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Today, we will mainly discuss Saturn.
Its orbit, and its place in our solar system, are shown on the next
two slides.
It is a little more than 9.5 times further from the sun than is the
earth, and almost twice as distant from the sun as Jupiter.
Its orbit, like the earth’s, lies pretty much in the ecliptic plane.
Saturn rotates in the same sense as its orbital motion, as does the
earth, and its rotational axis is inclined relative to the plane of
its orbit, much like the earth’s.
Saturn: Diameter, 120,000 km; Mass, 95.2 Earth masses;
Density, 0.7 (density of water is 1.0);
Rotation Period, 10 hours, 14 minutes;
Axis Inclination, 26° 44’; Oblateness, 0.1;
Surface Gravity, 1.15 (Earth = 1.0).
The inclined rotation axis of Saturn means that Saturn has seasons,
like we do on earth, although it is amusing to think how the
seasons on Saturn might be affected by the presence of Saturn’s
ring system.
The following tables, repeated from last time, give comparisons of
some of the physical properties of the planets.
The most important columns of these tables to see for the moment
are those that compare the sizes and material compositions of
the planets.
These data show that Saturn, like Jupiter, is a giant gaseous planet
entirely different from the earth.
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Sizes of the Giant Gas Planets
Saturn seen with Lick Observatory’s 120-inch telescope
(using the Coudé focus)
Voyager-1 image of Saturn from 76
million km. Moons Mimas &
Enceladus are at the bottom, Titan at
right,
and Dione & Tethys are at the top
left. Image obtained 9/17/80.
Saturn, like Jupiter, rotates very rapidly.
For this reason, it is visibly oblate.
Saturn has just about the same radius and rotation period as
Jupiter, but it is less than a third as massive. Therefore Saturn
exerts much less gravitational force than Jupiter does near its
surface. Consequently, Saturn has less gravity than Jupiter to
counterbalance the large centrifugal force generated near its
q
surface byy its rapid
p rotation. The result is that Saturn
equatorial
is visibly much more oblate than Jupiter.
Remember that this situation, where Saturn is much less massive,
yet nevertheless not very much smaller than Jupiter, arises
because both planets are made out of gases rather than rocks or
ice. If you were to add more mass to Jupiter, its gases would
become more compressed by the increased gravity, and the
planet might not become much larger. If you added enough
mass, Jupiter might even become smaller!
Figure 11.3 (a) Gravity alone makes a planet spherical, but rapid
rotation flattens out the spherical shape by flinging material near
the equator outward. (b) Saturn is clearly not spherical.
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The rapid rotation of Saturn, just as on Jupiter, inhibits circulation
of any parcel of gas in the atmosphere over a wide range of
latitudes.
If a parcel of gas in the atmosphere near the equator were to travel
very far toward either pole, the principle of conservation of its
angular momentum about the planet’s rotation axis would force
it to spin up to a tremendous velocity. The gas therefore prefers
to organize its circulation into bands,
bands so that no part of the
atmosphere travels over too wide a range of latitudes.
The atmospheric circulation therefore breaks up into a series of
“belts” and “zones,” horizontal bands in which prevailing wind
velocities alternate.
As on Jupiter, these motions are driven by convection. Convection
carries heat outward from the interior to the surface, where it is
radiated into space.
Zonal (east-west) wind
velocity for the giant
planets as a function of
latitude. There are gaps
where we lack contrasts
suitable for tracking winds
or where the atmosphere
was in darkness (on the
night side or shadowed by
Saturn’s rings).
From P. Gierasch and B.
Conrath (1993),
J. Geophys. Res. 98, 54595469.
Despite the similarities of Saturn and Jupiter just mentioned,
Saturn does not exhibit the garish atmospheric features that are
so striking on Jupiter.
Because of its overall lower temperatures, Saturn’s clouds lie
deeper in its atmosphere than those of Jupiter.
As a result, a layer of tan haze overlies the clouds and washes out
Saturn’s appearance.
Saturn has a
powerful
equatorial jet (very
rapid winds in the
direction of the
planet’s rotation).
(from Encyclopedia of the Solar System)
Saturn's atmosphere and its rings are shown here in a false color
composite made from Cassini images taken in near infrared light
through filters that sense different amounts of methane gas. Portions
of the atmosphere with a large abundance of methane above the
clouds are red, indicating clouds that are deep in the atmosphere.
Grey indicates high clouds, and brown indicates clouds at
intermediate altitudes. The rings are bright blue because there is no
methane gas between the ring particles and the camera.
View of Saturn from the Cassini Spacecrft, 2005.
The complex feature with arms and secondary extensions just above
and to the right of center is called the Dragon Storm. It lies in a
region of the southern hemisphere referred to as "storm alley" by
imaging scientists because of the high level of storm activity
observed there by Cassini in the last year. Radio bursts from
lightning associated with this storm have been observed by the
Cassini probe.
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Saturn's atmosphere
and its rings are shown
here in a false color
composite made from
Cassini images taken
in near infrared light
through filters that
sense different
amounts of methane
gas. Portions of the
atmosphere with a
large abundance of
methane above the
clouds are red,
indicating clouds that
are deep in the
atmosphere. Grey
indicates high clouds,
and brown indicates
clouds at intermediate
altitudes. The rings are
bright blue because
there is no methane
gas between the ring
particles and the
camera.
Spectra of Saturn’s atmosphere reveal a mysterious depletion of
helium to levels well below that in Jupiter’s atmosphere or in
the sun (helium is 4 times less abundant relative to hydrogen in
Saturn’s visible atmosphere than on the sun).
It is believed that the helium originally present in the outer
atmospheric layers of Saturn has separated from the hydrogen,
much like oil and vinegar separate naturally, and has settled
toward the deep interior of the planet.
This settling of the helium releases gravitational potential energy
as heat.
Without the heat input from the settling of helium, model
calculations indicate that Saturn should have lost the heat it
would have gained in its original accretion and arrived at its
presently observed temperature in only 2 billion years, less than
half of the age of the solar system. The heating from helium
settling seems to resolve this contradiction.
Although the structure of Saturn’s interior and its atmospheric
circulation patterns are fascinating, by far the most interesting
feature of this planet is its magnificent ring system.
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Artist’s concept of Saturn’s rings and major icy moons.
The outer rings, labelled G and E, are diffuse.
The E ring, the largest in the solar system, extends from the orbit of
Mimas to that of Titan, a distance of 1 million km.
The main rings (A,B,C) are less than 100 meters thick.
They are perhaps only a few hundred million years old.
Saturn’s rings orbit the planet inside what is called the Roche
zone. This is the region so close to the planet (within 2 or 3
Saturn radii) that the tidal forces caused by the planet prevent
the accumulation of sizable bodies as a result of their selfgravity.
Within the Roche zone, the tidal forces tending to pull an object
apart are comparable to the gravitational forces tending to hold
it together.
together
Therefore only relatively small bodies, especially bodies held
together by non-gravitational forces, as your body is, can
survive inside the Roche zone.
Saturn’s rings may have formed according to one of the following
2 scenarios:
1. They are remnants of the original nebula from which Saturn
formed. They were prevented from accumulating into a single
moon by the tidal forces of gravity.
2. They are the result of the tidal disruption of one or more
objects that wandered into the Roche zone. If the object did
not approach
pp
Saturn closely
y enough
g to be completely
p
y
disrupted, its disintegration may have been aided by cometary
or meteoroid impacts.
One can object to the first scenario by arguing that over a period of
perhaps 100 million years (a relatively short period compared
to the age of the solar system) the particles now in Saturn’s
rings will be ground into dust by collisions or spiral into the
planet. However, these particles may be continually resupplied
from the progressive disintegration of small moons.
Appearance of Saturn’s rings under different lighting conditions
An artist’s representation of particles in Saturn’s A ring
(from Sky & Telescope Jan., 1981, p. 10)
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Saturn’s rings look very different when viewed in reflected light or
transmitted light.
Tiny dust particles scatter light, but mostly do not change its
direction very much. (Just think about how dust on your car’s
windshield scatters sunlight.)
Therefore when the sun is behind Saturn’s rings, we see mostly
light scattered by small dust-sized particles.
MarbleM
bl or boulder-sized
b ld i d particles
ti l mostly
tl scatter
tt light
li ht back
b k in
i the
th
general direction that it came from.
Therefore when we view Saturn’s rings lit by sunlight which is
reflected back toward us, we see the regions populated by these
larger particles as the brightest, and the regions populated
mainly by smaller particles appear dark.
Here we see light reflected off the large particles in the rings.
Here we see light transmitted through ring regions containing primarily
small particles.
Voyager-1 view of Saturn in its crescent phase
Voyager-2 view of Saturn in its crescent phase
Saturn’s rings lit from behind.
Cassini’s division is bright (small particles), while the B ring is dark.
Voyager-1 image of Saturn’s rings from below at 717,000 km distance.
Cassini’s division appears bright in this image.
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Voyager-1 view of
Saturn’s rings from
30º above the ring
plane and 720,000
km distance.
In this image, we
see Cassini’s
division as a dark
band, because we
are looking at light
reflected by the
rings, and only the
large particles
reflect much light.
Instruments aboard the Cassini spacecraft analyze the spectra from
Saturn’s rings to determine the size of the ice particles on the
surfaces of the boulder-sized snowballs (mainly of water ice) that
are thought to make up the rings.
Evidence from the visual and infrared mapping spectrometer on
the Cassini spacecraft indicates that the grain sizes in Saturn’s
rings grade from smaller to larger, related to distance from Saturn.
g ) are shown next to a corresponding
p
g picture
p
of the
Those data ((right)
rings taken by Cassini's narrow angle camera.
Saturn's rings are thought to be made up of boulder-size snowballs.
By looking at the rings with the visual and infrared mapping
spectrometer, the size of the ice crystals, or grains, on the surfaces
of those boulders can be determined.
Saturn’s rings show a wealth of internal structure.
Many of the phenomena that cause this structure are complex, and
we will not discuss them.
However, some of the processes are easy to understand.
The rings are exceedingly thin, as can be observed on the
occasions when the earth (or a spacecraft) crosses the ring
plane.
The ring particles are composed mainly of water ice, and when
these particles collide with one another, the collisions are not
elastic. Such particles tend to chip each other rather than to
bounce like superballs (or like the atoms in a gas).
Instruments aboard the Cassini spacecraft analyze the spectra from Saturn’s rings
to determine the size of the ice particles on the surfaces of the boulder-sized
snowballs (mainly of water ice) that are thought to make up the rings.
The inelastic collisions of the ring particles have caused the
extremely low degree of random velocities compared to the size
of the orbital velocities. The small random velocities make the
rings so thin.
Natural color mosaic of Saturn’s rings at high resolution taken by the
Cassini spacecraft in 2004. Gaps, gravitational resonances and wave
patterns are all present, and the delicate color variations across the system
are clearly visible.
This mosaic of six images covers a distance of approximately 62,000
kilometers along the ring plane, from a radius of 74,565 kilometers to
136,780 kilometers (46,333 to 84,991 miles) from the planet's center. This
view is from Cassini's vantage point beneath the ring plane. The rings are
tilted away from Cassini at an angle of about 4 degrees.
Images taken using red, green and blue spectral filters were used to create
this natural color mosaic. The images were acquired using the Cassini
spacecraft narrow angle camera on Dec. 12, 2004, at a distance of
approximately 1.8 million kilometers (1.1 million miles). The image scale is
10.5 kilometers (6.5 miles) per pixel.
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Here we see Enceladus hovering past the B Ring, with 4 faint bands
visible within the Cassini Division below and to the right of this moon.
The rings are only tens of meters thick.
Here we do not see them precisely edge on, but it is nevertheless quite
clear that they are extremely thin and almost perfectly flat.
One of the most striking features of the rings are the prominent
gaps within them. The most prominent of these is the Cassini
Division.
The
Cassini
Division
in
Saturn’s
rings,
seen
from 13
million
km.
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Voyager-1 image of
the Cassini Division in
Saturn’s rings from
6 million km.
The Cassini Division occurs at 120,000 km from Saturn’s center.
At this location, a ring particle orbits Saturn in exactly half the
time that it takes the moon Mimas to orbit. Thus every two
orbits such a particle receives the same gravitational nudge from
Mimas, which tends to make the ring particle’s orbit eccentric.
Cassini recently
took a far more
detailed photo of
the outer edge of
the Cassini
Division,
leading into the
inner edge of the
A ring, at the
g The
right.
structures seen
are due to the
influence of the
co-orbital
moons Janus
and Epimetheus.
A mosaic image of Saturn’s
rings, viewed by Voyager-1
from 8 million km.
The growing eccentricity of the ring particle’s orbit leads it to
collide with other ring particles, and this process clears out a
gap in the rings – the Cassini Division.
In the following image, the Enke gap in the outer A-ring is clearly
visible. It has a different cause than the Cassini Division.
Saturn’s complex rings are both an intriguing scientific puzzle and a
supreme natural wonder. This view shows, from upper right to lower
left, the thin C ring, multi-toned B ring, the dark Cassini Division, the
A ring and narrow F ring.
At the bottom, Saturn’s moon Mimas (398 kilometers, or 247 miles
across) orbits about 45,000 kilometers (28,000 miles) beyond the
bright core of the F ring. The little moon is heavily cratered and is
thought to be largely composed of water ice. The bright speck just
outside of (below) the F ring is the shepherd moon Pandora (84
kilometers, or 52 miles across).
The image was taken in visible light with the Cassini spacecraft
narrow angle camera on Jan. 19, 2005, at a distance of 1.8 million
kilometers (1.1 million miles) from Saturn. The image scale is 11
kilometers (7 miles) per pixel. Pandora was brightened by a factor of
seven to aid visibility.
M
i
m
a
s
&
P
a
n
d
o
r
a
9
Reflected light from Saturn dimly illuminates the night side of the
cratered moon Mimas in this Cassini image. Above, the outer edges
of the planet's main rings show some interesting details. Mimas is
398 kilometers (247 miles) across.
Several thin ringlets comprising the F ring are nicely visible here,
and the bright core of the ring displays a few twisted knots. Perhaps
less noticeable are kinks in one of the thin ringlets of material
visible within the Encke Gap near the upper left corner. The outer
edge of the A ring appears notably brighter than the ring material on
the other side of the narrow Keeler Gap. Finally, numerous
gravitational resonances give the A ring a grooved or striped
appearance in this view.
The image was taken in visible light with the Cassini spacecraft
narrow angle camera on Jan. 17, 2005, at a distance of
approximately 1.2 million kilometers (746,000 miles) from Saturn.
The image scale is 7 kilometers (4 miles) per pixel.
Mimas as
viewed by
the Cassini
spacecraft.
Mimas is
seen here
with the thin
F ring in the
background.
The Enke gap in the outer A ring is 320 km wide.
It is believed that it is caused by the gravitational disturbances of
the 20-km-diameter satellite Pan orbiting within it.
The gravitational tug of Pan on nearby ring particles pushes them
into orbits with greater separation from Pan, thus clearing out
the Enke gap in the ring.
This counter
counter-intuitive
intuitive behavior of the ring particles is a result of
the special nature of Keplerian motion, which is illustrated on
the following slides.
Solid body rotation compared with motion due to gravity.
Pair of satellites near Saturn’s F ring -- Sky & Telescope, 1/81, p. 12
10
The inner moon shown orbits faster than the ring particles outside
it. Therefore its gravitational attraction tends to speed up the
orbital motion of these ring particles.
As the ring particles begin to orbit faster, they experience a greater
centrifugal force away from the planet, and they therefore move
to more distant orbits.
The outer moon shown orbits slower than the ring particles inside
its orbit.
orbit Therefore its gravity tends to slow down the orbital
motion of these particles.
As these ring particles begin to orbit more slowly, they experience
a reduced centrifugal force.
As a result, Saturn’s gravity pulls them closer to it, and they move
to orbits closer to the planet.
These effects of a moon on the ring particles inside and outside its
own orbit explain the clearing of the Encke gap.
Two co-orbiting satellites at Saturn’s F ring, viewed from 25 million km
Two images of Saturn’s eleventh moon, a trailing co-orbital satellite,
viewed from 177,000 km.
Saturn’s fourteenth satellite, just inside the F ring,
viewed from 7 million km
Saturn’s braided F ring
from 750,000 km.
Prometheus (63 miles across) and Atlas (12 miles across) orbit between
Saturn’s A and F rings. (view from Cassini spacecraft)
11
Saturn’s moon
Pandora (52 miles
across) is viewed by
Cassini with the F
ring in the
foreground. Pandora
is lit by reflected light
from Saturn.
Saturn’s moon Prometheus (63 miles across) is seen here making a
new diagonal gore in the tenuous material inside Saturn’s F ring.
Prometheus creates a new gore each time it comes closest to the F
ring, and the memory of these is preserved from previous passes.
Saturn’s moon Prometheus (63 miles across), followed by Pandora, is
seen here making a new diagonal gores in the tenuous material inside
Saturn’s F ring. (This is a movie clip that was shown in class.)
Moons visible in this image: Mimas (398
kilometers, or 247 miles across) at right,
Pandora (84 kilometers, or 52 miles across)
near center and Janus (181 kilometers, or 113
miles across) in the lower left corner. Mimas'
orbit inclination of 1.6 degrees relative to
Saturn's equator is enough to make it appear as
if it orbits just beyond the F ring when viewed
from this vantage point of 5 degrees below the
rings. In fact, it is 34,000 kilometers (21,000
miles) more distant than Janus.
One of the shepherd satellites of Saturn’s rings
Cassini confirms that a small moon is orbiting within the narrow
Keeler gap, near the outer edge of Saturn’s A ring.
(This is a movie clip that was shown in class.)
12
Cassini
view of
Titan with
its normal
haze.
Cassini view
of Titan on
its closest
flyby, April
16, 2005.
False color:
green areas
are on the
surface, and
red areas are
high in the
atmosphere.
A huge annular feature with an outer diameter of approximately
440 kilometers (273 miles) appears in this image taken with
Cassini's Titan radar mapper. It resembles a large crater or part of
a ringed basin, either of which could be formed when a comet or
asteroid tens of kilometers in size slammed into Titan. This is the
first impact feature identified in radar images of Titan.
The surface of Titan appears to be very young compared to other
Saturnian satellites. In Titan's case, debris raining down from the
atmosphere or other geologic processes may mask or remove the
craters. The pattern of brightness suggests that there is
topography associated with this feature; for example, in the center
of the image there appear to be mounds each about 25 kilometers
(15 miles) across. Since they are dark on their lower edges that
face away from the radar and bright on the opposite face, they
must be elevated above the surrounding terrain.
Cassini sees a
river on Titan.
First impact feature found on Titan by Cassini’s radar mapper.
First color view of Titan’s surface.
Initially thought to be rocks or ice
blocks, they are more pebble-sized.
The two rock-like objects just below
the middle of the image are about 15
centimeters (about 6 inches) (left)
and 4 centimeters (about 1.5 inches)
(center) across respectively,
respectively at a
distance of about 85 centimeters
(about 33 inches) from Huygens. The
surface is darker than originally
expected, consisting of a mixture of
water and hydrocarbon ice. There is
also evidence of erosion at the base
of these objects, indicating possible
fluvial activity.
13
A radar map
of the north
polar region
of Titan
made by the
Cassini
spacecraft in
7 flybys.
It rains
ethane and
methane,
making
hydrocarbon
lakes and
seas, some as
large as Lake
Superior on
Earth.
A spectacular landslide within the low-brightness region of
Iapetus’s surface known as Cassini Regio is visible in this image
from Cassini. Iapetus is one of the moons of Saturn.
The landslide material appears to have collapsed from a scarp 15
kilometers high (9 miles) that forms the rim of an ancient 600
kilometer (375 mile) impact basin. Unconsolidated rubble from
the landslide extends halfway across a conspicuous, 120-kilometer
diameter (75-mile) flat-floored impact crater that lies just inside
the basin scarp
scarp.
Iapetus close-up
Saturn’s
irregularly
shaped
moon
Iapetus
The most unique, and perhaps most remarkable feature
discovered on Iapetus in Cassini images is a topographic ridge
that coincides almost exactly with the geographic equator. The
ridge is conspicuous in the picture as an approximately 20kilometer wide (12 miles) band that extends from the western
(left) side of the disc almost to the day/night boundary on the
right. On the left horizon, the peak of the ridge reaches at least
13 kilometers (8 miles) above the surrounding terrain. Along the
roughly 1,300 kilometer (800 mile) length over which it can be
traced in this picture, it remains almost exactly parallel to the
equator within a couple of degrees. The physical origin of the
ridge has yet to be explained. It is not yet clear whether the
ridge is a mountain belt that has folded upward, or an extensional
crack in the surface through which material from inside Iapetus
erupted onto the surface and accumulated locally, forming the
ridge.
Cassini view
of Dione,
with Saturn
and its rings
in the
background.
The color in
this image,
g ,
including that
of Dione, is
roughly as it
would appear
to the human
eye.
14
Saturn’s
moon Rhea in
natural color.
15