Q: What trends can you see from your graphs?
Q: Compare these graphs for the Jovian system to the ones you made in part 1 for the solar system. What similarities are there?
A: size increases with distance from center, mass increases, density decreases
Q: Why do you think this might be?
Scientific context: Remember the discussion on the formation of the solar system from a cooling disk of gas and dust. Scientists call the Jovian system a "mini solar system" because they think that these four large satellites could have formed in a similar way, through the cooling of a disk of leftover material surrounding Jupiter right after it formed. The same decrease in density with increased distance from the center is there: Io, like Mercury, is dense and consists of lots of rock, while Ganymede and Callisto are farther out and less dense, and are probably made of mostly ice.
Q: Compare your plot to the plots you made in activity 2. What
similarities are there?
A: The planets or satellites closer to the center take less time to orbit.
Q: Io is the closest satellite to Jupiter, and Mercury is the closest planet to the sun, but Io only takes 1.7 days to go around Jupiter once, while Mercury takes 88 days to go around the sun once. Why do you think this might be?
A: Look at the orbital distance: Io is much closer to Jupiter than Mercury is to the sun, so Io has much less distance to cover in one orbit than Mercury does. So it takes a lot less time.
Q: If desired, calculate how many kilometers Mercury travels in one orbit (2*[[pi]]*r), compare to how far Io travels. Use orbital period to get orbital velocity. What's the orbital velocity for the Earth?
Interpretation II: Satellites and Planets
Q: Which planet is smaller than one of Jupiter's moons?
A: Ganymede is bigger than Mercury.
Q: The Moon orbits the Earth. Compare its size to the sizes of Jupiter's satellites. What is surprising about this?
A: Jupiter is many times larger than the Earth, yet Earth's Moon is comparable in size to Jupiter's moons!
Q: Look at the orbital period of the Moon (how long it takes to go around the Earth once). What is this close to?
A: This is about 1 month: the original definition of a month was one lunar orbit, and some calendars still use this definition (the Jewish calendar uses lunar months).
Q: Compare the density of the Moon to the densities of Jupiter's satellites. Which is it closest to?
Q: Remember what Jupiter's satellites were probably made of. Do you think the Moon is made mostly of rock, or of ice?
Q: Find the ratio of the mass of the Moon to the mass of the Earth. Compare this to the ratio of the mass of Ganymede, the largest satellite of Jupiter, to the mass of Jupiter.
Q: Find the ratio of the mass of Charon to the mass of Pluto.
Q: Compare the distance between Pluto and Charon to the distance between the Earth and the Moon. Some people call Pluto / Charon a "double planet". Can you see why?
A: They are so close in size, and close together in distance.
Scientific context: How do planets get moons? It depends on the planet. As discussed above, scientists think that Jupiter's four largest satellites formed in place around it out of leftover material, like the planets formed around the sun. Not all moons were formed this way, however. Mars has two tiny satellites that look a lot like members of the asteroid belt (an area full of small bodies (house-sized to city-sized) that orbits the sun between Mars and Jupiter). Since Mars is located right next to the asteroid belt, it seems likely that it captured these two satellites from there when their orbits got too close to Mars. Jupiter, on the other side of the asteroid belt, has two groups of moons that orbit outside the four major ones. Once group orbits prograde (forwards), the other retrograde (backwards). Scientists think these bodies might have been captured, like Mars' satellites. They might even have been captured as two larger bodies, each of which then broke apart into a number of pieces.
The Earth's Moon is harder to explain. It's so large in comparison to the Earth (as you showed above) that it's very unlikely that the Earth could have captured it unchanged, or formed it in place from leftover material. We also know from rocks brought back from the Moon by the Apollo astronauts that the Moon's composition is quite similar to Earth's in most respects, but that it has much less metal than the Earth does. This makes the capture theory even more unlikely, since a body that formed somewhere else in the solar system is unlikely to have a similar composition at all. The currently favored theory is that a giant impact soon after the Earth formed splattered a large amount of molten and rocky material into orbit. Most of the material would have fallen back to the Earth, but if the size and direction of the impact were within a certain range, enough material could have remained in orbit to clump together and form the Moon. Since metals are heavier than plain rocks, the metal would be more likely to fall back to Earth, while the less dense rock could stay in orbit long enough to form the Moon. So this could explain why the Moon's composition is so similar to the Earth's in most respects, but depleted in metals. Scientists are still working on this theory, and all the details have yet to be fully understood.
Pluto, and its moon Charon, are another question. As described above, the planets can roughly be divided into the four inner terrestrial planets, which are small and rocky, and the four outer gas giants, which are huge and gaseous. Pluto doesn't fit either category: it's far from the sun, near the gas giants, but it seems to be a small body made of rock and ice, rather than a huge ball of gas. Some scientists have thought that Pluto might originally have been a satellite of Uranus or Neptune, which escaped somehow (maybe due to a collision) and began orbiting the sun on its own. More recently, scientists have begun discovering icy bodies out beyond the orbit of Pluto. These bodies make up something called the Kuiper Belt. Not much at all is known about them, since they're so small, dark, and far from the sun, but some scientists think that these objects might be leftover remnants of rock and ice from the formation of the solar system, which were too far out to become part of a planet. Pluto might be one of the largest and closest of this class of objects (it's been called the "King of the Kuiper Belt"), and would therefore be a very interesting sample of what the material which formed the solar system was like over 4 billion years ago. We've never sent a space probe to Pluto, so we know very little about it. There have been some recent proposals for such a mission, however, and if one of these is selected for funding, we may soon know much more about this tiny, cold world so far from the sun.
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Galileo Solid State Imaging Team Leader: Dr. Michael J. S. Belton
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