Download The Voyager pictures show four additional faint rings. Saturn`s rings

Saturn ... The Ringed Planet
Introduction to Saturn
Saturn is considered the most beautiful of the planets, with its graceful rings and
soft yellow color. There is really nothing like it, and although the other three gas
planets have ring systems, none is as significant as Saturn's. Saturn is gaseous,
like Jupiter, but far less massive and less dense. In fact, if it were possible, Saturn
would float on an ocean of water. Besides the wonderful rings, Saturn has 62
moons (latest count), one of which (Titan) holds a significantly thick atmosphere.
Titan is one of the largest moons in the Solar System and offers some intriguing
possibilities for hydrocarbons and potential biomolecules on its cold surface.
Saturn has been visited by Pioneer 10 and 11, as well as by Voyager 1 and 2.
Presently, the Cassini spacecraft is en route to Saturn for an extended orbital
stay and will drop a probe into the atmosphere of Titan in July, 2004.
Planetary Data
Mass (kg), and mass 56.86x10^25 kg = 95.159
relative to Earth
earths
Equatorial diameter
120,536
(km)
Mean density
(gm/cm^3)
.69
Acceleration of
gravity (m/s^2)
10.59
Velocity of escape
35.6
(km/s)
Period of rotation
10.233 hours
Period of revolution 29.4577 years
Aphelion (AU)
10.042
Aphelion (km)
1,502,300,000
Perihelion (AU)
8.9900
Perihelion (km)
1,344,900,000
Mean orbital
distance from the
sun (AU)
9.5162
Mean orbital
distance from the
sun (km)
1,423,600,000
Orbital velocity
(km/s)
9.64
Eccentricity
.0553
Oblateness
0.107
Inclination to the
ecliptic
2.488 degrees
Inclination of the
equator to the orbit
26.73 degrees
Number of natural
satellites
62
Names of natural
satellites
Pan, Prometheus,
Pandora, Epimetheus,
Janus, Mimas,
Enceladus, Tethys,
Telesto, Calypso, Dione,
Helena, Rhea, Titan,
Hyperion, Iapetus,
Phoebe, 13 recently
discovered moons have
recently been named,
and 17 unnamed moons,
and 13 more announced
since 2006.
More Information on the Planet Saturn from the Nine
Planets Website
Much of the information below is direct from the Nine Planets Website. Some
material has been altered by me for this course, while other items and comments
are directly copied. I hope to maintain a continuous update of this material to
keep up with the findings from space satellites and telescopes. Additionally,
Saturn will be visited by the space satellite Cassini. This giant space craft was
launched in 1997 and arrived in the summer of 2004. Please click on the Cassini
website to learn more about its mission, design, and a great wealth of
information about Saturn.
Saturn is the sixth
planet from the Sun
and the second
largest:
orbit: 1,429,400,000
km (9.54 AU) from
Sun
diameter: 120,536 km
(equatorial)
mass: 5.68e26 kg
In Roman mythology,
Saturn is the god of
agriculture. The associated Greek god, Cronus, was the son of Uranus and Gaia
and the father of Zeus (Jupiter). Saturn is the root of the English word
"Saturday" (see Appendix 4).
Saturn has been known since prehistoric times. Galileo was the first to observe it
with a telescope in 1610; he noted its odd appearance but was confused by it.
Early observations of Saturn were complicated by the fact that the Earth passes
through the plane of Saturn's rings every few years as Saturn moves in its orbit.
A low resolution image of Saturn therefore changes drastically. It was not until
1659 that Christiaan Huygens correctly inferred the geometry of the rings.
Saturn's rings remained unique in the known solar system until 1977 when very
faint rings were discovered around Uranus (and shortly thereafter around
Jupiter and Neptune).
MISSIONS TO SATURN
Saturn was first visited by Pioneer 11 in 1979 and later by Voyager 1 and
Voyager 2 in 1980.
Cassini, now in orbit,
arrived July 1, 2004. So
much of our understanding
of Saturn will be deepened
greatly by this Cassini
spacecraft. It is the last of
the great planetary
exploratory satellites, being
nearly as large as a school
bus. A diagram of this
spacecraft is seen to your
left. Toward the right side of
the spacecraft is the Huygens
probe that will release from
the Cassini spacecraft and
plunge through the atmosphere of Saturn's largest moon Titan, and perhaps give
us an indication of what lies beneath the orange clouds of this moon. The final
inspection of Cassini is seen directly below, and an artist's depiction of this probe
release is seen below and in the center.
SATURN'S GENERAL APPEARANCE
Saturn is visibly flattened (oblate) when
viewed through a small telescope; its
equatorial and polar diameters vary by
almost 10% (120,536 km vs. 108,728 km).
This is the result of its rapid rotation and
fluid state. The other gas planets are also
oblate, but not so much so. I talk about
this in my college course. All of the
planets in the solar system are widest at
the equatorial regions due to the spinning
of the planet. The faster a planet spins,
the more force will be generated outward, and this force expresses itself most
strongly at the equator. Additionally, the more solid a planet is, the less likely
that outward force will act to bulge the planet. Therefore, planets like Saturn
and Jupiter which are made of gas and fluids, AND that also spin very rapidly
will have a very pronounced flattening at the polar regions and bulging at the
equatorial regions. Planets like Uranus and Neptune that are also made of gas
and fluids, BUT that spin more slowly will have less noticeable flattening.
Finally, rocky planets like Earth and Venus will be somewhat flattened in spite of
their solid nature because the spin more quickly than Mercury and Venus that
spin so slowly as to have an imperceptible equatorial bulging. Oh, click on the
image to your left for a larger version of this beautiful image.
SATURN'S PHYSICAL CHARACTERISTICS
Saturn is the least dense of the planets; its specific gravity (0.7) is less than that of
water. As I mentioned above, this means that Saturn would float on water if such
a large ocean could be found.
Like Jupiter, Saturn is about 75% hydrogen and 25% helium with traces of
water, methane, ammonia and "rock", similar to the composition of the
primordial Solar Nebula
from which the solar system
was formed.
Saturn's interior is similar to
Jupiter's consisting of a
rocky core, a liquid metallic
hydrogen layer and a
molecular hydrogen layer.
Traces of various ices are
also present.
Saturn's interior is hot
(12000 K at the core) and Saturn radiates more energy into space than it receives
from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz
mechanism as in Jupiter. But this may not be sufficient to explain Saturn's
luminosity; some additional mechanism may be at work, perhaps the "raining
out" of helium deep in Saturn's interior. When I am talking about a planet's
"luminosity" I am referring to the radiation of the planet of energy into space.
While Saturn does not produce visible light or make gamma radiation as stars
do, mechanisms within the planet cause electromagnetic radiation to be released,
often in the form of heat or Infrared radiation. Luminosity is a measure of ALL
of the wavelength energy emitted by an object. You will learn more about this in
the Star Unit. For now, Saturn reflects light from the Sun, but emits heat from
interior processes giving Infrared telescopes an
opportunity to measure its total energy output,
or "luminosity."
SATURN'S CLOUDS
The bands so prominent on Jupiter are much
fainter on Saturn. They are also much wider
near the equator. Details in the cloud tops are
invisible from Earth so it was not until the
Voyager encounters that any detail of Saturn's atmospheric circulation could be
studied. The image to the left is a "false color" image that allows variations in the
bands of the planet to be more readily seen.
Saturn also exhibits long-lived ovals and other
features common on Jupiter. In 1990, HST
observed an enormous white cloud near
Saturn's equator which was not present during
the Voyager encounters; in 1994 another,
smaller storm was observed (left).
Original
Caption Released with Image seen to your left:
A large, bright and complex convective storm that appeared in Saturn's southern
hemisphere in mid-September 2004 was the key in solving a long-standing mystery about
the ringed planet.
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.
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.
The Dragon Storm was a powerful source of radio emissions during July and September of
2004. The radio waves from the storm resemble the short bursts of static generated by
lightning on Earth. Cassini detected the bursts only when the storm was rising over the
horizon on the night side of the planet as seen from the spacecraft; the bursts stopped when
the storm moved into sunlight. This on/off pattern repeated for many Saturn rotations over
a period of several weeks, and it was the clock-like repeatability that indicated the storm
and the radio bursts are related. Scientists have concluded that the Dragon Storm is a giant
thunderstorm whose precipitation generates electricity as it does on Earth. The storm may
be deriving its energy from Saturn's deep atmosphere.
One mystery is why the radio bursts start while the Dragon Storm is below the horizon on
the night side and end when the storm is on the day side, still in full view of the Cassini
spacecraft. A possible explanation is that the lightning source lies to the east of the visible
cloud, perhaps because it is deeper where the currents are eastward relative to those at
cloud top levels. If this were the case, the lightning source would come up over the night
side horizon and would sink down below the day side horizon before the visible cloud. This
would explain the timing of the visible storm relative to the radio bursts.
The Dragon Storm is of great interest for another reason. In examining images taken of
Saturn's atmosphere over many months, imaging scientists found that the Dragon Storm
arose in the same part of Saturn's atmosphere that had earlier produced large bright
convective storms. In other words, the Dragon Storm appears to be a long-lived storm deep
in the atmosphere that periodically flares up to produce dramatic bright white plumes
which subside over time. One earlier sighting, in July 2004, was also associated with strong
radio bursts. And another, observed in March 2004 and captured in a movie created from
images of the atmosphere (PIA06082 and PIA06083) spawned three little dark oval storms
that broke off from the arms of the main storm. Two of these subsequently merged with
each other; the current to the north carried the third one off to the west, and Cassini lost
track of it. Small dark storms like these generally get stretched out until they merge with
the opposing currents to the north and south.
These little storms are the food that sustains the larger atmospheric features, including the
larger ovals and the eastward and westward currents. If the little storms come from the
giant thunderstorms, then together they form a food chain that harvests the energy of the
deep atmosphere and helps maintain the
powerful currents.
INTRODUCTORY COMMENTS
ABOUT SATURN'S RINGS
Two prominent rings (A and B) and one
faint ring (C) can be seen from the Earth.
The gap between the A and B rings is
known as the Cassini division. The much
fainter gap in the outer part of the A ring
is known as the Encke Division (but this is somewhat of a misnomer since it was
very likely never seen by Encke). The Voyager pictures show four additional
faint rings. Saturn's rings, unlike the rings of the other planets, are very bright
(albedo 0.2 - 0.6).
Though they look continuous from the Earth, the rings are actually composed of
innumerable small particles each in an independent orbit. They range in size
from a centimeter or so to several meters. A few kilometer-sized objects are also
likely.
Saturn's rings are extraordinarily thin: though they're 250,000 km or more in
diameter they're less than one kilometer thick. Despite their impressive
appearance, there's really very little material in the rings -- if the rings were
compressed into a single body it would be no more than 100 km across. The ring
particles seem to be composed primarily of water ice, but they may also include
rocky particles with icy coatings.
Voyager confirmed the existence of
puzzling radial inhomogeneities in the
rings called "spokes" which were first
reported by amateur astronomers (left).
Their nature remains a mystery, but may
have something to do with Saturn's
magnetic field.
Sa
tur
n's
outermost ring, the F-ring, is a
complex structure made up of several
smaller rings along which "knots" are
visible. Scientists speculate that the
knots may be clumps of ring material,
mini moons. The strange braided
appearance visible in the Voyager 1
images (left) is not seen in the Voyager
images perhaps because Voyager 2
imaged regions where the component
rings are roughly parallel.
or
2
There are complex tidal resonances between some of Saturn's moons and the
ring system: some of the moons, the so-called "shepherding satellites" (i.e. Atlas,
Prometheus and Pandora) are clearly important in keeping the rings in place;
Mimas seems to be responsible for the paucity of material in the Cassini division,
which seems to be similar to the Kirkwood gaps in the asteroid belt; Pan is
located inside the Encke Division. The whole system is very complex and as yet
poorly understood.
The origin of the rings of Saturn (and the other jovian planets) is unknown.
Though they may have had rings since their formation, the ring systems are not
stable and must be regenerated by ongoing processes, probably the breakup of
larger satellites.
SATURN'S MAGNETIC FIELD
Like the other jovian planets, Saturn has
a significant magnetic field, and thus it
too exhibits auroral displays like Earth
and Jupiter. Click on the image to your
left for a larger version.
When it is in the nighttime sky, Saturn is
easily visible to the unaided eye. Though
it is not nearly as bright as Jupiter, it is
easy to identify as a planet because it
doesn't "twinkle" like the stars do. The
rings and the larger satellites are visible
with a small astronomical telescope.
There are several Web sites that show the
current position of Saturn (and the other
planets) in the sky. More detailed and customized charts can be created with a
planetarium program such as Starry Night.
Saturn's Satellites/Moons
Saturn has 18 named satellites plus 42 newly announced moons, some of which
have names and some which do not:
Of those moons for which rotation rates are known, all but Phoebe and Hyperion
rotate synchronously with the planet.
The three pairs Mimas-Tethys, Enceladus-Dione and Titan-Hyperion interact
gravitationally in such a way as to maintain stable relationships between their
orbits: the period of Mimas' orbit is exactly half that of Tethys, they are thus said
to be in a 1:2 resonance; Enceladus-Dione are also 1:2; Titan-Hyperion are in a
3:4 resonance.
In addition to the 18 named satellites, a 13 more have been reported and given
provisional names, and 17 more were found in 2004 but have not yet been
named, and 13 more were announced since 2006.
Saturn's Moons
Satellite Name
Distance
Radius
Mass
Discoverer
Date
Pan
134,000 km
10 km
?
Showalter
1990
Atlas
138,000 km
14 km
?
Terrile
1980
Prometheus
139,000 km
46 km
2.70e17 kg
Collins
1980
Pandora
142,000 km
46 km
2.20e17 kg
Collins
1980
Epimetheus
151,000 km
57 km
5.60e17 kg
Walker
1980
Janus
151,000 km
89 km
2.01e18 kg
Dollfus
1966
Mimas
186,000 km
196 km
3.80e19 kg
Herschel
1789
Enceladus
238,000 km
260 km
8.40e19 kg
Herschel
1789
Tethys
295,000 km
530 km
7.55e20 kg
Cassini
1684
Telesto
295,000 k m
15 km
?
Reitsma
1980
Calypso
295,000 km
13 km
?
Pascu
1980
Dione
377,000 km
560 km
1.05e21 kg
Cassini
1684
Helene
377,000 km
16 km
?
Laques
1980
Rhea
527,000 km
765 km
2.49e21 kg
Cassini
1672
Titan
1,222,000 km
2575 km
1.35e23 kg
Huygens
1655
Hyperion
1,481,000 km
143 km
1.77e19 kg
Bond
1848
Iapetus
3,561,000 km
730 km
1.88e21 kg
Cassini
1671
Phoebe
12,952,000 km
110 km
4.00e18 kg
Pickering
1898
13 newly found
Here is a list of the recently discovered moons and their new
moons
Satellite
Name
Distance
Kiviuq
11,365,000
16 km
km
2000
Ijiraq
11,440,000
12 km
km
2000
Paaliaq
15,199,000
23 km
km
2000
Skadi
15,647,000
3 km
km
2000
Albiorix
16,040,000
32 km
km
2000
Erriape
17,616,000
10 km
km
2000
Siarnaq
18,160,000
40 km
km
2000
Tarvos
18,247,000
15 km
km
2000
Mundilfari
18,709,000
7 km
km
2000
S/2003 S1
18,719,000
7 km
km
2003
Suttung
19,463,000
7 km
km
2000
Diameter Year
Thrym
20,382,000
7 km
km
2000
Ymir
23,096,000
18 km
km
2000
To learn more about these new moons, go to Sam Sheppard's website at Saturn's
New Moons.
Saturn's Rings
Distance to Inner
Distance to Outer
Width
Type
67,000 km
74,500 km
7,500 km
ring
C-Ring
74,500 km
92,000 km
17,500 km
ring
Maxwell Division
87,500 km
88,000 km
500 km
divide
B-Ring
92,000 km
117,500 km
25,500 km
ring
Cassini Division
115,800 km
120,600 km
4,800 km
divide
Huygens Gap
117,680 km
n/a
285-440 km
subdivide
A-Ring
122,000 km
136,800 km
14,600 km
ring
Encke Minima
126,430 km
129,940 km
3,500 km
29%-53%
Encke Division
133,580 km
325 km
78%
F-Ring
140,210 km
30-500 km
ring
G-Ring
165,800 km
173,800 km
8,000 km
ring
E-Ring
180,000 km
480,000 km
300,000 km
ring
D-Ring
Mass
Guerin Division
Notes:
* distance is kilometers from Saturn's center
* the "Encke Minima" is a slang term used by amateur astronomers, not an
official IAU designation
This categorization is actually somewhat misleading as the density of particles
varies in a complex way not indicated by a division into neat regions: there are
variations within the rings; the gaps are not entirely empty; the rings are not
perfectly circular.
To learn more about Saturn and to see more incredible pictures, go to the JPL
webpage devoted to Saturn.
1.1e18
2.8e19
6.2e18
1e7?
VOYAGER 1 VISITED SATURN AND
THEN LEFT THE SOLAR SYSTEM
After Voyager 1 flew by up through the
rings of Saturn, it continued in a
northerly trajectory relative to the
ecliptic, and is continuing to travel out of
our solar system. There are two things
which control the future of the Voyager
spacecraft ... the amount of nuclear fuel
as well as the amount of thruster engine
fuel. The spacecraft uses the decay of
Plutonium to generate heat, which in turn
is converted into electricity, that can be used to send signals to the JPL facility in
Pasadena, as well as receive signals from Earth. The thruster fuel is used to point
the Voyager receiver/transmitter antenna at the Earth. Scientists believe that
there is enough of both fuels to keep Voyager 1 operational until about 20152020. It is hoped by that time, that Voyager will have escaped the heliosphere
(the bubble or gas blown out by the Sun), cross the heliopause (where the Sun's
wind-blown bubble ends and interstellar space begins), and enter interstellar
space. The mission has been renamed Voyager Interstellar Mission, and scientists
are continuing to maintain contact with both Voyager 1 and 2.
This information in the last paragraph of this page is found in several other
places within my course because I believe it to be incredibly important,
interesting, and relevant to a potential creative writing assignment of yours :)
Before you move forward to the Saturn quiz, I just wanted to include this
absolutely fantastic picture of Saturn taken by the Cassini spacecraft and
borrowed from the APOD (Astronomy Picture of the Day) website.
There are three webpages you are asked to look through in the Saturn lesson:
Saturn's Moons, Saturn's Rings, and Titan. After working through these three
pages, there are 10 questions I would like you to respond to:
1) Give a brief description of the physical appearance of the planet Saturn?
2) What is the most distinguishing feature of the planet Saturn?
3) What is the name of the satellite that is presently orbiting Saturn, when was it
launched, how long did it take to arrive at Saturn, and what is the name of the
probe that dropped to the surface of Titan?
4) What are the wind speeds of Saturn's clouds?
5) Why is there so much scientific interest in the moon Titan?
6) What are Saturn's rings made of?
7) What is the effect of Mimas on the rings of Saturn?
8) Why is Saturn flattened at the poles in such a pronounced manner?
9) What do astronomers believe the high albedo of Enceladus is caused by?
10) Can you think of a reason why a manned mission to Saturn would be
challenging that is different from any other challenge to the other eight planets?
When you believe you know the answers to these questions, please go forward to
the Saturn Quiz page and submit them to me.
Once you have completed that reading, then move forward to Uranus, back to
the Gas Giant Introduction, or the Syllabus.
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