Earth. The Model Planet. If you encountered another planet, what would you want to learn about it?. Basic physical parameters How old is the planet? How was it formed? How is it structured internally? Externally?. Learning more.
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Earth The Model Planet
If you encountered another planet,what would you want to learn about it? • Basic physical parameters • How old is the planet? How was it formed? • How is it structured internally? Externally?
Learning more • Does it have a magnetic field? How strong is it? How is it structured? What does it tell us about the planet’s interior? • How is its atmosphere structured? Of what is it made? What are its weather patterns? How does the atmosphere help control the planet’s energy budget?
Learning more (2) • Has the atmosphere always been the same as it is now? How has the atmosphere interacted with the surface? • What kinds of physical processes have produced the planet’s landforms? • Is there life on the planet? How does it interact with the various ecosystems?
Physical Properties Diameter: 12,756 km at the equator Why specify “at the equator”? Because the polar diameter is only 12,697 km
Physical Properties The bulging at the equator and flattening at The poles is called OBLATENESS. It’s due to the rotation of the planet. Earth is the most spherical (least oblate) of all the planets.
Physical Properties • Volume: 1.1 trillion cubic kilometers (km3) • Mass: 5.97 x 1024 kilograms Mt. Everest is about than 2400 km3http://experts.about.com/q/Geography-1729/Volume-Mount-Everest-1.htm This is equal to about 81 moons!
Physical Properties • Density: 5500 kg/m3 or 5.5 g/cm3 • We compare the density of materials like rocks & metals to the density standard: WATER! • Water’s density is 1.0 g/cm3
Physical Properties • Most rocks have densities between 2.5 and 4.0 g/cm3 • Most metals have densities greater than 6.0 g/cm3 • Iron is 7.8 g/cm3 • Nickel is 8.9 g/cm3
Physical Properties • The density of an object reflects its composition. • What does Earth’s density tell you about its composition? Earth must be made of a combination of rocks and metals.
How old is the Earth? • About 4.6 billion years. • The oldest rocks are found on the north slope of Canada, the Canadian Shield. These rocks are 4.0 billion years old. They were dated from the radioactive decay of uranium into lead. • The other 0.6 billion years is an estimate of how long it took the earth to form.
How was the earth formed? • Accretion • Differentiation • Interior Structure • Evidence, or “How sure are we?”
A rotating cloud of gas & dust: • a nebula
A star ignites in the center and a • temperature gradient forms.
Solid chunks, called planetissimals, • begin to condense close to the star.
Accretion • Planetissimals (dust-sized to small moon-sized solid bodies) begin to form near the sun. • Gravity attracts planetissimals into larger and larger bodies. Planets begin to grow. • Most of the “stuff” of planetissimals is rock (silicates: Na, Ca, Mg, O, Si) and heavy metals (Fe, Ni).
Differentiation • As earth grows in size, its gravity grows too • It begins to pull in other planetissimals, which impact on its surface. • What happens when you repeatedly hit a piece of metal with a hammer?
Differentiation • All the heat generated by planetissimals impacting on the surface, plus the heat generated by radioactive decay in the young earth’s interior causes the entire earth to melt.
Differentiation • When the entire earth is molten, the heavy elements (iron, nickel) sink to the interior. • The lighter materials (granite-type rocks) rise to the surface. • The medium density rocks (basalt-type) ends up in the middle. • Layers form: core, mantle, crust.
Molten Earth, heated by impacts & radioactive decay. Differentiated (layered) Earth after cooling.
Interior Structure • Inner Core (kept solid by the immense pressure of all the material on top of it.) • Outer Core (less pressure allows it to be a liquid.) • Mantle • Asthenosphere • Lithosphere • Crust
Solid Inner Core 2400 km diameter Iron & Nickel Liquid Outer Core 2270 km thick Iron & Nickel Crust 20-100 km thick Granitic rocks: Feldspars Mantle 2900 km thick Basaltic rocks: Olivine, Pyroxene Interior section of mantle is a thick fluid called the asthenosphere. The outer mantle + the crust are rigid and are collectively called the lithosphere.
More about the layers • The difference between the mantle and the crust is based on chemical composition. • The difference between the asthenosphere and the lithosphere is based on viscosity (the ability to flowunder pressure.)
How do we know? What evidence do we really have that the interior of the earth is the way we think it is? • Deep mines are hot! • Heat and molten material escape from volcanos and geysers. • Earthquake waves
Earthquake Waves • When an earthquake occurs it produces 2 types of waves: • P or primary waves. These are waves of compression of the rock. They travel fastest. • S for secondary or shear waves. The rock moves up & down or sideways.
Earthquake waves • P waves are capable of traveling through both liquids and solids, so they travel through mantle and cores. • S waves can’t travel through liquids, so they stop when they hit the outer core. • Look closely at the next diagram.
How big’s that core? • With the right placement of seismometers around the earth’s surface, we get a good estimate of the size of the outer core. • The size of the inner core is calculated from theory.
Earth’s Magnetic Field • Why does Earth have a global or world-wide magnetic field, while other similar planets either have no magnetic fields or very different kinds of fields? • Why should we care about Earth’s magnetic field? What does it do for us?
Earth’s Magnetic Field • Magnetic fields are made wherever there is an electric current, that is the movement of electrons. • In a regular bar magnet, the magnetic field comes from the electrons orbiting around the nuclei of the iron atoms. All the electrons orbit in the same direction.
Earth’s Magnetic Field • In the earth, electrical currents run through the molten iron core. Friction within the molten, flowing iron knocks electrons off iron atoms. • The molten iron flows at about 0.8 inches per second, but the electrical currents can flow faster.
Earth’s Magnetic Field • The electrical currents within the earth cause earth to act like a gigantic electromagnetic generator. • This is called the Dynamo Theory of magnetic field generation.
A little more info… • The earth’s magnetic field isn’t strong enough for us to feel, but many animals can sense it and even use it to navigate. It’s only about 0.4 Gauss, much weaker than a small magnet you can hold in your hand. • On average, the North & South poles “flip” every 390,000 years. There have been 9 flips in the past 3.5 million years.
The poles “flip” ? • No one knows how long the process takes, maybe a few years, maybe a few minutes. • Every so often, what was the North magnetic pole suddenly becomes the South magnetic pole. • Lava that cools quickly on the sea floor records these flips and lets us date them.
Stripes of different magnetic polarityform in the rocks as the lava from themid-ocean ridge cools.
Strange Things Going On • Earth’s magnetic field is NOT aligned with its rotational axis. • The magnetic field is tilted 12o to the rotational axis, and doesn’t even pass directly through the center of the earth. • Does this mean that the electrical currents don’t flow evenly and uniformly inside the earth? Is there turbulence inside?
Magnetic Fields in Space • Earth’s magnetic field extends 7-10 times the earth’s diameter outward from the earth. • The earth’s magnetic field would be spherical, but the solar wind compresses it on the side closest to the sun, and stretches it out into a long tail on the side opposite the sun. • Overall, it’s kind of tadpole shaped.
Magnetic Field Structure • The whole magnetic field is called the magnetosphere. • On the side closest to the sun, where the solar wind compresses the field, there is a “bow shock”, just like a boat pushes some water out of the way at its bow as it sails forward.
Why do we care? • Earth’s magnetic field isn’t just “there” with no purpose. Without it, you and I and every living thing on this planet would be dead (including the cockroaches!) • The magnetic field channels away the solar wind. • It also prevents erosion of the atmosphere.
Solar Wind • So what is the solar wind anyway? • It’s radiation: extremely hot, high-energy, fast-moving charged particles (ions) given off by the sun. Most of these particles are protons. • If you were exposed to it for just a few hours without protection, your skin and every organ in your body would be burned, and you’d have a fatal dose of radiation poisoning.
How does the magnetic field protect us? • The magnetic field captures the solar wind and channels much of it into a donut of radiation around the earth. • This donut (actually 2 layers – one inside the other) is called the Van Allen Radiation Belt (V.A.R.B.)
Van Allen Radiation Belts • Satellites must orbit either below or above the V.A.R.B., or their electronics would be fried. • The problem is even worse when we send manned missions into space. The ship must pass through the radiation belts as quickly as possible or the crew is toast !