April 4, 2006Astronomy 20101 Chapter 7 Earth as a Planet What do we know about the structure,...

April 4, 2006 Astronomy 2010 1

Chapter 7Earth as a Planet

What do we know about the structure, composition, and history of the planet Earth

that we can use in the study of other planets?

How have extraterrestrial factors influenced the Earth?


Chapter 7 Outline• The overall structure of Earth

– Below the surface– At the surface– Atmosphere

• Life on Earth• Extraterrestrial influences


7.1 The Global Perspective

Semimajor axis 1.00 AU

Period 1.00 year

Mass 1.00 Mearth = 5.98 x 1024 kg

Diameter 12,756 km

Escape velocity 11.2 km/s

Rotation period Sidereal day = 23h 56m 4s

Surface area 5.1 x 108 km2

Atmospheric pressure 1.00 bar

The Earth is a terrestrial planet (composed primarily of heavyElements) with a temperature suitable for liquid water.


7.1.1 Earth’s Interior• The interior is difficult to study.

– What’s the deepest hole bored in the Earth?


Seismic Waves• We learn about the structure

of the Earth’s interior by observing the behavior of seismic waves.– Earthquakes– Impacts– Explosions

• Two types of waves propagate in the Earth– P wave: compression wave

(like sound)– S wave: transverse wave (like

vibrating string)• Evidence for a liquid core.• Evidence of layers implies

that the Earth differentiated.


7.1.2 Magnetic Field and Magnetosphere

• The Earth’s magnetic field is the result of1. The Earth’s rotation

2. Electric charges moving within the core core must be molten.

• The north and south magnetic poles lie near the north and south poles.


The Magnetosphere

• The magnetosphere is the region where the Earth’s magnetic field dominates other magnetic fields (primarily the Sun’s).

• Charged particles from the Sun become trapped in the magnetosphere.

• The flow of charged particles from the Sun is called the solar wind.


The Magnetosphere (cont’d)


Aurora

• Charged particles spiral down and excite atoms of gas in the upper atmosphere. The atoms then give off light of particular colors:

• red – hydrogen, green – helium• The particles are able to spiral down at the North and South magnetic poles, hence the aurora borealis and aurora australis.


Aurora Borealis: The Northern Lights


7.2 The Crust of the Earth

• Igneous: rock that has cooled from a molten state.

• Sedimentary: rock built up by layering.

• Metamorphic: igneous or sedimentary rock chemically altered by pressure or temperature.

• Primitive: unaltered from formation of the solar sys.

• The Earth’s crust is the outer layer we live on, extending down about 10km.

• The crust is composed of 4 types of rock:


7.2.2 Plate Tectonics• Theory that explains the

motions of the Earth’s crust produced by heat-driven currents in the mantle.

• Also known as continental drift.

• The Earth’s surface can be divided into regions (plates) that tend to slide, push, or pull against each other, producing earthquakes, volcanoes, mountains, and rifts.


7.2.3 Rift and Subduction Zones• Rift zone: where 2 plates are pulling apart,

forming new crust in the gap.• Subduction zone: where 2 plates push together,

one sliding beneath the other.


7.2.4 Fault Zones and Mountain Building• Fault zone: where 2 plates are sliding past each

other. Plates can move at several cm per year. Usually active earthquake areas.

• San Andreas fault is one of the most famous fault zones.


Mountains• When two crustal plates

collide, high mountains can be formed by lifting and folding of the crust.– Alps

– Himalayas

– Rocky mountains

– Andes

• On Earth, erosion will sculpt the mountains to sharp peaks.


7.2.5 Volcanoes• Volcanoes occur when molten

magma pushes up through the crust to the surface.

• Sometimes volcanoes involve dramatic explosions.

• Often they are less dramatic, involving a more or less constant release of lava.

• Hawaiian islands lie over a hot spot that changes position over time as the ocean floor moves.


7.3 The Earth’s Atmosphere

• We live in the troposphere.• Above that are the

stratosphere, mesosphere, and ionosphere.

• The ozone layer in the stratosphere absorbs UV radiation.

• “Ocean of air” in which we live.• The pressure of the atmosphere is produced by the weight of

the air above us.• Pressure at surface is 1 bar.


7.3.2 Atmospheric Composition and Origin

Nitrogen (N2) 78%

Oxygen (O2) 21%

Argon (A) 1%

Water vapor (H2O) Trace

Carbon dioxide (CO2) Trace

• The composition is summarized in the table below.• This isn’t the whole story.

• Water in the oceans • Carbon dioxide locked in carbonaceous matter.


7.3.3 Weather and Climate

• I’ll skip this section.


7.4 Life and Chemical Evolution• Earth is the only place

we know has life.• Life on Earth dates

back to at least 3.5 billion years ago.

• Early life was plant-like, taking in CO2 and releasing O2.

• When enough oxygen was present, animals could evolve.


7.4.3 The Greenhouse Effect andGlobal Warming

• The surface of the Earth is heated by light from the Sun, mostly visible light.

• The hot surface radiates back infrared radiation, some of which escapes into space, cooling the Earth.

• Carbon dioxide absorbs infrared radiation.

• Like a blanket, CO2 in the atmosphere can stop the infrared radiation from escaping, trapping heat and causing temperatures to rise.


7.5 Cosmic Influences on the Evolution of Earth

• If the Earth has been influenced by extra-terrestrial phenomena, where is the evidence?

• Why isn’t the Earth cratered like the Moon?– Could the impacts have missed the Earth and hit the

Moon?– Maybe the objects burned up in the atmosphere?

• Possible for small meteors, but not the large objects that produce large craters.


7.5.2 Recent Impacts• On June 30, 1908, a huge explosion occurred in the

region of Tunguska River, Siberia.

• The shock wave flattened more than 1000 sq. km of forest and herds of animals were killed.

• The blast was equivalent to 15 megatons of TNT.

• No crater was formed.

• This event is believed to be due to a stony projectile of about 100,000 tons that disintegrated and exploded in the atmosphere.


Arizona – Meteor Crater• Northern Arizona • Impact 50,000 years ago• Iron-nickel meteorite• Weighing several hundred

thousand tons• Estimated size ~ 150 feet across • Hurtling at about 40,000 miles per

hour• Explosive force greater than 20

million tons of TNT. • Crater 700 feet deep, 4000 feet

across. • Today: crater is 550 feet deep,

and 2.4 miles in circumference.


Impacts

• There is evidence that the Earth has been and continues to be hit by large objects.

• Over periods of about 100 million years, the Earth’s crust is (almost) completely recycled, removing evidence of old craters.

• Over shorter periods, erosion of craters makes them harder to spot. With modern imaging, more than 150 craters have been found.


Known Craters on Earth


7.5.3 Extinction of the Dinosaurs• Very large impacts can have global

consequences.• 65 million years ago an impact on

the Yucatan coast occurred.• Evidence of the impact:

– Outline of a large crater (200km in diameter) near Chicxulub, Mexico of the right age

– Global sediment layers of that age with an excess of the element iridium.

• Hypothesis: This impact put enough material in the atmosphere to block the Sun and kill much of the plant and animal life on Earth (mass extinction).

• Explosive power of 5 billion Hiroshima nuclear bombs, about 50,000 trillion tons of TNT!


Chicxulub Gravity Profiles

• These images show the small changes in gravity (few parts per 100,000) outlining the crater.


7.5.5 Impacts in Our Future?• Periodic large impacts may be the reason for other

past mass extinctions.

• The evidence is that impacts have occurred in the past, and that some were large enough to cause global consequences.

• There is reason to believe it can happen again!– Small sized impact could destroy a city– Large impact could have global effects; could destroy

human civilization.


What’s Being Done• In the 1990’s Congress funded a program to search

for and track potentially dangerous objects, objects in so-called Earth-crossing orbits.– Objects bigger than 10km have been identified.– About half of the objects bigger than 1km have been

found. (Estimated total is 1100 objects.)


Discussion Question• Rate the importance (in $) of

finding objects with Earth-crossing orbits?

• For comparison, U.S. government spends:– Space station: $10 billion/yr– All NASA: $16 billion/yr– Medical research: $100

billion/yr– War on terrorism: $100

billion/yr

• How much taxpayer money should be spent?

April 4, 2006Astronomy 20101 Chapter 7 Earth as a Planet What do we know about the structure,...

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