Earth Sciences 2240F/G Lecture 14: Chapter 14- Impacts from Space- The Basics
11 Jun 2018
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Chapter 14: Impacts from Space- The Basics
1.0 Introduction
• one million years ago
o asteroid impacted at Zhamanshin, Kazakstan
o crater 13.5 km in diameter
o did not produce enough disruption to produce a ‘nuclear
winter’ and not big enough to result in a mass extinction
• 1989: astronomers noticed an asteroid moving away from earth,
having come within 700,000 km
o moon is 380,000 km away
o asteroids approach not detected at all
o if it had approached two hours later, its and earth’s orbits
would have intersected
• projections: expect one Zhamanshin size (1km diameter) impact
ever 1 million years
2.0 Asteroids and the Asteroid Belt
• earth receives most of its large space debris from the asteroid belt
• located between mars and Jupiter, consists of material that was
never incorporated into a planet
o not the debris of either an exploded small planet or two
planets that crashed together
• asteroids concentrated in a number of distinct belts, separated by
gaps that related to the orbit frequency of Jupiter
• asteroids have same/ similar composition as parent ingredients
from which planets were assembled
• collected many meteorites in this category of primitive material,
refer to them as chondrite meteorites
o because the contain 1mm, bead like spherules called
chondrules
• large asteroids underwent differentiation
o had enough radioactive material that they melted and formed
a core and mantle
• collisions within the belt spray debris out, sometimes into the gaps
form which it is whipped out in an eccentric orbit, with a looping
trajectory toward the sun and potential intersection with earth
• large number of asteroids have impacted the planets and their
satellites, evidence from cratering on moon
• Near Earth Objects: those asteroids and comets whose orbits
interest Earth’s orbit or comes very close
o Those that actually intersect earth’s orbit are referred to as
Apollos
o Those that intersect the orbit of Mars are referred to as
Amors
• Largest impact on moon had a diameter of 2200 km
o Asteroid measuring 50-150 km would have created that
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Chapter 14: Impacts from Space- The Basics
• If earth struck by object larger than 5 km in diameter, estimated
that the atmosphere would be temporarily destroyed
• Prior to 3.8 bya, objects also vaporized our oceans
• Largest crater preserved on land is Sudbury, Ontario
o 200 km diameter, 1.85 billion years old
• Second is Chicxulub, Mexico
o 170-180 km, some suggest as wide as 300 km, 66 mya
3.0 Comets
• solar system is surrounded by a trillion comets in a crude envelope
called the Oort cloud
• only a few comets in that cloud ever get diverted into orbits that
bring them close to earth
o those with frequency of 200 years of less appear to have
originated in a close zone of the Oort cloud called the Kuiper
Belt
• most famous is Halley’s Comet
o comes past earth every 74 to 79 years, next time in 2061
• Hale Bopp, went through solar system in 1997, nucleus was 40 km,
large enough to sterilize earth if impacted
4.0 Cosmic Dust, Shooting Stars and ‘Others’
• everyday earth receives between 100 and 1000 tonnes of space
junk- most in the form of dust
• dust particles so small that they slip through atmosphere without
undergoing change
• shooting stars:
o junk about 1 mm diameter, just large enough that the friction
of entering the atmosphere raises the temperature to the
melting point
o melt that forms as we see the meteor streak across the sky
cools to little glass droplets that are added to earth’s surface
• several large, icy bodies were discovered in outer solar system
o a mix of asteroid-like and comet-like objects out there
o a class of objects unidentified
o low-density, fluffy balls of ice-> termed ‘space balls’
5.0 Impact
• doesn’t require a massive object to get through atmosphere
• dust gets through, objects between dust and 1 gm do not, anything
more than 1 gm will land on surface most likely
• if meteorite travelling at the speed of sound or greater ( 1200
km/hr) or greater, it will be accompanied by a sonic boom
o the result of a violent compression of air
o if object greater than the size of a basketball and travelling at
speed of sound, sonic boom will be heard on earth surface
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Chapter 14: Impacts from Space- The Basics
• most objects slow because atmosphere density increases and
friction increases
• most start out at speed of sound and then impact at 300 km/hr
because get slowed down
• friction slows object, but temperature induced by friction can reduce
the size through melting, temperature can reach 3000 C
• very large objects (greater than 350 tonnes) plunge through
atmosphere with little effect on them
6.0 Classification of Meteorites
• Aerolites or stones (92.8% of all meteorites)
o Primitive chondrites: mostly composed of silicate materials-
particularly plagioclase, pyroxene and olivine, contain
chondrules, amino acids formed from primitive materials in
the solar system
o One type of chondrites is called “carbonaceous chrondrites”
▪ Characterized by a high volatile content and carbon
compounds
▪ Thought to be the most primitive of chondrites
o Differentiated chondrites (aka “Achondrites”): silicate
minerals similar to those ound in terrestrial rocks, some are
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