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Chapter 7

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John Lester

Astronomy Ch. 7 - Earth and Terrestrial Planets (Oct. 26) Earth as a Planet - Earth is being studied in tremendous detail = the science of geology - For astronomy - Earth’s general properties = our closest example of a major planet (some from Ch. 6) - We will use Earth to guide our investigation of other planets - particularly to other terrestrial planets How old is Earth? - We use Earth’s age to learn this - Chemical elements occur as different isotopes (from Ch. 5) - Some isotopes are naturally “radioactive” - they spontaneously change to different elements - the rate of change does NOT depend on the environment (temp, pressure, ...), only on the properties of the atomic nucleus - rate expressed as the “half-life” = time for 1/2 the isotopes to present to change - Examples of radioactive isotopes: - Carbon-14 → Nitrogen-14 with a half-life of 5730 years - this is very useful for archeology, but not for Earth’s age - Potassium-40 → Argon-40 (a gas) with a half-life of 1.25 billion years - For a rock measure the ratio of Potassium-40/Argon-40 = age of rock ▯ → oldest rocks = 4.5 billion years = age of Earth - Moon rocks also give the same age of 4.5 billion years = the age of the Solar System - When we get Mars rocks or Venus rocks, etc, we will determine their ages using the same method (Note: the age of the Universe is about 13.7 billion years) What is Earth’s Composition? - Surface rocks are mostly silicon, with a mixture of all the other chemical elements - Cannot analyze directly the interior or Earth - Note: deepest drill hole is only 10 km, but Earth’s radius is 6378 km - Method: compare the average density (mass/vol) of surface rock, about 3 g/cm^3, with Earth’s average density = 5.5g/cm^3 - Conclusion: Earth’s interior must be much denser than rock - Most common heavy element is iron - Therefore, Earth’s interior is likely to be mostly metallic iron - Note: using logic not direct evidence - Conclude Earth is NOT the same throughout, it has a differentiated = separated structure How did Earth become differentiated? - Solids are very hard to separate - Fluids separate naturally - heavier matter (metal) sinks - lighter matter (rock) “floats” to the surface - Conclude Earth must have been entirely molten when it was young - using logic - As Earth aged it has cooled and solidified, starting at the surface Is Earth’s Interior Still Molten? - Cooling has been taking place for 4.5 billion years, but - volume of Earth’s interior is huge, holds a lot of heat - Earth’s solid crust is like a blanket, insulating the interior, keeping the heat in - Radioactive elements (uranium) give off heat with a half-life of billions of years ▯ → prediction: interior is still molten Tests of Earth’s Interior - “Shake” Earth’s to learn what is inside - Earthquakes naturally shake Earth ▯ 1. P-waves (P = primary, pressure, pushing) ▯ ▯ - travel through solid/liquid/gas ▯ 2. S-waves (S = secondary, slower, shaking, side-to-side) ▯ ▯ - travel only though solids - For every earthquake, instruments around Earth measure the speed and strength or the P- and S- waves - Near the earthquake, both P- and S- waves are detected - Farther from the earthquake, no waves are detected - On the other side of Earth, only P- waves are detected - Analysis of the P- and S- waves finds that Earth has: - a surface that is a solid, rigid, rocky crust - no surprise, but we do learn its depth - below the crust is a more dense, “plastic-like” mantle - below the mantle an even denser, molten, metallic core - at the centre is a very dense, solid metallic inner core Earth’s Magnetic Field - Earth’s magnetic field is NOT permanent - Over millions of years, Earth’s magnetic N and S poles have flipped many times (measured and dated in hardened lava) - Caused by three key factors: - metallic core that conducts electricity - molten core that has currents - rotation of Earth creates currents in the core Oct. 28 Earth’s Changing Surface - Earth’s surface is constant over our lives - Over astronomical times, Earth’s surface has changed tremendously: - Impacts from space - Erosion by wind, ice and water - Eruptions by volcanoes - Movement of continents = continental drift or “plate tectonics” Why do Continents Move? - Earth’s core and mantle are still very hot - To cool down, the interior’s heat must reach the surface to escape into space - Heat can reach the surface in several ways: - radiation - NO, Earth is solid - conduction - NOT efficient - convection = slow like motion of hot rock toward the surface - like boiling water - Surface crust (lighter rock) is floating on the denser tock of the mantle - The flowing rock moves the crust with it, like wood chips on a current of water Earth’s Atmosphere - Composition - measured directly from the surface to the boundary with space - Nitrogen molecules (N ) 2 77% - Oxygen molecules (O ) =221% - Everything else = 2% Why is the sky blue? - The gases of the atmosphere are colourless and transparent - White sunlight contains all colours - violet through red - Air scatters blue light (short wavelength) more strongly than red light
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