L11GH09_Small_SS_Bodies_V_Comets

L11GH09_Small_SS_Bodies_V_Comets - Comets Lecture topics:...

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February 5, 2009 comets Comets •Lecture topics: orbits, meteor showers, tails, comae, nuclei •Text reading: Chapter 7
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February 5, 2009 comets Midterm Info/Reminder • Date: Thursday February 26 •L o c a t i o n : P 1 4 5 • Time: normal class period = 1:00-2:20PM • Topics: lecture material through ~February 12 • Questions: mix of MC + short answer + quantitative • Calculators: bring yours • Equations, Constants. ..: will be provided
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February 5, 2009 comets comets • Comets are distinguished from all other SS bodies by their appearance which includes a bright coma and long tails • The appearance of a comet changes as it moves through its orbit, becoming brighter as it approaches the Sun and fainter again as it moves away.
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February 5, 2009 comets Asteroid or Comet? • The only distinction between asteroids and comets is the presence of a Coma and tail – These features get dimmer as a comet moves farther away from the Sun. – E.g. comet Wilson-Harrington, discovered in 1949, was “rediscoverd” in 1979 as an asteroid. – Similarly, the asteroid 2060 Chiron moves in a cometlike orbit, and in 1988 it came closer to the Sun and became brighter and more cometlike. • Coma and tail are caused by sublimation of ice. • Thus appearance distinction is simply one of ice content.
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February 5, 2009 comets Comet composition • Comets become visible as such at a distance of about 2.5-3 AU. What temperature does this correspond to? () 177 5 . 2 280 / 280 1 1 2 / 1 4 / 1 = = AU r K A A T IR V • At this temperature, ice can sublimate to form water vapour 2 / 1 / 280 AU r K T = Equilibrium T (for water ice) without sublimation
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February 5, 2009 comets Sublimation •s u b l i m a t i o n : the process by which a solid passes directly to a vapour (without the intermediate melting. .) • vapour pressure of a given substance at temperature T is given by : = kT H kT H p p L L v 0 0 exp where H L is the latent heat of vaporization, and p 0 is the vapour pressure at some temperature T 0 . • The sublimation rate (number of molecules per unit time per unit area) depends on the vapour pressure and temperature: H v m kT p Z μ 12 =
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February 5, 2009 comets Energy Balance 1. Heating: radiation absorbed from the Sun, with efficiency (1-A v ) 2. Cooling: a) Reradiation in the thermal infrared, with efficiency (1-A IR ) b) Sublimation carries off an energy 4 π R 2 ZH L (Z=sublimation rate; L=energy needed/mole…) • To calculate the temperature at distance R, and the sublimation rate Z, must solve the energy balance equation by setting the heating rate equal to the cooling rate.
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February 5, 2009 comets Sublimation • calculations of the gas outflow rate as a function of heliocentric distance, for different ices.
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L11GH09_Small_SS_Bodies_V_Comets - Comets Lecture topics:...

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