Transcript Lecture Outlines Natural Disasters, 5th edition

Chapter 16
Impacts with Space
Objects
Lecture PowerPoint
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Impact Scars
• Surface of the Moon: intense
bombardment of first few
million years of Solar System
history recorded on surface of
Moon – tens of millions of
ancient impact craters
• Flood basalts on Moon 3.8 –
3.2 billion years ago created
maria  few impact scars, so
bombardment over by 3.8
billion years ago
• On geologically dead Moon,
craters are preserved
Figure 16.3
Impact Scars
• On dynamic Earth, craters are destroyed by plate tectonics
Insert table 16.1
Sources of Extraterrestrial Debris
•
•
•
•
Primarily from fragmented asteroids
Secondarily from comets
Pieces of asteroids and comets orbiting Sun: meteoroids
Meteoroids blazing through Earth’s atmosphere: shooting
stars or meteors
• Meteors that hit the Earth’s surface: meteorites
• Irons: metallic meteorites (most of collected meteorites)
• Stones: rocky meteorites (most of meteorites)
Sources of Extraterrestrial Debris
Asteroids
• Solar System: four small, rocky inner planets + four large, gaseous
outer planets (+ Pluto)
• Between Mars and Jupiter: asteroid belt of small (under 1,000 km
diameter) rocky, metallic and icy masses
• Most meteorites come from inner Solar System, from asteroid belt
Figure 16.6
Sources of Extraterrestrial Debris
Asteroids
Insert table 16.2
Sources of Extraterrestrial Debris
Asteroids
• All asteroids together would
have formed planet less than
half Moon’s diameter – too
strongly influenced by
Jupiter’s gravitational pull
• Many asteroids held together
with other asteroids by gravity
(Ida and Dactyl)  multiple
impact sites
Figure 16.7
Sources of Extraterrestrial Debris
Comets
• Short-period (orbit less than 200 years) or long-period
• Solar System surrounded by:
– About one billion comets
in Kuiper belt – flattened
disk (in plane of Solar
System) from near
Neptune to about 50
astronomical units (93
million miles, distance
from Earth to Sun)
– About one trillion comets
in Oort cloud – spherical
orbits far beyond planets
Figure 16.10
Sources of Extraterrestrial Debris
Comets
• Few comets from
Kuiper belt or Oort
cloud with very
elliptical orbits
enter Solar System
 may impact
planet
– Near Sun
– perihelion
– Farthest from Sun
– aphelion
Figure 16.12
Sources of Extraterrestrial Debris
Comets
• Described as “dirty snowballs”: composition of ice and
rocky debris
• When passes Saturn toward Sun, affected by sunlight and
solar win  sublimation releases gas and dust to form tail
• Nearer Sun, tail becomes larger, always pointing away
from Sun
• Halley’s comet: orbit from 74 to 79 years, from Sun to
beyond Neptune, last visible near Sun in 1986
• Comets contain carbon compounds (CHON – carbon,
hydrogen, oxygen, nitrogen), building blocks of life –
brought to Earth by comets?
In Greater Depth: Shoemaker-Levy 9
Comet Impacts on Jupiter
• Comet named for discoverers (9th co-discovered comet)
• Flew too close to Jupiter in 1992, broke into 21 pieces
Figure 16.14
In Greater Depth: Shoemaker-Levy 9
Comet Impacts on Jupiter
• 1994: Impacted Jupiter’s atmosphere, at up to 60 km/sec
– Initial flash at collision
– Superheated gas fireball, thousands of kilometers above clouds
– Radiation as plume crashed back down at high speed
• Largest (1 km) fragment G
 impact scar larger than
Earth
Figure 16.15
Rates of Meteoroid Influx
• 100,000
million or
more
meteoroids
enter Earth’s
atmosphere
every day
• Smaller
meteoroids 
greater
abundance
Figure 16.13
Rates of Meteoroid Influx
• At 115 km above ground, atmosphere is dense enough to heat
meteoroids to glowing (shooting star)
• Meteoroids typically visible 100 km above ground, vaporized
before reaching 60 km above ground
• Adds 100 to 1,000 tons of material to Earth’s surface each day
• Speeds of 11 – 30 km/sec  atmosphere behaves like solid
– Most meteoroids destroyed on impact, deflected back into space
or slowed down by friction
– Meteoroids larger than 350 tons largely unaffected by
atmosphere
Rates of Meteoroid Influx
Cosmic Dust
• Smallest meteoroids unaffected by atmosphere  settle
on surface as gentle rain
Shooting Stars
• Sand grain sized debris (1 mm diameter)
• Friction-generated flash about 35 km above ground as
debris melts to tiny droplets of glassy rock spheres
Rates of Meteoroid Influx
Meteorites
• Meteoroids 1 gram or more pass
through atmosphere to Earth’s
surface
• Frictional resistance of atmosphere
melts away exterior, protecting
interior  glazed, blackened crust
• Violently compresses air  minisonic boom
• Atmospheric frictional heat may
raise surface temperature to
3,000oC, creating tail to fireball
Figure 16.16
Rates of Meteoroid Influx
Meteorites
• 1954: stony meteorite crashed through roof of Alabama
home, bounced off walls then hit woman in hip, severely
bruising her
• October, 1996: meteorite streaked over New Mexico and
Texas, bounced into space, pulled back and fell in
California
• March, 2003: meteorite flashed over midwest as it broke
apart – more than 60 pieces found, hitting homes, cars and
fire department
• September, 2003: large meteorite broke apart over Orissa,
India, raining debris that injured three people and set
house afire
The Crater-Forming Process
• Energy release of
impact depends on
object’s speed,
mass
– Asteroids impact
at 14 km/sec,
comets at 70
km/sec
• Impacts of smaller
meteorites form
simple craters
(Meteor Crater,
Arizona)
Figure 16.17
The Crater-Forming Process
• Impacts of larger bodies form complex craters
– Examples: buried Chicxulub crater on Yucatan
Peninsula, Mexico; Manicouagan crater in Canada;
Yuty crater on Mars
– Miniature corollaries: falling drop of water hitting still
body of water, bullet into soft sand
The Crater-Forming Process
Impacts of larger bodies form complex craters
• Central uplifts, collapsed and fractured outer rims
• Much of asteroid and crater rock is melted and vaporized
• In first instant: shock wave with temperatures thousands
of degrees, pressures more than 100 gigapascals  new
minerals created (diamond, stishovite), ground pushed
downward and outward
• Still initial second: release wave deflects material upward
and outward  forms central uplift, transient crater
• Later: fractured walls of empty crater slide in  final
enlarged crater with central peak, circular trough,
outermost fractured rim
The Crater-Forming Process
Impacts of larger bodies form complex craters
Figure 16.18
The Crater-Forming Process
Impacts of larger bodies form complex craters
Figure 16.18
The Crater-Forming Process
Impacts of larger bodies form complex craters
Figure 16.18
The Crater-Forming Process
Impacts of larger bodies form complex craters
Figure 16.18
Crater-Forming Impacts
• Meteoroids greater than
350 tons in weight not
slowed down by
atmosphere
– Hit ground at nearly
original speed,
explode and excavate
craters
• Craters are erased by
erosion, destroyed by
plate tectonics and buried
under sediments
• 164 known impact
craters, including 57 in
U.S. and Canada
Insert Figure 16.21
Figure 16.21
Crater-Forming Impacts
Meteor Crater, Arizona
• World’s classic meteorite crater – over 1 km wide, excavated 175 m
below Colorado Plateau, rim 35 to 60 m above Colorado Plateau
• Evidence that formed by meteorite impact:
– Steep-sided, closed crater
– Surrounding rock rim of uplifted, tilted sedimentary rock layers
of region
– Surrounded by hills of inverted regional sedimentary sequence,
limestone blocks
– 265 m of shattered rock on crater floor
– Nearly 30 tons of nickel-iron metallic meteorite collected in
area
– Evidence of high temperature and pressure: coesite, stishovite,
cooled droplets of metal, fused sand grains, shatter cones
Crater-Forming Impacts
Meteor Crater, Arizona
• Evidence against anything other than meteorite impact formation:
– No volcanic material nearby  not volcanic explosion pit
– No solutional features  not sinkhole
Figure 16.22
Crater-Forming Impacts
Meteor Crater, Arizona
• Formed about 50,000 years ago: nickel-iron metallic
meteorite with 40 m diameter, weighing around 110,000
tons, traveling about 12 km/sec
• Impact melted 80% of meteorite and surrounding ground
in less than 1 second
• Shock wave leveled all trees in region, started wildfires,
released enough dust to darken sky
Impact Origin of Chesapeake Bay
• Impact 35.5 million years ago
formed 90 km diameter crater,
25 km diameter central peak,
400 m deep trough
• Spread tektites (glassy
spherules formed by in-air
cooling of impact-melted rock)
over southeastern U.S., Gulf of
Mexico, Caribbean Sea –
9,000,000 km2 area
• Impact crater formed
topographic low spot  rivers
flowed toward crater, drowned
today by Chesapeake Bay
Figure 16.23
The Cretaceous/Tertiary Boundary Event
• Study of Cretaceous/Tertiary (K/T) age rocks near
Gubbio, Italy by father and son Alvarez team disclosed
high levels (300 times normal) of iridium in K/T
limestone layer
• Iridium is associated with iron – abundant in core, very
sparse in crust, abundant in meteorites – could have been
supplied by meteorite
• Popular theory: asteroid with diameter equivalent to
height of Hawaii (from seafloor to peak of Mauna Kea)
hit Earth 65 million years ago, causing mass extinctions
and leaving excess iridium deposited in global clay layer
The Cretaceous/Tertiary Boundary Event
Figure 16.24
The Cretaceous/Tertiary Boundary Event
Evidence of the K/T Impact
• K/T boundary clay layers around world searched for evidence of
Earth-like vs. meteorite-like components
– Clay layer found on continents  iridium enrichment
– K/T boundary clay minerals have different composition than
clays in limestones above and below in rock sequences –
explained by mix of one part asteroid to ten parts Earth crust
– Shocked quartz grains present
– Spherules present, suggesting melting and resolidification
– Microscopic diamonds (found in some meteorites) found in K/T
boundary clay layer
– Carbon-rich grains with ‘fluffy’ structures indicative of fire are
abundant in K/T boundary clay layer
The Cretaceous/Tertiary Boundary Event
Site of the K/T Impact
• Evidence compelled scientists to agree that impact had
occurred, but crater had to be found
• Could have been subducted, buried under glacier,
hidden under continental or oceanic sediment, covered
by flood basalt, eroded and erased, destroyed by
continent collision
• 65 million year old sediment layers with shocked
quartz grains, spherules, huge angular blocks, and
tsunami deposits found throughout eastern North
America and Caribbean
The Cretaceous/Tertiary Boundary Event
Site of the K/T Impact
• Mexican national petroleum company (PEMEX) in
Yucatan Peninsula:
– Discovered layer of shattered rock with shocked quartz
and spherules in exploratory wells
– Ground surface with circular pattern of sinkholes
– Circular patterns of gravity and magnetic anomalies
– Seismic survey shows 80 km diameter raised inner ring
and 195 km diameter outer ring
• Chicxulub structure formed 64.98 (+/- 0.06) million
years ago when asteroid slammed in shallow tropical sea
The Cretaceous/Tertiary Boundary Event
Angle of Impact
• Subsurface features show opening to northwest – result of asteroid
coming in from southeast
• Oblique impact of 20 to 30o – concentrated energy into vaporizing
surface rocks  mammoth dust cloud
• Worldwide effects  significant role in end Cretaceous extinctions
Figure 16.25
Problems for Life from Impacts
• Earthquake of monumental magnitude with numerous
aftershocks – extrapolated impact magnitude of 11.3
• Wildfires rage regionally or even globally
• Huge amounts of nitrogen oxides in atmosphere falls as
acid rain, acidified surface waters
• Dust and soot in atmosphere block sunlight, inhibiting
photosynthesis
• Dust settles but water vapor and CO2 remain in
atmosphere  global warming for years
Problems for Life from Impacts
Figure 16.26
Problems for Life from Impacts
• Tsunami up to 300 m high
• Bubble of steam up to 500 km3 volume blows rock and
asteroid debris into upper atmosphere
• K/T asteroid landed in shallow sea underlain by
limestone – vaporized to contribute even more CO2 to
atmosphere, raised temperature as much as 10oC
Biggest Event of the Twentieth Century
Tunguska, Siberia, 1908
• Massive fireball from east exploded about 8 km above
ground in blast heard 1,000 km away
• Killed many reindeer in area (no humans nearby)
• Nearly ignited shirt of man 60 km away before air blast
threw him 2 m
• Knocked people off their feet 480 km away
• 20 km high column of fire was visible 650 km away
• Ground shaking was registered in Russia and Germany
• Barometric anomalies from air blast traveled around
world twice
• Years of speculation before expedition to remote area
Biggest Event of the Twentieth Century
Tunguska, Siberia, 1908
• Forest in more than 1,000 km2 area destroyed, 80 million trees
knocked down over 5,000 km2 area
• No impact crater or broken ground – only globules of once-melted
metal and silicon-rich rock (collected 1958)
• Meteoroid (fragment
of icy comet Encke or
large, stony meteorite)
racing through
atmosphere at 15
km/sec broke up and
exploded 8 km above
ground
• Fortunately occurred
over uninhabited area
Figure 16.27
Biggest “Near Events” of the 20th Century
• March 1989: asteroid 1989FC
missed Earth by six hours, less
than 700,000 km (impact
would have created 7 km
crater)
• May 1996: 150 m asteroid
missed Earth by 453,000 km
• March 1998: reports that
asteroid 1997XF11 might hit
Earth in 2028
• Also March 1998: movies
Deep Impact and Armageddon
• Torino scale developed
Insert table 16.4
Frequency of Large Impacts
• Determined by examination of Moon’s maria: one major
impact every 110 million years
• Extrapolated to Earth’s 80 times larger surface area: 2,400
impacts leaving craters bigger than 25 km diameter (720
of them on land)
• More than 160 craters discovered so far, most smaller
than 25 km diameter (remainder probably buried or
destroyed)
• Extremely small odds that Earth will be hit by large
asteroid during human lifetime
• Very large numbers of people killed when impact occurs
Frequency of Large Impacts
Lifetime Risks of Impact
• Risks from space objects bigger than 1 km diameter:
– 90% are near-Earth asteroids, short-period comets
– 10% intermediate- or long-period comets
Insert table 16.5
Frequency of Large Impacts
Lifetime Risks of Impact
• Over 200 Near-Earth
Objects: 25-50% will
eventually hit Earth
• Average interval of time
between impacts is great,
but tremendous number of
people (1.5 billion) killed
Insert table 16.6
Frequency of Large Impacts
Lifetime Risks of Impact and Prevention of Impacts
• Locate NEOs, determine orbits, learn which ones present threat
• Congress spent $40 million for NASA to find 90% of near-Earth
asteroids greater than 1 km diameter by 2008
• By February 2010, 6780 NEOs discovered – 803 large NEAs and
1086 PHOs
• Could alter NEO’s collision course by:
– Blowing apart with nuclear explosion
– Attaching rocket engine to drive it away
– Using big mirror to focus sunlight to vaporize rock
– Scooping rock mass and tossing it away
• Practice opportunity will come in 2029, 2036: 330 m diameter
asteroid 99942 Apophis might hit Earth
End of Chapter 16