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Lecture 21.docx

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Victor Aurora

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Lecture 21 Recap & Preview  Mars was not able to sustain a significant greenhouse effect because its interior cooled too quickly, but evidence suggests liquid water flowed on its surface in the distant past  Jupiter is a liquid hydrogen giant with weather features driven by strong convection  Be able to describe how small bodies can undergo geologic activity  Understand how natural satellites create and maintain planetary ring systems o another version: Jupiter's Moons  67 natural satellites: o four Galilean moons are the largest and all orbit in Jupiter's equatorial plane o the others are smaller and mostly non-spherical → probably captured asteroids Key Observations: Io and Europa  density: 3.6 × water  rock and metal, differentiated  hundreds of volcanoes  continual resurfacing from ash → craterless  density: 3 × water  rocky w/ thin, clean icy crust  smooth areas and long ridges, few craters, “ice rafts” Why are Io and Europa Active?  few craters → moons must be geologically active  but both are too small to retain heat from formation  Step 1: resonance o Europa is exactly twice as far from Jupiter as Io; Ganymede is exactly four times as far from Jupiter as Io; o because they “sync up”, Io gets a periodic gravitational tug from the other two moons at the same spot every → Io's orbital shape is stretched into an ellipse Animation: Why are Io and Europa Active?  Step 2: Tidal Stress  Mini-experiment: o break a piece of putty into two pieces o repeatedly stretch and expand one of the pieces o place the stressed putty on your upper lip, and then place the unstressed putty on your upper lip  What accounts for the difference? Where did the energy come from? Tides can Warm Satellite Interiors  Io's elliptical orbit, brings it closer to and further from Jupiter  recall: gravity pulls more strongly on the parts of Io that are facing Jupiter (tides); Io is repeatedly stretched when it makes its closest approach o friction warms the interior, heat flows to the cool exterior  tides also warm the interior of Europa: o cracks in surface → evidence for tidal flexing o Enough warmth for a subsurface liquid water ocean? Formation of Moons  Ganymede/Callisto: density: 1.9 and 1.8 × water → half rock, half ice  cratered / dark surface → old o dark: surface ice evaporated by “micrometeroids”  bright patches from recent craters → clean ice  observations: moons have decreasing density with increasing distance from Jupiter; all are in same orbital plane  hypothesis: moons formed in a disk of gas and dust around a warm young Jupiter, like a mini-solar system Jupiter's Rings  forward-scattering: rings are brighter when illuminated from behind o suggests composition is particles 10–7 m in size  pressure from th
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