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ASTB21H3F
Planetary Ring Systems
Piranavan Satkunanathan
Joseph Zhang
Jonguk Yoo
Seyed Hossein Hosseini Nassab
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Introduction:
Planetary ring systems are rings of cosmic dust and other small particles orbiting
around planets in extremely thin and flat circular disc-shapes. There are four planets that
we know so far with their individual ring systems in our Solar system, and they are:
Jupiter, Saturn, Uranus and Neptune. All of them belong to the Jovian planets and they
are all gas giants. The discoveries of these rings were fairly recent, other than the rings of
Saturn due to its spectacular appearance and size. Saturn’s rings were discovered by
Galileo during early 17th century and the other three ring systems were not found for
more than three and half centuries. Uranus’s rings were found in 1977, Jupiter’s rings
were found in 1979 and Neptune’s ring “arcs” were found in early 1980s. There are many
interesting facts and questions about the Planetary ring systems, so in this paper we are
going to introduce details of these Jovian ring systems with Saturn in most focus, and
explain their History/Formation, Properties/Composition/Dimensions and Historical
Observations/Questions.
History and Formation:
As a matter of fact: Saturn, Uranus, Jupiter and Neptune are the only planets that
have planetary rings in solar system, and among these planets, rings of Saturn appear
most clearly. So, what are the rings? During 1610s, the existence of the rings was first
discovered by Galileo, but he didn’t have precise explanation to what they were. He
could only describe them as Saturn’s “ears”. In 1655, Christiaan Huygens observed and
suggested that Saturn was surrounded by a ring with improved telescope. After few years,
in 1675, Giovanni Domenico Cassini determined that Saturn's rings were composed of
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multiple smaller rings with gaps between them. Pierre-Simon Laplace, in 1787, suggested
that the rings were composed of a large number of solid ringlets. In 1859, however,
British mathematician, James Maxwell, claimed that if the rings of Saturn were made of
solid material, the rings should have broken by Saturn’s tidal force. He also said that they
were made of number of small particles such as Dust or Ice but it just seemed like plate
with solid materials from the Earth because of the large distance. It is like a beach that
consists of large quantity of grains of sand but when viewed from a distance, it looks like
a piece of solid land.
So then, which theory was right? In 1850, French astronomer, Eduard Roche, had
studied that what would happen if the Moon gets too close to the Earth. Roche found that
the tidal force between the Earth and the Moon is inversely proportional to the cube of
distance between the Earth and the Moon. In other words, if you half the distance
between the Earth and the Moon, the tidal force will become 8 times larger. Also, he
found that if a satellite comes within a certain distance to a primary object, the tidal force
between them becomes greater than the force of gravity of the satellite itself, and the
satellite will not be able to maintain its physical form. After the satellite has been
disrupted by the tidal stress, its debris will end up spinning around the planet forming
plate-like structure, and this is what created the ring systems. This distance limit is called
Roche limit, and it is described as,
d = 2.44R (ρM / ρm) 1/3
-where R is the radius, ρm is the density of the primary object, and ρm is the density of the
satellite. This is the theory of Ring Formation. However, the exact causes of creation of
rings, dynamic evolution, and the ingredients of them are still not fully revealed. For
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example, some of Saturn’s rings are located outside of its Roche limit; so therefore, the
creation of rings cannot be fully explained by this theory. Another explanation for
existence of the rings is that the material of the proto-planetary disk that was within the
Roche limit of the planet could not coalesce to form moons. So far, there isn’t enough
evidence to conclude with a perfect theory with full explanation. The amount of
observations by space probes such as Voyager 1, 2 and Cassini-Huygens are very limited.
Gaps: If we look closely at the rings, there are gaps between them. The main
cause is believed to be the orbital resonances with outer satellites. Orbital periods of the
moons and the particles within the rings are explained by Kepler’s laws, and if an outer
moon has a period that is an integer fraction of the particles in the ring, then the moon
gives an extra pull on the particles each time they line up and this throws the particles
into a new orbit. Orbital resonances would, after billions of years, eventually sweep all
the particles out of a ring, but the effects of the inner moons counteract the pull from the
outer moon. This is how gaps formed and its relation to the existing rings.
For a long time, astronomers had the question “Is Saturn the only planet with
rings?” This question was eventually answered by a discovery, which found Uranus also
had the rings in 1977. However, Uranus’ rings were too thin and dim to observe from
Earth since it couldn’t reflect enough sun light. The discovery accidently happened, when
a nearby star was passing by. Astronomers saw observed the light from Uranus darkened
briefly, and it was found to be caused by the ring system of Uranus. In 1986, space probe,
Voyager 2, on observational mission to the giant planets, made clear observation as
evidence of Uranus’s ring system with total number of eleven rings discovered including
very faint ones. With the improved technology, the Hubble Space Telescope detected two
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more rings, but they were located at twice the distance from the previously found rings.
So, they are also called, ‘outer’ ring system of Uranus.
2 years after the discovery of Uranus’ rings in 1979, Jupiter’s rings were
discovered by the Voyager 1. Unlike the rings of Saturn, the rings of Jupiter are
composed of small bits of dust, therefore having less reflectivity.
In 1989, Neptune’s rings were also found by Voyager 2. However, Neptune's ring
system is very different from the ring system found at Saturn, they were much darker
than that of Saturn's. This was caused by the differences of their composition; Saturn's
rings are mostly composed of ice where as Neptune's appear to be made of small rocks
and dust much more similar to the rings of Uranus’s.
Observations by NASA:
Many questions have risen since the rings of Saturn were discovered with an
Earth based 20-power telescope by Galileo, who was puzzled by its odd appearance.
From what he saw, the rings looked like bumps on the planet and he wasn’t able to
imagine these bumps as rings we know today. Later in 1655, Christian Huygens observed
Saturn with an improved telescope, where he was able to describe the bumps observed by
Galileo to be a disk surrounding Saturn. However, only from ground base observations it
was not possible to fully understand the true nature of these planetary rings.
As technology improved, the image and characteristics of the rings became more
and more clear and understandable. From the missions by NASA such as Voyager 1/2,
Cassini Mission to Saturn, and Pioneer 11, astronomers were able to collect data to study
the nature of the rings. However these new samples of data raised even more questions.
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The first spacecraft to visit the Saturnian system was Pioneer 11. Pioneer 11 reached its
closest approach to the planet on September 1st 1979. Unfortunately Pioneer 11 was
designed primarily as a “fields and particles” spacecraft. The visible light sensor that was
equipped was not suited for high resolution imaging. Even though some of the images
revealed Saturn’s rings, the machine wasn’t able to provide a more detailed spectrum of
data for astronomers in order to expand their study on planetary rings. However, it was
able to discover a faint, narrow F and G ring, but its complexity became evident only
when Voyager 1 and 2 took closer looks.
Moreover, Pioneer 11 played an essential role in the discovery of the Jupiter’s
ring system. “In 1974, the processed data taken from the Pioneer 11 mission revealed an
unexpectedly low energetic charged particle around Jupiter’s little satellite, Am althea.
Scientists suggested that an unseen ring or undiscovered satellite might be responsible
for absorbing some of the charged particle flux inward of Am althea’s orbit, but little
attention was given to the suggestion until after the voyager 1 Jupiter
encounter”(Ellis D. Miner, Randii R, 2007). During the voyager flyby of Jupiter it
discovered a faint ring of matter surrounding Jupiter in the plane of the planet’s equator.
In the summer of 1977 from Cape Canaveral, Florida NASA launched Voyager 1 and
Voyager 2 on August 20 and September 5. Voyager 1’s primary target was to conduct
close up studies of Jupiter and Saturn including Saturn’s rings, and the larger moons of
the two planets, while Voyager 2 was launched to study the gas giants. Voyager 1 and 2
flew by Saturn on November 1980 and August 1981 respectively, and photographs
collected from them allowed scientists to confirm that the main rings (A, B and C) were
composed of tens of thousands of narrow ringlets. Two more rings were discovered by
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Voyager 2, the D and E ring, these were more faint rings that are completely invisible
from Earth. One mysterious part was that “there were changes in the appearance and
location of clumps within the F rings in the time period between the Voyager 1 encounter
(November 2 1980) and voyager 2 encounter (August 6, 1981)” (Ellis D. Miner,
Randii R, 2007). Some of the structures couldn’t be explained by the gravitational effects,
and additional data from the Cassini Obiter was needed for further understanding.
Voyager 2 was also able to capture one of the special features of the ring called “spoke”,
but the exact cause of the formation is still unknown. Voyager images also showed that
the largest gap in the rings, the Cassini division, is not actually completely empty of
matter. Voyager 2 encountered with Jupiter starting on April 24, 1979, for approximately
76 days before the closest approach (July 9, 1979). “From forward scattering, Voyager 2
cameras were able to detect a new faint ring structure that was all but absent from the
Voyager 1 image” (Ellis D. Miner, Randii R, 2007). The analysis of Voyager 2 data
showed that Jupiter’s ring is made of three major components known as main ring, halo
and gossamer rings. Moreover, the data from Voyager 2 states that the main ring
encompasses the orbits of two small moons, Adrastea and Metis.
On November 4th 1985 after a very long journey from Saturn, Voyager 2 entered
the Uranian system. During that time period Voyager 2 was able to discover a new ring
between the Epsilon and the Delta rings as well as another ring from the planet's center.
Importantly, sensors on Voyager 2 revealed what may be a great number of narrow rings,
or perhaps incomplete rings or ring arcs. In order to understand these unique narrow
rings, another mission to Uranus must be planned, until then they remain as mystery.
“From the Uranus encounter Voyager 2 was able to detect all nine previously known
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rings of Uranus and other new rings and ringlets in the Uranian system. These
observations revealed that Uranus's rings are distinctly different from those at Jupiter
and Saturn” (View of the Solar System 2001. Voyager Uranus Science Summary from
http://www.solarviews.com/eng/vgrur.htm)
Voyager 2’s encounter with Neptune began on June 5, 1989. On August 11th,
Voyager’s camera was able to detect two ring arcs. Like Jupiter and Uranus, Neptune's
rings are very dark, however, their composition is still not found.
Cassini Huygens is the first sophisticated spacecraft ever been sent into space and
the first to explore the Saturn’s ring system and moons from the orbit. On July 1st 2004,
after a nearly 7 year cruise, Cassini arrived at Saturn and immediately began sending
back intriguing images and data. Some of the unknown facts about the rings began to be
resolved, such as the rapid changes in the F ring are caused by small moonlets colliding
with the ring. Cassini Huygens was also able to find new elements of the ring system,
moon with rings! Recently NASA announced that “Cassini spacecraft has found
evidence of material orbiting Rhea, Saturn's second largest moon. The discovery was a
result of a Cassini close flyby of Rhea in November 2005. This is the first time rings may
have
been
found
around
a
moon”
(Cassini-Huygens
2008,
from
http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=820). However, some
of the data collected from this spacecraft regarding to ring properties are still in process.
Astronomers hope the data collected from Cassini Mission will resolve many mysteries
of this spectacular corner of our solar system.
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Composition:
To an observer looking at the planetary rings, at first it might appear as a
composition of “continuous sheet of matter, broken by at most a single division” (Beatty,
1999, p. 222), but more scientific approaches have indicated that they are actually a
collection of unconnected small particles in a disk shaped ring around planets.
Observations of planetary rings, especially from the Saturn’s rings, have shown
that the size of ring particles vary from fine dust to moonlets kilometres across.
Formation of smaller particles in rings is generally know to be caused by collisions of
larger particles with each other and their break up under the huge tidal forces to form
smaller particles. Because of their smaller masses, smaller ring particles tend to have
greater relative velocities with respect to larger particles and therefore extending to
higher altitudes of the ring while largest particles are narrowed to a single layer.
Spectral analyses of the solar system planetary rings have indicated that the
dominant chemical composition of planetary rings is water ice. Based on discoveries by
Voyager 1 and 2, more than 90% of Saturn’s ring is composed of water ice, which is
“contaminated by a small amount of organic contaminants” (Esposito, 2006, p. 161),
particularly with methane, tholin and silicate. This composition of organic contaminants
with the water ice removes the transparency of water particles and gives a dark red colour
to the ring particles, similar to “dirty snowballs” (Beatty, 1999, p. 236).
In comparison with Saturn’s ring particles, Jupiter’s ring particles are much
smaller, mainly micrometers across, giving Jupiter’s ring its unique dusty characteristics.
This abundance of dust in Jupiter’s rings is thought to have been ejected from the moons
Metis, Adrastea, and other unobserved objects. Spectral analyses of Jupiter’s rings have
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shown that its particles have red colour but have no indications of the chemical
composition. However, there are some suggestions based on the colour of the rings, that
Jupiter’s rings are a mixture of water ice, silicate, and carbon-rich material.
Rings of Uranus on the other hand, are composed of bigger particles mainly in the
10 cm-10 m range. Uranus’ rings are extremely dark, thus suggesting the composition to
be a mixture of water ice with some dark material, possibly organic compounds and
heavy processed material.
Neptune’s rings are characterized as most faint and dusty particles, with dark red
colour, indicating its composition to be a mixture of ice with radiation-processed
organics.
Properties:
Saturn’s rings: Saturn has the greatest ring system in our solar system. Saturn’s
spectacular rings are clearly visible, highly prominent and have the greatest complexity
of all the outer planetary rings. The rings lay in the planet’s equatorial plane, so therefore,
the appearance changes as seen from Earth as Saturn orbits the sun: Saturn’s axis of
rotation is tilted 27degrees with respect to its orbital plane so therefore, depending on
Saturn’s position on the rotation plane, its ring system reflects different amount of sun
light. The rings are very thin, about 250,000km in diameter and less than 1km in
thickness, so ratio wise it is thinner than: a piece of paper spread across a football field;
and the very tip of a razor. The rings are spread between the planet’s surface and the
approximate Roche limit, which is about 2.5 times the planet’s radius, and are composed
of particles ranging from a millimetre to tens of meters. There are many different lanes of
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rings, ranking from the inner most, there are the; D ring, C ring, B ring, A ring, F ring, G
ring and outer most, E ring. They have widths of 7700km, 17300km, 25500km,
14500km, 100km, 8000km and 300000km respectively. There are also gaps between
these rings; the two visible ones are Cassini division and Encke gap which are 4800km
and 300km in width respectively. Cassini division is between the B ring and the A ring,
and Encke gap lies within the A ring. Later observations have shown that these gaps were
actually not entirely empty, they also contained particles and dusts just in lower
reflectivity and density. The total mass of the Saturn’s ring system is estimated about
10^15 tons which is about enough to make a satellite about 250km in diameter. Most of
these masses are concentrated in the B ring and the A ring, these two rings have the
highest surface densities ranging from 20~100 g/cm^2 and also therefore these two lanes
of rings are most prominent.
Other Jovian Planetary Ring Systems:
Jupiter’s ring: Jupiter’s ring is much simpler, thin and less visible than that of
Saturn’s. It only consists of one lane of main ring, which lies between roughly 50000km
above the top cloud layer of Jupiter and the orbit of the innermost moon, with a width of
about 7000km and thickness of few tens of kilometres. The ring itself is almost
impossible to be observed with naked eye from Earth since it only has a surface density
of about 5*10^-6 g/cm^2, about 10^7 times less dense than Saturn’s main rings.
Uranus’s rings: Unlike Saturn’s bright and wide rings with narrow gaps between
them, Uranus’s rings are all very dim and narrow, and also widely spaced. There are total
of nine rings around Uranus, and except the most outer Epsilon ring with width of
20~100 km, all other rings are less than 10km in width, and the spacing between them
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ranges from few hundred to about a thousand kilometres. The densities of particles with
in Uranus’s rings are comparable to those found in Saturn’s A and B rings but the
particles are much smaller, spread out, and also considerably less reflective, causing them
to be extremely hard to detect.
Neptune’s rings: Neptune is surrounded by five dark rings. Three of them are
Leverrier, Arago, and Adams, which are narrow rings like those of Uranus’s, having
widths of 100km, 100km, and 50km respectively; and two of them are Galle, Lassell,
which are broad and diffuse rings similar to that of Jupiter’s, having widths of 2000km
and 4000km respectively. They all lie within the Roche limit of the planet except the
most outer Adam’s ring, which lies very near the Neptune’s Roche limit. This ring is
noticeably clumped in places and from Earth we can only observe partial arcs of the ring.
The unseen parts are simply too thin to be detected.
Conclusion:
It has been nearly 400 years since the discovery of the planetary ring was first
made by Galileo. During those years of observation, astronomers were able to discover
new forms of these elegant structures.
We know that Saturn, Uranus, Jupiter and
Neptune all have individual ring systems, and all these systems differ from one another.
Even though we had come long way since Galileo’s discovery, many questions about
these rings still remain unanswered. Questions such as: “What is the origin of the rings,
and why are they still unique among known planetary ring systems, (Ellis D. Miner,
Randii R, 2007)” cannot be answered with any evidence or theory at this point.
Astronomers believe most of these questions especially that of Saturn’s will be answered
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when the data from the Cassini mission has been fully digested. Cassini data might be
able to prove the causes of the so called irregular structure, found primarily in the inner
half of Saturn’s B ring”(Ellis D. Miner, Randii R, 2007), as well as the radial structure in
Saturn’s D and C ring. On the other hand, it’s also possible that Cassini mission will raise
even more questions about Saturn. Moreover, we can confidently say that in the next few
decades, missions to gas giants will provide new hints about the physical properties of
these spectacular parts of the universe. Maybe finding a ring in exo-planetary systems
might able answer some of these questions.
Although there are still many unanswered questions about the planetary rings,
many of the previously unanswered ones have been resolved, and are continuing to be
answered through intelligent observations and analyses, and we can expect to find out
more and more dynamic and interesting fasts about these spectacular objects.
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References:
Beatty, J.K. (1999). The New Solar System (4th Ed.). Cambridge: Cambridge
University Press
Chaisson, E. (2005). Astronomy Today (5th Ed.). Addison-Wesley
Ellis D, Miner (2007). Planetary Ring Systems. Praxis
Esposito, L. (2006). Planetary Rings (1st Ed.). Cambridge: Cambridge University
Press
NASA 2008. http://www.nasa.gov/
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