Saturn … and Planetary Ringsjweirich/PTYS_206_2/Lectures/Lec21-040507.pdf · – There will be a sample “planet fact sheet” ... • Mass – 95.2 Earth Masses ... Planetary

PTYS/ASTR 206 Saturn / Rings4/5/07

Saturn … and Planetary Rings


Announcements

• Reading Assignment– Chapter 15

• 5th 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.


Saturn – Basic Facts

• 6th 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


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/m3

• lowest of all the planets

• Acceleration of Gravity is about the same as that at Earth– You would weigh the same!


Saturn is noticeably oblate


Saturn is noticeably oblate


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)


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 favorableInstructor’s photo: Saturn opposition 2003


Amateur photo: Christopher GoNear Opposition -- 2006


Amateur photo: Christopher GoMarch, 2006 (1.5 months after opposition)


Cassini image of Saturn’s shadow cast on its rings


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


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


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


• 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


The Changing Appearance of Saturn’s Rings



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)


• 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 H2 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


Cassini/VIMS image


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


• 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


A recent “Astronomy Picture of the Day” showing unusual hexagonal features in Saturn’s north polar region


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


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


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)


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?


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


Rings Have to be Very Flat (Thin)

• Inclined orbits will result in collisions and destruction of the ring


Saturn’s rings are very thin (about 10 km). “As thin as a sheet of tissue paper spread across a football field”


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


A tilted ring won’t stay flat very long

• Particle orbits will precess at different rates• Collisions will soon destroy the ring


Saturn: Ring plane crossingRings are clearly aligned with the planet’s equator

The same is true for Uranus


All Rings Have Gaps:Why?


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 Cassinidivision 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


• 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


Cassini Found Density Waves in Saturn’s Rings

Cassini images of spiral density waves in Saturn’s Rings

Documents

Saturn … and Planetary Ringsjweirich/PTYS_206_2/Lectures/Lec21-040507.pdf · – There will be a sample “planet fact sheet” ... • Mass – 95.2 Earth Masses ... Planetary