PTYS/ASTR 206Saturn / Rings 4/5/07 Saturn … and Planetary Rings

PTYS/ASTR 206Saturn / Rings 4/5/07 Announcements Reading Assignment –Chapter 15 5 th homework due next Thursday, April 12 (currently posted on the website). Reminder about term paper – due April 17. –There will be a sample “ planet fact sheet ” (that you are required to attach to your term paper) posted on the website this weekend –There will also be details about posting it to turnitin.com this weekend as well Next study-group session is Wednesday, April 11, from 10:30AM- 12:00Noon – in room 330.

PTYS/ASTR 206Saturn / Rings 4/5/07 Saturn – Basic Facts 6 th planet from the Sun Avg. distance from the Sun –9.5 AU Orbital Period –29.5 years Rotational Period –10.2 hours –Only slightly longer than Jupiter’s which is the shortest of all the planets Over 40 moons

PTYS/ASTR 206Saturn / Rings 4/5/07 Saturn – Basic Facts Diameter (Equatorial) –9.4 Earth Diameters –Oblateness = 0.09 largest of all the planets Mass –95.2 Earth Masses Density –687 kg/m 3 lowest of all the planets Acceleration of Gravity is about the same as that at Earth –You would weigh the same!

PTYS/ASTR 206Saturn / Rings 4/5/07 Saturn is noticeably oblate

PTYS/ASTR 206Saturn / Rings 4/5/07 Saturn is noticeably oblate

PTYS/ASTR 206Saturn / Rings 4/5/07 Saturn visits Flybys –Pioneer-Saturn (formerly known as Pioneer 11) (1979) –Voyager 1 (Nov. 1980) –Voyager 2 (Aug. 1981) Orbiters/Landers –Cassini (still in orbit) –Huygens (Jan. 2005)

PTYS/ASTR 206Saturn / Rings 4/5/07 Saturn seen from Earth Saturn is at opposition with Earth every 12.4 months –About 2 weeks later each year. The last opposition occurred on Feb. 10, 2007 Spectacular (again) this year. –High in the sky (i.e. winter oppositions are best) –Orientation of the rings is still favorable Instructor’s photo: Saturn opposition 2003

PTYS/ASTR 206Saturn / Rings 4/5/07 Amateur photo: Christopher Go Near Opposition

PTYS/ASTR 206Saturn / Rings 4/5/07 Amateur photo: Christopher Go March, 2006 (1.5 months after opposition)

PTYS/ASTR 206Saturn / Rings 4/5/07 Cassini image of Saturn’s shadow cast on its rings

PTYS/ASTR 206Saturn / Rings 4/5/07 Galileo’s “Ears” Galileo first looked at Saturn through his (primitive) telescope and discovered what he called the “ears” of Saturn –They “disappeared” for a period and then returned –He was baffled by this and did not know what was going on –He died not knowing the answer –“I have observed the highest planet tri-form” We now know them to be Saturn’s rings “Discovered” by Giovanni Cassini and Christian Huygens

PTYS/ASTR 206Saturn / Rings 4/5/07 Galileo’s “Ears” Galileo first looked at Saturn through his (primitive) telescope and discovered what he called the “ears” of Saturn –They “disappeared” for a period and then returned –He was baffled by this and did not know what was going on –He died not knowing the answer –“I have observed the highest planet tri-form” We now know them to be Saturn’s rings “Discovered” by Giovanni Cassini and Christian Huygens

PTYS/ASTR 206Saturn / Rings 4/5/07 Ring-Plane Crossings: Saturn’s “disappearing” rings 2 HST images just before and during the ring-plane crossing The dark band in the lower image is the shadow of the ring on the planet Also seen are two of Saturn’s moons

PTYS/ASTR 206Saturn / Rings 4/5/07 The tilt of Saturn’s equator is about 27 degrees relative to the ecliptic plane As Saturn moves about its orbit, the orientation of its rings are seen to be different Orientation of Saturn’s Rings

PTYS/ASTR 206Saturn / Rings 4/5/07 The Changing Appearance of Saturn’s Rings

PTYS/ASTR 206Saturn / Rings 4/5/07

PTYS/ASTR 206Saturn / Rings 4/5/07 Saturn’s Atmosphere The atmosphere is primarily composed of hydrogen with small amounts of helium and methane. –Deficient in Helium compared to solar abundances –Saturn – 3.3% He (cloud tops) –Jupiter – 13.6% He (cloud tops) Saturn's hazy yellow hue is marked by broad atmospheric banding –can be seen through small telescopes, but not as noticeable as Jupiter –Much less color contrast –Have to look close to see storms! (they are there, but hard to spot)

PTYS/ASTR 206Saturn / Rings 4/5/07 Because Saturn’s atmosphere is quite cool, Helium may condense in the atmosphere and “rain” down through the atmosphere into the interior –This creates a deficiency of helium in the upper atmosphere –It also creates a form of friction with H 2 molecules in the interior –This friction creates heat which is then radiated into space accounts for Saturn’s larger internal heat source compared to Jupiter Saturn’s Helium Deficiency and Internal Heat Source

PTYS/ASTR 206Saturn / Rings 4/5/07 Cassini/VIMS image

PTYS/ASTR 206Saturn / Rings 4/5/07 Saturn’s Atmospheric Dynamics Saturn has a strong internal heat source –Releases more energy than it receives from the Sun –about 25% more than does Jupiter (on a per mass basis) Storms are sometimes seen –Similar to those seen on Neptune and Jupiter Much shorter lived –Typically last for several months to a year

PTYS/ASTR 206Saturn / Rings 4/5/07 Note that this banding is not easily seen from Earth through normal visible light The bands are generally broader near the equator (thicker) –This is probably related to the fact that Saturn (being less massive) cannot create the large pressures than Jupiter can The wind speeds on Saturn are faster than on Jupiter. –The reason for this is unknown. –The strongest winds (nearly 1100 miles per hour) are found near the equator. In False color, Saturn reveals banding like Jupiter

PTYS/ASTR 206Saturn / Rings 4/5/07 A recent “Astronomy Picture of the Day” showing unusual hexagonal features in Saturn’s north polar region

PTYS/ASTR 206Saturn / Rings 4/5/07 In the south pole, there is a large storm with a well-defined eye wall (which has not been seen on any other planet except Earth

PTYS/ASTR 206Saturn / Rings 4/5/07 Saturn’s Interior Outer layers –primarily molecular hydrogen and helium –Also contains ices of ammonia, methane, and water Interior –Liquid hydrogen Outer core –Liquid metallic hydrogen Inner core –rock →

PTYS/ASTR 206Saturn / Rings 4/5/07 Saturn’s Magnetic Field Not as strong as Jupiter’s –less mass, and smaller liquid metallic hydrogen core –larger than Earth’s, however, and has a huge magnetosphere Generates radio waves (like Jupiter), but they cannot be detected at Earth –Observed first by Pioneer 11 –Observations of radio signals used to determine rotation rate To the accuracy of measurement, Saturn’s magnetic field is aligned with its rotation axis – VERY UNUSUAL (only planet like this)

PTYS/ASTR 206Saturn / Rings 4/5/07 Planetary rings Which planets have them ? –All of the gas giants Jupiter, Saturn, Uranus, Neptune –No terrestrial planets (presently) Mars may have a ring system in about 40 million years Mercury, Venus, and Earth are unlikely to have rings in the future – but they probably did in the past What are they made of?

PTYS/ASTR 206Saturn / Rings 4/5/07 Rings are swarms of orbiting particles They cannot be solid “sheets” because the forces of gravity would vary from the inner to outer parts of the ring –This is observationally confirmed using Doppler imaging of Saturn’s rings Orbits have to be very circular Elliptical orbits will result in collisions, destroying the ring

PTYS/ASTR 206Saturn / Rings 4/5/07 Rings Have to be Very Flat (Thin) Inclined orbits will result in collisions and destruction of the ring

PTYS/ASTR 206Saturn / Rings 4/5/07 Saturn’s rings are very thin (about 10 km). “As thin as a sheet of tissue paper spread across a football field”

PTYS/ASTR 206Saturn / Rings 4/5/07 Rings have to be Equatorial Tilted orbits precess; the plane of the orbit rotates due to the planet’s equatorial bulge, other satellites, and the Sun’s gravity

PTYS/ASTR 206Saturn / Rings 4/5/07 A tilted ring won’t stay flat very long Particle orbits will precess at different rates Collisions will soon destroy the ring

PTYS/ASTR 206Saturn / Rings 4/5/07 Saturn: Ring plane crossing Rings are clearly aligned with the planet’s equator The same is true for Uranus

PTYS/ASTR 206Saturn / Rings 4/5/07 All Rings Have Gaps: Why?

PTYS/ASTR 206Saturn / Rings 4/5/07 Rings, Gaps, and Resonances Gaps can occur in rings because the positions of particles in that ring are in a resonance with the planet and a moon Example: The Cassini division is located at a point that is in a 2:1 resonance with Saturn’s moon Mimas The Cassini division between Saturn’s A and B rings

PTYS/ASTR 206Saturn / Rings 4/5/07 The gaps in rings are similar to Kirkwood gaps –Gaps in the number of asteroids in the asteroid belt –We will discuss this more in two weeks Distribution of asteroid periods showing gaps, like in planetary rings

PTYS/ASTR 206Saturn / Rings 4/5/07 Cassini Found Density Waves in Saturn’s Rings Cassini images of spiral density waves in Saturn’s Rings