Published 3 years ago
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.
- In Chapter 2 we learned:
- Aristotle noted evidence that Earth is a sphere.
- Eratosthenes measured Earth’s radius.
- 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.
- Radius of Earth is about 6,356.8 km
- Planets, including Earth, also have equatorial bulges caused by their rotation which makes the radius different at certain parts of the planet.
- 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.
- 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.
- 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 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 Earth formed almost 4.6 billion years ago - “deep time”.
- 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 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 S N 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.
- 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.
- 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 O2
- Over billions of years Earth’s atmosphere gradually changed to its current composition.