Disclaimer: machine translated by DeepL which may contain errors.
The Ring Observed by Cassini
Eiichiro Kokubo (Adjunct Professor, Department of Astronomy, UTokyo / The National Astronomical Observatory of Japan)
The Saturn probe Cassini-Huygens, named after the 17th century astronomer Giovanni Domenico Cassini who discovered the void in Saturn's rings, was launched in 1997 and arrived at Saturn in 2004. It arrived at Saturn in 2004 and continued its long exploration of the planet until 2017. It showed us the amazingly beautiful rings of Saturn.
Saturn and its rings photographed by CassiniSaturn with its rings has been special to me since I was a child. I think I first saw it in a picture book. I thought the rings circling Saturn floating from the main body of Saturn was very mysterious and cool. I still feel the same way.
Saturn's rings are made up of water ice particles ranging in size from a few centimeters to 10 meters. This is a size that is easy for us to understand. The spatial distribution of these particles creates beautiful patterns in Saturn's rings. Incidentally, it was the physicist James Clerk Maxwell who theoretically proved that the rings are made of particles. What determines the motion of particles is, first of all, Saturn's gravity. This is dominant, and the particles are in orbital motion. Next is the gravity from the satellite. This is especially important when there is orbital resonance (the ratio of the orbital period of the particle to that of the satellite is a simple integer ratio). Then, gravity and collisions between particles. Although it is a very simple dynamical system, the structures formed are diverse. The bright parts of the rings are called A-, B-, and C-rings from the outside, and the Cassini void is between the A and B rings.
I think it was in junior high school when I first saw Saturn in the flesh with my own eyes. I assembled a paper refractor kit and pointed it at Saturn. Looking through the eyepiece, I saw a small blotchy Saturn floating fleetingly. In a moment when the atmosphere stabilized, I saw a clear view of Saturn and its rings. I exclaimed, "Wow, it's the rings! I exclaimed. My dog, who was sitting next to me, stood up in surprise. The rings were so small and faint that I could not compare them with the picture in the picture book, but they were still real light. Saturn's rings still make me wonder and inspire me to study them to discover their secrets.
Cassini made many discoveries about the rings and satellites during its mission. For example, the non-axisymmetric (spiral) density waves of the rings, the propeller-shaped structure, and the satellites with UFO-like wings. All of these things tickle my intellectual curiosity. When I was tired from work, I was cheered up by seeing the beautiful pictures of the rings on the Cassini Project Web site.
The most surprising discovery of Cassini was the "mountain range" at the outer edge of the B ring. Near Saturn's vernal equinox, the sun shines on the rings from almost directly across. The average thickness of the B ring is 10 m. Frequent collisions between particles have thinned the rings. Cassini's photograph of the outer rim of the B ring shows a long line of shadows. Calculations show that the "mountains" that make up the shadows are more than 2 km high. What and how do these huge mountains form? It is still a great mystery.
On September 15, 2017, Cassini entered Saturn's atmosphere and completed its mission. I opened my computer and sat upright watching the internet coverage of the grand finale. I had always felt that Cassini, which had been on Saturn since I got my degree, was a friend of mine. When the communication from Cassini ceased, I felt an inexpressible mixture of loneliness, as if I had lost a close friend, and emotion, as if a great project had come to an end. Theorists have much homework left to do. I will continue to challenge the discoveries and further mysteries brought by Cassini.
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Published in The Rigakubu News May 2024