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--- courses:ast100:4 [2024/12/14 03:51] – [5. Classification of Planets] asad
+++ courses:ast100:4 [2024/12/14 09:41] (current) – [4. Detecting Planets] asad
@@ Line 105: / Line 105: @@
 **Socrates:** Let’s go, everyone.
+==== Saturn's Rings ====
+{{https://upload.wikimedia.org/wikipedia/commons/thumb/2/2d/Saturn_diagram.svg/1280px-Saturn_diagram.svg.png?nolink}}
+**Hermes:** Galileo first observed these rings with a telescope, but it was Huygens from the Netherlands who first understood that they were indeed rings. From here, it’s clear that these rings are not continuous disks but rather a collection of countless ice particles ranging from a few centimeters to several meters in size. Besides water, many of these particles also contain various carbon compounds. These rings extend from about 30,000 km above Saturn’s surface to nearly 150,000 km. Despite a diameter of 300,000 km, the thickness of these massive rings ranges from about 10 meters to a maximum of a few hundred meters. The A, B, C, D, and F rings can be seen here, and the gaps between the rings are named after different scientists. For instance, the gap between the A and B rings is called the Cassini Division and the Huygens Gap; between the B and C rings is the Coulomb Gap, and between the C and D rings is the Maxwell Gap. Maxwell, the founder of electromagnetic theory, was the first to understand that Saturn’s rings are not a single disk but rather a collection of countless small objects.
+**Socrates:** No need for further descriptions. Just tell me how these rings formed.
+**Hermes:** If a moon or asteroid comes too close to a planet, the planet’s gravitational force pulls the near side of the object more strongly than the far side, as gravity decreases with distance. This results in the object being stretched and eventually torn apart into fragments due to the gravitational pull. These fragments then form a ring around the planet. Saturn’s rings were formed in this way. Moreover, if you observe closely, you will see small rocky fragments, about 10–20 km in size, scattered in certain places within the rings. These are called moonlets. Due to these moonlets, spiral structures, similar to spiral galaxies, form within Saturn’s rings. Saturn’s rings and its 62 moons can be compared on one hand to an entire solar system and on the other hand to a spiral galaxy like the Milky Way, with the spiral patterns of the rings resembling those of a galaxy.
+**Socrates:** Hold on, hold on. During the stellar age, we should have understood how spiral arms are formed in galaxies. That wasn’t possible then. Now, do you want to explain spiral arms in galaxies using Saturn’s rings?
+**Hermes:** Why not? Because of the gravity of these moonlets in Saturn’s rings, small waves sometimes form in the ocean of the rings. Waves spread evenly outward from a moonlet. However, since the inner rings of Saturn rotate faster than the outer rings, the inward-moving waves outpace the outward-moving waves. The perfectly circular wave (technically called a __density wave__) becomes spiral due to this uneven velocity, just as a circular ring can be twisted into a spiral pattern. In the case of galaxies, replace the moonlets with nebulas and stars, and replace the ocean of rings with the gases and stars of a galaxy. Since the velocity of stars and gas in galaxies also decreases with distance, the density waves created within galaxies similarly give them their spiral shape.
+**Socrates:** Fascinating. It’s through such comparisons of one era with another, or one object with another, that we can progress. How are the pinkish-purple auroras seen around Saturn’s north pole formed?
 ===== - Earth =====
 **Hermes:** If you understand how auroras form on Earth, you’ll understand Saturn’s as well. The solar system is now about 1 billion years old. If we travel from Saturn to Earth, we can see how auroras formed near Earth’s poles even 3.6 billion years before the emergence of humans.
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 {{https://upload.wikimedia.org/wikipedia/commons/4/48/Hr8799_orbit_hd.gif?nolink&500}}
-**Socrates:** Ah, so the "+" symbol at the center indicates where the star would have been. Here’s another question about the transit method: If the total amount of light includes both starlight and the planet’s reflected light, wouldn’t the total light decrease slightly when the planet moves behind the star?
+**Socrates:** Ah, so the "★" symbol at the center indicates where the star would have been. Here’s another question about the transit method: If the total amount of light includes both starlight and the planet’s reflected light, wouldn’t the total light decrease slightly when the planet moves behind the star?
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