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Wilfrid Laurier University
Victor Aurora

Lecture #22 Recap & preview  different processes can provide the thermal energy that is needed for internal activity o radioactive decay (e.g. Earth) o impacts/heat of formation (e.g. Jupiter) o tidal stressing (e.g. Io) o helium rain (e.g. Saturn)  explained the observed planetary characteristics in the outer Solar System  understand how we can determine the composition of an extrasolar planet Revisiting the Roche limit  Moons are responsible for the formation and maintenance of planetary ring systems  gravity tries to pull matter into a sphere, but tidal stress (“stretch”) from the planet tries to pull it apart  inside the a boundary called Roche limit, the tides always win  most of Saturn's rings are inside the Roche limit  particles cannot grow by gravitation (they just break apart again) Correction: clouds on Jupiter  recall that temperatures rise at lower depths in Jupiter's atmosphere  ammonia, ammonium hydrosulfide, and water all condense at different depths → layers of clouds Ice giants vs. Jupiter and Saturn  model: deep hydrogen, helium atmospheres but relatively less than the other Jovians  similarly sized rocky cores o forming U + N took longer to grow massive enough for direct gas capture phase o perhaps nudged outward by J + S to region w/less material  traces of methane absorbs red light  lower pressure in interior → slushy mantle (ice + rock / methane / ammonia) o ammonia slush is electrical conductor + circulation or convection in water + fast rotation = magnetic field Comparing the ice giants  Uranus: no internal heat; Neptune: yes o this may be why U is so bland compared to N o some clouds also form at lower altitudes, below methane haze  Uranus has an extreme axial tilt: 98° o early collision with a planetesimal o seasons 21 years long  Uranus' rings discovered by occultation o as Uranus moves in orbit, the dark rings may temporarily block out light from a star in the background Pluto and Dwarf Planets  Distance from Sun: 39 AU Radius: 0.19 × Earth Mass: 0.002 × Earth Density: 1.8 × water → mixed icy/rocky surface o too small to be Jovian, not dense enough
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