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The Real Lord of the Rings
Four hundred years after they were discovered, Saturn's
breath-taking rings remain a mystery.
NASA
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*see caption*
*February
12, 2002:* Galileo Galilei was accustomed to extraordinary discoveries.
Using his primitive telescope he had found new worlds orbiting Jupiter,
watched planet-sized spots crossing the Sun, and explored craters on the
Moon. But when Galileo turned his telescope toward Saturn in 1610, even
he was amazed.
The planet looked nothing like others in the solar system. Through 17th
century optics, Saturn appeared to be one bright star closely flanked by
two dimmer ones -- a blurry suggestion of the planet's magnificent rings.
What Galileo did next was nearly as unusual as Saturn itself.
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He wanted to tell everyone what he had seen, but he also wanted to keep
his work secret while he studied the puzzling planet. So, he published
his discovery in code
: /smais mr
milmep oet ale umibunen ugttauir as/. Unscrambled, the anagram means "I
have observed the highest planet tri-form."
*Above*: Hubble Space Telescope images of Saturn and its rings. [more
]
Nowadays anyone with a department store telescope can get a better view
of Saturn's rings than Galileo did. Otherwise, matters stand much as
they did four hundred years ago. First-time observers of the planet
still step back from their telescopes speechless. And scientists are
still puzzled.
"After all this time we're still not sure about the origin of Saturn's
rings," says Jeff Cuzzi, a planetary scientist at the NASA Ames Research
Center. Astronomers once thought that Saturn's rings formed when Saturn
did: 4.8 billion years ago as the Sun and planets coalesced from a
swirling cloud of interstellar gas. "But lately," Cuzzi says, "there's a
growing awareness that Saturn's rings can't be so old."
see caption
Cuzzi
speculates that some hundreds of millions of years ago -- a time when
the earliest dinosaurs roamed our planet -- Saturn had no bright rings.
Then, he says, something unlikely happened: "A moon-sized object from
the outer solar system might have flown nearby Saturn where tidal forces
ripped it apart. Or maybe an asteroid smashed one of Saturn's existing
moons." The debris encircled the planet and formed the rings we see today.
*Left*: Saturn's rings might have formed only a few hundred million
years ago when dinosaurs and their cousins roamed our planet. Credit:
Humboldt State University [more
]
Saturn's ring particles range in size from microscopic dust to
barn-sized boulders. If you assembled them all in one place, notes
Cuzzi, you would have enough material to make an icy satellite one or
two hundred kilometers wide -- much like Saturn's present-day moon Mimas.
The debris layer is extraordinarily thin, he marvels. "Saturn's rings
are 250,000 km wide, but only a few tens of meters thick. A sheet of
paper the size of San Francisco would have about the same ratio of width
to depth." Indeed, if you made a 1-meter-wide scale model of Saturn, the
rings would be 10,000 times thinner than a razor blade.
Cuzzi says there are two reasons to believe the rings are young:
First, they are bright and shiny like something new. It's no joke, he
assures. The wide-spanning rings sweep up space dust (bits of debris
from comets and asteroids) as Saturn orbits the Sun. Rings much older
than a few hundred million years would be darkened by accumulated dust.
"The fact that they're bright suggests they're young," he says.
see caption
*Above*: Saturn's rings are very thin. Astronomers using the Hubble
Space Telescope captured this image of the rings edge-on in 1995.
Star-like objects in the ring plane are icy satellites. [more
]
Second, small moons that orbit through the outermost regions of the ring
system are gaining angular momentum at the expense of the rings. "During
the next few hundred million years," explains Cuzzi, "the outer half of
the rings will fall toward the planet, and the little moons -- called
shepherd satellites
-- will be flung away. This is a young dynamical system."
The first argument (shiny rings) is less certain than the second
(angular momentum), he cautions, "because we're not sure there's enough
dust at the orbit of Saturn to pollute and blacken the rings." NASA's
Cassini spacecraft will measure the dust population when it reaches
Saturn in 2004. Then, perhaps, there will be no doubt.
see caption
*Right:*
Saturn's 200 km-wide moon Mimas, also known as the "Death Star"
satellite because of its distinctive impact crater, is about as massive
as Saturn's rings. [more
]
Cuzzi hopes Cassini will solve other ring-mysteries, too. "In the early
'80's," he recalls, "the Voyager spacecraft visited Saturn and took
close-up pictures that revealed many strange things in the rings,
including spokes, braids
and waves.
"Some of the waves have a spiral shape, like the spiral arms of
galaxies," says Cuzzi. To an astronaut floating among the rings, such
waves would appear to be gentle swells, a few kilometers high and
hundreds of kilometers wide. They move around the rings every few days
or weeks. "We understand these spiral waves," he added. They're
triggered by gravitational tugs from Saturn's moons -- the same ones
that are sapping the rings' angular momentum.
Other structures, like spokes
and irregular ripples, are puzzling. Some of them might be signs of
space rocks plunging through the ring system. Others might be spawned by
tiny moonlets, as yet undiscovered, plowing through Saturn's rings.
"Cassini, which will orbit Saturn for years, should provide some
answers," he says.
see caption
*Left*:
Voyager 2 spacecraft images of "spokes
"
and other irregular features in Saturn's rings. [Quicktime movie
]
In many science fiction tales, alien visitors are amazed by Saturn as if
there were no ringed planets back in their own solar system. According
to Cuzzi, Saturn's rings might be rare indeed. "If they are as
short-lived as we think, we're lucky to be here at just the right time
to see them."
Actually, other giant planets in our solar system do have rings, but
they are very dark and millions of times less massive than the rings of
Saturn. Jupiter's rings are made of bits of dust that fly off Jupiter's
moons when they are struck by meteorites. No one is sure what made the
black rings of Neptune and Uranus, although Cuzzi notes they could be
debris from kilometer-sized moonlets that were struck by asteroids.
In another few hundred million years, if Cuzzi is right, Saturn's rings
will sag inward and our solar system will become a little more ordinary.
Perhaps by then star-faring humans will have seen countless ringed
planets elsewhere in the Galaxy and won't care much what happens to
Saturn. On the other hand, maybe Saturn's rings really are a Galactic
wonder, and super-engineers of the distant future will take measures to
preserve them.
No one knows.
We can only be sure that Saturn's rings are lovely now. And if they are
indeed fleeting, as such ages are reckoned for stars and planets, their
short life makes them even more wonderful. Don't miss them!
*SEND THIS STORY TO A FRIEND*
_Editor's Note_: This month is a good time to see Saturn.
Northern-hemisphere sky watchers can find it
high overhead after sunset; it is one of
the brighter stars in the sky. Southern-hemisphere observers should look
near the northern horizon. On Feb. 20th,
Saturn will have a close encounter in the sky with Earth's Moon. Stay
tuned to Science at NASA for details....
*Credits & Contacts
*Author: Dr. Tony Phillips
Responsible NASA official: Ron Koczor
Production Editor: Dr. Tony Phillips
Curator: Bryan Walls
Media Relations: Steve Roy
/The Science and Technology Directorate at NASA's Marshall Space Flight
Center sponsors the Science at NASA web sites. The mission of Science at NASA
is to help the public understand how exciting NASA research is and to
help NASA scientists fulfill their outreach responsibilities./
*Web Links and more...*
*Why are Saturn's rings so thin?* Cuzzi explains: In the beginning, the
rings might have been "fat" -- like a bagel or doughnut -- but they
would have flattened over time because collisions between ring particles
are inelastic. (An inelastic collision is like two lumps of clay
colliding; they don't bounce.) "If there were large vertical motions
among the ring particles, those motions would be damped by collisions."
Ring particles would "settle" into their average orbital plane, and the
rings would become more like an old-fashioned phonograph record than a
bagel.
*Cassini-Huygens Mission* -- (JPL) learn
more about the Cassini spacecraft, which will reach Saturn in 2004 to
explore the ringed planet and its intriguing satellite Titan.
*Saturn Links* -- Frequently Asked Questions About Saturn's Rings
(JPL); Saturn's Ring System
(NASA Ames); Saturn
(The
Nine Planets); Saturn
(Windows
to the Universe)
*see caption* *What are the Names of
Saturn's Rings? * (Yahoo!)
*Galileo Discovers Saturn's Rings*
-- (Rice
University) learn more about Galileo's observations of Saturn and the
anagram he used to report his discovery of the rings, from Rice University
*Rings around other planets*: Jupiter
(NASA Ames);
Jupiter
(JPL); Uranus
(NASA Ames); Uranus
(JPL);
Neptune (NASA
Ames); Neptune
(JPL)
*Jupiter's rings formed from dust blasted off tiny moons*
-- (Astronomy
Now) NASA's Galileo spacecraft solves the mystery of Jupiter's moons.
*Timelines*: Prehistoric World Images
(prehistory.com); The Dinosaur
Interplanetary Gazette
(dinosaur.org); Important Dinosaur Dates
(San
Francisco University)
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