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Earth Science - Lecture 37 Notes.docx

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McMaster University
Earth Sciences
Sergei Basik

IMPACTS OF ASTEROIDS AND COMETS The Ultimate Catastrophe? • Asteroid 10-15km in diameter struck Yucatan peninsula of eastern Mexico, 65 million years ago • Formed Chicxulub crater (80-110km in diameter) • Crater walls collapsed to form195-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 • Huge tsunami waves swept over continental margin • Widespread acid rain Wildfires • Darkness and abrupt cooling would last several years, 
from atmospheric dust • Extinctions of plant and animal life, including dinosaurs Projectiles from Space  Space objects that cross Earth’s path include: o Asteroids o Comets o Smaller pieces of rock that cause meteors Asteroids • One planet of solar system is missing where asteroid belt is located between orbits of Mars and Jupiter • Asteroids: 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 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: o Lies in plane of solar system o Extends to
20,000 times distance from Earth to sun o Contains trillions of 
comets • Comets travel up 60-70km/s (making impacts catastrophic) • 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 • Comets have highest chance of coming close to Earth 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 
 Identification of Meteorites • Types of meteorites are somewhat similar to rocks of Earth’s deep interior • Iron meteorites: o 6% of all meteorites: o Consist mostly of nickel-iron alloy, with density of 7.7-8 grams per cubic cm o Distinctive in external appearance and have intersecting sets of parallel 
lines in interior • Stony-iron meteorites: o Less than 1% of meteorites o Magnesium and iron-rich silicate minerals in nickel-iron matrix • Chondrites: o Stony meteorites o 93% of all meteorites o Consist mostly of magnesium-iron-rich minerals o Density of 
3.3 grams per cubic cm o Similar to Earth’s mantle • Achondrite: o Meteorites are similar to basalt • Stony meteorites are distinctively heavy and may have 
inclusions of nickel- iron 
 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 
 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: o Is converted to heat and vaporization of asteroid and target o Melts rock, excavates crater o Blasts out rock and molten glass o Huge fireball heats and melts rock, burns everything 
 Energy, Mass and Velocity 
 ke = 0.5mv2 
 ke = kinetic energy m = mass
 V = velocity 
 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 • Blast
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