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Physics 320 Lecture 19: Planetary Ring Systems. Bin Chen NJIT Physics Department. Saturn’s Rings. Io. A short video about this image: https :// www.youtube.com / watch?v = KuTukEleqvo. T rue-color photo from Cassini in 2013. Galileo’s Discovery.
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Physics 320 Lecture 19: Planetary Ring Systems Bin Chen NJITPhysics Department
Saturn’s Rings Io A short video about this image: https://www.youtube.com/watch?v=KuTukEleqvo True-color photo from Cassini in 2013
Galileo’s Discovery • In 1610, Galileo wrote a letter to the Duke of Tuscany: “…planet Saturn is not alone, but is composed of three, which almost touch one another and never move nor change with respect to one another.” • In 1612, Galileo observed Saturn again, and found the rings appeared to vanish. Mystified, Galileo wondered, "has Saturn swallowed his children?"
A brief history • 1610: first discovery by Galileo. • 1655: Christiaan Huygens became the first person to suggest that Saturn was surrounded by a ring. Huygens observed Saturn and wrote that “It [Saturn] is surrounded by a thin, flat, ring, nowhere touching, inclined to the ecliptic”. • In 1675, Giovanni Cassinidetermined that it was composed of multiple smaller rings with gaps between them, the largest of which was later named the ”Cassini Division”. • In 1859, James Maxwell proposed that the rings must be composed of numerous small particles, which were later proven to be correct using spectroscopic observations. • 1970s–present: the space age. Pioneer 11 discovered the F ring in 1979. Voyager discovered the G ring. Cassini entered the orbit of Saturn in 2004 and is still observing (until mid 2017), which provided the most detailed images to date.
Saturn’s rings: physical properties • Consist of numerous small particles from ~1 cm to ~10 m • Composed of mostly water ice (>99%) • Clumpy and dynamic Artist concept of close-up view of Saturn’s ring particles (NASA/JPL)
Saturn’s rings are ridiculously thin! • When viewed edge-on, they practically disappear! • Saturn’s rings are 250,000 km wide, but only a few x 10 m thick in some places. A sheet of paper the size of San Francisco would have about the same ratio of width to depth. • A letter-sized paper is 100–10,000 x fatter than Saturn’s rings!
Why are Saturn’s rings so thin? • Particles with tilted orbits collide with each other inelastically, losing kinetic energy and angular momentum. • Orbits flatten out due to these collisions. • This continues to occur until the particles spread very thin to a few times the size of the largest particles. Artist concept of close-up view of Saturn’s ring particles (NASA/JPL)
Other phenomena of Saturn’s rings Pan, Encke gap, and edge waves Dark Spokes
Saturn’s Shepherd Moon Prometheus
Interactions between Saturn’s moons and rings Prometheus enters Saturn’s F ring
To recap: Phil Plait’s Crash Course on Saturn • https://www.youtube.com/watch?v=E8GNde5nCSg
Jupiter’s tenuous ring system Jupiter and its ring in infrared Galileo images (NASA/JPL)