Our Home: The Earth

! Why is the sky blue (+ the Moon red during eclipses)?! Why is this the Earth’s 3rd atmosphere? What

happened to the Earth’s first two atmospheres? How does this compare to other planets?

! Is the Earth geologically active? What determines if a planet is geologically active? How do plate tectonics relate to volcanic activity?

! How old is the Earth’s surface and its oldest rocks?! Does the Earth have a magnetic field? What are the

two main causes of a planet’s magnetic field? What is the aurora and how does it relate to the Earth’s magnetic field?

Learning Objectives

Radius 6378 kmSurface gravity 9.8 m/s2

Mass 6.0x1024 kgDistance from Sun 1.5x108 km(in AU) (1AU)Year 365.26 daysSolar day 1 dayRotational Period (Sidereal Day) 23h 56m

Earth Facts…

Earth’s Atmosphere! Earth’s atmosphere is

a mixture of gases! Nitrogen (N2) 78%! Oxygen (O2) 21%! Argon (Ar) 0.9%! Water Vapor (H2O) 0–3%

(varies by location)! Carbon Dioxide (CO2)

0.04%! Trace gases (Helium, Neon, Ozone, Methane, etc.)

Why is the Sky Blue?! The daytime

sky is blue because molecules in the air scatter blue light from the Sun more than they scatter red light

! When we look towards the Sun at sunset, we see red and orange because the blue light has been scattered out and away from the line of sight

Origins of Our Atmosphere! Our atmosphere is the Earth’s third

atmosphere! The Earth’s first atmosphere was from

the formation of the Earth! It was composed of hydrogen and helium! It gradually escaped into outer space

! The Earth’s second atmosphere was spewed out from volcanoes!Carbon dioxide and water, some nitrogen

What Happened to the Carbon Dioxide (CO2)?

! The Earth’s surface, after its formation, was comprised of a lot of water !The water precipitated out of the air, into

the oceans!The CO2 then dissolved into those oceans

! Life was another factor!Photosynthetic organisms converted

Carbon Dioxide into Oxygen (O2)! What remained was N2, O2, and H2O

The Earth's Surface!~70% of the Earth's

surface is covered with water!The ocean floors are

young, none more than 200 million years old

!~30% is dry land – Continents!A mixture of young rocks and old rocks!The land is about 4.2 to 4.4 billion years old

Geologically Active Surface!The young rocks on

the Earth's surface indicate that the Earth is geologically active

!Where do these rocks come from?!They bubble up from

beneath the surface (e.g., volcanoes)!Air, water erode rocks!Much of the Earth’s surface constantly changes

How to find the age of rocks?! Some atoms, called isotopes are unstable

! They have too few or too many neutrons! e.g., iron has 30 neutrons, but some iron atoms have 31! Some elements, then, are not stable, e.g., uranium

! Unstable atoms decay into stable atoms...this is called radioactivity! Decay time is measured by “half-life” (the time taken for

half of the atoms to decay)! If we compare numbers of unstable atoms to stable

decay products, we can measure the age of a rock! Radioactive dating has established that the Earth’s

oldest rocks are about 4.4 billion years old

How Plate Tectonics Works

!Currents in the denser rock squeeze, pull, break, and stretch the Earth's surface

!Stresses build up between the plates!Released in the form of earthquakes

!Breaks in the plates allow magma to flow up!Creates volcanoes

Plate Tectonics! The Earth’s surface is broken up into

about a dozen “plates”! Plates “float” on denser rock below

Volcanoes

http://www.volcano.si.edu/world/location.cfm

Earthquake Activity

Differentiation! The average density of

the Earth is 5.5 g/cm3

! But the average densityon the surface is 3 g/cm3

! So, something dense must be inside the Earth

! When the Earth formed it was molten!Heavy materials (e.g.

iron, nickel, gold) sank!Lighter materials (e.g. silicon, oxygen)

floated to the top

The Magnetic Field! As you know from

using a compass**, the Earth has a magnetic field

! Liquid iron outer core and the Earth’s rotation create an electrical current

! Acts like a big magnet inside the Earth

**it’s amazing how quickly these slides get old

Magnetic North

!Magnetic North is offset from the geographic North Pole (90oN latitude)

!Magnetic North can move as much as ~150 meters per day

Protection from the Solar Wind! The magnetic field protects Earth from

the solar wind! Charged particles get trapped in the

Van Allen belts

The Aurora! When the Van Allen

belts overload with charged particles, they leak through at the poles and cascade down into the Earth’s upper atmosphere

! The charged particles collide with particles in the air, make the air glow

Geomagnetic Reversal! The Earth’s magnetic field is

not constant! It strengthens and fades…

and even flips north-to-south!On average, flips occur about

every 450,000 yrs!~780,000 yrs since the last flip!Alternating orientations of iron in the ocean floor

suggest ~180 flips in the last ~80 million years!The field has been fading for the past 300 years

! We might be overdue for a flip…

Next Time

Our Companion: The Moon