Solar System Astronomy

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Solar System Astronomy

• Lecture Survey of the Solar System, Part II

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Night Sky Focus of the WeekSeptember 20-24
Scorpius and Sagittarius (Zodiac constellations)
Moon is waxing quarter tomorrow night! (What time
will it rise?)
Antares (from the Greek rival of Mars) red
supergiant, distance 520 light-yrs,
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Looking S-SW at 8pm tonight
Center of galaxy
Tea kettle
Antares
1st qtr Moon
Scorpion
Sun
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Physics of gases (planetary atmospheres)

• Temperature atoms in motion higher temperatures
  imply higher average speed

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Exam 1 questions

• Suppose you are an inhabitant of Jupiter and are
  observing the Earth. At what location in Earths
  orbit would its angular size be smallest?
• Inferior conjunction
• Superior conjunction
• Opposition
• Greatest elongation
• Least elongation
• 2. An observer sees the Moon rising at 900 pm.
  What phase is it?
• Full
• New
• Waxing crescent
• Waning crescent
• Waning gibbous

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• 4. An observer on the Arctic circle (67.5? N
  latitude) sees the Sun at the zenith at noon.
  What date is it?
• 21 June
• 21 September
• 21 March
• 21 December
• This is impossible.

• 7. Suppose the Earths rotation axis were exactly
  perpendicular (at right angles) to the ecliptic
  plane. What statement would be incorrect? (Ignore
  effect of elliptical orbit)
• There would be no seasons
• Observers at the South Pole would never see the
  Sun rise above the horizon
• In Iowa City there would be 12 hours of daylight
  every day
• Observers everywhere on the equator would see
  the Sun at zenith every day at noon
• Observers everywhere on the Arctic Circle would
  never see the Sun set below the horizon.

• 13. When Mars is closest to the Earth, which is
  not true?
• It is in orbit around the Sun
• It is at inferior conjunction
• It is at opposition
• It is in the ecliptic
• It is in a line with the Sun and Earth

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AnnouncementsWed Sept 22

• PRS
• First quiz today
• Must be registered prior to quiz!
• PRS results will on special website shortly(?)

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Escape speed of gases

• For Earth, the escape speed is Vesc is 11.2
  km/s (6.8 miles/s or 25,000 mph)
• Since gases have a range of speeds, a fraction
  of the particles will always be above the average
  speed
• Eventually all particles with V Vesc
  particles will escape.
• Over billions of years, all gases with average
  speed V Vesc will be lost
• In practice, (rule of thumb) only gases with
• Vavg
• will be retained in the atmosphere

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Planetary atmosphere escape velocity speed of a
gas particle exceeds escape speed of the planet
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Escape velocity

• Depends on temperature and mean molecular weight

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• Planetary atmospheric gases escape if

N.B. Mars cannot retain water!
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Density and Pressure

• Density Mass/Volume

About 5.5x density of water (1,000 kg/m3)
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Mean density and escape speed of the planets
Note Average density of terrestrial planets is
3x - 5x density of water Jovian planets nearly
same as water
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Gas pressure , density, temperature The ideal
gas law
P Pressure N density k constant
(Boltzmann) T temperature
Demonstration of pressure, temperature match
lighting
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AnnouncementsFriday Sept 24

• PRS quiz results delayed posted this weekend (I
  hope)
• Finish Chap 7 today, Chap 8 (Earth as a planet)
  Monday

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Heat in planetary interiors

• The interiors of planets are extremely hot
  compared with the surfaces
• For Earth, the core temperature is extremely
  high, about 5000?K (9,000?F)
• Three sources of heat
• Accretional heating material falls in to body
  during formation (most important for larger
  planets)
• Radioactive heating Unstable isotopes (mainly
  26Al, 40K, 235U, Th) produce heat during decay.
  (most important for terrestiral planets)
• Sunshine only heats surface.

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Fission, Fusion and radioactive decay

• Fusion light elements fuse inot heavier elements
  (e.g. Hydrogen - Helium in core of Sun,
  hydrogen bomb)
• Fission heavy element split into lighter
  elements (e.g. Uranium - lead, First Atomic
  bomb)
• Radioctive decay unstable isotope releases
  radiation, decay particle (e.g. radium - radon
  (gas) - polonium) produces heat in planetary
  interiors

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For Fusion and Fission, Mass loss Energy
released
E mc2
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Side bar Where does radon gas come from?

• All soil contain Uranium (few parts per million)
• U238 (92 protons, 146 neutrons) has ½ life of
  4.5 billion yrs
• U238 decays to thorium (Th)
• Thorium decays to Radium
• Radium decay to radon (inert gas)
• Radon eventually decays to lead (Pb)
• Radon causes lung cancer concentration in
  basements in health hazard in Iowa

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Temperature of Earth versus depth and melting of
rocks
Granite melts
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Interior heating by radioactive decay in
planetary interiors
Now
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Most important radioactive decays heating the
interior of terrestrial planetsAluminum (Al,
early)Potassium (K)Uranium (U)
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Heating planetary interiors by accretion (note
much more important for Jovian planets)
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Chemical composition of Solar System

• 93.6 Hydrogen!
• 6.3 Helium
• 0.1 all other elements
• Note This is NOT true for the Earth

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Radiation Properties of thermal (black body)
objects (for example all planets, Sun)

• Spectrum of radiation has a peak dependent of
  temperature (Wiens law)
• colder bodies are redder, hotter are bluer
• Total amount of radiation dramatically increases
  with temperature Stefan-Boltzmann law
• Spectrum and luminosity of a black body depends
  only of the temperature and surface area

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Wiens law (equation 7.3)
or
Example The peak observed wavelength of Plutos
spectrum is ? 58 ? (1 ? 10-6 meters). What is
the surface temperature?
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Surface temperature of the planets
Comfort zone for humans
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Stefan-Boltzmann LawThe luminosity of a black
body increases as the 4th power of the temperature
or
Example Asteroids A and B differ in temperature
by a factor of 2. What is the ratio of their
luminosities?
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Heat transport

• Radiation
• EM radiation
• Propagates in a vacuum
• Sun- Planet
• Convection
• Requires liquid or gas flow
• Dominant form of heat motion in atmospheres
• Conduction
• Motion of molecules in solids
• Speed