Impacts and Mass Extinctions

The impact of a space object with a size greater than (.62+ miles) would be expected to be felt over the entire surface of the Earth. 

Smaller objects would certainly destroy the ecosystem in the vicinity of the impact, similar to the effects of a volcanic eruption, but larger impacts could have a worldwide effect on life on the Earth.

Regional and Global Effects

As humans, we have no firsthand knowledge of what the effects of an impact of a large meteorite or comet would be.

However, science has studied this, and from calculations and experiments, has come to a general consensus. The impact of a large object with the Earth would cause:

1. A massive earthquake, up to Richter Magnitude 13, and numerous large magnitude aftershocks.
2. Large quantities of dust blasted into the atmosphere.
   
   This would block incoming solar radiation. The dust could take months to settle back to the surface.  Meanwhile, the Earth would be in a state of continual darkness, and temperatures would drop throughout the world, generating global winter like conditions.
   
   Blockage of solar radiation would also diminish the ability of plants, to photosynthesize. Since photosynthetic organisms are the base of the food chain, this would seriously disrupt all ecosystems.
3. Widespread wildfires ignited by radiation from the fireball as it passed through the atmosphere.
   
   Smoke from these fires would further block solar radiation to enhance the cooling effect and further disrupt photosynthesis.
4. A large steam cloud to rise - if the impact was in the oceans - produced by the sudden evaporation of the seawater.
   
   This water vapor and CO₂ would remain in the atmosphere long after the dust settled. Because both of these gases are greenhouse gases, solar radiation would be scattered and create a warming effect.  Thus, after the initial global cooling, the atmosphere would undergo global warming for many years after the impact.
5. A giant tsunami - if the impact was in the oceans.
   
   For a 10 km-diameter object the leading edge would hit the sea floor of the deep ocean basins before the top of the object had reached sea level.  The tsunami from such an impact is estimated to produce waves from 1 to 3 km high.  These could easily flood the interior of continents.
6. Large amounts of nitrogen oxides produced from combined Nitrogen and Oxygen - due to the shock produced by the impact.
   
   These nitrogen oxides would combine with water in the atmosphere to produce nitric acid which would fall back to the surface as acid rain, resulting in the acidification of surface waters.

The Geologic Record of Mass Extinction

The suggested causes of these mass extinctions have been:

• Large volcanic eruptions,
• Changes in climatic conditions,
• Changes in sea level,
• Meteorite impacts.

While the meteorite impact theory of mass extinctions has become accepted by many scientists for particular extinction events, there is still considerable controversy among scientists.

However, many accept the possibility that an impact with a large object could have caused at least some of the mass extinction events.

So, let's look at the known extinctions, shown in the Geologic Record.

Major extinction events occurred at the

• End of the Tertiary Period, 1.6 million years (m.y.) ago.
• Boundary between the Cretaceous and Tertiary periods 65 m.y. ago.
• End of the Triassic, 208 m.y. ago.
• End of the Permian, 245 m.y. ago (estimated that over 96% of the species alive at the time became extinct).
• End of the Devonian, 360 m.y. ago
• End of Ordovician, 438 m.y. ago
• End of the Cambrian period, 505 m.y. ago

So, from just this overview, many of the world's plants and animals, have at times, gone extinct.

Let's look at some of these.

The most well known extinction was at the Cretaceous - Tertiary boundary. This is called the K-T boundary. ( Geologists use the letter K to stand for the Cretaceous Period and the letter T for the Tertiary.)

Thus, at 65 million years ago evidence shows impact event, which resulted in the extinction of over 50% of the species living at the time, including the dinosaurs.

In 1978 a group of scientist led by Walter Alvarez of the University of California, Berkeley, were able to locate the K-T boundary very precisely in layers of limestones near Gubbio, Italy.

At the boundary they found a thin clay layer.  Chemical analysis of the clay revealed that it contains a high concentration of the rare element Iridium (Ir).  Ir has extremely low concentrations in most crustal rocks; however, it reaches very high concentrations in meteorites.  The only other possible source of high concentrations of Ir is in basaltic magmas. 

Over the next several years, the K-T boundary was located at several other sites throughout the world, and also found to have a thin clay layer with high concentrations of Ir.  Although a large eruption of basaltic magma could not immediately be ruled out as the source of the high concentration of Ir, other evidence began to accumulate that the fallout of impact ejecta had been responsible for both the thin clay layers and the high concentrations of Ir. 

Among the evidence found at different localities where the K-T boundary is exposed is:

• Clay layers at some localities have a high proportion of black carbon that could have originated as soot produced by wildfires set off by an impact.
• Some of the clay layers contain grains of quartz with a crystal structure that shows evidence that the quartz was severely strained by a large shock.
• In some clay layers tiny grains of the mineral stishovite is found. Stishovite is a high pressure form of SiO₂ that is not found at the Earth's surface except around known meteorite impact sites.  The mineral can only be produced as a result of extremely deep burial in the Earth, or by high pressure generated by an impact.
• Other clay layers contain tiny spherical droplets of glass, called spherules.  The glass is not basaltic in composition, but could represent droplets of melt formed during an impact event.

At the time of these discoveries, there was no known impact structure on the Earth with an age of 65 million years.  This is not unexpected, since 71% of the Earth's surface is covered by water, and is largely unexplored. 

But, in the late 1980s attention started to be focused on a buried impact site near the tip of the Yucatan Peninsula, in Mexico.  Here oil geologists had drilled through layers of brecciated rock and found  impact melt rock.  Further geophysical studies revealed a circular structure about 180 km in diameter. 

Radiometric dating reveals that the structure, called the Chicxulub Crater,  formed about 65 million years ago. 

Although the crater itself is now filled and buried by younger rocks, drilling throughout the Gulf of Mexico has revealed the presence of shocked quartz, glass spherules, and soot in deposits the same age as the crater. 

In addition, geologists have found deposits from the tsunami that was generated by the impact all along the Gulf of Mexico coast extending considerable distance inland from the current shoreline. The size of the crater suggests that the object that produced it was about 10 km in diameter. 

While there is still some debate among geologists and paleobiologists as to whether or not the extinctions that occurred at the K-T boundary were caused by the impact that formed Chicxulub Crater, it is clear that an impact did occur about 65 million years ago, and that it likely had effects that were global in scale.

Here's another even larger extinction.

It was only recently that paleontologists, like hikers stumbling upon an unmarked grave in the woods, noticed a startling pattern in the fossil record:

Below a certain point in the accumulated layers of earth, the rock shows signs of an ancient world teeming with life. In more recent layers just above that point, signs of life all but vanish.

Somehow, most of the life on Earth perished in a brief moment of geologic time roughly 250 million years ago. Scientists call it the Permian-Triassic extinction or "the Great Dying."

No class of life was spared from the devastation. Trees, plants, lizards, proto-mammals, insects, fish, mollusks, and microbes -- all were nearly wiped out. Roughly 9 in 10 marine species and 7 in 10 land species vanished.

Life on our planet almost came to an end.

Scientists have suggested many possible causes for the Great Dying:

• Severe volcanism,
• A nearby supernova,
• Environmental changes wrought by the formation of a super-continent,
• The devastating impact of a large asteroid -- or some combination of these.

Proving which theory is correct has been difficult. The trail has grown cold over the last quarter billion years; much of the evidence has been destroyed.

Undaunted, Luann Becker, a geologist at the University of California, Santa Barbara led a NASA-funded science team to sites in Hungary, Japan and China where 250 million year old rocks still exist and have been exposed.

There they found telltale signs of a collision between our planet and an asteroid 6 to 12 km across -- in other words, as big or bigger than Mt. Everest.

Deep inside Permian-Triassic rocks, Becker's team found soccer ball-shaped molecules called "fullerenes" (or "buckyballs") with traces of helium and argon gas trapped inside. The fullerenes held an unusual number of ³He and ³⁶Ar atoms -- isotopes that are more common in space than on Earth. Something, like a comet or an asteroid, must have brought the fullerenes to our planet.

Right:  The carbon atoms in a fullerene molecule are arranged in a spherical pattern similar to a geodesic dome. (Geodesic domes were invented by Buckminster Fuller, hence the name of the molecules.) This shape allows the fullerenes to trap gases inside. Image courtesy Luann Becker.

Becker's team had previously found such gas-bearing buckyballs in rock layers associated with two known impact events:

• The 65 million-year-old Cretaceous-Tertiary impact and
• The 1.8 billion-year-old Sudbury impact crater in Ontario, Canada.

They also found fullerenes containing similar gases in some meteorites. Taken together, these clues make a compelling case that a space rock struck the Earth at the time of the Great Dying.

Many scientists believe that life was already struggling when the putative space rock arrived. Our planet was in the throes of severe volcanism. In a region that is now called Siberia, 1.5 million cubic kilometers of lava flowed from an awesome fissure in the crust. (For comparison, Mt. St. Helens unleashed about one cubic kilometer of lava in 1980.) Such an eruption would have scorched vast expanses of land, clouded the atmosphere with dust, and released climate-altering greenhouse gases.

World geography was also changing then. Plate tectonics pushed the continents together to form the super-continent Pangea and the super-ocean Panthalassa. Weather patterns and ocean currents shifted, many coastlines and their shallow marine ecosystems vanished, sea levels dropped.

"If life suddenly has all these different things happen to it," Becker says, "and then you slam it with a rock the size of Mt. Everest -- boy! That's just really bad luck."

What would happen if another such event occurred while we humans dominate the surface of the Earth? What could we do, if anything to prevent such a catastrophic disaster?

Human Hazards

It should be clear that even if an impact of a large space object did not cause the extinction of humans, the effects would cause a natural disaster of proportions never witnessed by the human race. 

Here we first look at the chances that such an impact could occur, then look at how we can predict or provide warning of such an event, and finally discuss ways that we might be able to protect ourselves from such an event.

• Risk - It is estimated that in any given year the odds that you will die from an impact of an asteroid or comet are about 1 in 20,000. 

The table below shows the odds of dying in the U.S. from various other causes.  Although 1 in 20,000 seem like long odds, you have about the same odds of dying in an airplane crash, and somewhat less risk of dying from other natural disasters likes floods and tornadoes.  In fact the odds of dying from an impact event are much better than the odds of winning the lottery.

Here's a few events to think about.

In March, 1989 an asteroid named 1989 FC passed within 700,000 km of the Earth, crossing the orbit of the Earth.  It was not discovered until after it had passed through the orbit of the Earth. 

Its size was estimated to be about 0.5 km.   Such a body is expected to hit the Earth about once every million years or so, and would release energy equivalent to about 10,000 megatons of TNT, a little greater than the energy released in a nuclear war, and enough to cause nuclear winter.  Although 700,000 km seems like a long distance, it translates to a miss of the Earth by only a few hours at orbital velocities.

On March 19, 2004, a 30 m diameter asteroid, named 2004 FH, passed within 26,500 miles (43,000 km) of earth, just beyond the orbit of weather satellites.  The object was small, and likely would have only caused a local effect if it had hit the earth's atmosphere, but it was discovered only 4 days before it passed.

• Prediction and Warning - It is estimated that over 90% of Near Earth Objects (NEOs) have not yet been discovered.  Because of this, with our present knowledge, there is a good chance that the only warning we would have is the flash of light from the fireball as one of these objects entered the Earth's atmosphere. 
  
  Scientists have proposed the "Spaceguard Survey" to find and track all of the large NEOs.  If such a survey is carried out, we could predict the paths of all NEOs and have years to decades to prepare for an NEO that could impact the Earth.

• Mitigation - Impacts are the only natural hazard that we can prevent from happening by either deflecting the incoming object or destroying it. 
  
  Of course, we must first know about such objects and their paths in order to give us sufficient warning to prepare a defense.  Sufficient time is usually thought to be about 10 years.  This would likely give us enough time to prepare a space mission to intercept the object and deflect its path by setting off a nuclear explosion or some other manner. 

Currently, however, there are no detailed plans.  But, even if we did not have the ability to destroy or deflect such an object, 10 years warning would provide sufficient time to store food and supplies, and maybe even evacuate the area immediately surrounding the expected impact site.

In 1993 and 1998 the U.S. Congress held hearings to study the hazards associated with NEOs.

In 1997 NASA formulated a plan to find 90% of the NEOs larger than 1 km in diameter within the next 10 years.  Still, at the current rate at which these objects are discovered and tracked, it will take over 100 years to achieve the 90% objective.

Currently NASA spends about $3 million per year on this survey of space objects.

How does this compare with funding for mitigation of less likely, but more local hazards in the Table of odds shown above?

Pages

The Mary Elizabeth Collection

Solar System
Before the Beginning
Our Beginning

Comets
    Stardust - A Robotic Mission

The Stones
    Abee - The Mystery
    Allende - A Blast
    Axtell
    Bonita Springs
    Cat Mountain
    Chergach (aka Mali)
    Claxton
    Gujba
    Kendleton
    Melrose - The Golden One
    Millbillillie
    Mundrabilla
    Murchison
    Saratov
    Vesta & Its Meteorites
        Bilanga
        Chaves
        Sioux County
Stony Irons
    Beautiful Esquel
    Brenham
    Pallas Iron
    Vaca Muerta
The Irons
    An American Icon
    Campo Del Cielo
    Cape of Good Hope
    Coahuila
    Gibeon
    Henbury
    The Mythic Kaalijarv
    Nantan
    Nelson County
    Sikhote-Alin
    Wolfe Creek
Historic Meteorites
    Orgueil - & the Comet
    Pultusk Shower
    Weston

Glossary

Impact Features
   Rocks
   Craters of the World
   Events
   Mass Extinctions

Moon Rocks FAQs

Links

Types of Meteorites
   Pallasites -- A Rare View
Meteor Showers
Interesting meteorite falls

NASA's Earth & Space Sciences

Near-Earth Object (NEO) Program
Basic Science II: Impact Cratering
Chesapeake Bay impact crater

Media

Peekskill N.Y. fireball video
London Natural History Museum video
Video of crater in Arizona
Understanding: Prehistoric Meteor Hit the Caribbean Sea

National Geographic News

• Planets Can "Ping Pong" From Star to Star
• Is This Russian Landscape the Birthplace of Native Americans?
• Elephants Took 24 Million Generations to Evolve From Mouse-Size
• Space Pictures This Week: Hubble Galaxy, Poet Nebula, More
• Giant Crack in Antarctica About to Spawn New York-Size Iceberg

Discover Space

• Hey, I can see my snow-covered house from here! | Bad Astronomy
• OK, a couple of more things about a Moon base | Bad Astronomy
• An astronomer’s paradise | Bad Astronomy
• Space caturday | Bad Astronomy
• Video of the lunar far side from GRAIL/Ebb | Bad Astronomy

If interested in meteorites, we are happy to link you to these outstanding sites:

• New England Meteoritical Services, Dir, Russell Kempton: www.meteorlab.com

• Aerolite Meteorites, Owner, Geoffrey Notkin: www.aerolite.org