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When is it possible to see Saturn in the night sky?
Configurations:ConjunctionOpposition
~12.4 months
Planet viewing in 2005 •behind Sun – July•at opposition – now•looks like a bright golden star
How the appearance of the rings changes as Saturn Orbits the Sun
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Hubble Space Telescope Images of Saturn’s Rings from 1996 to 2000
SATURN
• Similar composition to Jupiter, mostly hydrogen and helium.
• 1/3 mass of Jupiter, but nearly same radius. – Lower density, 700 kg / m3 – Less massive planet, lower gravity, lower density. – similar 11-hour rotation period.
• Less gravity, same rotation speed, so even more oblate (flattened).
• Atmospheric structure similar to Jupiter, but less visually dramatic.
• Interior structure similar to Jupiter, with less metallic hydrogen.
• Also generates heat by gravitational contraction.
• One large moon (Titan) and many smaller ones.
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Internal Structure
Saturn’s Rings
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Saturn’s Rings
SATURN'S RINGS
• Discovered by Galileo (1610)
• Recognized as rings by Huygens (1659)
• Edouard Roche (1850) : the rings are very close to the planet, tidal force would destroy any solid object at that distance
• Maxwell (1857): rings cant be solid: tidal gravity (stronger pull on inside than outside) would tear them apart
• James Keeler (1895) proved that the rings are not solid
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How a solid ring should rotate:
other part must move faster than the inner part
Saturn’s ring:other part rotates slower than the inner part
SATURN'S RINGS• Overall characteristics:
– Very thin (200 m), very wide (200 000 km across)
• Composed of particles centimeters to meters in size. – Each particle orbits Saturn like tiny moon, held by gravity. – Composed of (or at least coated by) water ice: highly reflective. – Size inferred by transparency to optical light, infrared light, radio
waves. – Occasional collisions between particles: bouncing, sticking,
breaking.
• Total mass comparable to small moon, 300 km in diameter.
• Other planets with rings– Jupiter (1979, Voyager 1)– Uranus (1977, from Earth)– Neptune (Voyager 2)
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SATURN'S RINGS
Hubble Space Telescope, 1995
Fig. 12.29
JUPITER'S RINGS
Voyager 1
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STRUCTURE OF SATURN'S RINGS• Complex structure• Giovanny Cassini, 1675, Cassini gap: rings A & B• Ring C (1848)
Fig. 12.36
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STRUCTURE OF SATURN'S RINGS
• Several major rings (identified with Earth-based telescopes)
• Voyager missions, Cassini mission – Thousands of ringlets. – Some rings are ultra-thin, less than 100 m. – Gaps are caused by gravitational perturbations
of small moons. – Some thin rings are maintained by gravity of
"shepherd" moons inside and outside.
Fig. 12.34
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Fig. 12.32
The detailed structure of Saturn’s B ringVoyager 2
Fig. 12.16
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Fig. 12.30
Fig. 12.35
Spokes
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• Consider an orbiting mass held together by gravity. Far from the planet the mass is practically spherical.
• Closer to the Roche limit the body is deformed
From Wikipedia
• The mass's own gravity can not longer withstand the tidal forces, and the body disintegrates.
• The varying orbital speed of the material eventually causes it to form a ring.
• Roche limit
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Formation and Evolution of Planetary Rings
• Roche Distance• 2.5 times the radius of the
planet• Rings are inside the Roche
distance• Ring systems fade away,
new are produced
• If ring material collected into a moon, planet's tidal gravity would destroy it.
• Rings may be material that was too close to Saturn to make a moon, or could be remains of moon that came too close and was torn apart.
Formation and Evolution of Planetary Rings
