ASTR 2310: Chapter 9

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ASTR 2310 Chapter 9

• Earth and Moon
• Earth
• Interior
• Atmosphere
• Magnetosphere
• Moon
• Interior and Surface
• Origin
• Radioactive Dating

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ASTR 2310 Chapter 9
One of the most significant pictures EVER!
Update http//www.youtube.com/watch?vTmaOcPYCGMA

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ASTR 2310 Chapter 9

• Earth's Interior
• Differentiation

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ASTR 2310 Chapter 9

• Earth's Interior
• How do we know this stuff? Dammit Jim, I'm an
  astronomer not a geologist...
• Like astronomy, there's a chain of indirect
  evidence that makes a consistent picture...
• Seismic waves
• P-waves (pressure, or sound)?
• S-waves (transverse waves, like in a rope)?

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ASTR 2310 Chapter 9

• Earthquakes
• Hypocenter vs. epicenter...
• Can triangulate to it
• P-Waves faster than S-Waves...
• Don't travel straight but bounce at interfaces
  where sound speed differs. Stifferfaster, at
  higher densities, refracted upward
• Details (which I won't test over) let us map the
  interior of the Earth. Take geology for more!

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ASTR 2310 Chapter 9

• Details of the Earth's Structure
• Lithosphere (solid crust plus solid upper
  mantle)?
• Asthenosphere (partially molten upper mantle)?
• Plate tectonics (about a dozen plates)?
• Rift zones (pulling apart)?
• Subduction zones (going under cue Evanescence)?
• Core (about 5000 K, cooler than Sun's surface)?
• Radioactive materials (decay and heat)?
• Geothermal heat 2.6x1013W (double human
  consumption)?
• Earth to remain geological active billions of yrs

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ASTR 2310 Chapter 9

• Earth's Atmosphere
• Not constant in time! Evolution here, too...
• Initially volatile gas H, He, methane,
  ammonia...
• As described, Earth can't hold H and He...
• Also, no ozone layer, so methane and ammonia
  broken down by UV light (photodissociation)?
• Outgassing by volcanoes brings up carbon dioxide
  and water (vapor). Solid carbonates made in
  oceans.
• Finish up with non-reactive heavy gas like N2.

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ASTR 2310 Chapter 9

• Earth's Atmosphere
• 3 billion years ago, life modifies the atmosphere
• Photosynthesis creates O2 as a byproduct
• O2 is very reactive, lots of consequences
• Today, by number of molecules, DRY air
• Nitrogen is 78.1
• Oxygen is 20.9
• Argon is 0.93
• Carbon Dioxide is 0.04
• Other trace elements.

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ASTR 2310 Chapter 9

• Earth's Atmosphere
• Hydrostatic Equilibrium and Scale Height
• Balance pressure and gravity
• Determines how thick an atmosphere is
• Derivation on the board...

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ASTR 2310 Chapter 9

• Earth's Atmosphere

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ASTR 2310 Chapter 9

• Earth's Atmosphere Hadley cells and winds

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ASTR 2310 Chapter 9

• Earth's Atmosphere Effects on Astronomy
• Rayleigh Scattering (particles smaller than the
  wavelength of the light you're studying).
  Effective cross section for scattering is
  inversely proportional to lambda to the 4th
  power. Visible light is scattered by oxygen and
  nitrogen in the atmosphere as well as aerosols.
  This is why the sky is blue! Sunsets red!
• Dust scattering, when the particle size is
  similar to the wavelength. Cross section is
  inversely proportional to wavelength.
• Particle size larger than lambda. Water
  droplets. All wavelengths scattered...rainbows!
• Also some dispersion red and blue light refract
  slightly differently. Can be an important effect
  in slit spectra.

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ASTR 2310 Chapter 9

• Earth's Magnetosphere

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ASTR 2310 Chapter 9

• Earth's Magnetosphere van Allen Belts

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ASTR 2310 Chapter 9

• Earth's Magnetosphere ISS flying through an
  aurora at 28,000 kilometers per hour

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ASTR 2310 Chapter 9

• Moon's Interior and Surface
• Ave. Density is 3370 kg per cubic meter, similar
  to Earth's crust. No iron core here, although
  slightly differentiated.
• Nearly dead geologically speaking smaller and
  cools faster (same reason most animals can't be
  scaled up to giant versions)
• Highlands and lowlands (mare or seas not
  quite!). Mountains, craters, etc.
• Radioactive dating indicates age is 4 Gyrs

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ASTR 2310 Chapter 9

• Moon's Interior and Surface
• Cratering
• Impact craters
• Escape velocity from sun at 1AU is 42 km/s, and
  Earth orbits at 30 km/s, with moon around Earth
  of 1km/s, so impacts can be up to 73 km/s.
• Deep penetration and vaporzation
• Can be VERY energetic (depends on mass and speed,
  kinetic energy)
• Formation of crater and ejecta explosions
  basically, and hence circular, ejecta can be
  glassy post melting
• Crater walls and central peak

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ASTR 2310 Chapter 9

• Moon's Interior and Surface
• Cratering

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ASTR 2310 Chapter 9

• Moon's Interior and Surface
• Differences between lunar and terrestrial rocks
• All lunar rocks are igneous solidified lava (no
  sedimentary or metamorphic rocks)
• No water in lunar rocks (although new results
  suggest that moon retains some water)
• Iron in lunar rocks is not oxidized so no oxygen
• Depleted in volatile elements (exposed to heat in
  the past)
• No large volcanoes on moon, but evidence of lava
  flows are seen (rilles)
• Surface dust, aka regolith, some meters deep

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ASTR 2310 Chapter 9

• Origin of Moon
• Our moon seems unusual compared to other
  terrestrial planets
• Different hypotheses in the past
• Fission
• Capture
• Co-creation
• And the winner is...
• giant impact!

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ASTR 2310 Chapter 9

• Origin of Moon
• http//www.youtube.com/watch?vm8P5ujNwEwM

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ASTR 2310 Chapter 9

• Radioactive dating appendix
• Skipping formal lecture given lab activity
• Moving on...