Lecture11

Lecture11 - Sublimation A sample of ice in a vacuum will...

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Sublimation A sample of ice in a vacuum will sublimate, and the vapour will fill the chamber until it reaches the vapour pressure, = kT H kT H p p L L v 0 0 exp , where H L is the latent heat of vapourization (the amount of energy required to remove one molecule). T 0 , p 0 are constants Gas p 0 (Pa) 0 (K) H L (10 -20 J/molecule) N 2 133 47 1.1 CH 4 133 67 1.5 CO 2 133 139 3.8 NH 3 133 164 5.0 H 2 O 133 256 7.6 In a vapour in thermal equilibrium, the molecules of mass m are moving randomly, with velocities v, and the kinetic energy is equal to the thermal energy: kT v m H 2 3 2 2 = µ The number of molecules per unit volume striking a surface, per unit time, is just nv Z = 3 1 2 1 , where the factor 1/6 is due to the fact that we only count particles moving in the (say) +x direction Using the ideal gas law to relate n and vapour pressure p V , we get an expression for the outflow rate (molecules per unit time per unit area sublimating): H v H v m kT p m kT kT p Z 12 3 6 1 = = Comet activity at >3 AU indicates the presence of volatile ices such as NH 3 (ammonia), CH 4 (methane), CO 2 , and N 2 . 1. We see the sublimated gas because of the photons the atoms absorb from solar radiation and then reemit.

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Comet Orbits Long-period comets have: ¾ Highly eccentric orbits ¾ Large semi-major axes and, therefore, aphelion distances ¾ Originate from all directions ¾ Postulated to originate from the Oort cloud, a spherical shell surrounding the SS at great distance (at least ~10 4 AU). ± Short-period comets: ¾ Represent about 20% of all known comets ¾ Eccentricities typically 0.6-0.8, lower than for long-period comets ¾ Most orbital inclinations within ~30 degrees of ecliptic ¾ Likely originated in the Kuiper belt. ¾ Prograde orbits ¾ Halley’s comet is an exception, with a high inclination, retrograde orbit, e=0.97, but P=76 yr. Halley and comets like it probably originated in the Oort cloud but had their orbit perturbed.
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Lecture11 - Sublimation A sample of ice in a vacuum will...

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