Lecture #34: Jupiter and its Moons

I. Planetary escape speeds -- calculate Jupiter's atmospheric escape speed relative to Earth -- 12 times larger radius, 5 times smaller density -- implies 6 times larger escape speed -- for comparison: escape speed from Sun from Jupiter's orbit, relative to the case of the Earth: 5.2 times farther, so about 1/5.2^(1/2) times 42 km/s, or about 18 km/s -- so from Jupiter, it's easy to escape the solar system if you can build a rocket strong enough to escape Jupiter! II. More atmospheres in the solar system -- Jupiter: dark belts and bright zones, Great Red Spot (unknown composition) surface rotation rate: 5 x10^5 km in 10 hours (= 3.6 times 10^4 s) is about 14 km/s ! (if Earth were rotating that fast, it would exceed escape speed and fly apart! -- such fast rotation has huge atmospheric effects: Coriolis is huge -- get dark bands, bright zones, moving at different speeds -- slide show -- Neptune: similar to Jupiter, Great Dark Spot -- Io: lots of sulfur and sodium -- Titan: only moon with thick atmosphere, similar N2 content to Earth, some methane, photochemical "smog" gives it orange color III. Atmospheric pressure -- weight of atmosphere creates pressure -- air pressure is not negligible on Earth: 15 pounds per square inch! -- force is everywhere: cancels out (but materials are under stress) -- spacesuit and deep-sea diver suit look similar: note they perform the opposite function! -- high-flying jets are in the bottom of the stratosphere, about six times smaller air pressure. Planes are "pressurized". What happens when the hull is breached? IV. Interesting Moons -- Galileo first saw the 4 big ones: Io, Europa, Ganymede, Callisto -- Moons are like a mini-solar system, formed similar to solar nebula -- compositions of moons vary like the compositions of the planets: the further out, the more icy and less dense (but there are no "gas giant" moons! Why not? H gas was captured by Jupiter instead.) -- Io is active, but is too small to hold heat since the beginning. So it must have been heated by tidal stresses from Jupiter. The other moons must play a role in disturbing Io's orbit so that its shape is warped like kneading bread. -- Europa is less active and has an icy crust that shows signs of plate tectonics, probably like an ice field floating on liquid water, so there might be some tidal heating melting the ice, and maybe the possibility for some kind of life! -- Ganymede and Callisto are similar: large, and very icy. Ganymede is larger than the planet Mercury, and almost as big as Mars! -- the density of the moons can only be determined by watching something that is affected by their gravity, like something orbiting them. But there isn't any such thing, except: man-made spacecraft like Voyager and Galileo!