1 Chapter 16 Impacts with Space Objects Lecture PowerPoint Copyright © The McGraw-Hill Companies,...

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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)

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

Sources of Extraterrestrial Debris

Figure 16.6

Asteroids


Insert table 16.2

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

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

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

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

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 mini-sonic boom

• Atmospheric frictional heat may raise surface temperature to 3,000oC, creating tail to fireball


Figure 16.16

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 ProcessImpacts 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 ProcessImpacts 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

Figure 16.21

Insert Figure 16.21

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

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


Figure 16.24

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


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


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


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 CenturyTunguska, 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 CenturyTunguska, 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

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

Frequency of Large Impacts

Insert table 16.5

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

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

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