A100 Oct. 6 The Earth

Today’s APOD

READ Chapter 5 – The EarthHomework 4 Due FridayQuiz on Friday

The Sun Today

A100 Oct. 6 The Earth

Loren Acton (1936 – )U.S. astronaut

Looking outward to the blackness of space, sprinkled with the glory of a universe of lights, I saw majesty – but no welcome. Below was a welcoming planet. There, contained in the thin, moving, incredibly fragile shell of the biosphere is everything that is dear to you, all the human drama and comedy. That’s where life is; that’s where all the good stuff is.

The Earth from Space

Understanding Earth helps us understand other planets

Understanding other planets helps us understand the Earth

How has Earth has changed over time Why does it change? How does it differ from other planets?

Size and Shape of the Earth

The Earth is a huge, rocky sphere spinning in space and moving around the Sun at a speed of about 100 miles every few seconds

Earth also has a blanket of air and a magnetic field that protects the surface from the hazards of interplanetary space

Earth Data

Distance from Sun 1 AU = 1.5 x 108 km

Orbital velocity 30 km / second

Orbital period 365.256 days

Rotation period 23.9345 hours

Inclination of equator to orbit: 23.26 degrees

Diameter: 12,756 km Mass 6 x 1024 kg Mean density

5.5 gm/cm3

Surface temperature -130 F to +140 F (avg.

70F)

An Average Planet

Our Earth is about average Earth is the largest and most massive of the

four terrestrial planets, but smaller and less massive than the four giant, or Jovian, planets.

Earth is third in distance from the Sun among the four terrestrial planets.

Earth has a moderately dense atmosphere; 90 times less dense than that of Venus but 100 times denser than that of Mars.

Unique Features of Earth Plate Tectonics – the only planet with a surface

shaped by this type of tectonics Atmospheric Oxygen – the only planet with

significant Oxygen in its atmosphere Surface Liquid Water – the only planet where

temperature & pressure allow surface water to be stable as a liquid

Climate Stability - differs from Venus & Mars in having a relatively stable climate

Life – The only world known to have life; it certainly has abundant & diverse life forms

Magnetic Field Large moon

Size and Shape of the Earth

The Earth is large enough for gravity to have shaped it into a sphere

More precisely, Earth’s spin makes its equator bulge into a shape referred to as an oblate spheroid – a result of inertia

Composition of the Earth The most common elements

of the Earth’s surface rocks are: oxygen (45.5% by mass), silicon (27.2%), aluminum (8.3%), iron (6.2%), calcium (4.66%), and magnesium (2.76%)

Silicon and oxygen usually occur together as silicates

Ordinary sand is the silicate mineral quartz and is nearly pure silicon dioxide

Density of the Earth Density is a measure of how much material (mass) is

packed into a given volume Typical unit of density is grams per cubic centimeter Water has a density of 1 g/cm3, ordinary surface rocks

are 3 g/cm3, while iron is 8 g/cm3

For a spherical object of mass M and radius R, its average density is given by

For Earth, this density is found to be 5.5 g/cm3

Consequently, the Earth’s interior (core) probably is iron (which is abundant in nature and high in density)

343

M

R

What’s Inside?

CrustAsthenosphereMantleOuter CoreInner Core

How do we know

this?

Earthquakes generate seismic waves that move through the Earth with speeds depending on the properties of the material through which they travel

These speeds are determined by timing the arrival of the waves at remote points on the Earth’s surface

A seismic “picture” is then generated of the Earth’s interior along the path of the wave

A sonogram of the Earth

Interior Structure of the Earth A solid, low-density and

thin crust made mainly of silicates

A hot, thick, not-quite-liquid mantle with silicates

A liquid, outer core with a mixture of iron, nickel and perhaps sulfur

A solid, inner core of iron and nickel

Differentiation

Differentiation occurs in a mixture of heavy and light materials if these materials are liquid for a long enough time

The Earth must have been almost entirely liquid in the past

The densest materials are at the center of the Earth and the least dense materials are at the surface – differentiation

Earth’s Interior

Earth’s interior gets hotter

towards the center (6500K, as

hot as the Sun’s surface)

The Earth’s inner nickel/iron core is solid because the high pressure (from overlying materials) forces it into a solid state – even at that high temperatureThe outer core is liquid nickel/iron

Why so hot??? The high

temperature in the Earth’s core

(6500K) is probably due to

two causes

Left-over heat from the impact of small bodies that eventually formed the Earth The radioactive decay of radioactive elements that occur naturally in the mix of materials that made up the Earth

The Earth cools very

slowly

Heat is trapped inside the Earth’s interior due to the long time it takes to move to the surface and escape

Pseudo-color infrared image of the Earth from

space. The red areas are warm water during an El

Nino event

Age of the Earth• Radioactive decay used to determine the

Earth’s age– Radioactive atoms decay into daughter atoms– The more daughter atoms there are relative to

the original radioactive atoms, the older the rock is

Age of the Earth Radioactive potassium has a half-life of 1.28 billion

years and decays into argon, which is a gas that is trapped in the rock unless it melts Assume rock has no argon when originally formed Measuring the ratio of argon atoms to potassium atoms gives

the age of the rock This method gives a minimum age of the Earth as 4 billion

years Other considerations put the age at 4.5 billion years

Early History of Earth

Earth formed 4.6 billion years ago from the inner solar nebula

Four main stages of evolution:

Differentiation

Bombardment

Cooling

Surface evolution

Most traces of ancient bombardment (impact craters) have been destroyed by later geological activity

Meteorite Impacts Still Occur

Over 150 impact craters found on Earth.

Famous example:

Barringer Crater near

Flagstaff, AZ:

1.2 km diameter

200 m deep

Formed ~ 50,000 years ago by a meteorite of ~ 80 – 100 m diameter

The Chicxulub CraterComet nucleus impact producing the Chicxulub crater ~ 65 million years ago may have caused major climate change, leading to the extinction of many species, including dinosaurs.

The impact of a large body formed a crater ~ 180 – 300 km in diameter in the Yucatán peninsula, ~ 65 million years ago

Evidence of the event is seen in rock layers around the world

ASSIGNMENTSthis week

Dates to Remember

READ Chapter 5 – The EarthHomework 4 Due FridayQuiz on Friday