out all of the mammals on Earth today

How and why?


One possible cause of this could have been the impact of a comet or an asteroid

What is the evidence for this possibility?


An important discovery by Berkeley scientists, including Noble laureate Luis Alvarez
deposits of Iridium at the cretaceous period
• Heavy elements. such as iridium, are rare near the surface of the Earth
• The Alvarez group found large amounts at the cretaceous boundary
  
  

  Iridium deposits found at the K-T boundary around the world

  Location Concentration relative to normal Gubbio, Italy 30 times Haiti 300 times New Zealand 120 times Caspian Sea 70 times Texas 43 times Deep in the North Pacific Ocean 330 times

  
  
  Comets and Asteroids contain relatively more heavy elements than the surface of the Earth
  
  Therefore, the Alvarez group concluded that 65 million years ago the Earth was struck by a cosmic body leading to the extinction of most of the species, including the Dinosaurs


Other minerals have also been found whose origin is clearly meteoritic
• shocked crystals of quartz found at the K-T (Cretaceous/Tertiary) boundary show evidence for very high pressures
• nickeliferous magnetites have been found in several K-T sediments; they are formed during the fall of a meteorite through the atmosphere

The crater?
• Earth is scarred with nearly 100 craters larger than 0.1 km in diameter
  • Barringer Crater (1 km)
  • Quebec's Manicouagan Reservoir
    • 70 km diameter
  • Tunguska event of 1908 (in Siberia)
    • exploded in Earth's atmosphere before reaching surface
• Major collisions of large meteoroids with the Earth occur every few hundred-thousand years



From the amount of iridium found in the K-T layer around the world we can estimate:
• The associated comet (or asteroid) was about 10 kilometers in diameter
• The crater would have been about 200 kilometers in diameter
• The debris from the impact would have been raining down for about ten thousand years

The crater has very likely been found
Chicxulub on the Yucatan Peninsula, Mexico
• NASA's Ames research center uncovered a 170km diameter semi-circle of sinkholes

And pieces of the candidate asteroid have been recovered


Other possible causes of the demise of the dinosaurs:


• volcanic eruptions that covered an area the size of a continent
• protracted ice ages
• changes in sea level
• epidemics
• other factors




Jupiter, 1994

In 1994 the Earth witnessed a catastrophic event on the surface of Jupiter when the Comet Shoemaker-Levy crashed into its surface


• Comet Shoemaker-Levy crashed into the surface of Jupiter in July, 1994
• Comet was discovered in March, 1993
  • composed of several pieces
  • "string of pearls"
• How did Comet get this way?
  • July, 1992 passed within Roche Limit of Jupiter -> broke apart
• Series of impacts
  • Huge fireballs (hundreds of km)
    • (like a billion nuclear explosions)
  • Jupiter disturbed over thousands of kilometers

Periodicity of mass extinctions?



Some scientists have concluded that the rate of extinctions show a 26 million year periodicity


A Scientific American "Ask the Experts" discussion on this issue


Could a periodic mechanism for extinction be produced by a cosmic phenomena?

Some evidence also suggests cratering on Earth shows a similar periodicity


(CAUTION: This is fairly speculative)


Two possible mechanisms have been considered
• Periodic passage of the Solar System through the plane of the Milky Way
• Companion star to the Sun (Nemesis)


Each idea causes a disruption of the Oort Cloud, increasing the number of comets penetrating the inner Solar System

Are either of these mechanisms at work? - We do not know

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Supernova

Suppose a star about 10 light years from us exploded as a supernova

Such a supernova would be billions of times as bright as the Sun, in absolute terms

Example: SN1987A in the Large Magellanic Cloud


However due to the distance of about a million times the Sun's distance, it would appear somewhat fainter than the Sun in the Earth's sky

Besides the visible light, a very intense flux of X-rays, gamma-rays, and cosmic rays would accompany the supernova
• Radiation death would be a risk on Earth
• Substantial long-term atmospheric changes (such as in the ozone layer) could result
• Could a nearby supernova explosion have been the cause of one of the Earth's mass extinction events?

A few stars explode as supernovae in the Milky Way every century

How often can we expect an event this close (10 light years) to Earth?
About once every few billion years or so

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Gamma-ray Burst

Gamma-ray Bursts are the most violent events known in the Universe


• Discovered in late 1960s by military satellites monitoring nuclear weapons tests
  
  
• Today these bursts are one of astronomy's deepest mysteries
  
  
• Now detected at rate of about 1 per day
  
  
  • Uniformly distributed in sky
    
    
  • Therefore, not distributed within the Milky Way Galaxy, or would follow distribution of stars
    
    
  • Must be at very great distances from us (say billions of parsecs), which makes their luminosity enormous (this is a conclusion of studying the optical counterparts, such as for GRB970228)
  
  
  
• Could they be they be merging binary neutrons stars?
  • This is one leading explanation, but we do not yet know what is their source of energy

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Ice Ages

From landforms, geologists conclude there have been great continental glaciation
ice on polar caps has extended far into what now is a temperate climate - like USA


Two very different time scales
• long, generally cool intervals of Earth history during which glaciers waxed and waned
  • tens to hundreds of millions of years
• shorter time periods during which glaciers were near their maximum extent.
  • tens of thousands of years
  • "glaciations"


Last retreat of ice about 10,000 years ago

During ice age - periods of advance and retreat

Present volume of ice on Earth
(25,000,000 cubic kilometers)
is much more than existed during most of history
If it melted the ocean would rise about 200 feet


Perhaps we are now in a period of retreat that will turn to a period of advances
This is a practical reason to understand the cause


Consider some possible explanations for the onset of the glacial epoch several million years ago
• Sun stopped radiating
  perhaps the fusion in the Sun goes through cycles with the Sun cooling periodically
• Volcanic ash from period of volcanic activity
  ash in the atmosphere would block some of the Sun's rays and cool the Earth
• Movement of the continents
  Suppose the landmasses that are near the poles were not there in the past
  If the poles had been oceans they might not have been as cold in the past
• Earth's wobble and shape of the Earth's orbit about the Sun
  • This is the most likely explanation
  • Three orbital parameters are especially important in causing ice sheet waxing and waning:
    1. Changes in the eccentricity of the Earth's orbit
       • Eccentricity measures how far the orbit is from being a circle
       • The eccentricity of the Earth's orbit has varied over time
    2. Changes in the tilt of the Earth's rotation axis
       • The tilt of the Earth's spin axis has varied over time
    3. The precession of the equinoxes
       • Precession in time
    These factors combine to influence the amount and distribution of solar radiation reaching the Earth
    • Changes vary with both latitude and season
    • Because of the different periodicities of variation for the three factors, the composite variations in solar radiation are very complex
  
  
  

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Summary

There are many hazards to life from cosmic events.


• Impacts of asteroids and comets have occurred and will occur in the future
  • The dinosaurs likely were killed off by such an event
  • Many other cataclysmic extinctions are candidates for this interpretation
  
  
• A nearby supernova could be quite harmful to life on Earth
  
  
• The most violent explosion known are the Gamma Ray Bursts
  • Fortunately they appear to be at too great a distance to be a threat
  
  
• What causes the ice ages in Earth's history?
  • Likely changes in the Earth's rotation and orbit