# Chapter 6: Planet Earth

## Earth Characteristics

### Earth as a Planet

• We know a tremendous amount about Earth - the study of geology.
• Earth is also the closest planet to us.
• We use Earth’s general properties, structures, and physical processes as a guide to understanding the other planets of the solar system.

### Earth’s Shape

• In Chapter 2 we learned:
• Aristotle noted evidence that Earth is a sphere.
• However, Newton’s laws of motion and gravity (Chapter 3) show that the spinning Earth is not a perfect sphere, but has a bulging equator.
• Planets, including Earth, also have equatorial bulges caused by their rotation which makes the radius different at certain parts of the planet.

### Earth’s Composition

• We can analyze Earth’s surface to learn its composition.
• Earth’s radius = 6,378 km, but the deepest we have drilled into Earth is only 12 km < 0.2% of Earth’s radius.
• Therefore, we have not explored inside Earth.
• We must use indirect methods to learn about the Earth’s interior.

### Average Density of Earth

• We know Earth’s radius, beginning with the method of Eratosthenes.
• We can measure the acceleration of gravity at Earth’s surface and use Newton’s laws of motion and gravity to find Earth’s mass.
• We combine Earth’s mass and radius to find Earth’s average density where Earth’s mass is in grams and Earth’s radius is in centimeters.
• To explain Earth’s high average density the interior of Earth must contain much denser matter.
• The calculation in the text finds that about 50% of Earth’s volume is iron and nickel.
• This is a conclusion from logic, not a direct measurement of Earth’s interior.

### Earth’s Interior

• The model that Earth’s interior is 50% iron needs to be tested - the Scientific Method outlined in the Preview.
• One way to test Earth’s interior is by shaking it - using earthquakes that send waves through Earth.

### Earthquakes

• Every earthquake sends out two kinds of seismic waves:
• P waves - “P” stands for “primary” because they travel fastest and arrive first at a distant location, but “P” can also represent “pressure” or pushing.
• S waves - “S” stands for “secondary” because they arrive after the “P” waves since they are slower.
• Detectors located around Earth record the arrival time and the strength of the seismic P and S waves of every earthquake.
• P waves travel through solids, liquids, and gasses (Ex. Knocking a door).
• S waves travel only through solids because the matter must be connected.
• The wave’s speed ﻿$\rightarrow$﻿ matter’s density.

### Earth’s Interior Structure

• Analyzing the P and S waves we learn the structure of Earth’s interior.

Crust - solid surface of Earth, extending to depths of 20 to 70 km.

Mantle - solid, denser rock below the crust to a depth of about 2,900 km.

• A molten outer metallic (iron + nickel) core.
• A solid inner metallic (iron + nickel) core, solid because of the greater pressure.

### Why is Earth’s Interior Hot?

• Earth’s core temperature is estimated to be 6,500 K - hotter than the Sun’s surface!
• Earth was very hot when it formed billions of years ago
• And radioactive elements in the core, like uranium, release heat over billions of years.
• And Earth’s crust acts like a blanket, slowing the interior’s heat from escaping to space.
• This could happen in other planets.
• Why is Earth’s Core Made of Iron?
• A solid Earth should be a mix of different elements - not separated rock and iron.
• However, the young Earth was so hot that it was completely molten - fluid.
• In a fluid, the denser matter sinks down and the less dense matter floats to the top - called differentiation.

### How Old is Earth?

• To measure Earth’s age we use isotopes (see Chapter 4) that are radioactive - change spontaneously from one chemical element into another.
• Radioactive potassium-40 (19 p+ + 21 n0)
• Calcium-40 (20 p+ + 20 n0)
• Argon-40 (18 p+ + 22 n0)
• Potassium’s rate of change = half-life = 1.28 billion years.
• Knowing the half-life, we find a rock’s age by measuring the amount of (argon-40/amount of potassium-40).
• This is the time since the rock cooled and became solid, trapping the argon gas.
• Oldest rocks ﻿$\rightarrow$﻿ Earth formed almost 4.6 billion years ago - “deep time”.

### Dynamic Earth

• Earth is so old that features we consider permanent may be quite “new” and have changed greatly over that time.
• What could cause Earth to change?
• Erosion by water, ice, and wind.
• Motion in Earth’s interior.
• Recall Earth’s interior is very hot.
• Fundamental laws of energy ﻿$\rightarrow$﻿ heat goes from hot regions to cooler regions.
• How does the heat from Earth’s core come to the surface?
• Radiation? NO - Earth is solid so it doesn’t go inside the Earth.
• Conduction? NO - Very inefficient.
• Convection? YES - Hotter material travels to a cooler region.
• Speed is about 2 cm/year. Takes 50-200 million years to go through one cycle.

### Continental Drift - “Plate Tectonics”

• Earth’s lower density crust “floats” on the higher density mantle.
• Convection in the mantle pushes the rigid crust, breaking it up into pieces - rifting.
• Moving crust causes earthquakes.
• Crust pieces can move apart.
• Crust pieces can collide.
• Mantle material can reach the surface as volcanic plumes.
• The idea of continental drift began long ago from the shape of South America and west Africa.
• The idea was rejected and revived many times over the centuries.
• The debate was settled when rocks from South America and west Africa were found to have the same ages and composition.
• We can measure the motions of the continents to be up to 10 cm/year. Using this speed we can work out the arrangement of the continents millions of years ago.
• We will look for evidence of plate tectonics on other planets.

### Earth’s Magnetic Field

• Earth has a global magnetic field with north and south magnetic poles, although they are not at the north and south rotation poles.
• Earth’s magnetic field is NOT permanent:
• The magnetic poles are moving constantly.
• Lava records that the direction to the poles has flipped many times over millions of years N ﻿$\rightarrow$﻿ S ﻿$\rightarrow$﻿ N ﻿$\rightarrow$﻿ S …
• Earth’s magnetic field is created by an electric current in the core - a dynamo.
• Earth’s core is made of iron so it conducts electricity.
• Earth’s outer core is molten so it flows.
• Earth rotates driving a current in the core.
• Why Earth’s magnetic field flips is still being studied.
• Do other planets have magnetic fields?
• Earth’s magnetic field protects Earth from charged cosmic particles.
• We observe this in the aurora.

### Earth’s Atmosphere

• We live in Earth’s atmosphere, so it is easy to study.
• At Earth’s surface each cm3 of air contains about 1019 molecules.
• At every altitude the air pressure exactly balances the weight of the air above it.
• Therefore the air density decreases with increasing altitude.
• Air temperature also varies with altitude, but it depends on the absorption and emission of energy from radiation and cosmic rays.
• Low atmosphere - “Troposphere”. The temperature decreases with altitude.
• Higher in the atmosphere - “Stratosphere” the temperature increases with altitude.
• Still higher in the atmosphere the temperature decreases and then increases.
• At Earth’s surface we can easily measure the composition of the air.
• Atmosphere made of mainly Nitrogen and Oxygen.
• At higher altitude other molecules become important - ozone = O3, which is important because it absorbs harmful ultraviolet sunlight and protects Earth’s surface.

#### Greenhouse Effect

• In chapter 4 we learned that energy is conserved: absorbed - emitted.
• Earth’s surface temperature is et by energy absorbed from sunlight = the energy emitted by the warmed Earth.
• Without an atmosphere Earth’s surface temperature would be -20o C = 253 K, the same as the Moon’s surface temperature.
• Earth’s surface is much warmer, average is about 15o C, because the atmosphere slows Earth’s infrared emission from escaping to space.
• This is also how a greenhouse works, so it is called the greenhouse effect.
• The carbon dioxide (CO2) and water vapor (H2O) molecules are major absorbers.
• Do other planets have the greenhouse effect?

### Origin of Earth’s Atmosphere

• Our atmosphere is 78% N2 and 21% O2.
• Is this Earth’s original atmosphere? No.
• There are several ideas about how Earth’s original atmosphere formed:
• Volcanic eruptions emitted gas trapped inside Earth.
• Object impacting Earth freed trapped gas.
• Impacting comets vaporize their frozen gas.
• All explanations form an atmosphere with carbon dioxide (CO2), methane (CH4), ammonia (NH3) … very different from now.
• What changed the composition?
• Ultraviolet sunlight broke molecules apart, some H drifted into space leaving C, N, and to form new molecules.
• Water vapor condensed into rain and washed molecules like CO2 out of the atmosphere.
• Planet life began in the ponds, lakes, and oceans (we don’t know how) and created oxygen.
• Plants use sunlight energy + CO2 ﻿$\rightarrow$﻿ O2
• Over billions of years Earth’s atmosphere gradually changed to its current composition.