7PTYS_206_light_heat_from_planets_stars

# Area that sunlight is spread over is 4 times larger

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Area that sunlight is spread over is 4 times larger Power at 2AU is: 1367 W m -2 / 4 = 342 W m -2 In general… Where R is the distance from the Sun in AU 4 * 10 26 W 1367 W m -2 Area increases with distance squared Solar power = 1367 W m -2 / R 2

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PYTS/ASTR 206 – Light and Heat from Planets and Stars 20 Another example. Mars is 1.52 AU from the Earth How much solar power does it receive per square meter? Distant planets receive less sunlight Colder temperatures Material behaves differently A gas on Earth (e.g. CO 2 ) is solid ice on the outer solar system Solar power = 1367 W m -2 / R 2 Solar power = 1367 W m -2 / 1.52 2 Solar power = 592 W m -2
PYTS/ASTR 206 – Light and Heat from Planets and Stars 21 Surface temperature of a planet is much lower than the Sun What wavelengths does it emit radiation at? Let’s take Earth for example – temperature is about 300 K Remember Wien’s Law Plugging in the temperature gives ~10 -5 m (10 μm) i.e. In the infra-red We can’t see Earth glowing with our Reflection

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PYTS/ASTR 206 – Light and Heat from Planets and Stars 22 Most of the radiation we detect in planetary astronomy is generated by the Sun The signal of the solar spectrum is stamped on the light being reflected Spectrum from a planet depends on: The solar spectrum What wavelengths the planet reflects best Reflectance is also called Albedo From 0 (totally black) to 1 (very bright) Varies with wavelength – gives color 400 nm 700 nm Solar Spectrum Planet’s Albedo Planet’s Spectrum (What we see)
PYTS/ASTR 206 – Light and Heat from Planets and Stars 23 Different planets reflect light differently Compositional differences Grain size of the soil Clouds in the atmosphere Etc…

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PYTS/ASTR 206 – Light and Heat from Planets and Stars 24 Different planets reflect light differently 400nm 700nm A - Planet’s Albedo 400nm 700nm B - Planet’s Albedo 400nm 700nm C - Planet’s Albedo Match up the Albedo with the planet….
PYTS/ASTR 206 – Light and Heat from Planets and Stars 25 Different planets reflect light differently 400nm 700nm A - Planet’s Albedo 400nm 700nm B - Planet’s Albedo 400nm 700nm C - Planet’s Albedo

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PYTS/ASTR 206 – Light and Heat from Planets and Stars 26 Earth’s atmosphere is a good example Look at a part of the sky away from the Sun Where’s that light coming from? Scattering in Earth’s atmosphere is mostly by gas molecules Very small - smaller than the wavelength of light Scatters blue light more easily than red Scattering Light can bounce of particles without getting absorbed
PYTS/ASTR 206 – Light and Heat from Planets and Stars 27 Why is the Sky on Mars not Blue?

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PYTS/ASTR 206 – Light and Heat from Planets and Stars 28 Why is the Sky on Mars not Blue? Most of the scattering is from reddish dust Dust is bigger than the wavelength of light
PYTS/ASTR 206 – Light and Heat from Planets and Stars 29 Shadows on Mars vs the Moon A difference from scattered light Which is which?

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PYTS/ASTR 206 – Light and Heat from Planets and Stars 30 Shadows on Mars vs the Moon A difference from scattered light Moon Mars
PYTS/ASTR 206 – Light and Heat from Planets and Stars 31 Spectra Atoms have small nuclei surrounded by orbiting electrons Nuclei have almost all the mass Occupy almost none of the volume Electrons occupy orbits with only certain energies

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• Spring '07
• ShaneByrne

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