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Astronomy & Astrophysics
Michael Reid

November 21, 2013 th AST101 – Lecture 19 (20 class) o planets that are closer to Jupiter = more geologically active  aren’t as many craters  wouldn’t expect Io to be geologically active b/c smaller than Mercury and Mercury isn’t o Io = most volcanically active planet in the solar system  infrared light shows hot spots (which are volcanoes) o in order of distance from Jupiter: Io (closest), Europa, Ganymede, Callisto (farthest) o tidal heating stretches the moon in different ways because of its eccentricity o eccentricity: deviation of orbit from ellipse (in this case, the orbit is more elliptical) o amount of stretching of the moon varies depending on where it is in its orbit  more stretching when closer  less when it’s farther  rocks rub against one another during the process, creating heat  enough to keep Io’s core liquid! o Io’s orbit is eccentric in the first place because of orbital resonance! o orbital resonance – planets cross paths on the same side during orbital periods o e.g. if the ratio is 1:2 for Io & Europa, Io orbits 2 times for every 1 time Europa does
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