24.Comets

Natural Hazards and Disasters Chapter 17 Impacts of Asteroids and Comets

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

Asteroid Impact 65 million yrs ago • • • •

Asteroid 10-15 km wide struck Yucatan Formed Chicxulub crater 80-110 km wide Crater walls collapsed to form 195-km basin Asteroid impact killed dinosaurs and majority of other species on Earth • Energy = 1 million 1980 eruptions of Mount St. Helens

Crater is not exposed on surface

Time Scale is based on extinctions • Geological time before impact is Cretaceous • Geological time after impact is Tertiary • K-T (Cretaceous-Tertiary) boundary defined by this extinction

End of the Mesozoic

Major extinction event: 85% of all species died.

CretaceousTyrannosaurus Rex

Side Effects • Huge tsunami waves left debris 50-100 m above sea level around Gulf of Mexico • Breakup of asteroid may have caused multiple impacts • 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

Meteor Crater, Arizona

A more recent impact

Asteroids • One planet of solar system is missing, where asteroid belt is located • Some asteroids are pulled out of normal orbits by asteroid collisions or gravitational influence of sun and planets • Majority are less than 3 km in diameter, most between 100 meters and 1 km diameter

Jupiter

Venus Sun

Saturn

Neptune

Earth

Pluto

Mercury Mars

Inner Asteroid planets belt

The inner planets are small and rocky.

Uranus

Outer planets

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 • Comets travel up 60-70 km/s, making impacts catastrophic

Comets • Comets spray off water, dust and volatiles to form glowing tails when hit by solar wind (tail points away from sun)

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 • Iron meteorites are 6% of all meteorites – Similar to Earth’s core

Identification of Meteorites • Chondrites are stony meteorites, 93% of all meteorites – similar to Earth’s mantle

Evidence of Past Impacts

(1) Why are there so few craters on Earth? (2) Why do some areas of the moon have many craters, and other areas have so few?

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 =mass * velocity2 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

Impact Craters

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

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

Consequences of Impacts with Earth • Impact of 1.5-2 km in diameter asteroid might kill up to 25% of people • Would threaten civilization as we know it

Immediate Effects of Impact • • • • •

fires smoke would linger in atmosphere ozone layer would be destroyed nitric acid and acid rain Dust in stratosphere would block sunlight and cause cooling, wiping out agriculture

Impacts as Triggers for Other Hazards • Earthquakes would be generated • Impact into ocean would form tsunami waves up to 200 m high, with 300 m runup • Impact might cause volcanic activity

Evaluating the Risk of Impact • Odds of huge asteroid impacting Earth are • 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

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

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

Chances of a Significant Impact on Earth • Major impacts occur about every 33 million years – Major extinctions seem to occur about every 26-31 million years

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

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

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

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

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

Case In Point • Formed only 50,000 years ago by iron meteorite about 60 m across • Eugene Shoemaker studied crater as graduate student and found mineral evidence for impact, finally convincing scientific community

Case In Point A Close Grazing Encounter: Tunguska, Siberia • Asteroid 50 m diameter blew down and charred about 1,000 square km of forest in Siberia, but no crater was formed • Asteroid 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