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Learning Objectives. Differentiate between asteroids, meteoroids, and comets. Describe the physical processes associated with airbursts and impact craters. Suggest the possible causes of mass extinction, and provide examples.
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Learning Objectives • Differentiate between asteroids, meteoroids, and comets. • Describe the physical processes associated with airbursts and impact craters. • Suggest the possible causes of mass extinction, and provide examples. • Evaluate the evidence for the impact hypothesis that produced the mass extinction at the end of the Cretaceous Period.
Learning Objectives, cont. • List the physical, chemical, and biological consequences of impact from a large asteroid or comet. • Analyze the risk of impact or airburst of extraterrestrial objects, and suggest how that risk might be minimized.
Chelyabinsk Meteor, February 2013 • Small asteroid traveling about 20 times faster than speeding bullet • Entered Earth’s atmosphere and exploded • People living in Chelyabinsk could feel the intense heat from the fireball as well as the shock wave from the explosion • Meteor did not impact Earth • Exploded in the stratosphere about 23 km (~15 mi) above the surface • Released energy equivalent to 20 to 30 World War II-era nuclear explosions • Largest known asteroid to enter Earth’s atmosphere since Tunguska airburst
Chelyabinsk Meteor, February 2013, cont. • Before entry and explosion, no one knew it was coming • Meteor astrophysicists were looking the other way tracking a much larger and completely unrelated asteroid • Chelyabinsk meteor damage • More than 7000 buildings in six cities damaged • About 1500 people required medical attention • People were confused and frightened about what was happening • Mobile phone networks were overloaded with calls • Office buildings evacuated and schools closed • Broken windows in homes required immediate attention due to weather
14.1 Earth’s Place in Space • Origins of universe begin with “Big Bang” 14 billion years ago • Explosion producing atomic particles • First stars probably formed 13 billion years ago • Lifetime of stars depends on mass • Large stars burn up more quickly ~100,000 years • Smaller stars, like our sun, ~10 billion years • Supernovas signal death of star • No longer capable of sustaining its mass and collapses inward • Explosion scatters mass into space creating a nebula • Nebula begins to collapse back inward on itself and new stars are born in a solar nebula
14.1 Earth’s Place in Space, cont. • Five billion years ago, supernova explosion triggered the formation of our sun • Sun grew by buildup of matter from solar nebula • Pancake of rotating hydrogen and helium dust • After formation of sun, other particles were trapped in rings • Particles in rings attracted other particles and collapsed into planets • Earth was hit by objects that added to its formation • Bombardment continues today at a lesser rate
Asteroids, Meteoroids, and Comets • Particles in solar system are arranged by diameter and composition • Asteroids • Found in asteroid belt between Mars and Jupiter • Composed of rock, metallic, or combinations • Meteoroids are broken up asteroids • Meteors are meteoroids that enter Earth’s atmosphere • Burn and create “shooting stars” • Comets have glowing tails • Composed of rock surrounded by ice • Originated in Oort Cloud beyond the Kuiper Belt
Diagram of Our Planetary System Showing Asteroid Belt and Kuiper Belt
14.2 Airbursts and Impacts • Objects enter Earth’s atmosphere at 12 to 72 km/s (~7 to 45 mi per second) • Metallic or stony • Heat up due to friction as they fall through atmosphere, produce bright light and undergo changes • Meteoroid will either • Explode into an airburst • Object explodes in atmosphere 12 to 50 km (~7 to 31 mi.) • Ex: Tunguska • Or collide with Earth as a meteorite • If the object strikes Earth • Concentrated in Antarctica
Impact Craters • Provide evidence of meteor impacts • Example: Barringer Crater in Arizona • Bowl shaped depressions with upraised rim • Rim is overlain by ejecta blanket, material blown out of the crater upon impact • Broken rocks cemented together into Breccia • Features of impact craters are unique from other craters • Involve high velocity, energy, pressure and temperature • Kinetic energy of impact produces shock wave into earth • Compresses, heats, melts, and excavates materials • Rocks become metamorphosed or melt with other materials
Extraterrestrial Impact Craters of the United States and Canada
Impact Craters, cont. • Can be grouped into two types • Simple craters • Typically small < 6 km (4 mi.) • Ex. Barringer Crater • Complex impact craters • Larger in diameter > 6 km (4 mi.) • Rim collapses more completely • Center uplifts following impact • Ancient impact craters difficult to identify • Usually eroded or filled with sedimentary depostis • Examples: Chesapeake Bay in U.S. and Manicouagan Crater in Quebec
Impact Craters, cont. • Craters are much more common on Moon • Most impact sites on Earth are in ocean where they are buried or destroyed • Impact craters on land are now generally subtle features because they have been eroded or buried by debris • Smaller meteoroids and comets tend to burn up and disintegrate in Earth’s atmosphere before impact
Uniformitarianism, Gradualism, and Catastrophism • Catastrophism • Those studying the formation of mountains, large river valleys, and other features had a hard time understanding how they could be formed in 6000 years • Based on Archbishop Ussher’s “young Earth” belief, concluded that the processes were catastrophic in nature • Gradualism or Uniformitarianism • James Hutton introduced concept in 1785, popularized by Charles Hutton • Present geological processes may be studied to learn the history of the past • Argued Earth must be much older than 6000 years
Uniformitarianism, Gradualism, and Catastrophism, cont. • Uniformitarianism lasted into the twentieth century and culminated with plate tectonics • Occasionally scientists also discover evidence of catastrophic events • Impact craters • Rapid extinctions • Lead to a new concept • Punctuated uniformitarianism • Although uniformitarianism explains the long geologic record of gradual mountain building, canyon erosion, and landscape construction, periodic catastrophic events do occur and can cause mass extinctions
14.3 Mass Extinctions • Sudden loss of large numbers of plants and animals relative to number of new species being added • Defines the boundaries of geologic periods or epochs • Usually involve rapid climate change, triggered by • Plate Tectonics • Slow process that moves habitats to different locations • Volcanic activity • Flood basalts produce large eruptions of CO2, warming Earth • Silica-rich explosions produce volcanic ash that reflects radiation, cooling Earth • Extraterrestrial impact or airburst
14.3 Mass Extinctions, cont. • Six major mass extinctions • Ordovician, 446 mya, continental glaciation in Southern Hemisphere • Permian, 250 mya, volcanoes causing global warming and cooling • Triassic-Jurassic boundary, 202 mya, volcanic activity associated with breakup of Pangaea • Cretaceous-Paleogene boundary (K-Pg boundary), 65 mya, Asteroid impact • Eocene period, 34 mya, plate tectonics • Pleistocene Epoch, initiated by airburst, continues today caused by human activity
K-Pg Boundary Mass Extinction • Dinosaurs disappeared with many plants and animals • 70 percent of all genera died • Set the stage for evolution of mammals • First question, What does geologic history tell us about K-Pg Boundary? • Walter and Luis Alvarez decided to measure concentration of Iridium in clay layer at K-Pg boundary in Italy • Fossils found below layer were not found above • How long did it take to form the clay layer? • Iridium deposits say that layer formed quickly • Probably extinction caused by single asteroid impact
K-Pg Boundary Mass Extinction, cont. • Alvarez did not have a crater to prove the theory • Crater was identified in 1991 in Yucatan Peninsula • Diameter approx. 180 km (112 mi.) • Nearly circular • Semi-circular pattern of sinkholes, cenotes, on land defining edge • Possibly as deep as 30 to 40 km (18 to 25 mi.) • Slumps and slides filled crater • Drilling finds breccia under the surface • Glassy indicating intense heat
K-Pg Boundary Mass Extinction, cont. • Sequence of events • Asteroid moving at 30 km (19 mi.) per second • Asteroid impacts Earth produces crater 200 km (125 mi.) diameter, 40 km (25 mi.) deep • Shock waves crush, melt rocks, vaporized rocks on outer fringe
K-Pg Boundary Mass Extinction, cont. • Sequence of events, cont. • Seconds after impact • Ejecta blanket forms • Mushroom cloud of of dust and debris • Fireball sets off wildfires around the globe • Sulfuric acid enters atmosphere • Dust blocks sunlight • Tsunamis from impact reached over 300 m (1000 ft.)
K-Pg Boundary Mass Extinction, cont. • Sequence of events, cont. • Month later • No sunlight, no photosynthesis • Continued acid rain • Food chain stopped • Several months later • Sunlight returns • Acid rain stops • Ferns restored on burned landscape
K-Pg Boundary Mass Extinction, cont. • Impact caused massive extinction, but allowed for evolution of mammals • Another impact of this size would mean another mass extinction probably for humans and other large mammals • However, impacts of this size are very rare • Occur once ever 40 to 100 my • Smaller impacts are more probable and have their own dangers
14.4 Linkages with Other Natural Hazards • Asteroid or comet impact or airburst direct cause for • Tsunamis • Most impacting objects land in the world’s oceans • Wildfires • Superheated clouds of gas and debris reach temperatures capable of drying out then igniting living vegetation • Earthquakes • Seismic waves created from impact • Mass Wasting • Earthquakes activate numerous landslides on land and under water • Climate Change • Inject large quantities of dust into atmosphere and causes cooling • Warming then follows from large amounts of greenhouse gases • Volcanic Eruptions • Impacts cause melting and instability in Earth’s mantle
14.5 Minimizing the Impact Hazard: Risk Related to Impacts • Risk related to probability and consequences • Large events have consequences will be catastrophic • Worldwide effects • Potential for mass extinction • Return period of 10s to 100s of millions of years • Smaller events have regional catastrophe • Effects depends on site of event • Return period of 1000 years • Likelihood of an urban area hit every few 10,000s years
Risk Related to Impacts, cont. • Risk from impacts is relatively high • Probability that you will be killed by • Impact: 0.01 to 0.1 percent • Car accident: 0.008 percent • Drowning: 0.001 percent • However, that is AVERAGE probability over thousands of years • Events and deaths are very rare!
Minimizing the Impact Hazard • Identify nearby threatening objects • Spacewatch • Inventory of objects with diameter > 100 m in Earth crossing orbits • 85,000 objects to date • Near-Earth Asteroid Tracking (NEAT) project • Identify objects diameter of 1 km • Use telescopes and digital imaging devices • Most objects threatening Earth will not collide form several 1000s of years from discovery
Minimizing the Impact Hazard, cont. • Options once a hazard is detected • Blowing it up in space • Small pieces could become radioactive and rain down on earth • Nudging it out of Earth’s orbit • Much more likely since we will have time to study object • Technology can change orbit of asteroid • Costly and need coordination of World military and space agencies • Evacuation • Possible if we can predict impact point • Could be impossible depending on how large an area would need to be evacuated
Chelyabinsk Meteor, February 2013 – Applying the 5 Fundamental Concepts • Largest known asteroid to have entered Earth’s atmosphere since Tunguska event • Chelyabinsk meteor, fireball, and blast was video-recorded by numerous people and sensors monitored by the U.S government • Explosion of meteor • Compression of atmospheric gasses generated massive amounts of heat • Meteor erupted into a fireball • Then exploded and produced a tremendous shock wave
Chelyabinsk Meteor, February 2013 – Applying the 5 Fundamental Concepts, cont. • Small meteorites collected from a hole found penetrating a frozen lake • Long meteorite fragment found at bottom of lake eight months later • Composition and density of the meteor was determined • Chondritic in compositions • Density about 3.6 g per cm (3.6 times that of water) • Total weight estimated to be 11,000 tons • Location of explosion was important • Catastrophic damage if closer to the ground • If over an urban area, millions of deaths could occur
Chelyabinsk Meteor, February 2013 – Applying the 5 Fundamental Concepts, cont. • How could the effects of an impact be minimized? • Predict when and where a strike may occur so there is enough time to evacuate • However, smaller extraterrestrial objects like the Chelyabinsk meteor are not easily identified • Scientists were tracking an unrelated larger meteor at the time of the Chelyabinsk meteor • Funds appropriated to identify 90 percent of the objects 150 m and larger are insufficient • Technology is available and as it improves, it could identify even smaller NEOs
