Transcript Lec21-040507

Saturn … and
Planetary Rings
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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 10:30AM12:00Noon – in room 330.
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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
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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!
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Saturn is noticeably oblate
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Saturn is noticeably oblate
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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)
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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
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Instructor’s photo:
Saturn opposition 2003
Amateur photo: Christopher Go
Near
Opposition
-- 2006
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Amateur photo: Christopher Go
March, 2006 (1.5 months after
opposition)
Cassini image of
Saturn’s shadow
cast on its rings
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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
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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
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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
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Orientation of Saturn’s Rings
• 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
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The Changing Appearance of Saturn’s
Rings
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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)
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Saturn’s Helium Deficiency and Internal Heat Source
• 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
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Cassini/VIMS
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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
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In False color, Saturn reveals banding like Jupiter
• 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
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equator.
A recent “Astronomy Picture of the Day” showing unusual
hexagonal features in Saturn’s north polar region
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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
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Saturn’s Interior
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→
• 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)
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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?
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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
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Rings Have to be
Very Flat (Thin)
• Inclined orbits will
result in collisions and
destruction of the ring
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Saturn’s rings are
very thin (about 10
km). “As thin as a
sheet of tissue paper
spread across a
football field”
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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
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A tilted ring won’t stay flat
very long
• Particle orbits will precess at different rates
• Collisions will soon destroy the ring
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Saturn: Ring plane crossing
Rings are clearly aligned
with the planet’s equator
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The same is true for Uranus
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All Rings Have Gaps:
Why?
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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
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The Cassini division between
Saturn’s A and B rings
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• 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
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Cassini Found Density Waves in Saturn’s
Rings
Cassini images of spiral density waves in Saturn’s Rings
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