Natural Hazards and Disasters Chapter 17 Impacts of Asteroids and Comets

1. Natural Hazards and Disasters Chapter 17 Impacts of Asteroids and Comets

2. The Ultimate Catastrophe? Asteroid 10-15 km in diameter struck Yucatan peninsula of eastern Mexico, 65 million years ago Formed Chicxulub crater 80-110 km in diameter Crater walls collapsed to form 195-km crater basin with central uplift Asteroid impact killed dinosaurs and majority of other species on Earth Energy equivalent to 1 million 1980 eruptions of Mount St. Helens

3. The Ultimate Catastrophe? Crater was later buried by limestone layers and is no longer visible at surface Record of impact is preserved in sedimentary deposited at that time around world Geological time before impact is Cretaceous Geological time after impact is Tertiary K-T (Cretaceous-Tertiary) boundary is interface between two periods

4. The Ultimate Catastrophe? K-T boundary rocks contain shocked quartz grains, extremely high pressure silica mineral coesite, glass spherules Huge tsunami waves left debris 50-100 m above sea level around Gulf of Mexico Manson impact structure in central Iowa is also 65 million years old, but only 35 km in diameter – too small to be main impact site Breakup of asteroid may have caused multiple impacts

5. The Ultimate Catastrophe?

6. Projectiles from Space Asteroids, comets and meteors cross Earth’s path Asteroids and comets are known as bolides

7. Asteroids One planet of solar system is missing, where asteroid belt is located between orbits of Mars and Jupiter Asteroids are chunks of rock that did not coalesce into planet when solar system formed Some asteroids are pulled out of normal orbits by asteroid collisions or gravitational influence of sun and planets Difficult to spot if on trajectory toward Earth Majority are less than 3 km in diameter, most between 100 meters and 1 km diameter

8. Comets Consist of ice and some rock: dirty snowballs Come from Oort cloud – vast spherical region extending more than 100,000 times Earth’s distance from sun, containing billions of comets Kuiper comet belt lies in plane of solar system, extending to 20,000 times distance from Earth to sun and contains trillions of comets Comets travel up 60-70 km/s, making impacts catastrophic

9. Comets Comets spray off water, dust and volatiles to form glowing tails when hit by solar wind (tail points away from sun) Eventually lose all their water, become rocky and difficult to distinguish from asteroids Probably is continuous gradation between asteroids and comets

10. Meteors and Meteorites Meteors: objects that form light streak in sky as they pass through Earth’s atmosphere Meteorites: same objects once they collide with Earth Most come from asteroid belt Small meteors burn up in upper atmosphere Large meteors become incandescent on outside (fireball) but cores remain cool Large rocks in atmosphere break up to form strewn field

11. Identification of Meteorites • Types of meteorites are somewhat similar to rocks of Earth’s deep interior • Iron meteorites are 6% of all meteorites • Consist mostly of nickel-iron alloy, with density of 7.7-8 grams per cubic cm • Distinctive in external appearance and have intersecting sets of parallel lines in interior • Stony-iron meteorites are less than 1% of meteorites • Magnesium and iron-rich silicate minerals in nickel-iron matrix

12. Identification of Meteorites • Chondrites are stony meteorites, 93% of all meteorites • Consist mostly of magnesium-iron-rich minerals, with density of 3.3 grams per cubic cm, similar to Earth’s mantle • Achondrite meteorites are similar to basalt • Stony meteorites are distinctively heavy and may have inclusions of nickel-iron

13. Evidence of Past Impacts Most impacts into oceans (2/3 of Earth’s surface) are undetected or destroyed by subduction Continental impact sites are broadly distributed, but more have been found in populated or better exposed areas

14. Evidence of Past Impacts

15. Impact Energy • Energy of moving object equals its mass times the square of its velocity • Asteroids have slower speeds but higher mass • Comets have lower mass but higher speeds • Kinetic energy of incoming object is converted to heat and vaporization of asteroid and target • Melts rock, excavates crater, blasts out rock and molten glass • Huge fireball heats and melts rock, burns everything

16. By the Numbers Energy, Mass and Velocity E = mC2 where: E = energy m = mass C = velocity

17. Impact Craters Craters provide evidence about size and date of past impacts Relatively small impacts form open craters Complex craters form when walls of broad, deep crater collapse inward Energy of impact causes object to explode violently Blasts nearly round hole regardless of angle of impact

18. Impact Craters

19. Shatter Cones and Impact Melt Energy output gives rocks distinctive features, such as shatter cones Glass spherules form from melted asteroid or target material Impact melt may contain small but extraordinary amounts of nickel, iridium, platinum and other metals that are abundant in iron meteorites

20. Fallout of Meteoric Dust • End-Cretaceous impact deposited thin, dark layer of clay which contains • Soot • Shocked quartz • Spherules • Anomalous amounts of iridium and other platinum-group elements: iridium anomaly

21. Multiple Impacts Asteroid would be likely to break up in atmosphere, so should expect multiple impacts in sequence Shoemaker-Levy 9 comet broke up into 21 fragments before impacting Jupiter in 1994 Fragments (less than 1 km in diameter) impacted one after another in arc across planet over six days

22. Consequences of Impacts with Earth Impact of modest-sized asteroid (1.5-2 km in diameter) is thought to be enough to kill up to ¼ of people on Earth Would threaten civilization as we know it Consequences would be disastrous for life

23. Immediate Effects of Impact Fireball or ejecta from impact would ignite fires within hundreds of kilometers of impact site Plume of smoke would linger in atmosphere for years Large portion of ozone layer would be destroyed Heat and energy would cause widespread reaction to form nitric acid and acid rain Dust in stratosphere would block sunlight and cause cooling, wiping out agriculture

24. Impacts as Triggers for Other Hazards • Earthquakes would be generated within hundreds of kilometers of impact site • Impact into ocean would form tsunami waves up to 200 m high, with 300 m runup • Impact might cause volcanic activity • Relationship between impacts and flood basalts • Flood basalts form around time of mass extinctions

25. Mass Extinctions • Impact of asteroid at 65 million years ago was associated with extinction of dinosaurs and 40-70% of all species • Mass extinction for most would occur from indirect causes • Acid rain would kill vegetation and sea life • Dust, soot and nitrogen dioxide would block sunlight, dropping land temperatures to freezing within few weeks • Toxic nickel deposits would kill vegetation

26. Evaluating the Risk of Impact Odds of huge asteroid impacting Earth are tiny Consequences of impact would be truly catastrophic, could wipe out civilization Small impacts are common, giant events are rare There is a 1% chance of a 6-meter diameter bolide impacting Earth in any year

27. Evaluating the Risk of Impact About 1,500 asteroids larger than 1 km across are known to be in Earth-crossing orbits Most cross Earth’s orbit at long intervals, so chance of collision is small

28. Your Personal Chance of Being Hit by a Meteorite • Only well-documented case of person hit by meteorite: • In 1954, Alabama woman hit by 3.8-kilogram meteorite on hip – badly bruised but OK • 2004: stony meteorite crashed through roof in Auckland, New Zealand • 1997: 24-kilogram meteorite hit garden outside Moscow, Russia • 1992: meteor shot across sky in fireball before hitting car in driveway in Peekskill, New York

29. Chances of a Significant Impact on Earth • Number of asteroids should decrease with time, as they impact Earth and other bodies • Collisions in asteroid belt create new asteroids • About 10% of impacts to Earth and moon are from comets • Major impacts occur about every 33 million years • Major extinctions seem to occur about every 26-31 million years

30. Chances of a Significant Impact on Earth • Hale-Bopp comet was seen by most people on Earth in early 1997 • Passed 320 million km from Earth • Collision would have been 10-100s times larger than dinosaur-killing collision • Asteroid 1997 XF11 is 1.5 km in diameter • Will pass Earth at 2.5 times distance to Moon in 2028 • Collision with Earth would expend energy of 2 million Hiroshima-size atomic bombs

31. What Could We Do about an Incoming Asteroid? • If very large asteroid was discovered on collision course with Earth: • Inside Moon’s orbit, would be three hours from impact • An hour from impact, would appear as bright as Venus • Fifteen minutes from impact, would appear as irregular mass • Would enter Earth’s atmosphere with blinding flash, then impact Earth three seconds later

32. What Could We Do about an Incoming Asteroid? • Suggestions for dealing with very large asteroid on collision course with Earth: • Blast asteroid into pieces with nuclear weapon • Might just pepper Earth with thousands of smaller pieces • Attach rocket to deflect its orbit so it misses Earth • Change amount of heat radiated from one side to change its orbit (if enough time)

33. What Could We Do about an Incoming Asteroid? NASA catalogs near-Earth objects larger than 1 km in diameter Sometimes there is not much warning before object comes close There will eventually be an object to impact Earth – do not know when No formal plan of action, national or international No mechanism for implementing any action

34. Case In Point A Round Hole in the Desert: Meteor Crater, Arizona Classic open-crater impact site 1.2 km across, 180 m deep, circular with raised rims

35. Case In Point • A Round Hole in the Desert: Meteor Crater, Arizona • Formed only 50,000 years ago by iron meteorite about 60 m across • Originally interpreted as volcanic crater or limestone sinkhole, even though recognizable iron meteorites pieces had been found throughout area • Mining engineer Barringer filed claim for crater, hoping to mine iron ore • Presented papers concluding that crater was formed by impact, but was dismissed • Eugene Shoemaker studied crater as graduate student and found mineral evidence for impact, finally convincing scientific community

36. Case In Point A Nickel Mine at an Impact Site: The Sudbury Complex, Ontario Intrusion is 140 km in diameter Widespread shatter cones indicate asteroid impact as cause Largest nickel deposit in world Considerable disagreement as to source of nickel (from impacting meteorite?) Shocked quartz represents pressures found in mantle

37. Case In Point An Impact Sprays Droplets of Melt: Ries Crater in Germany Crater is 15 million years old and 24 km in diameter Formed in sedimentary rocks over crystalline basement Impact ejected sedimentary rock fragments with droplets of explosively melted crystalline rock (tektites) Glass droplets and shock features are widespread over region Crater is circular but damage pattern suggests impactor arrived at low angle from west

38. Case In Point A Close Grazing Encounter: Tunguska, Siberia Asteroid of about 50 m diameter blew down and charred about 1,000 square km of forest in Siberia, but no crater was formed Asteroid was traveling at 15 km/s when it exploded with energy of 1,000 Hiroshima-sized atomic bombs, 8 km high in atmosphere Huge fireball across sky was followed by bright flash, loud bangs, shaking ground, blasts of hot air 1921 expedition to area found trees blown radially outward from explosion site, but no meteorite fragments Later microscope examination of soil discovered iron oxide meteoritic dust – object probably was stony meteorite