Spitzer Discovers Saturn's Largest Ring
October 7, 2009: NASA's Spitzer Space Telescope has discovered an enormous and previously unknown infrared ring around Saturn.
"This is one supersized ring," says Anne Verbiscer, an astronomer at the University of Virginia, Charlottesville. "If you could see the ring in the night sky, it would span the width of two full Moons."
Verbiscer is co-author of a paper about the discovery to be published online tomorrow by the journal Nature. The other authors are Douglas Hamilton of the University of Maryland and Michael Skrutskie of the University of Virginia.
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This artist's conception shows a nearly invisible ring around Saturn -- the largest of the giant planet's many rings. It was discovered by NASA's Spitzer Space Telescope.
The artist's conception simulates an infrared view of the giant ring. Saturn appears as just a small dot from outside the band of ice and dust. The bulk of the ring material starts about six million kilometers (3.7 million miles) away from the planet and extends outward roughly another 12 million kilometers (7.4 million miles). The ring's diameter is equivalent to roughly 300 Saturns lined up side to side.
The inset shows an enlarged image of Saturn, as seen by the W.M. Keck Observatory at Mauna Kea, Hawaii, in infrared light. The ring, stars and wispy clouds are an artist's representation.
Image credit: NASA/JPL-Caltech/Keck
The new belt lies at the far reaches of the Saturnian system, with an orbit tilted 27 degrees from the main ring plane. The bulk of its material starts about six million kilometers (3.7 million miles) away from the planet and extends outward roughly another 12 million kilometers (7.4 million miles). It would take about one billion Earths stacked together to fill the voluminous ring. One of Saturn's farthest moons, Phoebe, circles within the newfound ring, and is likely the source of its material.
The ring is tenuous, consisting of widely-dispersed particles of ice and dust. Spitzer's infrared eyes were able to spot the glow of the cool dust, which has a temperature of only about 80 Kelvin (minus 316 degrees Fahrenheit).
The discovery may help solve an age-old riddle of one of Saturn's moons. Iapetus has a strange appearance — one side is bright and the other is really dark, in a pattern that resembles the yin-yang symbol. The astronomer Giovanni Cassini first spotted the moon in 1671, and years later figured out it has a dark side, now named Cassini Regio in his honor.
Saturn's supersized ring could explain how Cassini Regio came to be so dark. The ring is circling in the same direction as Phoebe, while Iapetus, the other rings and most of Saturn's moons are all going the opposite way. According to the scientists, some of the dark and dusty material from the outer ring moves inward toward Iapetus, slamming the icy moon like bugs on a windshield.
"Astronomers have long suspected that there is a connection between Saturn's outer moon Phoebe and the dark material on Iapetus," said Hamilton. "This new ring provides [the missing link]."
Right: Saturn's moon Iapetus. One side of the moon is darkened as the moon plows through the dust of Saturn's newly-discovered infrared ring. [more]
Verbiscer and colleagues used Spitzer's longer-wavelength infrared camera, called the multiband imaging photometer, to scan through a patch of sky far from Saturn and a bit inside Phoebe's orbit. The astronomers had a hunch that Phoebe might be circling around in a belt of dust and, sure enough, when the scientists took a first look at their Spitzer data, a band of dust jumped out.
The ring would be difficult to see with visible-light telescopes. The relatively small numbers of particles in the ring wouldn't reflect much visible light, especially out at Saturn where sunlight is weak.
"The particles are so far apart that if you were to stand in the ring, you wouldn't even know it," said Verbiscer. "By focusing on the glow of the ring's cool dust, Spitzer made it easy to find."
For additional images relating to the ring discovery and more
information about Spitzer, visit http://www.spitzer.caltech.edu.
Editor: Dr. Tony Phillips | Credit: Science@NASA
more information
Spitzer Space Telescope -- Home Page
The Phoebe ring's huge extent dwarfs the main rings.
Artist's conception of the nearly invisible Phoebe ring.
On 2009 October 6, the discovery of a tenuous disk of material in the plane of and just interior to the orbit of Phoebe was announced. This disk can be loosely described as another ring. Measurements place the inside of the ring at 128 times the radius of Saturn and the outside of the ring at 207 times,[60] with calculations indicating that it may extend up to 300 Saturn radii;[61] Phoebe orbits the planet at an average distance of 215 radii. The ring is about 20 times as thick as the diameter of the planet.[62] Since the ring's particles are presumed to have originated from impacts (micrometeoroid and larger) on Phoebe, they should share its retrograde orbit,[61] which is opposite to the orbital motion of the next inner moon, Iapetus. This ring is tilted 27 degrees from Saturn's equatorial plane (and the other rings) which is similar to Phoebe's inclination, but as Phoebe has retrograde orbital motion, Phoebe's inclination is usually measured from 180 degrees in the opposite direction, i.e. an inclination of 152 degrees rather than 28 degrees. Ring material migrates inward due to reemission of solar radiation,[60] and would thus strike the leading hemisphere of Iapetus. This could contribute to the two-tone coloration of that moon,[63] either directly or in a catalytic sense (the dark material covering much of Iapetus may derive primarily from Iapetus itself, via a positive feedback self-segregation process of ice sublimation from warmer regions, followed by vapor condensation onto cooler regions[64]). Although very large, the ring is virtually invisible—it was discovered using NASA's infra-red Spitzer Space Telescope.[65] The existence of the ring was proposed in the 1970s by Joseph Burns of Cornell University.[61] The discovery was made by Anne J. Verbiscer and Michael F. Skrutskie (of the University of Virginia) and Douglas P. Hamilton (of the University of Maryland, College Park) and published in Nature.[60][66]
FROM NEWS.SOFTPEDIA.COM
Astronomers have recently discovered that the motion and behavior exhibited by Saturn's rings could be very similar to the same traits displayed by the entire spiral arm of a galaxy.
In other words, it could be that the same physical principle underlie the action of both bodies, which is an amazing finding, because studying spiral arm physics is really difficult.
Furthermore, we now have an orbiter around the gas giant. The NASA Cassini spacecraft has been revolving around the planet since July 2004, and it has thus far provided vast amounts of photographic material to cover the rings.
If the same studies were to be carried out on galactic spiral arms, then researchers would have to look at the Andromeda galaxy, or other such structures, even further away.
This is a lot more difficult to do than simply browsing through some high-resolution images of Saturn's rings, experts involved in the research say.
Scientists first determined that the rings have potential for this kind of research when they discovered the peculiar oscillations that affect the Saturn's massive B Ring.
It was determined that the motion were not caused by interactions the ring may have had with a Saturnine moon, or with the planet itself, which meant that another kind of physics had to be at work.
After careful studies, the experts realized that the ring has sufficient density to allow for unforced “free” waves to self-develop. This was made easy by the fact that the ring's edges are very sharp.
This type of physical reaction to a shape is very common in disk systems in general, say astronomers, which means that any findings that are made around Saturn can be translated in planetary disks and galactic spiral arms.
“We have found what we hoped we'd find when we set out on this journey with Cassini nearly 13 years ago: visibility into the mechanisms that have sculpted not only Saturn's rings, but celestial disks of a far grander scale, from solar systems, like our own, all the way to the giant spiral galaxies,” explains researcher Carolyn Porco.
She holds an appointment with the Cassini imaging team, and at the Space Science Institute in Boulder, Space reports.
The expert is also the co-author of a new paper detailing the findings, which appears in the November 1 online issue of the the Astrophysical Journal.
“These oscillations exist for the same reason that guitar strings have natural modes of oscillation, which can be excited when plucked or otherwise disturbed,” adds researcher Joseph Spitale.
“The ring, too, has its own natural oscillation frequencies, and that's what we're observing,” explains the SSI scientist, who is also the lead author of the new paper.
SATURN's RING ANGLE AT 10 DAY INTERVALS FROM 2012 FEB 5 - 2012 NOV 21
.
NOTE THAT SATURN'S RING ANGLE WAS DECREASING IN 2010 JANUARY FROM 4.903 DEG TO A MINIMUM OF 1.670 DEG ON MAY 26 BEFORE OPENING UP TO OVER 10 DEG BY THE END OF 2010 AND 14.8 DEGREES BY THE END OF 2011.
THIS IS DUE TO THE EARTH'S VARYING ANGULAR POSITION IN ITS OWN ORBIT THROUGHOUT THE YEAR. ALSO NOTE THAT SATURN'S DECLINATION ALMOST REACHED THE CELESTIAL EQUATOR GOING NORTH TO SOUTH ON 2010 JAN 7 AT 8:43 AM EST WITH A DECLINATION OF +0.2936 DEGREES. SATURN'S DECLINATION APPEARED TO MOVE NORTH DUE TO THE EARTH'S MOTION REACHING A MAXIMUM IN 2010 ON MAY 25 AROUND 7:50 PM EDT OF ALMOST 3.1 DEGREES NORTH ( NEAR THE TIME OF SATURN'S MINIMUM RING ANGLE FOR 2010).
THE ACTUAL CELESTIAL EQUATOR CROSSING FOR SATURN FROM NORTH TO SOUTH OCCURRED AT 9:56:54.7 AM EDT ON 2010 SEPTEMBER 8. THE CALCULATIONS IN THIS PARAGRAPH ARE FOR THE TRUE EQUATOR OF DATE.
THE CHART BELOW CALCULATES THE EQUATOR FOR THE YEAR 2000 SINCE MOST STAR CHARTS ARE USING THE EPOCH FOR THE YEAR 2000.
HOW TO READ THE CHART BELOW - THE FIRST LINE FOR EXAMPLE SHOWS THE YEAR IS 2011, THE MONTH IS JAN, THE DAY IS THE FIRST,
THE TIME IS MIDNIGHT STARTING JAN 1ST IN UNIVERSAL TIME (5 HOURS AHEAD OF EST), THE RIGHT ASCENSION COORDINATE (LIKE LONGITUDE IS 13.075568 HOURS
AND THE DECLINATION (LIKE LATITUDE) IS -4.26239 DEGREES (BELOW THE CELESTIAL EQUATOR),
SATURN'S RING ANGLE IS OPEN TO US ON EARTH BY 10.141 DEGREES, SATURN'S EQUATORIAL RADIUS IS 8.615 ARC SECONDS, SATURN'S DISTANCE FROM THE SUN IS
1,433,900,000 KILOMETERS (890,984,000 MILES), SATURN'S DISTANCE FROM EARTH IS 1,443,000,000 KILOMETERS ( 896,640,000 MILES) AND
SATURN IS 42.897 DEGREES SEPARATED FROM OUR MOON IN THE SKY.
THE CHART BELOW GIVES DATA ON SATURN AND ITS RINGS FOR 2011 at 5 DAY INTERVALS
THE TIME IS O hrs UT, THE RING ANGLE IS IN DEGREES, AND THE SUN & EARTH DISTANCES ARE IN BILLIONS OF KILOMETERS
YEAR MO DAY HR MIN RA DEC RING ANG SAT.RADIUS SUNDIST EARTHDIST ANGLE FROM MOON
2011 1 1 0 0 13.075568 -4.26239 10.141 8.615 1.4339E+09 1.4430E+09 42.897
2011 1 6 0 0 13.088479 -4.31752 10.205 8.690 1.4341E+09 1.4306E+09 107.065
2011 1 11 0 0 13.098757 -4.35583 10.250 8.766 1.4344E+09 1.4181E+09 165.099
2011 1 16 0 0 13.106339 -4.37717 10.277 8.843 1.4346E+09 1.4058E+09 131.178
2011 1 21 0 0 13.111185 -4.38151 10.285 8.920 1.4348E+09 1.3936E+09 61.757
2011 1 26 0 0 13.113273 -4.36887 10.274 8.997 1.4350E+09 1.3817E+09 13.516
2011 1 31 0 0 13.112585 -4.33933 10.244 9.073 1.4353E+09 1.3701E+09 78.070
2011 2 5 0 0 13.109129 -4.29323 10.195 9.148 1.4355E+09 1.3589E+09 139.057
2011 2 10 0 0 13.102963 -4.23113 10.129 9.220 1.4357E+09 1.3483E+09 159.565
2011 2 15 0 0 13.094186 -4.15385 10.045 9.289 1.4359E+09 1.3383E+09 96.553
2011 2 20 0 0 13.082921 -4.06234 9.946 9.354 1.4361E+09 1.3290E+09 23.642
2011 2 25 0 0 13.069305 -3.95759 9.832 9.414 1.4364E+09 1.3206E+09 50.059
2011 3 2 0 0 13.053502 -3.84080 9.704 9.468 1.4366E+09 1.3130E+09 113.099
2011 3 7 0 0 13.035716 -3.71345 9.564 9.516 1.4368E+09 1.3063E+09 169.909
2011 3 12 0 0 13.016194 -3.57724 9.414 9.558 1.4370E+09 1.3006E+09 125.888
2011 3 17 0 0 12.995217 -3.43401 9.255 9.592 1.4373E+09 1.2960E+09 58.496
2011 3 22 0 0 12.973069 -3.28562 9.091 9.618 1.4375E+09 1.2925E+09 19.298
2011 3 27 0 0 12.950032 -3.13391 8.922 9.636 1.4377E+09 1.2901E+09 87.256
2011 4 1 0 0 12.926413 -2.98088 8.751 9.645 1.4379E+09 1.2888E+09 147.587
2011 4 6 0 0 12.902540 -2.82867 8.580 9.646 1.4382E+09 1.2887E+09 150.945
2011 4 11 0 0 12.878746 -2.67938 8.413 9.638 1.4384E+09 1.2898E+09 88.838
2011 4 16 0 0 12.855363 -2.53503 8.250 9.622 1.4386E+09 1.2920E+09 19.019
2011 4 21 0 0 12.832690 -2.39743 8.095 9.597 1.4388E+09 1.2953E+09 57.764
2011 4 26 0 0 12.810999 -2.26826 7.948 9.565 1.4390E+09 1.2996E+09 122.839
2011 5 1 0 0 12.790567 -2.14921 7.813 9.526 1.4393E+09 1.3050E+09 171.860
2011 5 6 0 0 12.771662 -2.04183 7.690 9.479 1.4395E+09 1.3114E+09 115.543
2011 5 11 0 0 12.754521 -1.94750 7.582 9.426 1.4397E+09 1.3188E+09 49.962
2011 5 16 0 0 12.739343 -1.86726 7.489 9.368 1.4399E+09 1.3269E+09 24.431
2011 5 21 0 0 12.726284 -1.80191 7.413 9.305 1.4401E+09 1.3359E+09 94.608
2011 5 26 0 0 12.715475 -1.75217 7.354 9.238 1.4404E+09 1.3456E+09 155.808
2011 5 31 0 0 12.707038 -1.71863 7.313 9.168 1.4406E+09 1.3559E+09 142.489
2011 6 5 0 0 12.701075 -1.70171 7.291 9.095 1.4408E+09 1.3668E+09 79.047
2011 6 10 0 0 12.697651 -1.70162 7.288 9.020 1.4410E+09 1.3782E+09 12.343
2011 6 15 0 0 12.696787 -1.71827 7.304 8.944 1.4412E+09 1.3899E+09 62.253
2011 6 20 0 0 12.698482 -1.75146 7.339 8.867 1.4415E+09 1.4019E+09 128.354
2011 6 25 0 0 12.702729 -1.80100 7.392 8.791 1.4417E+09 1.4141E+09 168.403
2011 6 30 0 0 12.709510 -1.86662 7.463 8.714 1.4419E+09 1.4265E+09 109.459
2011 7 5 0 0 12.718795 -1.94789 7.552 8.639 1.4421E+09 1.4390E+09 42.172
2011 7 10 0 0 12.730525 -2.04423 7.658 8.565 1.4423E+09 1.4513E+09 29.851
2011 7 15 0 0 12.744608 -2.15493 7.780 8.493 1.4426E+09 1.4636E+09 97.425
2011 7 20 0 0 12.760961 -2.27931 7.918 8.424 1.4428E+09 1.4757E+09 158.841
2011 7 25 0 0 12.779502 -2.41670 8.071 8.357 1.4430E+09 1.4876E+09 140.006
2011 7 30 0 0 12.800145 -2.56643 8.237 8.293 1.4432E+09 1.4991E+09 75.992
2011 8 4 0 0 12.822791 -2.72769 8.417 8.231 1.4434E+09 1.5102E+09 8.377
2011 8 9 0 0 12.847325 -2.89955 8.608 8.174 1.4437E+09 1.5209E+09 65.565
2011 8 14 0 0 12.873615 -3.08114 8.810 8.120 1.4439E+09 1.5310E+09 129.478
2011 8 19 0 0 12.901549 -3.27160 9.022 8.069 1.4441E+09 1.5406E+09 168.113
2011 8 24 0 0 12.931021 -3.47015 9.243 8.023 1.4443E+09 1.5495E+09 109.981
2011 8 29 0 0 12.961918 -3.67594 9.472 7.980 1.4445E+09 1.5578E+09 41.878
2011 9 3 0 0 12.994111 -3.88802 9.708 7.942 1.4448E+09 1.5653E+09 32.589
2011 9 8 0 0 13.027463 -4.10542 9.949 7.907 1.4450E+09 1.5721E+09 98.782
2011 9 13 0 0 13.061840 -4.32722 10.196 7.877 1.4452E+09 1.5781E+09 158.839
2011 9 18 0 0 13.097118 -4.55257 10.446 7.852 1.4454E+09 1.5832E+09 140.898
2011 9 23 0 0 13.133180 -4.78064 10.699 7.831 1.4456E+09 1.5875E+09 78.910
2011 9 28 0 0 13.169900 -5.01053 10.953 7.814 1.4458E+09 1.5909E+09 9.122
2011 10 3 0 0 13.207129 -5.24124 11.208 7.802 1.4461E+09 1.5934E+09 66.459
2011 10 8 0 0 13.244721 -5.47183 11.463 7.794 1.4463E+09 1.5950E+09 129.460
2011 10 13 0 0 13.282540 -5.70142 11.716 7.791 1.4465E+09 1.5956E+09 169.101
2011 10 18 0 0 13.320453 -5.92919 11.967 7.792 1.4467E+09 1.5953E+09 111.556
2011 10 23 0 0 13.358328 -6.15427 12.215 7.798 1.4469E+09 1.5941E+09 46.277
2011 10 28 0 0 13.396023 -6.37575 12.458 7.809 1.4471E+09 1.5919E+09 29.226
2011 11 2 0 0 13.433371 -6.59267 12.696 7.824 1.4474E+09 1.5888E+09 98.339
2011 11 7 0 0 13.470217 -6.80414 12.928 7.844 1.4476E+09 1.5848E+09 158.164
2011 11 12 0 0 13.506421 -7.00936 13.153 7.869 1.4478E+09 1.5798E+09 141.600
2011 11 17 0 0 13.541838 -7.20755 13.369 7.898 1.4480E+09 1.5740E+09 80.245
2011 11 22 0 0 13.576319 -7.39786 13.577 7.931 1.4482E+09 1.5674E+09 12.883
2011 11 27 0 0 13.609703 -7.57942 13.775 7.969 1.4484E+09 1.5600E+09 62.830
2011 12 2 0 0 13.641816 -7.75139 13.962 8.011 1.4486E+09 1.5517E+09 128.477
2011 12 7 0 0 13.672504 -7.91303 14.138 8.057 1.4489E+09 1.5428E+09 169.880
2011 12 12 0 0 13.701624 -8.06364 14.302 8.108 1.4491E+09 1.5332E+09 111.276
2011 12 17 0 0 13.729034 -8.20259 14.453 8.162 1.4493E+09 1.5230E+09 46.379
2011 12 22 0 0 13.754582 -8.32915 14.591 8.220 1.4495E+09 1.5123E+09 25.458
2011 12 27 0 0 13.778108 -8.44262 14.714 8.282 1.4497E+09 1.5010E+09 95.682
2012 1 1 0 0 13.799467 -8.54244 14.822 8.346 1.4499E+09 1.4894E+09 157.637
2012 1 6 0 0 13.818532 -8.62814 14.916 8.414 1.4501E+09 1.4775E+09 141.769
2012 1 11 0 0 13.835198 -8.69932 14.993 8.484 1.4504E+09 1.4653E+09 78.247
National Geographic (Link) - Ker Than (August 11, 2009)
A mystery object that punched through one of Saturn's thin outer rings created a glittering spray of ice crystals and pulled some material along in its wake, as seen in this rare image recently released by NASA's Cassini orbiter.
The puncture, which Cassini snapped on June 11, is among the many marvels that have been revealed in the weeks leading up to Saturn's equinox, which happens today.
Equinoxes occur at the point in a planet's orbit when the sun shines directly on the planet's equator. Earth has two equinoxes each year, one in spring and another in autumn.
Because of Saturn's comparatively long orbit around the sun, the ringed planet has an equinox just once every 15 Earth years.
This celestial alignment lowers the angle at which sunlight strikes Saturn, causing objects higher or lower than the ring plane to cast long shadows—and offering scientists a completely new look at the gas giant and its rings.
"We're finally seeing the rings in three dimensions," said Carolyn Porco of the Space Science Institute in Boulder, Colorado. "This is the fist time in history that we've been there watching as this happens."
The object is most likely a small satellite, or moonlet, that is zipping around Saturn on an inclined orbit, said Porco, who leads Cassini's imaging team.
The moonlet appears to be about 0.6 mile (a kilometer) wide, but a comet-like haze of ice crystals and dust surrounding it may be obscuring its true size, Porco added.
Scientists think Saturn's moonlets are fragments of larger moons that were shattered by asteroids or comets eons ago.
It's possible that the object is a small comet or asteroid barreling through the ring, said Nicole Albers, a Saturn-ring scientist at the University of Colorado at Boulder. But such occurrences are rare.
"Since we suspect that there are moonlets in the F ring," Albers said, "it's really not unlikely that it is one."
Posted at 11:52 PM in Science | Permalink
The full set of rings, photographed as Saturn eclipsed the sun from the vantage of the Cassini spacecraft on September 15, 2006 (brightness has been exaggerated in this image). The "pale blue dot" at about the 10 o'clock position, outside the main rings and just inside the G Ring, is Earth.
The rings of Saturn are the most extensive planetary ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometres to metres[1], that form clumps that in turn orbit about Saturn. The ring particles are made almost entirely of water ice, with some contamination from dust and other chemicals.
Although reflection from the rings increases Saturn's brightness, they are not visible from Earth with unaided vision. In 1610, the year Galileo Galilei first turned a telescope to the sky, he became the very first person to observe Saturn's rings, though he could not see them well enough to discern their true nature. In 1655, Christiaan Huygens was the first person to describe them as a disk surrounding Saturn.[2] Although many people think of Saturn's rings as being made up of a series of tiny ringlets (a concept that goes back to Laplace),[2] true gaps are few in number. It is more correct to think of the rings as an annular disk with concentric local maxima and minima in density and brightness.[citation needed] On the scale of the clumps within the rings there is a lot of empty space.
There are several gaps within the rings: two opened by known moons embedded within them, and many others at locations of known destabilizing orbital resonances with Saturn's moons. Other gaps remain unexplained. Stabilizing resonances, on the other hand, are responsible for the longevity of several rings, such as the Titan Ringlet and the G Ring.
Galileo's work
Galileo portrait by Sustermans (c. 1637).
Galileo Galilei was the first to observe the rings in 1610 using his telescope, but he was unable to identify them as such. He wrote to the Duke of Tuscany that "The planet Saturn is not alone, but is composed of three, which almost touch one another and never move nor change with respect to one another. They are arranged in a line parallel to the zodiac, and the middle one (Saturn itself) is about three times the size of the lateral ones [the edges of the rings]." He also described Saturn as having "ears." In 1612 the plane of the rings was oriented directly at the Earth and the rings appeared to vanish. Mystified, Galileo wondered, "Has Saturn swallowed his children?", referring to the myth of the god Saturn eating his own children to prevent them from overthrowing him.[3] Then, in 1613, they reappeared again, further confusing Galileo.[4]
Early astronomers used anagrams as a form of commitment scheme to lay claim to new discoveries before their results were ready for publication. Galileo used smaismrmilmepoetaleumibunenugttauiras for Altissimum planetam tergeminum observavi ("I have observed the most distant planet to have a triple form") for discovering the rings of Saturn.[5]
Ring theory and observations
Robert Hooke noted the shadows (a and b) cast by both the globe and the rings on each other in this 1666 drawing of Saturn.
In 1655, Christiaan Huygens became the first person to suggest that Saturn was surrounded by a ring. With a telescope far superior to those available to Galileo, Huygens observed Saturn and wrote that "It [Saturn] is surrounded by a thin, flat, ring, nowhere touching, inclined to the ecliptic."[4] Robert Hooke was an another early observer of the rings of Saturn, and noted the casting of shadows on the rings.[6]
In 1675, Giovanni Domenico Cassini determined that Saturn's ring was composed of multiple smaller rings with gaps between them; the largest of these gaps was later named the Cassini Division. This division is a 4800 km-wide region between the A Ring and B Ring.[7]
In 1787, Pierre-Simon Laplace suggested that the rings were composed of a large number of solid ringlets.[2]
In 1859, James Clerk Maxwell demonstrated that the rings could not be solid or they would become unstable and break apart. He proposed that the rings must be composed of numerous small particles, all independently orbiting Saturn.[8] Maxwell's theory was proven correct in 1895 through spectroscopic studies of the rings carried out by James Keeler of Lick Observatory.
Cassini space probe view of the unilluminated side of Saturn's rings (May 9, 2007).
The rings can be viewed using a quite modest modern telescope or with good binoculars. The dense main rings extend from 7 000 km to 80 000 km above Saturn's equator, with an estimated local thickness of only 10 meters[9], and are composed of 99.9 percent pure water ice with a smattering of impurities that may include tholins or silicates.[10] The main rings are primarily composed of particles ranging in size from 1 centimeter to 10 meters.[11]
The total mass of the rings is about 3 x 1019 kg. This is a small fraction of the total mass of Saturn (about 50 ppb) and is just a little less than the moon Mimas.[12] There have been recent claims, as yet unverified, that this is an underestimate due to clumping in the rings and the mass may actually be three times this figure.[13]
While the largest gaps in the rings, such as the Cassini Division and Encke Gap, can be seen from Earth, both Voyager spacecraft discovered that the rings have an intricate structure of thousands of thin gaps and ringlets. This structure is thought to arise, in several different ways, from the gravitational pull of Saturn's many moons. Some gaps are cleared out by the passage of tiny moonlets such as Pan,[14] many more of which may yet be discovered, and some ringlets seem to be maintained by the gravitational effects of small shepherd satellites (similar to Prometheus and Pandora's maintenance of the F ring).[citation needed] Other gaps arise from resonances between the orbital period of particles in the gap and that of a more massive moon further out; Mimas maintains the Cassini division in this manner.[citation needed] Still more structure in the rings consists of spiral waves raised by the inner moons' periodic gravitational perturbations at less disruptive resonances.[citation needed]
Data from the Cassini space probe indicate that the rings of Saturn possess their own atmosphere, independent of that of the planet itself. The atmosphere is composed of molecular oxygen gas (O2) produced when ultraviolet light from the Sun interacts with water ice in the rings. Chemical reactions between water molecule fragments and further ultraviolet stimulation create and eject, among other things O2. According to models of this atmosphere, H2 is also present. The O2 and H2 atmospheres are so sparse that if the entire atmosphere were somehow condensed onto the rings, it would be on the order of one atom thick.[15] The rings also have a similarly sparse OH (hydroxide) atmosphere. Like the O2, this atmosphere is produced by the disintegration of water molecules, though in this case the disintegration is done by energetic ions that bombard water molecules ejected by Saturn's moon Enceladus. This atmosphere, despite being extremely sparse, was detected from Earth by the Hubble Space Telescope.[16]
Saturn shows complex patterns in its brightness.[17] Most of the variability is due to the changing aspect of the rings,[18][19] and this goes through two cycles every orbit. However, superimposed on this is variability due to the eccentricity of the planet's orbit that causes the planet to display brighter oppositions in the northern hemisphere than it does in the southern.[20]
In 1980, Voyager 1 made a fly-by of Saturn that showed the F-ring to be composed of three narrow rings that appeared to be braided in a complex structure; it is now known that the outer two rings consist of knobs, kinks and lumps that give the illusion of braiding, with the less bright third ring lying inside them.[citation needed]
A 2007 artist impression of the aggregates of icy particles that form the 'solid' portions of Saturn's rings. These elongated clumps are continually forming and dispersing. The largest particles are a few meters across.
Saturn's rings may be very old, dating to the formation of Saturn itself. There are two main theories regarding the origin of Saturn's rings. One theory, originally proposed by Édouard Roche in the 19th century, is that the rings were once a moon of Saturn whose orbit decayed until it came close enough to be ripped apart by tidal forces (see Roche limit).[21] A variation of this theory is that the moon disintegrated after being struck by a large comet or asteroid.[22] The second theory is that the rings were never part of a moon, but are instead left over from the original nebular material from which Saturn formed.[citation needed]
It seems likely however that they are composed of debris from the disruption of a moon about 300 km in diameter, bigger than Mimas. The last time there were collisions large enough to be likely to disrupt a moon that large was during the Late Heavy Bombardment some four billion years ago.[23]
The brightness and purity of the water ice in Saturn's rings has been cited as evidence that the rings are much younger than Saturn, perhaps by 100 million years, as the infall of meteoric dust would have led to darkening of the rings. However, new research indicates that the B Ring may be massive enough to have diluted infalling material and thus avoided substantial darkening over the age of the Solar system. Ring material may be recycled as clumps form within the rings and are then disrupted by impacts. This would explain the apparent youth of some of the material within the rings.[24]
The Cassini UVIS team, led by Larry Esposito, used stellar occultation to discover 13 objects, ranging from 27 meters to 10 km across, within the F ring. They are translucent, suggesting they are temporary aggregates of ice boulders a few meters across. Esposito believes this to be the basic structure of the Saturnian rings, particles clumping together, then being blasted apart.[25]
The densest parts of the Saturnian ring system are the A and B Rings, which are separated by the Cassini Division (discovered in 1675 by Giovanni Domenico Cassini). Along with the C Ring, which was discovered in 1850 and is similar in character to the Cassini Division, these regions comprise the main rings. The Main Rings are denser and contain larger particles than the tenuous dusty rings. The latter include the D Ring, extending inward to Saturn's cloud tops, the G and E Rings and others beyond the main ring system. The word "dusty" used to characterize these diffuse rings refers to the small size of the particles (often about a micrometre); their chemical composition is, like the main rings, almost entirely of water ice. The narrow F Ring, just off the outer edge of the A Ring, is more difficult to categorize; parts of it are very dense, but it also contains a great deal of dust-size particles.
Natural-color mosaic above of Cassini 's narrow-angle camera images of the UNilluminated side of
Saturn's D, C, B, A and F rings (left to right) taken on May 9, 2007. Slide bar to the right to see the
entire ring cross-section.
The illuminated side of Saturn's rings with the major subdivisions labelled
Notes:
(1) distance is to centre of gaps, rings and ringlets that are narrower than 1000 km
(2) unofficial name
(3) Names as designated by the International Astronomical Union, unless otherwise noted. Broader separations between named rings are termed divisions, while narrower separations within named rings are called gaps.
(4) Data mostly from the Gazetteer of Planetary Nomenclature, a NASA factsheet and several papers.[27][28][29]
Oblique (4 degree angle) Cassini images of Saturn's C, B, and A rings (left to right; the F ring is faintly visible in the full size upper image if viewed at sufficient brightness). Upper image: natural color mosaic of Cassini narrow-angle camera photos of the illuminated side of the rings taken on December 12, 2004. Lower image: simulated view constructed from a radio occultation observation conducted on May 3, 2005. Color in the lower image is used to represent information about ring particle sizes.
A Cassini image of Saturn's D ring processed to show faint ripples within it; the much brighter C ring appears in the upper left.
The D Ring is the innermost ring, and is very faint. In 1980, Voyager 1 detected within this ring three ringlets designated D73, D72 and D68, with D68 being the discrete ringlet nearest to Saturn. Some 25 years later Cassini images showed that D72 had become significantly fainter and moved planetward by 200 kilometres. Present in the gap between the C ring and D73 is finescale structure with waves 30 kilometres apart.[30]
The C Ring is a wide but faint ring located inward of the B Ring. It was discovered in 1850 by William and George Bond, though William R. Dawes and Johann Galle also saw it independently. William Lassell termed it the "Crepe Ring" because it seemed to be composed of darker material than the brighter A and B Rings.[31]
Its vertical thickness is estimated at 5 metres, its mass at around 1.1 × 1018 kilograms, and its optical depth varies from 0.05 to 0.12.[citation needed] That is, 5 and 12 percent of light shining through perpendicular to the ring is blocked, so that when seen from above or below, the ring is close to transparent.
The Maxwell Gap and the Maxwell Ringlet on its right side are above and right of center. The Bond Gap is above a broad light band towards the upper right; the Dawes Gap is within a dark band just below the upper right corner.
The Colombo Gap lies in the inner C Ring. Within the gap lies the bright but narrow Colombo Ringlet, centered at 77 883 kilometers from Saturn's center, which is slightly elliptical rather than circular. This ringlet is also called the Titan Ringlet as it is governed by an orbital resonance with the moon Titan.[citation needed] At this location within the rings, the time period of a ring particle's apsidal precession is equal to the time period of Titan's orbital motion, so that the outer end of this eccentric ringlet always points towards Titan.[citation needed]
The Maxwell Gap lies within the outer part of the C Ring. It also contains a dense non-circular ringlet, the Maxwell Ringlet. In many respects this ringlet is similar to the ε ring of Uranus. There are wave-like structures in the middle of both rings. While the wave in the ε ring is thought to be caused by uranian moon Cordelia, no moon has been discovered in the Maxwell gap as of July 2008.[25]
The B Ring is the largest, brightest, and most massive of the rings. Its thickness is estimated as 5 to 15 metres, its mass at 2.8 × 1019 kg, and its optical depth varies from 0.4 to 2.5, meaning that well over 99% of the light passing through some parts of the B Ring is blocked.[citation needed] The B Ring contains a great deal of variation in its density and brightness, nearly all of it unexplained. These are concentric, appearing in the form of narrow ringlets, though the B Ring does not contain any gaps.[citation needed]
Dark spokes mark the B ring's sunlit side in low phase angleCassini images
Up until 1980, the structure of the rings of Saturn was explained as being caused exclusively by the action of gravitational forces. Then images from the Voyager spacecraft showed radial features in the B ring, known as spokes, which could not be explained in this manner, as their persistence and rotation around the rings was not consistent with orbital mechanics.[32] The spokes appear dark in backscattered light, and bright in forward-scattered light (see images in gallery). The leading theory regarding the spokes' composition is that they consist of microscopic dust particles suspended away from the main ring by electrostatic repulsion, as they rotate almost synchronously with the magnetosphere of Saturn. The precise mechanism generating the spokes is still unknown, although it has been suggested that the electrical disturbances might be caused by either lightning bolts in Saturn's atmosphere or micrometeoroid impacts on the rings.[33]
The spokes were not observed again until some twenty-five years later, this time by the Cassini space probe. The spokes were not visible when Cassini arrived at Saturn in early 2004. Some scientists speculated that the spokes would not be visible again until 2007, based on models attempting to describe their formation. Nevertheless, the Cassini imaging team kept looking for spokes in images of the rings, and they were next seen in images taken on September 5, 2005.[34]
The spokes appear to be a seasonal phenomenon, disappearing in the Saturnian midwinter/midsummer and reappearing as Saturn comes closer to equinox. Suggestions that the spokes may be a seasonal effect, varying with Saturn's 29.7-year orbit, were supported by their gradual reappearance in the later years of the Cassini mission.[35]
The Cassini Division imaged from the Cassini spacecraft. The Huygens Gap lies at its right border; the Laplace Gap is towards the center. A number of other, narrower gaps are also present.
The Cassini Division is a 4,800 km (2,980 mile) wide region between the A Ring and B Ring. It was discovered in 1675 by Giovanni Cassini. From Earth it appears as a thin black gap in the rings. However, Voyager discovered that the gap is itself populated by ring material bearing much similarity to the C Ring.[25] The division may appear bright in views of the unlit side of the rings, since the relatively low density of material allows more light to be transmitted through the thickness of the rings (see second image in gallery).[citation needed]
The inner edge of the Cassini Division is governed by a strong orbital resonance. Ring particles at this location orbit twice for every orbit of the moon Mimas.[citation needed] The resonance causes Mimas' pulls on these ring particles to accumulate, destabilizing their orbits and leading to a sharp cutoff in ring density. Many of the other gaps between ringlets within the Cassini Division, however, are unexplained.[citation needed]]
Huygens Gap
The Huygens Gap is located at the inner edge of the Cassini Division. It contains a dense eccentric ringlet named Huygens Ringlet in the middle. This ringlet demonstrates irregular azimuthal variations of the geometrical width and optical depth, which can be caused by the nearby strong 2:1 resonance with Mimas and by the influence of the eccentric outer edge of the B-ring. There is an additional narrow ringlet just outside the Huygens Ringlet.[25]
The central ringlet of the A ring's Encke Gap coincides with Pan’s orbit, implying its particles oscillate in horseshoe orbits.
The A Ring is the outermost of the large, bright rings. Its inner boundary is the Cassini Division and its sharp outer boundary is close to the orbit of the small moon Atlas. The A Ring is interrupted at a location 22% of the ring width from its outer edge by the Encke Gap. A narrower gap 2% of the ring width from the outer edge is called the Keeler Gap.
The thickness of the A Ring is estimated as 10 to 30 metres, its mass as 6.2 × 1018 kg (about the mass of Hyperion), and its optical depth varies from 0.4 to 1.0.[citation needed]
Similarly to the B Ring, the A Ring's outer edge is maintained by an orbital resonance, in this case the 7:6 resonance with Janus and Epimetheus.[citation needed] Other orbital resonances also excite many spiral density waves in the A Ring (and, to a lesser extent, other rings as well), which account for most of its structure. These waves are described by the same physics that describes the spiral arms of galaxies. Spiral bending waves, also present in the A Ring and also described by the same theory, are vertical corrugations in the ring rather than compression waves.[citation needed]
Waves in the Keeler gap edges induced by the passage of Daphnis.
Near Saturn's equinox, Daphnis and its waves cast shadows on the A Ring.
The Encke Gap is a 325-kilometre-wide gap within the A Ring, centered at a distance of 133,590 kilometers from Saturn's center.[36] It is caused by the presence of the small moon Pan,[37] which orbits within it. Images from the Cassini probe have shown that there are at least three thin, knotted ringlets within the gap.[25] Spiral density waves visible on both sides of it are induced by resonances with nearby moons exterior to the rings, while Pan induces an additional set of spiraling wakes (see image in gallery).[25]
Johann Encke himself did not observe this gap; it was named in honour of his ring observations. The gap itself was discovered by James Edward Keeler in 1888.[38] The second major gap in the A Ring, discovered by Voyager, was named the Keeler Gap in his honor.[39]
The Encke Gap is a gap because it is entirely within the A Ring. There was some ambiguity between the terms gap and division until the IAU clarified the definitions in 2008; prior to that, the separation was sometimes called the "Encke Division".[citation needed]
The Keeler Gap is a 42-kilometre-wide gap in the A Ring, approximately 250 kilometres from the ring's outer edge. The small moon Daphnis, discovered May 1, 2005, orbits within it, keeping it clear.[40] The moon induces waves in the edges of the gap.[25] Because the orbit of Daphnis is slightly inclined to the ring plane, the waves have a component that is perpendicular to the ring plane, reaching a distance of 1.5 km "above" the plane.[41][42]
The Keeler gap was discovered by Voyager, and named in honor of the astronomer James Edward Keeler. Keeler had in turn discovered and named the Encke Gap in honor of Johann Encke.[38]
Location of the first four moonlets detected in the A ring.
In 2006, four tiny "moonlets" were found in Cassini images of the A Ring.[43] The moonlets themselves are only about a hundred meters in diameter, too small to be seen directly; what Cassini sees are the "propeller"-shaped disturbances the moonlets create, which are several km across. It is estimated that the A Ring contains thousands of such objects. In 2007, the discovery of eight more moonlets revealed that they are largely confined to a 3000-km belt, about 130 000 km from Saturn's center.[44] Over 150 "propeller" moonlets have now been detected.[45]
The Roche Division (passing through image center) between the A Ring and the narrow F Ring. Atlas can be seen within it. The Enke and Keeler gaps are also visible.
The separation between the A Ring and the F Ring has been named the Roche Division in honor of the French physicist Édouard Roche.[1] The Roche Division should not be confused with the Roche limit, a physical concept that describes when a large object gets so close to a planet (such as Saturn) that the planet's tidal forces will pull it apart.[citation needed] Lying at the outer edge of the main ring system, the Roche Division is in fact close to Saturn's Roche limit, which is why the rings have been unable to accrete into a moon.[citation needed]
Like the Cassini Division, the Roche Division is not empty but contains a sheet of material.[citation needed] The character of this material is similar to the tenuous and dusty D, E, and G Rings.[citation needed] Two locations in the Roche Division have a higher concentration of dust than the rest of the region. These were discovered by the Cassini probe imaging team and were given temporary designations: R/2004 S 1, which lies along the orbit of the moon Atlas; and R/2004 S 2, centered at 138,900 km from Saturn's center, inward of the orbit of Prometheus.[citation needed]
The shepherd moons Pandora (left) and Prometheus (right) orbit on either side of the F ring; Prometheus is followed by dark channels that it has carved into the inner strands of the ring.
The F Ring is the outermost discrete ring of Saturn and perhaps the most active ring in the Solar system, with features changing on a timescale of hours.[46] It is located 3000 km beyond the outer edge of the A Ring.[47] It was discovered in 1979 by the Pioneer 11 imaging team.[48] It is very thin, just a few hundred kilometers wide, and is held together by two shepherd moons, Prometheus and Pandora, which orbit inside and outside it.[37]
Recent closeup images from the Cassini probe show that the F Ring consists of one core ring and a spiral strand around it.[49] They also show that when Prometheus encounters the ring at its apoapsis, its gravitational attraction creates kinks and knots in the F Ring as the moon 'steals' material from it, leaving a dark channel in the inner part of the ring (see video link and additional F Ring images in gallery). Since Prometheus orbits Saturn more rapidly than the material in the F ring, each new channel is carved about 3.2 degrees in front of the previous one.[46]
In 2008, further dynamism was detected, suggesting that small unseen moons orbiting within the F Ring are continually passing through its narrow core due to perturbations from Prometheus. One of the small moons was tentatively identified as S/2004 S 6.[46]
A mosaic of 107 images showing 255° (about 70%) of the F Ring as it would appear if straightened out. The radial width (top to bottom) is 1500 km.
The outer rings seen back-illuminated by the Sun
The Anthe Ring Arc. The bright spot is Anthe.
The backlit E ring, with Enceladus silhouetted against it. The moon's south polar jets erupt brightly below it.
A faint dust ring is present around the region occupied by the orbits of Janus and Epimetheus, as revealed by images taken in forward-scattered light by the Cassini spacecraft in 2006. The ring has a radial extent of about 5000 km [50]. Its source is particles blasted off the moons' surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.[51]
The G Ring (see last image in gallery) is a very thin, faint ring about halfway between the F Ring and the beginning of the E Ring, with its inner edge about 15000 km inside the orbit of Mimas. It contains a single distinctly brighter arc near its inner edge (similar to the arcs in the rings of Neptune) that extends about one sixth of its circumference, centered on the moonlet S/2008 S 1, which is held in place by a 7:6 orbital resonance with Mimas.[52][53] The arc is believed to be composed of icy particles up to a few meters in diameter, with the rest of the G Ring consisting of dust released from within the arc. The radial width of the arc is about 250 km, compared to a width of 6000 km for the G Ring as a whole.[52] The arc is thought contain matter equivalent to a small icy moonlet about a hundred meters in diameter.[52] Dust released from larger source bodies within the arc by micrometeoroid impacts drifts outward from the arc due to interaction with Saturn's magnetosphere (whose plasma corotates with Saturn's magnetic field, which rotates much more rapidly than the orbital motion of the G Ring). These tiny particles are steadily eroded away by further impacts and dispersed by plasma drag. Over the course of thousands of years the ring gradually loses mass; at some point it will eventually disappear.[54]
A faint ring arc, first detected in Sept. 2006, covering a longitudinal extent of about 10 degrees is associated with the moon Methone. The material in the arc is believed to represent dust ejected from Methone by micrometeoroid impacts. The confinement of the dust within the arc is attributable to a 14:15 resonance with Mimas (similar to the mechanism of confinement of the arc within the G ring).[55] Under the influence of the same resonance, Methone librates back and forth in its orbit with an amplitude of 5° of longitude.
A faint ring arc, first detected in June 2007, covering a longitudinal extent of about 20 degrees is associated with the moon Anthe. The material in the arc is believed to represent dust knocked off Anthe by micrometeoroid impacts. The confinement of the dust within the arc is attributable to a 10:11 resonance with Mimas. Under the influence of the same resonance, Anthe drifts back and forth in its orbit over 14° of longitude.[55]
A faint dust ring shares Pallene's orbit, as revealed by images taken in forward-scattered light by the Cassini spacecraft in 2006.[50] The ring has a radial extent of about 2,500 km. Its source is particles blasted off Pallene's surface by meteoroid impacts, which then form a diffuse ring around its orbital path.[51]
The E Ring is the outermost ring, and is extremely wide, beginning at the orbit of Mimas and ending somewhere around the orbit of Rhea. It is a diffuse disk consisting mostly of ice, with silicates, carbon dioxide and ammonia.[56] Unlike the other rings, it is composed of microscopic rather than macroscopic particles. In 2005, the source of the E Ring's material was determined to be cryovolcanic plumes[57][58] emanating from the "tiger stripes" of the south polar region of the moon Enceladus.
Main article: Rings of Rhea
Saturn's second largest moon Rhea may have a tenuous ring system of its own consisting of three narrow bands embedded in a disk of solid particles.[59][60] These rings have not been imaged, but their existence has been inferred from Cassini observations in November 2005 of a depletion of energetic electrons in Saturn's magnetosphere near Rhea. The Magnetospheric Imaging Instrument (MIMI) observed a gentle gradient punctuated by three sharp drops in plasma flow on each side of the moon in a nearly symmetric pattern. This could be explained if they were absorbed by solid material in the form of an equatorial disk containing denser rings or arcs, with particles perhaps several decimeters to approximately a meter in diameter. However, not all scientists are convinced that the observations were caused by a ring system.
Mosaic of Cassini images of the unlit side of the inner B Ring (top) and outer C Ring (bottom), taken near Saturn's equinox, showing multiple views of the shadow of Mimas. The shadow is easily visible through most parts of the C ring, but is attenuated by the denser B ring. The Maxwell Gap is below center.
Dark B Ring spokes are visible in this low-phase-angle Cassini image of the unilluminated side of the rings. Left of center, two dark gaps (the larger being the Huygens Gap) and the bright (from this viewing geometry) ringlets between and immediately beyond them comprise the Cassini Division.
Cassini image of the sun-lit side of the rings taken in 2009 at a phase angle of 144°, with bright B Ring spokes.
The motion of Pan (not visible) through the A Ring's Encke Gap induces edge waves and a set of spiraling wakes (which are not self-propagating) ahead of it in its orbit on the gap's inner side. The other more tightly wound bands are spiral density waves.
Close-up view of Prometheus and the F Ring. A movie of Prometheus at its apoapsis drawing a streamer of material out of the ring, leaving a dark channel, may be viewed here or here.
Prometheus (at center) and Pandora are the inner and outer F Ring shepherds.
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Édouard Roche - French astronomer who described how the breakup of a satellite could form the rings, when it comes within the Roche limit of a celestial body.
Galileo Galilei - the first person to observe Saturn's rings, in 1610
Christian Huygens - the first person to propose that there was a ring surrounding Saturn, in 1655
Giovanni Cassini - discovered the separation between the A and B rings, in 1675 - (Cassini Division)
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